Author(s): RUGGIERO, L., GRUBER, M., BOCCIA, G., Institution: UNIVERSITY OF PADUA, Country: ITALY, Abstract-ID: 586

INTRODUCTION:
Explosive strength is the neuromuscular capacity to produce joint torque as rapidly as possible. This quality is critical for sport performance, physical activity, and clinical function. When measured as an absolute value (Nm·s⁻¹), explosive strength depends on athletic background (e.g., endurance vs. power), sex, and fatigability status. However, these differences almost completely disappear as soon as explosive strength is normalised to maximal strength (peak torque per second; PT·s⁻¹), indicating the existence of an upper limit of explosive strength imposed by maximal torque capacity. By combining experimental measures and computational modelling, we tested the existence of such a limit, and checked whether it can be reached for physiological ranges of motor unit (MU) recruitment rates.
METHODS:
Isometric ankle dorsiflexion torques were recorded in 13 participants during trains of supramaximal electrical stimuli delivered to the common peroneal nerve at 10 and 100 Hz, including catch-like contractions. Torque traces were used to extract parameters for a model of electrically evoked contractions, and to derive mean MU force. The discharge rate of MU was modelled using a bi-exponential system to account for spike-frequency adaptation (Spielmann et al., 1993), from experimental discharge rates of 158 MUs identified in the tibialis anterior during explosive contractions (Grootenhuis et al., 2025). Simulations included 150 motor units, consistent with anatomical estimates for the tibialis anterior (Debenham et al., 2025). Explosive isometric contractions were simulated across physiological MU recruitment rates, from 200 to 12000 MU·s⁻¹ (Del Vecchio et al., 2022). Explosive strength was quantified as the peak value of the rate of torque development function (RTD), computed with progressively increasing time windows (by 1 ms) from torque onset.
RESULTS:
The effect of MU recruitment rate on RTD was non-linear. Increasing recruitment rate from 200 to 4000 MU·s⁻¹ produced a disproportionate rise in RTD from 1.1 to 5.0 PT·s⁻¹. Beyond this range, progressively larger increases in recruitment rate yielded only marginal RTD gains. At high but physiologically plausible MU recruitment rates (> 6000 MU·s⁻¹), RTD approached a plateau of ~6 PT·s⁻¹, consistent with maximal values reported in the literature for the ankle dorsiflexors.
CONCLUSION:
These findings support the existence of an upper physiological limit to explosive strength determined by maximal torque capacity and provide an explanation for similar normalized explosive strength across populations and conditions. Although maximal strength constrains explosive performance, its precise role as a confounder, mediator, or collider needs still to be determined.

References:
Del Vecchio et al., 2022, J Appl Physiol, 132(1): 84–94.
Debenham et al., 2025, J Electromyogr Kinesiol, EPub ahead of print.
Grootenhuis et al., 2025, Scand J Med Sci Sports, 35(5) ; e70065.
Spielmann et al., 1993, J Physiol, 464(1):75-120.