FORCE-VELOCITY AND FORCE-POWER PROPERTIES OF SINGLE MUSCLE-FIBERS FROM ELITE MASTER RUNNERS AND SEDENTARY MEN

Gastrocnemius muscle fiber bundles were obtained by needle biopsy from five middle-aged sedentary men (SED group) and six age-matched endura nce-trained master runners (RUN group). A single chemically permeabili zed fiber segment was mounted between a force transducer and a positio n motor, subjected to a series of isotonic contractions at maximal Ca2 + activation (15 degrees) and subsequently run on a 5% polyacrylamide gel to determine myosin heavy chain composition. The Hill equation was lit to the data obtained for each individual fiber (r(2) greater than or equal to 0.98). For the SED group, fiber force-velocity parameters varied (P < 0.05) with fiber myosin heavy chain expression as follows : peak force, no differences; peak tension (force/fiber cross-sectiona l area), type IIx > type IIa > type I; maximal shortening velocity (V- max defined as y-intercept of force-velocity relationship), type IIx = type IIa > type I; a/P-o (where a is a constant with dimensions of fo rce and P-o is peak isometric force), type IIx > type IIa > type I. Co nsequently, type IIx fibers produced twice as much peak power as type IIa fibers, whereas type IIa fibers produced about five times more pea k power than type I fibers. RUN type I and IIa fibers were smaller in diameter and produced less peak force than SED type I and IIa fibers. The absolute peak power output of RUN type I and IIa fibers was 13 and 27% less, respectively, than peak power of similarly typed SED fibers . However, type I and IIa V-max and a/P-o were not different between t he SED and RUN groups, and RUN type I and IIa power deficits disappear ed after power was normalized for differences in fiber diameter. Thus the reduced absolute peak power output of the type I and IIa fibers fr om these fibers and a corresponding reduction in their peak power prod uction. This impairment in absolute peak power production at the singl e fiber level may be in part responsible for the reduced in vivo power output previously observed for endurance-trained athletes.