During growth, the increase in muscle PCSA in rats and rabbits is accompanied by an increase in aponeurosis length and width (Woittiez et al., 1989; Willems and Huijing, 1992; Böl et al., 2017; Siebert et al., 2017), accommodating the greater fCSA. Tendons of rats and rabbits also increase in length and thickness (Alder et al., 1958; de Koning et al., 1987; Woittiez et al., 1989; Böl et al., 2017; Siebert et al., 2017). The increase in tendon CSA occurs by the increase in CSA of tendon fibers (Woittiez et al., 1989; Nakagawa et al., 1994; Nakagawa, 1996). The longitudinal and radial tendon growth affects their mechanical properties, such as tensile strength, and stiffness (Haut and Little, 1972; Walker et al., 1976; Woo et al., 1980; Shadwick, 1990; Nakagawa, 1996). However, tendon and muscle belly do not grow proportionally. The growth rate of tendons and aponeuroses are muscle-specific (Böl et al., 2017; Siebert et al., 2017). As the growth ratio differs considerable between muscles, even from the same species, the tendon-muscle belly or fascicle length ratio differ between muscles and are dependent on the growth stage (Böl et al., 2017; Siebert et al., 2017). For example, the GM and GL of rabbits showed high tendon-muscle fascicle length ratios 3weeks after birth, the ratio had almost doubled by the age of 14weeks (Siebert et al., 2017). This was explained by a lack of longitudinal muscle fascicle growth and a large increase in aponeurosis, as well as tendon length during growth. In contrast, for tibialis anterior muscle of rabbits at the age of 3weeks, a low tendon-muscle fascicle length ratio was shown, which was slightly decreased by the age of 14weeks (Siebert et al., 2017). The tibialis anterior showed a linear increase of the fascicle lengths during growth, while the growth ratio of the tendon and aponeurosis were low.

Mechanical loading of skeletal muscle may occur by (active) contraction at high resistance or by applying external tensile forces onto the muscle. Different models cause high mechanical loading and stimulate skeletal muscle hypertrophy, for example, weightlifting and synergist denervation. In addition, mechanical loading of muscle may also originate from surrounding muscle tissue via myofascial force transmission (Huijing, 2009). Muscle fiber hypertrophy in the flexor carpi radialis muscle has been reported in cats after a period of weight lifting which was accompanied by an increase in power generating capacity (Gonyea, 1980). Increased load, due to resection of synergistic muscles (soleus and GM) showed that muscle fibers from the plantaris muscle increased their CSA (Vaughan and Goldspink, 1979). Van der Meer et al. (2011) showed that denervation of the plantaris synergists muscles caused a considerable addition of myonuclei, which was followed by hypertrophy in the plantaris of young-adult and old rats. As a result of increased CSA, caused by increased mechanical loading, passive force increases as well, and thus affects the length-force relationship of the muscle.

The effects of activity on muscle length seem to be highly dependent on the length at which the muscle is maintained or the length range over which the muscle is operating. Low frequency stimulation of the low pennate soleus muscles of rabbits (i.e., 10–13 degrees PA (Siebert et al., 2017; Papenkort et al., 2020)) showed increases in their number of sarcomeres in series only when the muscles were kept at high length, the opposite occurred at short length (Williams et al., 1986). The reduction of the serial sarcomere number was higher than that of muscles immobilized at short muscle length without activation. These findings indicate that the serial sarcomere adaptation is enhanced by contractile activity. Moreover, activation seems to prevent connective tissue accumulation caused by immobilization (Williams et al., 1988).

In addition to morphological changes, activation seems to affect muscle phenotype. For instance, low-frequency muscle stimulation was reported to stimulate the adaptation towards a slow-contracting muscle (i.e., higher amount of slow muscle fibers; Pette and Vrbová, 1999). This should be considered when applying as a treatment since these fibers have a lower power output and smaller fCSA. Specific consequences of this change in phenotype, for muscle force in parallel fibered muscles and muscle length in pennate muscles are yet not known.

In summary, based on the data of different animal models, it is concluded that mechanical overloading of muscle leads to muscle fiber hypertrophy (i.e., increase of the fCSA and PCSA) in both parallel fibered and pennate fibered muscles. However, longitudinal muscle growth, due to increased muscle activity has only been shown in parallel fibered muscles when placed in a lengthened position. Moreover, the activity as a sole intervention regulates the addition and subtraction of sarcomeres in series depending on the muscle length during muscle activity.

To the best of our knowledge, there are no studies showing the effects of isometric contraction training on muscle function or morphology of animal muscles. Therefore, only the effects of concentric and eccentric exercise interventions on muscle growth of animal muscles are summarized in the following sections.

Concentric contractions are characterized by muscle shortening while force is actively produced. Uphill walking that is characterized by concentric activity at short muscle length, and promotes the loss of serial sarcomeres in the vastus lateralis (VL) and intermedius of rats (Butterfield et al., 2005). Furthermore, Lynn et al. (1998) showed that the vastus intermedius of rats running uphill had less sarcomeres in series than that of rats running downhill. Moreover, the range of the torque-angle curves of the animals performing concentric exercise was smaller than that of animals which performed eccentric exercise.

Although these studies reported slightly different results, it seems that animal hind limb muscles reduce their number of sarcomeres in series within 5 to 10days after starting concentric exercises at short muscle length.

Eccentric contractions are characterized by MTC lengthening while muscles are actively exerting a resistance to lengthening. Effects of eccentric contractions are determined by the uniformity of the muscle architecture and the length range over which the muscle is eccentrically contracted.

Eccentric exercise at short muscle length was shown to cause only a regional addition of sarcomeres in series in the tibialis anterior (TA), but no addition in the extensor digitorum longus of rabbits (Koh and Herzog, 1998; Butterfield and Herzog, 2006). Moreover, downhill running with a 20° decline did not increase the number of sarcomeres in series in the soleus and extensor digitorum longus in rats (Chen et al., 2020). Neither did downhill running with a 14° decline in the GM, vastus medius and lateralis in mice (Morais et al., 2020). In contrast, the vastus intermedius (low PA) of rats who ran downhill with a 16° decline on a treadmill showed a substantial increased number of sarcomeres in series compared to sedentary rats and rats that ran uphill (i.e., concentric contractions; Lynn and Morgan, 1994). Similar results were found by Butterfield et al. (2005) in the vastus intermedius and lateralis. These results suggest that a non-uniform muscle architecture causes the lack of adaptation to eccentric exercise, as the non-uniformity results in differences in local strains.

Furthermore, eccentric exercises performed at high muscle length of the TA of rabbits did not affect the number of serial sarcomeres in the lateral muscle regions, which was similar to control muscles (Butterfield and Herzog, 2006). Whereas in the medial, central superficial, and deep regions, the number of serial sarcomeres increased.

Eccentric contractions performed over the full muscle length range of the TA resulted in an increased number of sarcomeres, whereas contractions performed over a small muscle length range reduced the number of sarcomeres in series compared to control muscles (Butterfield and Herzog, 2006).

Eccentric exercises also stimulate the addition of parallel arranged sarcomeres within the muscle fibers resulting in increases in both fCSA and the PCSA, moreover, when applied to the muscle at long lengths showed higher increases in PCSA than those performed at smaller muscle lengths (Butterfield and Herzog, 2006). Eccentric exercises improve the length force-characteristics by improving both the maximum force and length range of active force exertion.

As in muscles with a low degree of pennation (i.e., parallel fibered), changes in optimum muscle length are only achieved by changes in the number of sarcomeres in series. Immobilization in a lengthened position (i.e., full dorsi flexion), results in a considerable addition of serial sarcomeres (Tabary et al., 1972; Williams and Goldspink, 1973; Williams and Goldspink, 1978; Soares et al., 2007), ensuring that the optimum muscle length is attained in the imposed immobilized joint position.

Lengthening immobilization of mouse soleus muscle was shown to induce an increase of the optimal muscle force and a shift of the optimum length to longer muscle lengths. Moreover, immobilized and control muscles showed similar passive length-force curves (Williams and Goldspink, 1978). In addition, lengthening immobilization of the soleus, GM and tibialis anterior muscles in rats leads to hypertrophy (Coutinho et al., 2004). Lengthening immobilization of the rabbit extensor digitorum, tibialis anterior and soleus muscles stimulates the IGF-1 expression within these muscles. IGF-1 is an anabolic growth factor, stimulating muscle hypertrophy by enhancing the rate of protein synthesis and inhibiting protein degradation (Yang et al., 1997; Glass, 2005). These results suggest that lengthening immobilization increases the PCSA of immobilized muscles.

At young age, in pennate rat muscles, optimum length is increased by both an increase in the number of sarcomeres in series and PCSA (Heslinga and Huijing, 1993), while in young adult rats the increase in optimum length is mainly due to an increase in PCSA (Heslinga et al., 1995). However, the effects of long length immobilization in those muscles have not been widely studied in animals. Though, there is evidence that pennate muscles immobilized in a lengthened position add sarcomeres in series and reduce their sarcomere lengths (Heslinga et al., 1995).

Culture of single fiber (iliofibularis muscle of Xenopus laevis) at low strain, comparable with short muscle length immobilization in vivo, did not show a reduced number of sarcomeres in series or atrophy. Immobilization at high strain did not show the expected trophic response (in neither longitudinal nor radial direction; Jaspers et al., 2004). The lack of the expected trophic adaptation suggests that either mechanical interaction with neighboring (muscles)-fibers is needed to trigger adaptation of muscle fiber size (Jaspers et al., 2006) or other factors in vivo (e.g., growth factor or cytokines) differ in vivo from that in vitro.

Immobilization in a lengthened muscle position in young (1week of age mice; 4weeks old rabbit) muscles resulted in a lack of increase in sarcomeres in series, also the length of the sarcomeres remained unchanged (Tardieu et al., 1977; Williams and Goldspink, 1978). Moreover, tendon length increased such that the mechanical stimuli (strain and force) on the sarcomeres remained low (Tardieu et al., 1977; Williams and Goldspink, 1978; Blanchard et al., 1985).

Studies investigating the effects of rabbit muscle immobilization at an extended length in combination with electrical stimulation showed increases in muscle length by an addition of serial sarcomeres after only 4days when compared to non-stimulated control muscles or to muscles which were only immobilized muscles (Cotter and Phillips, 1985; Williams et al., 1986). Immobilized soleus muscles of cats in extended position increased their number of sarcomeres in series, irrespectively whether the muscle was innervated or not, however, the rate of increase in number of sarcomeres in series was lower in denervated muscle (Goldspink et al., 1974). These results suggest that neural drive is not necessary for longitudinal muscle adaptation and that the number of sarcomeres in series may be regulated predominantly by passive force.

Furthermore, young denervated cat soleus muscle (pennate muscles) immobilized in an extended position had serial sarcomere addition and less tendon length growth than in innervated muscles (Blanchard et al., 1985). Treatments aiming to lengthen muscles or to increase the range of force exertion in children with neuromuscular disorders may benefit from such effects.

Stretching is often defined as a form of physical exercise that includes several sets of stretching stimuli applied to target muscles during a relative short amount of time to target muscles. Static passive stretching has been found to slightly increase the number of sarcomeres and the fCSA in a fully extended soleus muscle of Wistar rats (Coutinho et al., 2004). A study with incremental static stretching of the latissimus dorsi of rabbits during 3 weeks (5% above the optimum length every week), resulted in an increase in serial sarcomere number, muscle mass, as well as fCSA of type 1 fibers (Cox et al., 2000). The collagen content, on muscular level, was also increased, however, returned to baseline values when the stretch was maintained for 6weeks. The tetanus force and the maximal muscle power decreased. This latter might be explained by the shift in MHC muscle fiber composition from fast fatigable towards more slow twitch fibers. Moreover, the fCSA of type I fibers increased by 30%, and the fCSA in type 2B fibers decreased slightly. The adaptations on collagen content are opposite to those of muscles subjected to electrical stimulation (Williams et al., 1988), and may emphasize the importance of activity to prevent an increase of collagen within the muscle.

Intermittent stretching is characterized by holding a muscle under stretch for a certain period interrupted by short breaks in which no force is applied. Passive intermittent static stretching of the soleus muscle of rabbits increased the number of sarcomeres in series and muscle mass (de Jaeger et al., 2015). No changes were observed in the number of sarcomeres or muscle mass of the gastrocnemius and plantaris muscles. The passive torque- ankle angle relationship and muscle stiffness remained unchanged. Moreover, intermittent stretching prevents the loss of sarcomeres in series and loss of range of force exertion in muscles immobilized in a shortened position (Williams, 1990). These results suggest that the effects of static stretching are dependent of the muscle architecture.

To date, no animal studies are known that investigated the effects of passive stretching combined with enhanced contractile activity on serial sarcomere addition. However, to the best of our knowledge, there is only one study that assessed the effects of stretching combined with enhanced contractile activity on muscle PCSA (Stauber et al., 1994). This study showed that muscle hypertrophy was dependent on stretch velocity. Slow stretching of soleus rat muscles (10mm/s) resulted in increased muscle mass and mean fCSA. In contrast, fast stretching (25mm/s) of the soleus induced an increase in muscle mass, while fCSA was reduced and connective tissue content was increased. The reduced muscle fiber size in combination with the increase in connective tissue suggests that fast stretching causes muscle fiber injury, an increase in variation in muscle fiber diameter, and concomitant fibrosis. These results indicate that slow velocity stretching increases muscle size and likely improves muscle force generating capacity, whereas fast stretching is not beneficial for muscle integrity and force generating capacity. Further studies are required to investigate the underlying mechanisms of muscle fiber injury caused by fast stretching, and the possible implications for human muscles.

In accordance with maturational processes in animals, human muscle growth is driven by bone elongation and related increases in body mass (Morse et al., 2008; Benard et al., 2011). Since human muscles already contain the full adult complement of muscle fibers by the time of birth (Carlson, 2019), existing muscle fibers increase in length and grow in diameter in proportion to the growth of the body. These growth mechanisms may be different from the onset of the pubertal age since sex hormones start to contribute, which results in a pronounced rise of whole-body muscle mass and strength during adolescence (Round et al., 1999). After adolescence, adult values are attained (van Praagh and Dore, 2002). These adaptations are also reflected in both increases in muscle CSA (Deighan et al., 2006) and muscle strength with age (Morse et al., 2008; Pitcher et al., 2012). Before puberty, increases in muscle force generating capacity during growth occur in parallel with increases in muscle CSA, whereas increases in strength exceed those in muscle CSA after puberty (see van Praagh and Dore, 2002 for summary).

Studies on gross muscle morphology of pennate muscles (knee extensors and plantar flexors) further reported smaller fascicle and muscle lengths, muscle volumes and PCSA in children when compared to adults (Morse et al., 2008; O’Brien et al., 2010b). Further investigations showed that increases in muscle volume likely result from an increase in fascicle length and PCSA (O’Brien et al., 2010b; Benard et al., 2011). Therefore, during childhood, an addition of sarcomeres in series and in parallel as reported for pennate animal muscles seems to occur. Besides, there is still no conclusive evidence about alterations of the fascicle PA with age (Binzoni et al., 2001; O’Brien et al., 2010b; Benard et al., 2011).

During pubescence, relatively higher gains in body mass than in body height and tibia length result in higher loads on the musculoskeletal system (Benard et al., 2011; Weide et al., 2015). Today, there is evidence that longitudinal growth of pennate muscles (e.g., GM muscle) during pubescence seems to be mediated by increase in PCSA solely (Weide et al., 2015) rather than by longitudinal fascicle growth, which is likely due to increased serum levels of growth factors (Rogol et al., 2002).

Although neglected for decades, recent studies in humans demonstrated that tendons also adapt to biological maturation processes by alteration of their properties (Kubo et al., 2001b; O’Brien et al., 2010a; Waugh et al., 2012; Mogi et al., 2018). Age-related increases in body mass and force generating capabilities during maturation are associated with an increase in tendon loading that stimulates adaptations in tendinous tissue and changes the tendon’s mechanical properties (Waugh et al., 2012).

Several studies have shown that tendon length and CSA increase during childhood until adulthood (O’Brien et al., 2009; Waugh et al., 2012; Mogi et al., 2018). It seems that before the onset of the adolescent growth spurt (i.e., peak height velocity, PHV), longitudinal growth of the MTC results from increases in both the muscle belly and tendon lengths (O’Brien et al., 2010a; Mogi et al., 2018). After PHV, the longitudinal MTC growth results from increases in muscle length only (Mogi et al., 2018). Increases in tendon CSA mainly appear before the adolescent growth spurt (Mogi et al., 2018), whereby the CSA is highly variable and may even decrease around PHV (Neugebauer and Hawkins, 2012). Moreover, the relative Achilles tendon CSA (to body mass) was reported to be greater in (pre-) pubertal children than in adults (Kubo et al., 2014). Both findings indicate the occurrence of critical phases of tendon development during adolescence (see Mersmann et al., 2017 for review).

Tendons of pre-pubertal children seem to be less stiff when compared to those of adults, which was reported for both the Patellar tendon (O’Brien et al., 2010a) and tendinous structures (i.e., combined deep aponeurosis and distal tendon (Kubo et al., 2001b)). As reported for animals, tendon stiffness increases during childhood (Waugh et al., 2012). Consequently, the difference in tendon stiffness between children and adults disappears during adolescence (Kubo et al., 2001b). It could be shown that a significant increase in both Achilles tendon stiffness and Young’s modulus occurred at and/or around the PHV with no further changes thereafter (Mogi et al., 2018). Therefore, it is assumed that tendon stiffness does not increase linearly from child- to adulthood but predominantly around the PHV due to alterations in the material property since tendon CSA did not change during that growth phase. This finding may be supported by the observation that the inner tendon core is highly metabolically active until adulthood (Heinemeier et al., 2013; Gumucio et al., 2015). Adaptations in adult tendon may predominantly occur in its outmost layers, evoking tendon CSA increases (Gumucio et al., 2015).

There is evidence that concentric-only strength training (CT) leads to increases in muscle strength and enhanced muscle size as reported for several muscles (e.g., VL, quadriceps, biceps brachii) after 8–12weeks of training (Higbie et al., 1996; Housh et al., 1996; Seger et al., 1998; Farthing and Chilibeck, 2003; Moore et al., 2012; Franchi et al., 2014; Farup et al., 2014b; Quinlan et al., 2021). For instance, Franchi et al. (2014) found VL muscle volume increases of 8% after 10weeks of CT and increases in VL ACSA in the muscle’s mid and distal portions (11 and 2%, respectively). Farup et al. (2014b) further reported larger enhancements when the VL CSA was measured at mid-muscle-level. In contrast, no significant change in mean ACSA of the elbow flexors was observed after 12weeks of CT (Vikne et al., 2006). Interestingly, both Franchi et al. (2014) and Seger et al. (1998) found regional hypertrophy in the central region of the VL ACSA (11%) and total quadriceps ACSA (3.4%) after CT, respectively. Increases in quadriceps muscle volume were also observed by Quinlan et al. (2021) after only 4weeks of submaximal CT performed for 8weeks with a leg press in young (mean age: 23.3years) and older males (mean age: 69.1years).

Concerning changes in fascicle length and PA, only a few studies are available. Despite observed increases in VL fascicle lengths of 2% after 4weeks of CT (Franchi et al., 2015) and around ~6% after 10weeks of CT (Blazevich et al., 2007; Franchi et al., 2014), decreases in fascicle lengths of 11.8% and~11.7% were reported for the biceps femoris and supraspinatus after the performance of isokinetic CT (Kim et al., 2015; Timmins et al., 2016). Regarding changes in fascicle PA after CT, studies consistently showed increases in the VL (ranging from 13–30%), biceps femoris (21.1%) and supraspinatus (~22.7%; Blazevich et al., 2007; Franchi et al., 2014; Kim et al., 2015; Timmins et al., 2016; Quinlan et al., 2021). In this context, Quinlan et al. reported significant increases in VL PA already after 2weeks of submaximal CT in both young and older males. Note that the magnitude of the observed changes was higher in the younger compared to the older group.

Regarding muscle fiber hypertrophy, increases in VL type 1 fiber area of 12.5% were reported after 12weeks of isotonic CT that were even higher with protein supplementation (Farup et al., 2014a). VL type 2 fiber CSA increased by 25.7% exclusively in the subjects who received the protein supplementation (Farup et al., 2014a). In contrast, no change in VL fiber area was observed following a cycle ergometer training for 8weeks (LaStayo et al., 2000) and only slight non-significant increases were displayed after 12weeks of progressive isokinetic CT (Hortobagyi et al., 1996).

Seger et al. (1998) investigated the effects of CT on VL muscle fiber CSA of fibers type 1, 2A, and 2B: the absolute areas tended to increase (9.9–23.5%). Concerning the elbow flexors, no difference in fiber type 1 proportion in the biceps brachii was found after 12weeks of CT (Vikne et al., 2006). Despite that, a significant reduction in 2X fibers of 2.8% and a trend for reduction of the 2AX fibers was observed. Please note that both fiber types are subtypes of the fast twitch fibers. They are classified based on differential myosin heavy chain gene expression and hybrid myosin heavy chain gene expression (Talbot and Maves, 2016).

Only a few studies have yet examined the effects of CT on tendon properties and tendinous structures. After 12weeks of CT, Patellar tendon stiffness and Young’s modulus were reported to increase by 75.7% (Malliaras et al., 2013). Similarly, Patella tendon stiffness and Young’s modulus were already significiantly increased after 4weeks of CT in young as well as older males (Quinlan et al., 2021). Quinlan et al. further showed that while the observed changes in tendon mechanical properties stayed constant in the young males, they continued to increase from week 4 to week 8 in the older males suggesting a delayed response. Accordingly, a significant decrease was observed in tendon elongation and strain by Malliaras et al. (2013). In contrast, no change in Achilles tendon stiffness was reported after 6weeks of a heel-drop regimen (Morrissey et al., 2011). In the previous studies, the Patella tendon CSA and length remained unchanged (Malliaras et al., 2013; Quinlan et al., 2021), while increases in Patella tendon CSA (+14.9%) were found after a 12-week CT performed with whey protein supplementation (Farup et al., 2014b).

In summary, studies that investigated the effects of CT mainly showed its potential to stimulate muscle hypertrophy in both upper and lower limb muscles at variable rates and locations. However, no clear conclusion can be drawn about the underlying mechanisms. Franchi et al. (2014) suggested that CT leads to hypertrophy mainly through addition of sarcomeres in parallel. This idea might be supported by the findings of larger muscle PA observed after CT in several studies and evidence of enhanced fiber areas. Furthermore, CT also stimulates an increase in tendon stiffness likely due to changed tendon material properties. As discussed by Quinlan et al. (2021), muscle and tendon adaptations may occur simultaneously to maintain the efficacy of the whole MTC irrespective of the contraction mode since synchronous adaptations were also found after eccentric-only strength training (see subsequent section).

Similar to the effects of CT, eccentric-only strength training (ET) also leads to increased muscle strength and muscle hypertrophy. For instance, increases in muscle volumes were reported for the VL, total quadriceps muscle, and separate quadriceps muscles after 10weeks of isokinetic and overload ET (Higbie et al., 1996; Norrbrand et al., 2008; Franchi et al., 2014; Quinlan et al., 2021). Similarly, increases in muscle volume, ACSA, and PCSA of different hamstring muscles were found after performing the Nordic hamstring exercise (NHE; Bourne et al., 2017; Seymore et al., 2017) or a hip extension exercise (Bourne et al., 2017) for several weeks. Elbow flexor ACSA increased by 11% after 12weeks (Vikne et al., 2006) and biceps brachii muscle CSA by 6.5% after 9weeks of ET (Moore et al., 2012). Increases in muscle thickness were also reported for different muscles (Farthing and Chilibeck, 2003; Duclay et al., 2009; Baroni et al., 2013; Guilhem et al., 2013; Cadore et al., 2014; Franke et al., 2014; Leong et al., 2014; Kim et al., 2015; Timmins et al., 2016; Alonso-Fernandez et al., 2018). Interestingly, the comparison of the effects of ET performed with submaximal and supramaximal intensity (80 and 110% of the concentric 1RM, respectively) showed no different effect on biceps brachii thickness (Krentz et al., 2017). By comparing the effects of CT and ET on muscle ACSA, only Higbie et al. (1996) and Vikne et al. (2006) found a superior effect for the latter. Further information on the comparison can be found in a previous review (Franchi et al., 2017).

As for CT, a location-specific response in muscle hypertrophy after ET was reported (Seger et al., 1998; Franchi et al., 2014). However, after ET, significant increases in muscle ACSA occurred in the distal and not the central portion of the muscles (Seger et al., 1998; Franchi et al., 2014, respectively).

Significant increases in VL and rectus femoris fascicle lengths were already observed after 4weeks of ET (Baroni et al., 2013; Franchi et al., 2015; Quinlan et al., 2021) with similar increases in VL fascicle lengths (3.1%) observed after 10weeks of ET (Blazevich et al., 2007). Even higher increases (12–17.6%) were found after 10–12weeks of ET (Baroni et al., 2013; Franchi et al., 2014). Guilhem et al. (2013) reported no change in VL fascicle lengths after a 9-week isoload or isokinetic ET. In contrast, two volume-matched ET regimen (isokinetic vs. isotonic) performed for 6weeks resulted in longer fascicle lengths (14.4 and 14.7%, respectively) in another study (Coratella et al., 2015). Longer fascicle lengths were also reported for the GM (Duclay et al., 2009) and the biceps femoris (Potier et al., 2009; Timmins et al., 2016; Bourne et al., 2017; Alonso-Fernandez et al., 2018; Presland et al., 2018) after different ET interventions. Besides increases in fascicle lengths, no superior effect of high volume NHE training could be found compared to low NHE training (Presland et al., 2018). Contrastingly, no change in biceps femoris fascicle length was observed by Seymore et al. (2017) likely caused by training at shorter hamstring muscle length when compared to other studies. Similarly, supraspinatus fascicle length did not change after 8weeks of isokinetic ET (Kim et al., 2015). In summary, the mentioned studies support the idea that ET leads to longer fascicle lengths. However, please note that the results concerning biceps femoris fascicle length changes reported after some ET interventions should be considered with caution due to methodological issues regarding the extrapolation methods used to assess fascicle lengths (Franchi et al., 2020; Sarto et al., 2021), i.e., overestimations and underestimations of fascicle length indicating, among other things, a subject dependent accuracy of the methods (Franchi et al., 2020).

Concerning the PA, the reported changes after ET deliver no clear trend. Varying increases in VL fascicle PA (+3% to 21.4%), rectus femoris (+31%) and GM PA (+7.6%) have been reported after 4–10weeks of different ET interventions (isokinetic, isoload, cycling; Blazevich et al., 2007; Duclay et al., 2009; Guilhem et al., 2013; Franchi et al., 2014, 2015; Leong et al., 2014; Quinlan et al., 2021), whereas others observed no changes (Potier et al., 2009; Baroni et al., 2013; Coratella et al., 2015) or even a decrease in PA in different muscles (Timmins et al., 2016; Alonso-Fernandez et al., 2018; Presland et al., 2018).

In contrast to their findings on the effects of CT, several authors reported significant increases in VL fCSA after 8–12weeks of ET (Hortobagyi et al., 1996; LaStayo et al., 2000). Similarly, mean CSA increases of both type 1 and 2A fibers of the biceps brachii were also observed (Vikne et al., 2006). These findings indicate that eccentric loading may favor in particular the increase of type 2 fiber CSA (Franchi et al., 2017). In addition, independent of protein supplementation, similar increases in VL fiber type 1 area (on average+15.7%) were reported after ET when compared to CT (Farup et al., 2014a). In contrast, only Mayhew et al. (1995) observed a greater change in fiber type 2 CSA after CT compared to ET, and Seger et al. (1998) observed a decrease in VL fiber type 2A area (−14.9%) after 10weeks of eccentric isokinetic knee extensor training.

Guex et al. (2016) assessed the effects of 3-week isokinetic ET at slow velocity (30°/s) on biceps femoris muscle architecture and hamstring function when performed with the hamstrings at short and long muscle length. Although no group-by-time interactions could be reported, higher effect sizes were found for changes in the long muscle length training group. This study may imply that the training of range of motion or the muscle excursion range might be important for fascicle length adaptations. More details about the underlying mechanisms can be found in the subsequent section. In contrast, Sharifnezhad et al. (2014) did not find a superior effect of isokinetic ET performed at longer muscle length of the VL or, in agreement with Krentz et al. (2017), higher intensity of the training after 10weeks (Sharifnezhad et al., 2014). However, the authors reported an increase in fascicle length of 14% after ET only when performed with a high (240°/s) but not a low fascicle lengthening velocity (90°/s).

Furthermore, there is evidence that ET leads to increases in tendon stiffness (Duclay et al., 2009; Malliaras et al., 2013; Geremia et al., 2018; Quinlan et al., 2021) and Young’s modulus (Malliaras et al., 2013; Geremia et al., 2018; Quinlan et al., 2021). In contrast, some studies have reported a lack of significant change (Mahieu et al., 2008) or even a decrease in stiffness (Morrissey et al., 2011). Since loading intensity seems to be key for tendon adaptation (e.g., Arampatzis et al., 2007, 2010), the differences between studies might be related to the applied loads or the duration of the training programs (Bohm et al., 2015). However, Quinlan et al. (2021) did not find a difference in neither absolute nor percentage change in tendon stiffness and Young’s modulus between ET and CT at any training duration (after 2, 4, 6, and 8weeks of training) or between age groups (younger vs. older adults). Their findings support the idea that the direction of strain applied to a tendon may not be decisive to evoke changes in its properties. Finally, increases in tendon CSA (+9.7%) were reported after 12weeks of ET (Farup et al., 2014b) with even higher increases (+15%) observed after 8weeks of high-load ET (Geremia et al., 2018).

Since isometric contractions allow for a controlled application of force within pain-free joint angles (Oranchuk et al., 2019), isometric strength training (IT) is often used in clinical settings. Therefore, it is important to know whether relevant adaptations in muscle-tendon properties occur.

Recent studies provided evidence that also IT leads to significant muscle hypertrophy. For instance, increases in muscle volume were reported for the quadriceps (Kubo et al., 2006b; Noorkõiv et al., 2014; Balshaw et al., 2016). Increases in VL muscle thickness were observed after 8weeks of IT depending on muscle site and training muscle length (Alegre et al., 2014) and after 14weeks of IT (~16.5%) performed with varying periodization (Ullrich et al., 2015). Similarly, increased volume of the triceps brachii (+12.4%) was reported after 10weeks of high-intensity IT (Kanehisa et al., 2002). Noteworthy, explosively performed IT did not result in significant changes in quadriceps muscle volume (Balshaw et al., 2016).

Two studies (Alegre et al., 2014; Noorkõiv et al., 2014) indicated a superior effect of IT performed at long muscle length (LML) to enhance VL muscle volume and thickness in comparison to training at short muscle length (SML). A shift of peak torque production to the training muscle lengths was also observed (Alegre et al., 2014), with a greater change found in the LML training group. As discussed by Alegre et al. (2014), the mechanisms underlying superior hypertrophic effects of training at LML could be influenced by several factors. For instance, IT performed at LML may have caused greater damage, therefore, greater adaptation by placing the muscle fascicles under greater stretch in comparison to training at SML. Furthermore, smaller moment arms resulting from greater flexed knee angles may induce greater mechanical fascicle stress. Noorkõiv et al. (2014) further highlighted the role of upregulation of mechanosensitive signaling mechanisms and augmentation of calcium signaling cascades that may only become critical when the target muscle was activated at long length. Finally, training-dependent releases of insulin-like growth factor or mechano growth factors may also be related to training muscle length. Furthermore, Oranchuk et al. (2019) deduced that training intensity seems to have a small effect on the hypertrophic responses, though it may have to reach a certain threshold (>20% of maximum voluntary contraction). In general, higher training volume had a superior effect on the hypertrophic responses when IT is performed.

Information about the effects of IT on muscle fascicle lengths and PA is scarce and inconsistent. Increases in VL fascicle length have been reported at the mid portion of the femur (+5.6%) after IT at SML, whereas increases in the distal portion (+5.8%) were observed after training at LML (Noorkõiv et al., 2014). This result may indicate specific adaptations in fascicle lengths regarding training muscle length chosen during IT. Furthermore, increases in VL fascicle lengths of ~16.5% have been observed after IT conducted with the knee joint at 70° (Ullrich et al., 2015), which lies between the angles chosen in the study of Noorkõiv et al. In contrast, Alegre et al. (2014) did not find changes in fascicle lengths after IT at LML and other muscles than the VL. Significant increases (+11.7%) in VL PA were only found after IT at LML (Noorkõiv et al., 2014) and in the long head of the triceps brachii (~15.5%) after high- and low-intensity IT (Kanehisa et al., 2002). No change in PA was observed after training at SML (Noorkõiv et al., 2014) and in the study of Ullrich et al. (2015).

Increases in VL tendon-aponeurosis stiffness were observed after 12weeks of IT at LML (Kubo et al., 2006b; Massey et al., 2018) with no significant change after training at SML (Kubo et al., 2006b). It was assumed that the higher mechanical stress applied in the LML training (due to shorter moment arm length) might have caused the differences between groups. Increased Patella tendon stiffness (+20% and+16%) and Young’s modulus (+22% and+16%) were observed after explosive IT and IT, respectively, with no significant differences between the interventions (Massey et al., 2018). VL aponeurosis hypertrophy (+7%), assumed to provide an enlarged attachment area for increased muscle CSA, was only found after non-explosively performed IT, which was in line with the greater volume observed in this study. In agreement, increases in Achilles tendon-aponeurosis stiffness (+36.0%), elastic modulus (~18.2%) and region-specific hypertrophy were found after 14weeks of high-strain, but not low-strain IT (Arampatzis et al., 2007), supporting the results of previous studies (Kubo et al., 2001a, 2006a). Arampatzis et al. (2010) also showed that a higher tendon strain duration per contraction may evoke superior adaptational responses. No change in Achilles tendon CSA, but increases in tendon-aponeurosis stiffness and Young’s modulus were also found after 12weeks of both isokinetic IT performed with short and long rest intervals between isometric repetitions performed at high-strain (Waugh et al., 2018). However, tendons trained with IT and shorter rest duration led to a reduction in collagen organization in the same study.

To gain information about the effects of long length immobilization, studies that reported immobilization of the knee joint in either 60 or 70 degrees of flexion and/or ankle joint angles in 90 degrees flexion were summarized.

In accordance with the results of short length immobilization, reductions in isometric force of −13% up to −28% after 7days (Deschenes et al., 2009, 2012) and~−25.5% after 14days (Mitchell et al., 2018) of unilateral lower limb suspension (ULLS) immobilization could be found for the knee extensors. The isometric force of the plantar flexors declined by −10% after 14days of a similar ankle joint immobilization (Seynnes et al., 2008b). Furthermore, decreases in isokinetic strength and muscle performance variables (e.g., total work) were reported (Deschenes et al., 2009).

Plantar flexor muscle volumes decreased after 2weeks of ULLS (~−5.3%) with higher decreases (−7.7%) present after 23days (Seynnes et al., 2008b). Furthermore, reductions of both GL fascicle length and fascicle PA were observed, which became significant after 23days (−4% and−5%, respectively). This was accompanied by declines (−3%) of GL PCSA after 14days, which did not further decrease until 23days due to a faster decrease in fascicle length then muscle volume (Seynnes et al., 2008b). Similar to short length immobilization, immobilization at long lengths resulted in decreased PCSA of the VL, rectus femoris, and total quadriceps in men and women (~−6.2%, ~−3.3% and~−5.8%, respectively) after 2weeks of knee bracing (Yasuda et al., 2005). Declines in fCSA were also observed, with similar reductions reported by Oates et al. (2010). These declines seem to be smaller than those observed after 2weeks of short length immobilization of the knee extensors (Hvid et al., 2010). However, similar values were also reported (Hortobagyi et al., 2000).

In conclusion, long length immobilization seems to result in reductions of sarcomeres in series and in parallel in humans. Future studies are needed for clarification.

Concerning the effects of immobilization on tendon properties, it is assumed that joint positioning and therefore tendon length may play a role for the amount of deterioration with disuse (Maganaris et al., 2006). Tendons may be more vulnerable to material and structural deterioration at shorter than longer lengths (Maganaris et al., 2006), however, to the best of our knowledge, this has not yet been verified. Since information on the joint position was not always retrievable from the studies, this assumption could neither be verified nor falsified in this review.

Studies showed that both short-term (2–3weeks, e.g., Kubo et al., 2004; Reeves et al., 2005; de Boer et al., 2007; Shin et al., 2008; Seynnes et al., 2008a; Couppe et al., 2012) as well as long-term (12weeks, Reeves et al., 2005) immobilization may affect the tendon mechanical and material properties. Patellar tendon stiffness decreases of 9.8 to 19.7% after 2 (de Boer et al., 2007; Couppe et al., 2012), ~30.7% after ~3 (Kubo et al., 2000; de Boer et al., 2007) and 58% after ~12weeks (Reeves et al., 2005) of immobilization have been reported. Comparing the results regarding differences in knee joint immobilization angle (10 degrees vs. 30 degrees by de Boer et al., 2007; Couppe et al., 2012, respectively), it could be suggested that immobilization at shorter patellar tendon length led to higher declines in tendon stiffness. However, due to the high variability between studies (e.g., duration of immobilization), further results cannot support this hypothesis. Nevertheless, the results show that disuse mainly affects the material properties rather than tendon CSA (de Boer et al., 2007; Shin et al., 2008; Kinugasa et al., 2010; Couppe et al., 2012). Most studies (except for Kinugasa et al., 2010) reported unchanged tendon length and CSA after short-term (Kubo et al., 2004; de Boer et al., 2007; Shin et al., 2008; Couppe et al., 2012) and prolonged unloading (Reeves et al., 2005). It is assumed that the material changes might be caused by adaptations in the collagen fibers (see Kinugasa et al., 2010 for summary) as reported in animal studies (Vailas et al., 1988; Nakagawa et al., 1989). The resulting changes in tendon stiffness may negatively affect, for example, force transmission and the electromechanical delay (Kubo et al., 2000; Reeves et al., 2005; de Boer et al., 2007) since the muscle has to shorten further to stretch the tendon (Maganaris et al., 2006).

Based on the findings of Maganaris et al. (2006) who investigated the effects of chronic unloading due to paralysis and found reductions of patellar tendon CSA of 17%, it can be assumed that changes in the tendon morphology may only occur after years of disuse (lesion duration: 1.5 to 24years).

Stretching is often used in sports for injury prevention and to speed up the recovery of the athletes. In clinical practice it is a common treatment approach to counteract, for instance, muscle contractures. Despite evidence that prolonged stretching for several weeks can improve joint range of motion (e.g., Blazevich et al., 2014), only trivial effects for reductions in muscle stiffness were reported (Freitas et al., 2018). Furthermore, it is still not clear if this approach can evoke muscle growth.

Only recently, Nunes et al. (2020) reviewed current studies on the effects of static stretching with the aim to elucidate whether stretch training is capable to elicit muscle hypertrophy in humans. Out of 10 studies only 3 reported positive effects in specific muscle size parameters (Nunes et al., 2020). Mizuno (2017) investigated the effects of 8-week static stretching only and static stretching combined with electrical stimulation (see section 4.4.1) on GM muscle architectural parameters. In contrast to the control group, GM muscle thickness was significantly increased (+5.8%) in the static stretching group (n=12), whereas no change in PA could be found (Mizuno, 2017). Increases in gastrocnemii muscle thickness were also found by Simpson et al. (2017) after 6weeks of loaded stretch training. However, this result was questioned by other authors (Jakobi et al., 2018; Nunes et al., 2018). Additionally, increases in muscle fascicle lengths, with differing amounts according to measurement site (distal or central region of the muscle), muscle (GM vs. GL), and time period (e.g., baseline, day 5, week 6), were also reported (Simpson et al., 2017). Decreases in PA were found in the GL muscle at both sites at week 6 and 1-week post-training, whereas the PA was increased (+4.9%) in the GM muscle 1-week post-training when measured distally. Increases in biceps femoris fascicle lengths of 13.6% were found after 8weeks of high-intensity stretching training in the pilot study of Freitas and Mil-Homens (2015). The training was performed 5 times a week with continuous stretching for 450s, in which torque was increased every 90s until discomfort was reported. Decreases in PA (−15.1%) did not reach statistical significance. Despite a small sample size, this finding implies a potential of static stretching to induce longitudinal muscle growth if applied for a longer time period with a high intensity (i.e., non-rest protocol; Freitas and Mil-Homens, 2015). This is supported by a cross-sectional study of Moltubakk et al. (2018) who showed that ballet dancers, accustomed to regular stretching training over years, have longer gastrocnemius fascicle and Achilles tendon lengths compared to healthy controls. Sarcomerogenesis due to chronical stretch stimulus in VL fascicles of a 16-year-old girl following 8month of surgical femur lengthening was also shown by Boakes et al. (2007).

Furthermore, the idea that stretching intensity (i.e., high strain placed upon the muscle) might be a key factor to evoke adaptations on muscular level is also supported by findings of recent studies (e.g., Longo et al., 2021; Moltubakk et al., 2021; Panidi et al., 2021; Yahata et al., 2021). In contrast to Freitas and Mil-Homens (2015) and Panidi et al. (2021), Yahata et al. and Moltubakk et al. did not find changes in muscle architectural properties (i.e., fascicle lengths) when static stretching was performed either with a high stretching volume (6 sets of 5min/day, 2days/week, 5weeks, in total 18.000s) or for a longer period of time (24weeks; in total 40.320s of static stretching), respectively. As concluded by Yahata et al. (2021), very high volumes of static stretching training may not substitute muscle strain applied with low intensity.

We finally emphasize that the studies which reported hypertrophic effects all used an apparatus that may have ensured a high stretching (over-) load (Nunes et al., 2020). Therefore, using an apparatus/machine might be advantageous when compared to self-applied stretching by inducing a higher stimulus/intensity. However, also other factors, e.g., the total amount of or rest duration between stretches, may be important.

Studies that have investigated the effects of long-term static stretching on tendon properties are scarce, with limited evidence supporting no changes in tendinous tissues, at least when performed up to 6weeks. In this context, 30 sessions of overloaded stretch training did not alter Achilles tendon length or tendon thickness (Simpson et al., 2017). Additionally, Konrad and Tilp (2014) reported no change in passive Achilles tendon stiffness after 6weeks of plantarflexor stretching (Konrad and Tilp, 2014). In agreement, Kubo et al. (2002a) investigated the effects of 3-week static stretching (two times a day, five stretches for 45s, 15s rest between stretches) on the viscoelastic properties of human tendon structures (i.e., stiffness and hysteresis). The stretching training did not result in changes in stiffness but in significantly decreased hysteresis. Based on this finding, the authors concluded that static stretching does not affect the serial elastic component but may affect the connective tissue elements in parallel with the muscle fibers (i.e., the endomysium, perimysium, and epimysium).

Mizuno (2017) also investigated if static stretching combined with electrical stimulation might be superior in altering the GM muscle architectural properties when compared to static stretching alone. In the combined group, the muscle was contracted during the stretches as well as during the resting period (Mizuno, 2017). The authors reported a similar increase in GM muscle thickness for the static stretching and static stretching/stimulation group, therefore demonstrating no increased benefit of the combined approach.

There are a few studies that have investigated the effects of stretching on muscle hypertrophy when combined with strength training or strength exercises (e.g., Kubo et al., 2002b; Junior et al., 2017; Ferreira-Júnior et al., 2019). A brief overview of the results can be found in Nunes et al. (2020). The studies either showed reductions or no impairment of the hypertrophic responses when stretching was performed immediately before (Junior et al., 2017; Ferreira-Júnior et al., 2019) or separated from the strength training sessions (Kubo et al., 2002b). Still, no clear conclusions can be drawn.

Interestingly, it appears that loaded interset-stretching, i.e., stretching performed between the sets of strength exercises, may have a positive additional effect on strength and extensibility (Souza et al., 2013), and the occurring hypertrophic responses (Silva et al., 2014; Evangelista et al., 2019). For instance, Silva et al. (2014) reported greater increases in GM muscle thickness in the interest- stretching group after calf strengthening for 5weeks. Additionally, Evangelista et al. (2019) found greater changes in VL thickness and the summed thickness of 4 muscles when compared to the results of a traditional strength training program. Future work has to verify the additional benefit of interest stretches on hypertrophic responses.

Proprioceptive neuromuscular facilitation (PNF) stretching is commonly used with the aim to increase the joint range of motion and to lengthen the MTC (Sharman et al., 2006). Compared to static stretching, PNF further involves a static isometric (maximal) contraction of the stretched target muscle and/or a concentric contraction of the opposing muscle in order to lengthen the target muscle applied during the stretching procedure (Sharman et al., 2006).

Konrad et al. (2015) investigated the effects of a 6-week PNF stretching performed 5 times a week on muscle and tendon stiffness as well as GM muscle architecture and observed a slightly enhanced PA but no changes in muscle fascicle lengths after the intervention (Konrad et al., 2015). Furthermore, no changes in MTC and GM muscle stiffness appeared, whereas decreases in Achilles tendon stiffness were observed. In contrast, Mahieu et al. (2009) demonstrated no alterations in Achilles tendon stiffness after 6weeks of PNF stretching (Mahieu et al., 2009).

In summary, no clear statement about the effects of PNF stretching on muscle growth can be deduced yet.