Random motor unit activation by electrostimulation.

Area of Science:

• Exercise Physiology
• Neuromuscular Physiology

Background:

• The recruitment order of motor units during voluntary contractions follows Henneman's size principle.
• It remains unclear if surface neuromuscular electrostimulation also adheres to this principle or recruits motor units nonselectively.

Purpose of the Study:

• To investigate whether motor unit activation during surface neuromuscular electrostimulation is selective or random.
• To compare motor unit activation patterns between electrostimulation and voluntary contractions.

Main Methods:

• Sixteen healthy men performed submaximal isometric quadriceps contractions at 20%, 40%, and 60% of maximal voluntary torque.
• The twitch interpolation technique was used to assess motor unit activation during both voluntary and electrically stimulated contractions.
• Time-to-peak torque was measured and compared between conditions.

Main Results:

• Time-to-peak torque was significantly longer during electrostimulation compared to voluntary contractions across all torque levels.
• Time-to-peak torque decreased with increasing torque during voluntary contractions but remained unchanged during electrostimulation.
• Electrostimulation did not follow the size principle or reverse the voluntary recruitment order.

Conclusions:

• Surface neuromuscular electrostimulation does not recruit motor units according to Henneman's size principle.
• Muscle fiber activation during electrostimulation appears to be nonselective and lacks a specific recruitment sequence.
• These findings suggest a fundamental difference in motor unit recruitment between voluntary activation and electrostimulation.