Author(s): SALVAGGIO, F., GROSSIO, L., BIZET, B., RAINOLDI, A., MONTE, A., BOCCIA, G., Institution: UNIVERSITY OF TURIN, Country: ITALY, Abstract-ID: 2245

INTRODUCTION:
Explosive strength, quantified as the rate of torque development (RTD), can be viewed as a composite function resulting from the interaction between maximal strength and quickness in achieving a given level of torque. Therefore, RTD can improve in two ways: by increasing maximal voluntary torque (MVT) through maximal strength training or by increasing the rapidity of torque production through explosive strength training. This study aimed to compare the effects of explosive (EXP) versus sustained (STR) isometric training on RTD of the knee extensor muscles.
METHODS:
Twenty-two young adults underwent eight weeks of isometric training, two sessions per week. Participants trained one leg in EXP and the other in STR after randomized allocation counterbalanced by leg dominance. Both training interventions involved a linear increase in training volume. Force and High-Density surface EMG signals (HD-sEMG) from the three superficial knee extensor muscles were collected. MVT, time-locked absolute and normalized RTD, evoked RTD at 50 ms, and neural efficacy (voluntary/evoked RTD at 50 ms) were computed. Muscle excitation was calculated as the RMS of the HD-sEMG channel-wise normalized to the M-wave at each time window, and averaged across rectus femoris, vastus lateralis, and vastus medialis. M-wave peak-to-peak amplitude and conduction velocity were computed as measures of muscle excitability.
RESULTS:
MVT increased by 21.9% in STR and by 12.3% in EXP, with significantly larger gains in STR (time×training, p < .001).
Absolute RTD increased by 11.9% to 24.0% in STR and by 16.5% to 24.6% in EXP, but no significant interaction between time × training, nor between time × training × epoch appeared.
However, normalized RTD decreased by −1.7% to −8.9% in STR but increased +1.9% to +17.5% in EXP, showing a significant time × training interaction (p < .001).
Evoked RTD at 50 ms significantly increased by 5.1% in both groups with no significant time × training interaction, whereas neural efficacy only improved in EXP with time × training interaction.
Across time windows, muscle excitation increased by 13.0% to 19.6% in STR and by 16.2% to 19.8% in EXP without differences between trainings, while M-wave maximum amplitude and conduction velocity did not improve.
CONCLUSION:
These findings indicate that both maximal and explosive strength training enhance rapid torque production.
Maximal strength training primarily increases the magnitude of torque that can be generated, thereby elevating the ceiling of torque available during contraction. In contrast, explosive strength training predominantly enhances the rate at which this available torque is developed.