Alterations in torque and hamstrings agonist and antagonist activity over repeated maximum effort, reciprocal isokinetic flexion-extension movements

Examining the effects of fatigue on hamstrings\u27 functioning can provide useful information regarding their role in stabilizing the knee joint. The purpose of the present investigation was to determine the effect of fatiguing, maximum effort, reciprocal isokinetic flexion-extension movements on peak torque (PT) and average torque (AVT) of the knee flexors and extensors, and agonist and antagonist medial hamstrings (MH) and biceps femoris (BF) surface electromyography (SEMG), as measured by the root mean square amplitude (RMS) and median frequency of the frequency spectrum (MDF). Ten healthy adult female university students (age = 22.5 +/- 2.0 years) performed 32 maximal effort contractions of the quadriceps (extension) followed by maximal effort contractions of the hamstrings (flexion) at an angular velocity of 400 degrees. s(-1). Data were normalized (as a percentage) and compared to maximum voluntary contractions (MVC) at repetitions 8, 16, 24, and 32. Separate repeated measures analyses of variance were performed on data to determine changes from MVC levels. Results indicated: ( 1) there were significant declines in PT and AVT from MVC levels across repetitions tested; ( 2) when acting as agonists MH and BF displayed increased RMS across repetitions tested, except from repetition 8-to-16; ( 3) when acting as antagonists MH and BF displayed no significant differences in RMS across repetitions tested; and, ( 4) there was a steady decline in MDF across repetitions tested in both MH and BF muscles when acting as agonists and antagonists, indicating a shift in motor unit recruitment from fast-to-slow twitch units. Based on the results, it appears that the MH and BF behave in unison when acting as either agonists or antagonists but as a muscle pair, they behave differently when acting in agonist or antagonist mode. As agonists, the MH and BF displayed increased amplitude and decreased spectral frequency over the course of a fatiguing, maximum effort, reciprocal isokinetic flexion-extension test performed at 400 degrees . s(-1). However, when acting as antagonists, they displayed stable amplitude with decreasing spectral frequency