# Are Torque-Driven Simulation Models of Human Movement Limited by an Assumption of Monoarticularity?
## 3 Results
### Work Done  
Concentric and eccentric joint work was calculated for each torque generator and summed to provide the net work done by each component at a joint. For the squat jump (Table 8), the net ankle work was similar between SJ and TJ models, with one third of the plantar flexor work for the TJ model coming from the biarticular component. At the knee joint, the net knee work was similar between SJ and TJ models, although there was substantial TJ biarticular knee flexor work that was negated by predominantly monoarticular knee extensor work. At the hip joint, there was 36% more work done by the TJ hip extensors, of which 63% was from the biarticular knee flexor-hip extensor torque generators, which performed as much work as the SJ hip extensors. For the countermovement jump (Table 9), the work calculations represent both descent and ascent phases. At the ankle joint, the SJ model produced 32% more net ankle work than the TJ simulation model. The TJ simulation had a 68% contribution to plantar flexor work from the biarticular component. A similar scenario existed for the knee work in the countermovement jump as for the squat jump, with small net knee joint work, but a large contribution at the knee from the biarticular knee flexor which was negated by the monoarticular extensor. With respect to the hip joint work, the TJ simulation performed 21% more work than the SJ simulation, and most of this contribution was from the biarticular knee flexor-hip extensor.  
By way of illustrating the nature of the differences in work and potential mechanical advantage from moment arms, the mean contractile component velocity for the plantar flexors and hip extensors, as a percentage of the joint velocity, was calculated for the entire simulation (Table 10). When examining the biarticular plantar flexor contractile component velocity for the squat jump and the biarticular hip extensor contractile component velocity for the squat and countermovement jumps, the biarticular velocities were substantially less than the joint velocity and their monoarticular and single-joint equivalents at 66%, 58% and 25% of joint velocity, respectively. According to the Hill hyperbola (force-velocity relationship), this would place them at a torque generating advantage. In contrast, the monoarticular and SJ torque generators had contractile component velocities that more closely reflected their joint velocities. In the case of the TJ squat jump model and when examining the full simulation, the biarticular component had an average contractile component velocity that was larger than the monoarticular equivalent and SJ version. This was because it started with a much lower activation level that ramped up continuously throughout the simulation, causing the series elastic component to be stretched and the contractile component to shorten, as compared to the monoarticular component, which had a constant activation of approximately 50% for the first 0.25 s, and therefore, had an almost constant series elastic component length. For the final 130 ms of the jump, the same situation occurs as for other jumps, with the biarticular component experiencing contractile  
Figure 7: Countermovement jump ascent; SJ (dashed black line), TJ (solid grey line), measured (solid black line); (**a**) orientation, (**b**) ankle, (**c**) knee, (**d**) hip.