Affiliations: [a] Motor Control and Biomechanics Laboratory, Department
of Kinesiology, University of New Hampshire, Durham, NH, USA | [b] Movement Studies Laboratory, University of Georgia,
Athens, GA, USA
Note: [] Address for correspondence. E-mail: rvc@cisunix.unh.edu
Abstract: Examining the effects of fatigue on hamstrings' 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°· 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°·s^{-1}. However, when acting as antagonists, they displayed
stable amplitude with decreasing spectral frequency.
