# Are Torque-Driven Simulation Models of Human Movement Limited by an Assumption of Monoarticularity?
## 4 Discussion
One such solution is to use a variety of experimental interventions to manipulate the demands of vertical jumping, for example, placing different emphasis on stability and jump height and then using data reduction and clustering methods, such as principle component analysis (PCA) and discriminant function analysis (DFA), to calculate variable weightings for an objective function [46]. The use of biparticular (TJ) torque generators in simulations of jumping goes some way to bridging the gap between the capability of torque-driven simulations, which have typically answered questions associated with whole-body performance, and those of individual muscle models, which have tackled questions associated with neuromuscular control and individual muscle properties. A TJ simulation provides an additional illustration of the neuromuscular system by providing a synergistic description of monoarticular and biparticular components. Using a TJ simulation, it is now possible to investigate the general role of biparticular torque generators by combining the evaluation of technique and EMG. The number of available activation parameters for a TJ simulation is larger than that for SJ, however it should be noted that were the limitation of the SJ simulation that of a control limitation, it might have been expected that the simulation would exploit all the activation parameters available to it for ramping up and down the activations. More broadly, the SJ simulation did not use all the available ramps in activation and fewer than the TJ simulation, suggesting that the predominant limitation of the SJ model relates to strength. In this study, a single participant design was used, and this could be considered to be a limitation of the study as the effect of different inertias and strengths have not been considered. However, using a mechanics-based computer simulation model, the inertias and strength parameters used have much less influence on model performance than the activity to be modelled. Furthermore, although the model, including TJ representations, has more parameters to vary compared with the SJ version, the overall strength at each joint for both models was equivalent. This limits any advantage of additional parameters, and could indeed lead to worse performance if such in-built relationships were not of advantage for the movement under consideration. The objective difference functions, Equations (3) and (4), assume that a \(1^{\circ}\) difference in an angle change was equivalent to a 1% difference in time of contact, the vertical velocity at take-off or vertical ground reaction force. This is a reasonable assumption that has been successfully used to evaluate previously developed simulation models [38], giving a balanced measure of how well the model can match an actual performance across multiple criteria. In conclusion, the use of subject-specific TJ representations of lower-limb joint torques rather than SJ representations, results in smaller differences between simulations and measured performances for both squat and countermovement jumps (examples of activities that start with minimal initial energy but require maximal effort). The magnitude of the difference between simulations and measured performances are small for the submaximal, weight assisted, slow speed downward countermovement and large for movements requiring larger joint torques and a requirement for large joint velocities. The outcome measure of jump height alone indicates a clear improvement in model accuracy when utilising a TJ representation. However, simple SJ models are equally effective at matching submaximal aspects of the countermovement jump. Overall a TJ torque generator representation is likely to be more appropriate for simulating dynamic tasks requiring large joint torques and near-maximal joint velocities. Conceptualization, all authors; writing--original draft preparation, M.G.C.L. ; writing--review and editing, all authors; supervision, M.R.Y. and M.A.K. All authors have read and agreed to the published version of the manuscript.