Influences of dynamic exercise on force steadiness and common drive.

OBJECTIVES: To investigate the influences of dynamic exercise on force steadiness and common drive to motor units.

METHODS: Seventeen men (age 24±4 years; height 181.7±5.7 cm; mass 89.6±14.9 kg) performed 6 sets of 10 repetitions of maximal isokinetic concentric (CON) or eccentric exercise (ECC) with their dominant elbow flexors on separate experimental visits. Before and after the interventions, maximal strength testing and submaximal trapezoid isometric contractions were performed. To quantify force steadiness, we calculated the amplitude of force fluctuations in the flat area of the submaximal trapezoid contractions. In addition, surface electromyographic (EMG) signals from the same portion where we calculated the force steadiness were decomposed into individual motor unit action potential trains. The mean firing rate curves of the detected motor units were then cross-correlated with one another to quantify the common drive.

RESULTS: Although both interventions induced similar strength losses, the ECC caused greater force fluctuations (p=0.002). In addition, unlike the CON, which did not cause any changes in the common drive, the ECC induced an increased common drive to motor units.

CONCLUSIONS: We believe that the increased common drive is an important factor causing greater force fluctuations following the ECC.