Discharge pattern of human motor units during dynamic concentric and eccentric contractions

Area of Science:

• Human physiology
• Neuromuscular control
• Biomechanics

Background:

• Understanding muscle force control is crucial for rehabilitation and performance.
• Motor unit (MU) behavior, including recruitment and discharge patterns, underlies muscle force generation.
• Isovelocity movements provide a controlled environment to study neuromuscular responses.

Purpose of the Study:

• To investigate the effects of movement velocity on motor unit (MU) recruitment and decruitment thresholds (RT and DT) in the human biceps brachii.
• To analyze the influence of movement velocity on MU discharge patterns during concentric and eccentric contractions.
• To determine differences in muscle force control between shortening and lengthening contractions.

Main Methods:

• Investigated 45 motor units (MUs) in the human biceps brachii during isovelocity concentric and eccentric movements.
• Utilized a device applying external torque proportional to elbow angle changes.
• Employed a wire branched electrode for subcutaneous single MU potential discrimination.

Main Results:

• Increased movement velocity led to recruitment at lower torque values for most MUs (91%).
• Recruitment thresholds (RT) significantly decreased for 47% of MUs with increased velocity.
• A consistent discharge pattern of initial short interspike intervals (ISIs) followed by longer ones was observed in 93% of MUs.

Conclusions:

• Significant differences exist in muscle force control between shortening and lengthening contractions.
• Decruitment thresholds (DT) were lower at faster movements for 82% of MUs, with significant decreases in 47%.
• A declining MU discharge rate with a long final ISI characterized 93% of MUs during the entire movement.