REAL-TIME SARCOMERE LENGTH MEASURED IN FROG MUSCLE DURING ECCENTRIC CONTRACTION

Abstract

185 Sarcomere length was measured by laser diffraction during elongation of activated frog tibialis anterior muscle fibre bundles (i.e., eccentric contraction) at fibre strain magnitudes of 10%, 25% or 35% (n=16). Control muscles receiving passive stretch (n=6) or isometric contraction (n=6) were also investigated. Peak muscle fibre stress during eccentric contraction for all three fibre strain groups was initially 311.5±3.6 kN/m2, did not vary significantly between strain groups (p>0.65) and only changed slightly over the 10 contraction treatment. In contrast, maximum isometric tension, measured just prior to eccentric contraction differed significantly among strain groups and changed dramatically during the 10 contraction treatment (p<0.01). Maximum tetanic tension was not significantly different between strain groups prior to treatment, but decreased to 180.3±3.8 kN/m2, 125.1±7.8 kN/m2, and 78.3±5.1 kN/m2, for the 10%, 25% and 35% strain groups respectively (p<0.0001) 30 minutes after the eccentric contraction sequence. Muscle fibre strain measured during eccentric contraction was strongly correlated with the magnitude of tetanic tension decline (r2=0.87, p<0.0001) while peak fibre stress was not at all correlated with tetanic tension decline (r2=.005, p>0.8) indicating that muscle fibre strain, not stress is responsible for the magnitude of muscle injury that occurs after eccentric contraction. This work was supported by NIH grant AR40050 and the VA.