Behavior of coactive muscles during fatigue

Area of Science:

• Neuromuscular Physiology
• Exercise Science
• Muscle Fatigue

Background:

• Coactivation, the activity of antagonist muscles during voluntary contraction, is a key factor in joint stability and movement control.
• Previous research indicated that resistance training of knee extensors can reduce coactivation in knee flexors.
• Understanding the dynamics of coactivation during fatigue is crucial for comprehending force production limitations.

Purpose of the Study:

• To investigate the temporal changes in knee flexor coactivation during fatiguing contractions of the knee extensors.
• To determine if fatigue protocols of varying intensity and duration influence the coactivation response.

Main Methods:

• Ten male subjects underwent two fatigue protocols: low-intensity/long-duration and high-intensity/short-duration static leg extensions.
• Maximal voluntary contraction (MVC), submaximal force, and surface electromyography (EMG) of vastus lateralis (agonist) and biceps femoris (antagonist) were recorded.
• Measurements were taken periodically until the time limit of endurance (Tlim) was reached.

Main Results:

• Vastus lateralis EMG (agonist activity) increased progressively throughout the fatigue protocols, showing a 38% change from baseline at Tlim.
• Biceps femoris EMG (antagonist coactivation) significantly increased by approximately 60% at Tlim in both fatigue protocols.
• A small but significant loss of force production was observed at Tlim.

Conclusions:

• Increased antagonist muscle activity (coactivation) contributes to force loss during sustained submaximal contractions leading to fatigue.
• The observed increase in coactivation occurs regardless of the fatigue protocol's intensity and duration.
• The strong correlation between agonist and antagonist EMG suggests a potential 'common drive' influencing both motoneuron pools.