Motor unit recruitment pattern during low-level static and dynamic contractions.

Motor unit (MU) recruitment patterns were studied during dynamic and static contractions at workloads corresponding to 10% of maximal voluntary contraction force. The dynamic contraction consisted of a 20 degrees flexion and extension of the elbow performed with a velocity of 10 degrees/s. Motor unit potential trains were recorded from the brachial biceps muscle of 6 healthy females using a quadripolar needle electrode and a computerized decomposition program. Properties of the identified MUs were derived from concentric needle EMG. A total of 119 MUs were identified during dynamic contractions, 107 MUs during static anisotonic contractions, and 96 MUs during static isotonic contractions. The main result was that MUs recruited during different contractions showed similar properties and may belong to the same part of the motoneuron pool. This indicates that MU recruitment patterns during dynamic contractions may be almost as stereotypical as during static contractions and may even activate the same MUs.