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Individual responses in creatine kinase (CK) release after eccentric exercise are divergent. This study aimed to identify whether this could be related to selected humoral or intramuscular inflammatory factors. Twenty-three subjects were divided into non-exercising (n = 5) and downhill run (DHR; n = 18) groups (12 × 5 min, 10% decline at 15 km/h). Blood samples were analyzed for white blood cell differential count, CK, myoglobin, tumor necrosis factor-α, granulocyte colony-stimulating factor, interleukin (IL)-1β, IL-6, and IL-10. Muscle biopsies were analyzed for signal transducer and activator of transcription-3 (STAT3), IκBα, and myeloperoxidase (MPO). DHR participants clustered as early (DHR1) recovery, biphasic response (DHR2), or classic delayed exaggerated CK response (DHR3), with a delayed CK peak (4784 ± 1496 U/L) on day 4. For DHR1 and DHR2, CK peaked on day 1 (DHR1: 1198 ± 837 U/L) or on day 1 and day 4 (DHR2: 1583 ± 448 U/L; 1878 ± 427 U/L), respectively. Immediately post-DHR, IL-6 increased in DHR2 and DHR3 whereas IL-10 increased in all DHR groups. STAT3 signaling increased for DHR1 and DHR2 at 4 h, but MPO at day 2 only in DHR2. Objective cluster analysis uncovered a group of subjects with a characteristic biphasic CK release after DHR. The second elevation was related to their early cytokine response. The results provide evidence that early responses following eccentric exercise are indicative of later variation.

Satellite cells (SCs) are responsible for muscle repair following strenuous exercise or injury. SC responses to intervention have been studied, but most studies do not discuss or take into account the substantial variability in SC number among young individuals. We hypothesized that an active lifestyle reflected in higher VO(2max) may be associated with greater SC number. As training alters basal p38-mitogen-activated protein kinase (MAPK) activity, which is associated with SC proliferation, SC count may also correlate with this stress signaling kinase. Muscle biopsies from vastus lateralis of eight male participants were analyzed for fiber type, myogenin, and p38/phospho-p38 MAPK using SDS-PAGE and Western blotting. Immunofluorescence was used to detect Pax7(+) SCs. Two weeks following the biopsy, subjects underwent an incremental treadmill test to determine VO(2max) . A strong positive correlation (P = 0.0087) was found between the number of Pax7(+) nuclei and VO(2max) . Pax7(+) cell number correlated negatively with phospho-p38/p38 MAPK (P = 0.0006), but had no correlation with fiber type or myogenin. SC number is proportional to VO(2max) , and hence it can be postulated that higher levels of physical activity activate SC proliferation but not fusion, underlining the relevance of exercise in stimulating SC pool size even without injury.