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Motor control and kinetics during low level concentric and eccentric contractions in man
K Søgaard1, H Christensen, B R Jensen
1National Institute of Occupational Health, Department of Physiology, Copenhagen O, Denmark.
Electroencephalography and Clinical Neurophysiology
|October 1, 1996
Summary
This study on motor unit (MU) recruitment during dynamic contractions found that firing rate, not MU type, changes between concentric and eccentric phases. This suggests limited MU pool activation in repetitive tasks, potentially leading to fatigue and muscle disorders.
Area of Science:
- * Neuromuscular Physiology
- * Biomechanics
- * Exercise Science
Background:
- * Understanding motor unit (MU) recruitment is crucial for explaining muscle force generation and fatigue.
- * Dynamic contractions involve both concentric (shortening) and eccentric (lengthening) phases, with distinct neural demands.
- * Low-level, repetitive tasks are common occupationally and may lead to work-related muscle disorders due to specific MU activation patterns.
Purpose of the Study:
- * To investigate motor unit (MU) recruitment patterns during low-level dynamic elbow flexion and extension in female subjects.
- * To compare MU activity, including firing rate and number of active MUs, between concentric and eccentric contractions at various velocities.
- * To determine the primary mechanism (firing rate modulation vs. selective recruitment) responsible for force control during dynamic contractions.
Main Methods:
- * Six female subjects performed dynamic elbow flexion and extension at 10% maximum voluntary contraction.
- * Contractions were executed at mean angular velocities of 10, 20, and 40 degrees/second.
- * Motor unit (MU) action potential trains were recorded from the brachial biceps muscle, with 119 MUs identified at 10 degrees/s.
Main Results:
- * Approximately 60% of identified MUs were active during both concentric and eccentric phases across all velocities.
- * Mean MU firing rate significantly decreased during the transition from concentric to eccentric contractions.
- * The number and properties of active MUs remained similar between concentric and eccentric phases, irrespective of velocity.
Conclusions:
- * Firing rate modulation is the primary mechanism for controlling force during low-level dynamic contractions, rather than selective recruitment of different MUs.
- * Dynamic tasks, especially monotonous occupational ones, recruit a limited pool of MUs.
- * Over-recruitment of this limited MU pool may contribute to muscle fatigue and the development of work-related muscle disorders.