Length-force characteristics of rat muscle during isometric contraction are determined by independent and cumulative effects of fatigue and potentiation

Net effects on length-force characteristics of several mechanisms active during sustained contraction were investigated in fully recruited rat gastrocnemius muscle (GM) in situ. Comparisons were made for an initial condition (at t=200 ms) to that after 1800 ms stimulation at several frequencies. The main results were: (1) for 15-, 30- & 100-Hz, skewing of the length-force curve to higher muscle length (i.e. shifts of optimum lengths) reflecting the predominance of fatigue-induced force decline and creep of sarcomeres. (2) for 40-, 50- & 100-Hz, shifted active slack lengths to lower length. The greater part of the ascending limb of length-force relationships was shifted to lower muscle lengths reflecting predominance of potentiation. If it is assumed that for fatigue and potentiation maximal length of force exertion is unchanged, use of operational definitions of fatigue (i.e., force decrease) and potentiation (i.e., force increase) are not tenable particularly for the descending limb because the effects may be opposite