Gymnastik- och idrottshögskolan, GIH

Humans respond differently to identical exercise interventions. Skeletal muscle fiber type distribution and sex differences have been proposed as potential mechanisms underlying this variability. This study aimed to improve our understanding of these factors by examining how muscle fiber type distribution and sex relate to individual adaptations following exercise. Thirty recreational runners completed an 8-week training intervention. Fiber type distributions were quantified using immunofluorescence microscopy and were analyzed alongside a range of physiological and performance measures. Correlations followed by principal component analysis (PCA), identified key contributing variables and produced a composite score termed “Adaptation Index”. This index incorporated maximal exercise duration, VO2 at threshold, session RPE, delta efficiency, and post-exercise lactate and glucose metabolism. The index was then used in a regression model to examine the explanatory power of baseline fiber type distribution. The model indicated a moderate to strong association between baseline fiber type distribution and training adaptation, with a negative correlation between aerobic-type response and the proportion of type I fibers. No significant sex differences were found, possibly due to the small sample size of female participants (n = 6). These findings suggest that the distribution of muscle fiber type is one contributing factor to the heterogeneous responses observed following standardized exercise interventions. Unexpectedly, Type I correlated negatively with aerobic-type response, suggesting a ceiling effect. In practical terms, this highlights the potential value of tailoring training programs based on individual muscle characteristics. Future research should explore sex-specific patterns in adaptation with larger and more balanced samples.