Fatigue effects on the bilateral deficit are speed dependent

Area of Science:

• Neuromuscular physiology
• Human motor control
• Exercise science

Background:

• The bilateral deficit describes reduced force during simultaneous bilateral muscle contractions compared to the sum of unilateral contractions.
• This phenomenon is potentially linked to the neural activation of high-threshold motor units.
• Understanding factors influencing the bilateral deficit, such as fatigue and contraction velocity, is crucial for exercise and rehabilitation.

Purpose of the Study:

• To investigate the influence of muscle fatigue, induced by different contraction speeds, on the magnitude of the bilateral deficit during knee extension.
• To examine the relationship between the bilateral deficit and motor unit activation under fatiguing conditions.
• To test the hypothesis that a unilateral fatigue protocol would result in a comparable bilateral deficit at both fast and slow contraction velocities.

Main Methods:

• The study involved 20 men performing isokinetic knee extensions at 30 and 150 degrees/s.
• The bilateral deficit was measured before and after a unilateral fatigue protocol at each contraction speed.
• Maximum voluntary contraction (MVC) force was assessed to quantify the bilateral deficit and muscle fatigue.

Main Results:

• The bilateral deficit was initially similar at both slow (30 degrees/s) and fast (150 degrees/s) contraction velocities.
• Muscle fatigue significantly reduced maximum voluntary knee extension torque at both speeds.
• The bilateral deficit was not significantly altered after fatigue at 150 degrees/s but significantly increased after fatigue at 30 degrees/s.

Conclusions:

• The findings contradict previous research suggesting that reduced high-threshold motor unit activation is the primary cause of the bilateral deficit.
• Muscle fatigue at a slow contraction velocity appears to exacerbate the bilateral deficit, indicating complex neural adaptations.
• Contraction velocity plays a significant role in modulating the bilateral deficit response to fatigue, challenging existing models.