An insult to the immature, developing brain before the age of two results in a condition clinically referred to as cerebral palsy (CP), characterized by motor impairment (Rosenbaum et al., 2007). Despite that the brain injury is non-progressive, motor function commonly deteriorates over time and a progressive contracture formation, i.e., a decrease in passive range of motion (pROM), is common (Hagglund and Wagner, 2011). For many children, surgical treatment is needed to restore and preserve musculoskeletal function. Clinical experience suggests that contracture formation is due to shortening/stiffening of the musculotendinous complex, as the range of motion of the joint often is practically normal when tendons of contracted muscles are cut during surgery. What causes the increased stiffness and shortness of the muscle is not known, but clinically we know that it affects joint biomechanics.

Children with CP generally have less skeletal muscle mass as compared to typically developing (TD) children (Barrett and Lichtwark, 2010). Recent reports have highlighted stunted growth in the lower limb as a contributing factor to contracture development (Gough and Shortland, 2012) and slowed growth rate has been detected as early as 15-months of age (Herskind et al., 2015). Several investigators have described increased passive stiffness of the calf muscle in children with CP and different tests have shown that the calf muscle is 22–120% stiffer in CP children compared to TD (Ross et al., 2011; de Gooijer-van de Groep et al., 2013; Geertsen et al., 2015). The process starts early and increased whole muscle passive stiffness in the calf has been described as early as 3 years of age (Willerslev-Olsen et al., 2013). Biochemical studies of muscle in CP have shown an increased content of intramuscular collagen (Booth et al., 2001), with an increased amount of connective tissue around fiber bundles i.e., a thickening of the perimysial extracellular matrix (de Bruin et al., 2014). The perimysium is considered to be in a physical continuum with the tendon (Passerieux et al., 2007). Thus, marked collagen deposition in the perimysium could potentially offer an increased resistance to passive stretch.

Less information is available on how skeletal muscle size correlates to joint biomechanics and muscle function for the upper limb. This is unfortunate as the clinical importance of accurate information on skeletal muscle function and biomechanical properties prior to for example tendon transfer surgery of the wrist is of great importance and a guide in clinical decision making. Therefore, we aimed to model the mechanical characteristics of the forearm flexors acting across the wrist joint in children with CP during passive stretch. Second, we aimed to correlate skeletal muscle size and biomechanics to age, body weight and strength.

No difference was seen between the groups with respect to age (CP 11.3 yrs ± 3.1 vs. TD 12.3 yrs ± 3.7, ns) and body weight (CP 46.8 ± 26.1 kg vs. TD 44.7 ± 14.9 kg, ns). FCR cross sectional area was 43% smaller in children with CP as compared to TD children (CP 0.84 cm² ± 0.38 vs. TD 1.47 cm² ± 0.75, p < 0.01). As expected, grip strength (10 s isometric contraction) was also significantly lower, −65% in CP as compared to TD children (CP 58.3N ± 32.1 vs. TD 167.5N ± 93.5, p < 0.01, Figure 2).

Skeletal muscle passive stiffness was increased 2-fold in CP as compared to TD (CP 4.65 ± 3.42 vs. TD 1.96 ± 0.62, p < 0.05) whereas skeletal muscle viscosity didn't differ significantly between the groups (CP 1.71 ± 1.22 vs. 1.27 ± 0.86, Figure 3).

Skeletal muscle size correlated to age (R² = 0.58, p < 0.01), body weight (R² = 0.92, p < 0.0001) and strength (R² = 0.58, p < 0.01) in TD children. Interestingly, the same relationship between skeletal muscle size did not correlate to age and body weight in children with CP, however still showing a significant correlation to strength (R² = 0.60, p < 0.05). Skeletal muscle size did not correlate to passive stiffness or viscosity in CP or in TD children (Figure 4).

This study shows that forearm flexor muscles in children with CP are stiffer as compared to TD children. Similar findings have been described for the lower limb, specifically the ankle flexors. Previous reports have estimated the passive tension to be 22–120% higher in CP for the ankle flexor muscles (Ross et al., 2011; de Gooijer-van de Groep et al., 2013; Geertsen et al., 2015). Thus, our finding of a 2-fold increase in skeletal muscle stiffness of the forearm flexors is comparable to data on weight bearing muscles of the lower limb. Information on the current status of the muscle is of great clinical importance when planning treatments, such as stretching, use of splints and surgical interventions e.g., tendon transfer surgery. Tendon lengthening, rerouting and transfer in the upper limb aim to balance the muscles of the wrist, so that the wrist can be maintained in a functional neutral position. All these surgical techniques have a component of dosing—how tight should the tendon transfer be, how long the tendon lengthening? This dosing is highly dependent on how stiff the transferred or lengthened muscle is. Long-term results are difficult to predict, as there often is a progression of contracture formation and stiffness. Abnormalities such as contracture and stiffness in even a few muscles will add yet another obstacle to smooth and efficient movements (Keenan et al., 2009; Kutch and Valero-Cuevas, 2011; Valero-Cuevas et al., 2015) which can progress very quickly in these children.

Spastic CP is characterized by a hyperexcitibility of the stretch reflex (Lance, 1980). Due to the fact that many children with spastic CP eventually develop skeletal muscle contractures, spasticity has been believed to be causative and clinical practice has up until recently been focused on spasticity reducing therapies for the preservation of motor function. However, recent reports have clearly demonstrated that despite good control of spasticity and reduction of muscle tone, by intramuscular botulinum toxin injections (chemical denervation) or selective dorsal rhizotomy that disrupts the reflex loop, skeletal muscle contracture formation progresses (Alhusaini et al., 2011; Tedroff et al., 2014). The poor effect of spasticity reduction on contracture development has raised the question of whether other factors such as growth disturbances and/or alterations in muscle composition are of greater mechanistic importance. Furthermore, recent findings suggest an overestimation of the contribution of spasticity to increased passive tension in muscle of young children with CP and at the same time an underestimation of the presence of contractures (Willerslev-Olsen et al., 2013; Herskind et al., 2015). In a large population study of children with CP, spasticity has been shown to peak around 4 years of age and thereafter decreases and level out at 12 years of age, whereas reductions in passive range of motion i.e., skeletal muscle contractures is progressive (Hagglund and Wagner, 2011). Therefore, we have chosen to focus on the instantaneous mechanical response of the muscle that is independent of reflex activity.

Muscle fiber size increases from 10–12 μm² at birth to 40–60 μm² after puberty (Oertel, 1988). Lexell et al have shown that skeletal muscle fiber size continuously increases with age up until and shortly after puberty. The same study also found a close relationship between fiber size and muscle cross sectional area (Lexell et al., 1992). Children with CP have reduced growth in general (Day et al., 2007). Skeletal muscle growth rate is reduced, evident as early as at 15-months of age (Herskind et al., 2015) and without showing any tendency to catch up later in life (Malaiya et al., 2007; Barber et al., 2011, 2016; Noble et al., 2014b). Just as for passive stiffness of skeletal muscles, deficit in skeletal muscle size in children with CP has so far only been described for the lower limb. The cross sectional area of the FCR muscle was significantly smaller in children with CP as compared to TD children. We have shown in previous studies that wrist flexors in children with CP have a greater fiber size variability and altered myosin composition compared to TD children (Ponten and Stal, 2007). In addition, the sarcomere, the smallest contractile unit of the muscle, has altered properties in CP (Ponten et al., 2007). By using laser diffraction methodology, we have measured sarcomere lengths of wrist flexors intra-operatively during tendon transfer surgeries. With the wrist held in neutral position, the sarcomeres were longer in CP compared to control, and sarcomeres were also longer—i.e., more stretched out- the worse the wrist contracture was (Ponten et al., 2007). This means that the wrist extension movement by the NeuroFlexor® likely results in extreme stress of the sarcomeres, with less overlap of the actin and myosin filaments and more stress on the perimyseal collagen surrounding the fiber bundles (de Bruin et al., 2014). In our study population the cross sectional area of the wrist flexor FCR, as determined by ultrasound, was significantly smaller in CP as compared to TD children and did not correlate to age or body weight. This implies that the differing brain damage the children with CP have, has a greater impact on muscle growth than age and body size. This further supports the impression of great heterogeneity within CP previously reported (Handsfield et al., 2016) and underscores the need for structured, individualized longitudinal follow-up.

Increased skeletal muscle echo intensity as measured by ultrasound has previously been used to infer altered skeletal muscle composition (Heckmatt et al., 1982; Nielsen et al., 2006). Pitcher et al. recently investigated the gastrocnemius muscle of 40 children with cerebral palsy and compared their findings to typically developed age matched controls (n = 12) (Pitcher et al., 2015). They show increased skeletal muscle echo intensity in CP and suggest that the underlying cause is due to increased content of non-contractile tissue e.g., collagen. Biochemical analysis of hydroxyproline content, a measure of intramuscular collagen content, correlates to spasticity severity (muscle tone as assessed by Modified Ashworth Scale, MAS) in children with CP (Booth et al., 2001). Staining muscle collagen using Sirius red has in CP shown increased amount of perimyseal collagen surrounding muscle fiber bundles (de Bruin et al., 2014). Similarity, MRI assessment of the lower limb indicates that young adults with CP have higher intramuscular fat content as compared to healthy controls (Noble et al., 2014a). Thus, it's without a doubt that skeletal muscle in individuals with CP with time contains an increased amount of non-contractile material. We interpret this as an indicator of the progressive remodeling in CP muscle. Despite that CP is defined as a non-progressive disease, skeletal muscle pathophysiology seems to continuously worsen over time.

We conclude that children with CP have weaker, thinner and stiffer forearm flexors as compared to typically developed children.

FvW: Performed experiments, analyzed data, drafted manuscript (MS), designed figures, finalized MS, approved final version of MS. KJ: Analyzed data, drafted MS, designed figures, finalized MS, approved final version of MS. BE: Performed experiments, designed figures, finalized MS, approved final version of MS. JF: Performed experiments, finalized MS, approved final version of MS. FJVC: Conception of study, analyzed data, approved final version of MS. EP: Conception of study, drafted MS, finalized MS, approved final version of MS.

The research reported herein was supported by grants from Sunnerdahls handikapp Stiftelse, Norrbacka-Eugeniastiftelsen and Stiftelsen Samariten to FvW; Linnea and Josef Carlssons Stiftelse, Stiftelsen Promobilia and by grants provided by the Stockholm County Council (ALF project) to EP. This material is in part based upon work supported by NIH Grant R01-052345, NIH Grant R01-050520 to FJVC. This project is also supported by the Postdoctoral Research Scholarship from Fonds de Recherche du Québec- Nature et Technologies to KJ.