The neuromechanical delay of the quadriceps shortens with increasing contraction intensity

Neuromechanical delay (NMD) quantifies the time lag between the neural drive to a muscle and muscle force production. While NMD varies with factors like contraction speed and muscle group, its response to different contraction intensities remains unclear. This study examined NMD in the quadriceps during isometric trapezoidal contractions at 10%, 30%, 50%, and 70% of maximum voluntary contraction (MVC) in 13 recreationally trained males (24.3 ± 2.8 years). High-density surface electromyography recorded motor unit firings in the vastus lateralis and medialis, with neural drive quantified via cumulative motor unit firings. NMD was determined by cross-correlating neural drive with force signals during steady-force phases. Results showed a significant NMD reduction (p < 0.001) with increasing contraction intensity, decreasing by 31% from 175.8 ms (± 44.1) at 10% MVC to 121.6 ms (± 34.6) at 70% MVC. This likely reflects changes in muscle mechanical properties due to higher-threshold motor unit recruitment. These findings enhance our understanding of neuromechanical coupling, revealing a force-dependent modulation of NMD that may have implications for neuromuscular function in health and disease.