Different strategies to compensate
for the effects of fatigue revealed by
neuromuscular adaptation
processes in humans
M. Bonnard, A.V. Sirin, L. Oddsson, A.
Thorstensson
Introduction
• In daily tasks, many times we need to
maintain stable performance levels, which can
gradually increase fatigue
• Compensatory mechanisms at various levels
of the neuromuscular system act to delay the
effects of fatigue so that the motor activity
can be prolonged
Task for this study
• Subjects were required to maintain an initially
submaximal hopping activity as long as
possible (until the task became the maximal
performance) at a constrained global power
output (hopping frequency and amplitude
were imposed)
Justification
• Can show within-joint versus between joints
compensating mechanisms:
• Ankle extensors are the main contributors to
the vertical propulsion in this task, and with
prolonged activity the ankle extensor muscles
will most likely be affected by fatigue, which
may lead to reorganization of neuromuscular
control between the joints of the leg.
Study Aim
• Analyze the differences in movement patterns
and EMG activity of the main extensor leg
muscles at the beginning of the hopping
activity and just before the subjects became
unable to maintain their activity any longer at
the constrained frequency and amplitude.
Methods
• 4 active and healthy adult subjects
• Instructed to maintain a two-legged hopping task
for as long as possible
• Hopping frequency was set at 2Hz
• Hopping amplitude was controlled at 30% of the
maximal hopping height
Methods Continued
• The four subjects were able to maintain the
hopping task for 23, 34, 37, and 44 minutes.
• Data was recorded for the first 2 minutes and
just before subjects stopped
• Electrogoniometer, force plate, 3-D motion
capture, and EMG
Results: Inter-Joint Compensation
• Ankle maximal negative moments increased
significantly
• Knee maximal negative moments were not
significantly different
• Ankle maximal positive moment increased,
but does not compensate for the increase in
the maximal negative moment at the ankle
• Knee maximal positive moment increased
Results: Hopping Cycle
• The total hopping cycle duration was not
different when comparing pre and postfatigue.
• The relative duration of the ground contact
phase slightly decreased
Results: Kinetics
• Peak vertical force increased and occurred
earlier during the contact phase when
fatigued
• Center of pressure location did not change as
a result of fatigue
Results: Kinematics
• When fatigued, subjects landed with knees more
flexed
• Knee and ankle joints reached a smaller minimum
angle during the ground contact phase
• Maximal flexion of the ankle occurred earlier with
fatigue
• With fatigue, the ankle began extension before
the knee and hip, compared to without fatigue,
where all 3 joints extended simultaneously
Results: EMG
• IEMG decreased in the medial gastrocnemious
during the eccentric phase and the preactivation phase, but vastus lateralis activity
increased
• No change in IEMG during the eccentric
phase, but there was an earlier activation of
the medial gastrocnemious prior to ground
contact
Discussion
• This study shows that hopping can be
maintained with the same global power
output for extended periods of time, with
relatively small changes in movement pattern
• All subjects increased IEMG in the biarticular
rectus femoris
Discussion
• This study suggests 2 different strategies are
effective to maintain a hopping task mainly
involving the ankle extensor muscles for
extensive periods of time
– Preactivation at the medial gastrocnemious before
ground contact
– Trade-offs between muscles across joints and
operates during the eccentric phase