Lack of increased rate of force development after strength training is explained by specific neural, not muscular, motor unit adaptations

Alessandro Del Vecchio, Andrea Casolo, Jakob Lund Dideriksen, Per Aagaard, Francesco Felici, Deborah Falla, Dario Farina

Research output: Contribution to journalArticlepeer-review

Abstract

While maximal force increases following short-term isometric strength training, the rate of force development (RFD) may remain relatively unaffected. The underlying neural and muscular mechanisms during rapid contractions after strength training are largely unknown. Since strength training increases the neural drive to muscles, it may be hypothesized that there are distinct neural or muscular adaptations determining the change in RFD independently of an increase in maximal force. Therefore, we examined motor unit population data acquired from surface electromyography during the rapid generation of force before and after four weeks of strength training. We observed that strength training did not change the RFD because it did not influence the number of motor units recruited per second or their initial discharge rate during rapid contractions. While strength training did not change motoneuron behaviour in the force increase phase of rapid contractions, it increased the discharge rate of motoneurons (by ~4 spikes/s) when reaching the plateau phase (~150 ms) of the rapid contractions, determining an increase in maximal force production. Computer simulations with a motor unit model that included neural and muscular properties, closely matched the experimental observations and demonstrated that the lack of change in RFD following training is primarily mediated by an unchanged maximal recruitment speed of motoneurons. These results demonstrate that maximal force and contraction speed are determined by different adaptations in motoneuron behaviour following strength training and indicate that increases in the recruitment speed of motoneurons are required to evoke training-induced increases in RFD.

Original languageEnglish
JournalJournal of Applied Physiology
Early online date18 Nov 2021
DOIs
Publication statusE-pub ahead of print - 18 Nov 2021

Keywords

  • Motor Neurons
  • Motor Units
  • Rate of Force Development
  • Muscle Force
  • Strength Training

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