The effects of length on fatigue and twitch potentiation in human skeletalmuscle

Fatigue is the decrease in active force that happens after repeated muscle stimulation, and post tetanic twitch potentiation (PTP) is the increase in twitch force observed after repeated muscle stimulation. This study investi gated the effects of length on the interaction between fatigue and PTP, as these two forms of force regulation are length-dependent and may coexist. A total number of 14 subjects were tested in 3 days, in which fatigue and PT P were induced in the knee extensor muscles in three different knee angles (30 degrees, 60 degrees and 90 degrees; full extension = 0 degrees). PTP wa s evaluated in rested and fatigued muscles with twitch contractions elicite d before and after 10 s maximal voluntary contraction (MVC), and fatigue wa s evaluated with nine 50 Hz electrically elicited contractions (5 s duratio n, 5 s interval between contractions). Fatigue was length-dependent, with f orce values that were (mean +/- SEM) 59 +/- 5, 56 +/- 3 and 38 +/- 1% of ma ximal force at 90 degrees, 60 degrees and 30 degrees, respectively. PTP was also length-dependent. Rested muscles showed PTP of 39 +/- 4, 47 +/- 2 and 68 +/- 5% at 90 degrees, 60 degrees and 30 degrees, respectively. Fatigued muscles showed PTP of 44 +/- 3, 55 +/- 6 and 68 +/- 5%, at 90 degrees, 60 degrees and 30 degrees, respectively. This study shows that fatigue and PTP may represent independent mechanisms, as they regulate force in opposite d irections and are both enhanced in short muscle lengths.