Author(s): SAMOZINO, P., MOREL, B., BOCCIA, G., VONDERSCHER, M., LAFARGE, C., BOWEN, M., Institution: UNIVERSITY SAVOIE MONT BLANC, Country: FRANCE, Abstract-ID: 2367

INTRODUCTION:
The ability to rapidly increase muscle force (explosiveness) and dynamic force production capacities across different velocities (force-velocity-power capacities) are known to be two different determinants of ballistic contractions. However, when the force-velocity relationship is obtained from averaged values over a dynamic contraction, these two determinants may interact. Indeed, at any given time of the contraction, the maximal force the neuromuscular system can produce depends on the muscle shortening velocity and length, and the time from the initiation of the contraction. Recently, Boccia et al. (2025) showed that explosiveness (Rate of Torque Development, RTD) was correlated to maximal velocity (V0) and power (Pmax), but not maximal torque (T0), during isoinertial knee extensions. However, the understanding of this interaction may have been limited by interindividual variability in force-velocity, -length and -time relationships. The aim was to clarify the sensitivity of T0, V0 and Pmax to change in RFD from simulations of a simple model of knee extension.
METHODS:
Considering intrinsic hyperbolic torque-velocity, polynomial torque-length and exponential torque-time relationships, forward simulations of the knee extensors’ (KE) external torque were conducted over time during maximal dynamic knee extensions against various loads. External Torque (T)-Velocity (V) relationships of KE were obtained from fitting averaged values (100-130° knee angle) of T and V using a Hill-type function to derive external T0, V0 and Pmax. The sensitivity of T0, V0 and Pmax to change in RTD was i) tested via simulations of external TV relationship using time constant of the force-time curve from 0.05 to 0.15s, ceteris paribus, and ii) quantified from various T0 (150 to 350 N.m) and V0 (600 to 1400°/s) values.
RESULTS:
For typical values of T0 (250 N.m) and V0 (1000°/s), a 100% change in RTD (usually observed within population) affect positively V0 (+10.4%) and Pmax (+19.4%), and negatively T0 (-5.3%). When T0 changes from 150 to 350 N.m, the sensitivities of T0, V0 and Pmax to change in RTD remain quite unchanged. In contrast, when V0 changes from 600 to 1400°/s, the sensitivities of V0 and Pmax to change in RTD increase from 3.1 to 15.3% and from 14.2 to 23.3%, respectively, while no main change observed for T0.
CONCLUSION:
During dynamic contractions, the explosiveness interacts with muscle intrinsic TV and force-length relationships, resulting in differences between intrinsic and external TV relationships. The latter, when obtained from contraction-averaged values, is thus affected by RTD, with a substantial positive sensitivity for V0 and Pmax: the lower RTD, the lower external V0 or Pmax for the same intrinsic muscle capacities. This sensitivity is higher for high V0 values, supporting that a decrease in RTD alters more V0 if V0 is high, due to the quickness of the contraction.

Boccia et al, SJMSS 2025 Mar;35(3)