Aim. It is well-established that an isolated, highly intensive strength training of the plantar flexors of the Achilles tendon increases its stiffness and region-specific hypertrophy. But it remains unclear which biomarkers of the extracellular matrix (ECM) are regulated by this kind of exercise and may therefore contribute to the described adaptation of the Achilles tendon. Thus, the aim of this study was to investigate ECM-specific biomarkers following highly intensive plantar flexor repetitions. Methods. 14 male, healthy subjects (age: 26.6 ± 1.2 yrs) participated, who performed repeated isometric plantar flexions (3 sec loading, 3 sec recovery) at 90% of the achieved moment during a maximum voluntary contraction, which causes an Achilles tendon strain of about 4.5-5.0%. This protocol was performed three to four times per week including five sets of four repetitions per intervention. Venous blood samples were taken before and after the 1st, 9th, and 56th training session, respectively. Proteoglycans (PGs), e. g. decorin, biglycan, and ECM-specific enzymes, e. g. matrix metalloprotease (MMP-1) were measured by ELISA or Western blot. Results. As determined by Western Blot analysis, decorin and biglycan were unaffected by the conducted training programs. Interestingly, decorin-specific bands were observed at around 30 kDa, but not at 39 kDa. Beside bands at 42 kDa for biglycan, Western blot analyses revealed biglycan-specific bands at around 30 kDa. Exercise had no effect on MMP-1. Conclusion. It is demonstrated that SLRPs seem to be further degraded within the circulation. This effect might be regulated by activated proteases different from MMP-1. It is known that SLRPs have degradation products of about 30 kDa, but those data were obtained from tissue instead of serum. This finding is important as it demonstrates that physical exercise induces the degradation of SLRPs in serum. Further studies are needed to verify these data and to investigate possible pathways.