Short-term training for explosive strength causes neural and mechanical adaptations

This study investigated the neural and peripheral adaptations to short-term training for explosive force production. Ten men trained the knee extensors with unilateral explosive isometric contractions (1 s fast and hard) for 4 weeks. Before and after training, force was recorded at 50-ms intervals from force onset (F50, F100 and F150) during both voluntary and involuntary (supramaximal evoked octet; eight pulses at 300 Hz) explosive isometric contractions. Neural drive during the explosive voluntary contractions was measured with the ratio of voluntary/octet force, and average EMG normalized to the peak-to-peak M-wave of the three superficial quadriceps. Maximal voluntary force (MVF) was also measured, and ultrasonic images of the vastus lateralis were recorded during ramped contractions to assess muscletendon unit stiffness between 50 and 90% MVF. There was an increase in voluntary F50 (+54%), F100 (+15%) and F150 (+14%) and in octet F50 (+7%) and F100 (+10%). Voluntary F100 and F150, and octet F50 and F100 increased proportionally with MVF (+11%). However, the increase in voluntary F50 was +37% even after normalization to MVF, and coincided with a 42% increase in both voluntary/octet force and agonist-normalized EMG over the first 50 ms. Muscletendon unit stiffness between 50 and 90% MVF also increased. In conclusion, enhanced agonist neural drive and MVF accounted for improved explosive voluntary force production in the early and late phases of the contraction, respectively. The increases in explosive octet force and muscletendon unit stiffness provide novel evidence of peripheral adaptations within merely 4 weeks of training for explosive force production.