Acute changes in motor unit discharge property after concentric versus eccentric contraction exercise in knee...

Area of Science:

• Neuromuscular Physiology
• Exercise Physiology
• Muscle Contraction

Background:

• Motor unit firing properties are crucial for muscle force production and fatigue.
• Understanding the differential effects of concentric and eccentric contractions on motor units is important for optimizing training strategies.
• Previous research has explored muscle adaptations to different contraction types, but immediate post-exercise motor unit firing properties require further investigation.

Purpose of the Study:

• To investigate the immediate effects of fatiguing concentric versus eccentric contraction exercise on motor unit firing properties.
• To compare changes in motor unit recruitment thresholds and discharge rates between concentric and eccentric exercise protocols.
• To examine associated changes in muscle size and evoked torque.

Main Methods:

• Eighteen healthy men performed maximal isokinetic knee extension exercises (concentric or eccentric) to fatigue.
• High-density surface electromyography (HD-sEMG) of the vastus lateralis was recorded before and after exercise during isometric ramp contractions.
• Individual motor units were identified, and their recruitment thresholds and discharge rates were analyzed. Muscle cross-sectional area and evoked torque were also measured.

Main Results:

• Motor units with moderate to high recruitment thresholds showed decreased thresholds post-exercise under both conditions.
• Motor unit discharge rates significantly increased after concentric contraction compared to eccentric contraction.
• Concentric contraction resulted in a greater muscle cross-sectional area, while evoked torque decreased similarly in both conditions.

Conclusions:

• Fatiguing concentric exercise leads to a greater increase in motor unit discharge rates compared to eccentric exercise.
• Concentric contractions may induce greater neural adaptations and metabolic responses immediately following fatiguing exercise.
• These findings highlight the distinct neuromuscular responses to different modes of muscle contraction during fatiguing exercise.