Metabolic enzymes and phenotypic expression among human locomotor muscles

Percutaneous biopsies were taken from the right vastus lateralis (VL), tibialis anterior (TA), soleus (Sol), and lateral gastrocnemius (LG) muscles of eight recreationally active adult males. Approximately 60 fibers in each sample were analyzed for their type (I, IIa, or IIx), cross-sectional area (CSA), and succinic dehydrogenase (SDH), alpha glycerol phosphate dehydrogenase (GPDH) and calcium-activated actomyosin adenosine triphosphatase (qATPase) activities. This was done to test the hypothesis that metabolic enzyme activities are more reflective of the functional diversity among human locomotor muscles than fiber type composition. The results showed that enzymatic characteristics differed more or less than expected between muscles of the same or different fiber type. For example, the relative CSA occupied by fast fibers was only about 50% greater in the mixed (LG and VL) than in the slow (Sol and TA) muscles (57 vs. 38%). At the same time, average fiber SDH activity and fiber type specific SDH:qATPase*%CSA, both used as estimates of fatigue resistance, were greater in Sol and LG than in TA and VL. As a result, the two slow muscles and the two mixed muscles had different values, and a mixed muscle (LG) had higher values than a slow muscle (TA). The findings suggest that differences in enzymatic profile, more than fiber type composition, afford human locomotor muscles the capacity to perform their purportedly divergent functional tasks.