Motor Unit Discharge Rates during Maximal Shortening and Lengthening Contractions at Three Angular Velocities in...

Area of Science:

• Human Physiology
• Motor Control
• Biomechanics

Background:

• Previous research indicated lower motor unit discharge rates (MUDR) during lengthening versus shortening contractions, but only at sub-maximal efforts and slow speeds.
• This limitation highlighted a gap in understanding motor unit behavior during maximal dynamic contractions.

Purpose of the Study:

• To investigate and compare MUDRs of the triceps brachii during maximal-effort shortening and lengthening contractions.
• To examine these differences across a range of angular velocities (30, 60, and 90 °/s).

Main Methods:

• Ten young males and five females performed maximal isokinetic elbow extensions (shortening and lengthening) at three velocities.
• Fine-wire electrodes recorded MUDRs from the triceps brachii, with surface EMG also collected.
• Data were analyzed for motor unit trains during each contraction phase.

Main Results:

• No significant differences in mean MUDRs were found between shortening and lengthening contractions at 30, 60, or 90 °/s.
• Surface EMG amplitude also showed no difference between contraction types across all velocities.
• MUDRs increased with velocity from 30 °/s to 60 and 90 °/s, but not between 60 and 90 °/s.

Conclusions:

• Maximal shortening and lengthening contractions exhibit comparable MUDRs in the triceps brachii.
• This finding suggests no deficit in motor unit activation between dynamic contraction phases at maximal effort.
• The results challenge previous assumptions based on sub-maximal contractions.