Motor unit discharge rate modulation during isometric contractions to failure is intensity- and modality-dependent

Area of Science:

• Human Physiology
• Neuroscience
• Exercise Science

Background:

• Muscle fatigue leads to a decline in maximal muscle torque production.
• Motor unit discharge rate (MUDR) is a key factor in maintaining muscle force.
• The task-specific nature of MUDR adjustments during fatiguing contractions remains unclear.

Purpose of the Study:

• To investigate how MUDR changes during different submaximal isometric knee extension tasks to task failure.
• To determine if MUDR adjustments are specific to contraction modality (sustained vs. intermittent) and intensity.

Main Methods:

• High-density surface electromyography (EMG) recorded from vastus lateralis and medialis during isometric knee extensions to failure.
• Individual motor unit discharge timings were decomposed from EMG signals and tracked.
• MUDR was quantified and analyzed across different contraction intensities (20-50% MVT) and modalities (sustained and intermittent).

Main Results:

• MUDR exhibited distinct patterns based on contraction type and intensity.
• In a sustained 20% MVT task, MUDR initially decreased then returned to baseline.
• In an intermittent 50% MVT task, MUDR remained stable before increasing towards task failure.
• Sustained contractions at higher intensities (30% and 50% MVT) showed earlier increases in MUDR compared to lower intensities.

Conclusions:

• Motor unit discharge rate adjustments during submaximal isometric contractions to failure are dependent on both the contraction modality and intensity.
• These findings provide insights into the neural control strategies employed by the neuromuscular system under different fatiguing conditions.