Tissues responsible for proper movement execution (e.g., muscles, tendons) underlie a perpetual process of adaptation in response to specific environmental stimuli. In particular, unaccustomed eccentric exercise is known to induce high mechanical stress, leading to myofibril damage and extracellular matrix (ECM) disruption (1). Many studies have focused on nutritional interventions aimed at improving recovery from locomotor impairments, which are often observed as reductions in functional movement abilities (2–8). In this context, interest in collagen peptide (CP) administration has increased in recent years since regular CP intake has been shown to possibly alleviate joint discomfort, enhance ankle and knee functionality and improve recovery from Achilles tendinopathy, therefore acting as a potential and safe nutritional supplement in sports related injuries (9). Moreover, several short-term studies have reported that CP ingestion is superior to placebo after exercise-induced muscle damage (10–13). Faster restoration of biomechanical parameters such as improved countermovement jump height at 24 h (12) and 48 h (10), improved post-exercise muscle strength at 48 h, and reduced muscle soreness immediately after a muscle-damaging exercise bout (13) have been demonstrated following CP intake in conjunction with exercise. A proteomic analysis illustrated a significantly higher degree of upregulated proteins associated with skeletal muscle fibers, mainly contraction-related changes and structural adaptations to training. This suggests greater changes in collagen-specific proteins after 12 weeks of CP supplementation combined with resistance training (RT) (14). A recent study investigating gene expression patterns following strenuous bouts of knee extensions prior to a single dose of CP showed significant upregulation of the PI3K/Akt and MAPK pathways, both known to be essential for (myofibrillar) protein synthesis (15). The PI3K/Akt pathway may also play a role in connective tissue protein synthesis by mediating TGF-β2 signaling and inducing mRNA expression of COL1A1 & COL1A2 (collagen type 1 genes) in human pigment cells (16). Whether CP supplementation also stimulates connective tissue protein synthesis rates in vivo is currently unknown (17), but seems plausible, as increased collagen concentration was measured in human in vitro-engineered ligament after a single dose of 15 g CP (18).

Recovery from severe lengthening contractions (e.g., squats or drop jumps) also causes biomarkers in serum blood to peak within the following days. Proteins and metabolic-related enzymes such as creatine kinase (CK), lactate dehydrogenase (LDH), myoglobin (MYO), alanine transaminase (ALT), c-reactive protein (CRP) and interleukin-6 (IL-6) have been considered metabolic indicators of muscle damage and concomitant inflammation, both of which occur after strenuous, mainly eccentric exercise (19). However, there are only two studies without exercise intervention that examined recovery-related blood markers after CP supplementation. Neither found differences between CP and placebo after nine and 42 days of supplementation (10, 11). Therefore, the importance of regular, long-term, and at least moderately intense physical activity in combination with CP ingestion may be of interest for proteomic outcomes related to recovery. Furthermore, the CP administered in this study have only been utilized in two long-term investigations without examining blood related recovery parameters so far (20, 21). Thus, the impact of these particular CPs is also noteworthy.

Based on this assumption, the present study aimed to investigate whether 12 weeks of CP supplementation together with a concurrent training intervention would significantly influence exercise-induced muscular stress. In addition, recovery should be confirmed by specific blood parameters. This could indicate either a reduced acute stress response due to improved functional or structural integrity of the muscles. Or, on the other hand, an optimized muscular regenerative capacity that alleviates both myofibrillar damage and inflammation after exercise-induced muscle damage, or both. Therefore, we hypothesized that the increase of biomarkers in blood after a second bout of muscle-damaging exercise would be lower in subjects taking specific collagen peptides (SCP).

After meeting the inclusion criteria and a positive medical examination, 75 participants (initially 76, but one subject declined further participation before day 1) were randomized and assigned to the SCP (n = 37) or PLA group (n = 38) (Figure 2). Participants lost to follow-up were excluded for non-compliance (missing more than 10 sessions) with the training protocol. No adverse events or side effects related to SCP or PLA supplementation were observed. 55 subjects successfully completed the study.

To the best of our knowledge, this is the first study to examine the longer-term impact of a daily intake of specific collagen peptides (SCP) in combination with a 12-week concurrent training (CT) program on muscle stress and recovery-related blood biomarkers. 15 g of SCP significantly decreased the area under the curve (AUC) as well as the ΔAUCs of myoglobin (MYO), creatine kinase (CK) and lactate dehydrogenase (LDH) activity, suggesting improved regenerative capacity within 48 h after muscle damage-inducing bouts of drop jumps.

The results showed better adaptation after eccentric exercise, suggesting a more pronounced repeated bout effect (RBE) after prolonged SCP ingestion. The RBE, a not yet fully understood mechanism that protects tissues involved in muscle lengthening exercise execution from subsequent damage, is thought to be due to neural adaptations, extracellular matrix remodeling, adaptations at the muscle-tendon junction, and inflammation (34).

Severe myofibrillar damage, particularly as it occurs after unaccustomed eccentric exercise bouts, usually results in significant increases in CK, LDH and MYO, which have been extensively used as indirect biomarkers to monitor the extent of acute stress as well as recovery (35). The activity of both cytosolic enzymes (CK, LDH) during a nine (10) and 33-day (13) CP supplementation period was not lower compared to a placebo after muscle-damaging exercise, which could have indicated either reduced acute muscle stress or accelerated reconstruction of damaged tissue. These two studies are the only ones to date that have examined recovery after CP intake by analyzing CK & LDH, albeit in a more short-term manner without prolonged exercise intervention. Therefore, the three-month training intervention in the present study may have improved the remodeling of the force-producing and transmitting tissues (ECM, muscle) with regular SCP administration, reducing the increase in CK & LDH. This hypothesis is supported by the results of a proteomic approach following a 12-week intake of 15 g CP and resistance training (14). The authors identified a 14-fold change in hypoxia-inducible factor alpha 1 (HIF-1α), the alpha subunit of the transcription factor HIF-1, which has shown its contribution to skeletal muscle regeneration in mice (36). HIF-1 exhibits upregulated expression in hypoxic situations, such as exercise, and has been found to play a role in collagen production in rodent tubular epithelial cells (37). Specifically, HIF-1 induces the expression of prolyl and lysyl hydroxylase, two enzymes essential for fiber formation, and promotes proper collagen biogenesis, ECM organization, stiffness, cell adhesion, and motility in human fibroblasts (38). Heat shock protein 90 (HSP90), also upregulated 14-fold in CP in (14), is an HSP chaperone group protein. HSPs are usually overexpressed when exposed to infection, heat-induced stress, hypoxia and thus heat treatment (used as a therapeutic approach), and exercise. Previous studies in mice indicated increased protein levels and markers of muscle hypertrophy/recovery (MHCneo) associated with higher HSP72 levels following downhill running (39). Data from healthy human subjects running downhill for 45 min have shown a negative correlation of ΔMYO and ΔCK along with increasing concentrations of ΔHSP90 (1 h minus baseline) 1 hour after the eccentric endurance bout (40). In addition, Heat shock factor 1 (HSF1), a major transcription factor of HSPs, was also upregulated in CP influenced by HIF-1α in (14), indicating a close interdependence of these proteins (41). It appears that HSPs are involved in protecting cells from damage (42). Still, caution is also warranted when persistently elevated levels are observed, as they indicate chronic inflammation (43). Finally, five and four-fold changes in MAPK and PI3K-Akt pathways have been observed after CP ingestion (14), which are important initiators of signaling cascades leading to protein synthesis. There is little evidence from in-vitro experiments that PI3K-Akt & MAPK may also trigger collagen type I gene expression via TGF-β2 activation (16, 44, 45). TGF-β cytokines generally induce intracellular signaling via specific Smad proteins (Smad2, 3 forming Smad 4 etc.) (46), the latter being responsible for the expression of COL1A2 and COL3A1 (47) which give rise to collagen types I & III, both of which are most abundant in intramuscular connective tissue (48, 49). Given that Smad2, 4 and COL1A1 & COL3A1 mRNA were significantly upregulated 27 (COL1A1) and 30 days after two muscle-damaging bouts of eccentric contractions separated by 27 days (50), the remodeling of muscle ECM, particularly force-bearing proteins such as COL I & III, represents a longer-term process (51). This functional recovery and adaptation process appears to be enhanced by CP supplementation induced by the mechanisms described above.

Based on the CK, LDH & MYO results, the findings suggest increased regenerative capacity throughout ongoing myocellular remodeling. While LDH typically peaks within hours, MYO concentrations and CK activity are highest on the first to third day or, in some cases, even on the fourth day after strenuous exercise, depending on the type of load, muscles used, and individual training history (52–57). Each biomarker leaves its original intracellular location due to cell damage-induced permeability and subsequently enters the bloodstream, where it can be measured in serum/plasma in chronological order (LDH > MYO > CK) (35, 58). In the present study, prolonged SCP intake was shown to ameliorate several cell damage markers and have a beneficial effect on muscle stress response, early-phase muscle recovery, and enhanced RBE. Whether adaptions occurred in surrounding structures such as intramuscular connective tissue and/or muscle fibers per se remains unknown.

Regarding inflammatory parameters, the acute-phase protein C-reactive protein (CRP) is a commonly used marker of local and systemic inflammation. In this trial, there was no significant difference between both groups in terms of hsCRP. Considering the measurement time points (baseline and after 24 h), significant CRP changes were already detected after a marathon race (59), a one-hour strenuous endurance run (52), a 90-min eccentric exercise protocol (60) and 100 drop jumps (61). SCP intake had no apparent significant effect on post-exercise inflammatory status in this study; among other factors, the high CV of 12.5% for hsCRP might be responsible for this.

The choice of concurrent training (CT) as the type of training intervention in the present study was based on the following considerations. In general, CT combines the benefits of both RT and ET, at least in untrained to recreational athletes. CT improves physical fitness, does not impair maximal strength and muscle hypertrophy, and is thus consistent with public health guidelines (62). In contrast, CT limits explosive strength gains compared to resistance-only training, offset in the current study by performing RT before ET (63). In agreement with (64), moderate to high ET intensity was prescribed to benefit cardiorespiratory fitness and muscle strength (64). As a recent systematic review mentioned, protein intake (whey, casein, soy, milk) after an acute CT bout increases myofibrillar protein synthesis to a similar extent as after RT (65). Moreover, phosphorylation of the signaling proteins Akt–mTOR-S6K increased significantly immediately after CT and protein intake compared with placebo, indicating enhanced translation of myofibrillar mRNAs (66). CT plus protein also resulted in an immediate post-exercise increase in specific micro RNAs, suggesting a more myogenic rather than oxidative adaptation, and an effect on longer-term CT responses (65, 67). Whether SCP supplementation in combination with CT also leads to the above results remains unknown but may be of interest for future studies.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by Ethics Committee of the University of Vienna (reference no. 00765). The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

KB: Writing – original draft, Writing – review & editing. SS: Writing – review & editing. LB: Writing – review & editing. SO: Writing – review & editing. AB: Writing – review & editing. DK: Writing – original draft, Writing – review & editing.