Early motor unit conduction velocity changes to HIIT versus continuous training

PURPOSE: Moderate-intensity continuous training (MICT) and high-intensity interval training (HIIT) are associated with different adjustments in motor output. Changes in motor unit (MU) peripheral properties may contribute to these adjustments, but this is yet to be elucidated. This study evaluated early changes in MU conduction velocity (MUCV) and MU action potential (MUAP) amplitude following two weeks of either HIIT or MICT.

METHODS: Sixteen men were assigned to either an MICT or HIIT group (n=8 each), and participated in six training sessions over 14 days. HIIT: 8-12×60-s intervals at 100% peak power output. MICT: 90-120min continuous cycling at ~65% VO2peak. Pre and post intervention, participants performed maximal voluntary contractions (MVC) and submaximal (10, 30, 50 and 70% of MVC) isometric knee extensions while high-density electromyography (HDEMG) was recorded from the vastus medialis (VM) and vastus lateralis (VL) muscles. The HDEMG was decomposed into individual MUs by convolutive blind-source separation and tracked pre-and post-intervention.

RESULTS: Both training interventions induced changes in MUCV, but these changes depended on the type of training (p<0.001). The HIIT group showed higher values of MUCV following training at all torque levels (p<0.05), MICT only displayed changes in MUCV at low torque levels (10-30% MVC, p<0.002). There were no changes in MUAP amplitude for either group (p=0.2).

CONCLUSIONS: Two weeks of HIIT or MICT elicit differential changes in MUCV, likely due to the contrasting load and volume used in such training regimes. This new knowledge on the neuromuscular adaptations to training has implications for exercise prescription.