Single fiber physiology differs between three human leg muscles

Abstract

While single fiber contractile function within a given fiber type population is largely similar between muscles, fiber type distribution can vary considerably. Because single fiber contractile function and fiber type distribution both contribute to whole muscle function, we generated composite profiles to compare three leg muscles of interest. Eight recreationally active males (5) and females (3) (23 ± 1 yr, 64 ± 3 kg, 51.8 ± 2.6 ml/kg/min) were biopsied from the vastus lateralis (VL) and soleus (S). To make further comparisons, seven recreationally active males (4) and females (3) (22 ± 1 yr, 68 ± 2 kg, 49.5 ± 2.6 ml/kg/min) were biopsied from the gastrocnemius (G). Single fibers were isolated and investigated for size and function, and subsequently processed for myosin heavy chain (MHC) composition (SDS‐PAGE). A composite value, which is a theoretical representation of the entire muscle, was then calculated for each contractile parameter based on the MHC composition of the studied fibers. The VL had higher specific tension (VL 66% > G and 28% > S), shortening velocity (40% > G; 48% > S), peak power (266% > G; 152% > S), and normalized power (276% > G; 221% > S), when compared to the gastrocnemius and soleus. Additionally, the gastrocnemius had lower peak tension (G 37% < VL; G 31% < S) and lower specific tension (40% < VL; 22% < S) when compared to the vastus lateralis and soleus. These findings highlight the unique intrinsic profile of these select muscles, which should be considered when interpreting how they adapt to exercise training.