The purpose of this study was to: (1) Assess the ability of an EMG-to-force model to accurately predict net joint torques in a broad range of lower extremity movements; (2) Determine the relative contribution of the muscle fibres and the tendon to the mechanical work done by the MTC in these movements; and (3) Determine the relative contribution of mechanical energy transfer via biarticular muscles to the work done at a joint in these movements.
Twelve subjects performed the semi-squat at 2 loading conditions, vertical jumps with and without a counter-movement, and walking, jogging and running trials within the same testing session. Sagittal plane trajectories of anatomical landmarks, ground reaction forces and electromyograms (EMG) from 6 lower extremity muscles were monitored as the subjects performed at least 5 repetitions at each experimental condition.
An EMG-to-force model was used to estimate the length, velocity and force in the contractile component (CC), series elastic component (SEC) and parallel elastic component (PEC) of 6 lower extremity muscles from input describing relative muscle stimulation (STEM) levels and muscle-tendon complex lengths (LMTC)- Normalised smoothed rectified EMG was used as measure of STIM and LMTC and moment arms for each muscle were determined using regression coefficients from the literature. Muscle specific parameter values for the maximum isometric force, tendon slack length and width of the isometric force-length relation were estimated by fitting modelled torque-angle curves to experimental torque-angle data for the hip, knee and ankle joint extensors.
As expected, a perfect agreement between model predictions and experimental measures of net knee and hip joint torques was not obtained. However errors in modelled and measured joint torques were relatively low compared to differences in joint torques between the tasks under investigation. For example, the mean %RMS error between modelled and measured torques averaged across all joints in the semi-squat was approximately 20%. It was therefore concluded that model predictions were adequate for examining the mechanical role of series elasticity and biarticular muscles in lower extremity movements.
On average subjects jumped 3 cm higher in the countermovement jump (CMJ) compared to the squat jump (SJ). Because the concentric angular displacements of the hip, knee and ankle were the same for both jumps, increased work production in the CMJ was explained by greater average force production in the CMJ, which was traced to the fibres and tendon of the hip and knee extensors. Differences m timing of force production of the MTC were noted between the CMJ and SJ. Peak muscle forces the SJ occurred during shortening of the MTC which meant that the CC performed work on the SEC during in the initial part of the push-off. In contrast, the CC and SEC shortened more as a single unit in the CMJ with all work by the CC and SEC being done on the skeleton.
Mechanical energy transfer (MET) via biarticular muscles in walking, jogging and running was examined using an EMG-to-force model as well as a model in which net joint torques were distributed amongst muscles spanning each joint assuming reciprocal inhibition and equal specific tension in synergistic muscles. Both models predicted MET via the biarticular rectus femoris from hip to knee (stance phase) and knee to hip (stance/swing transition) during walking, jogging and running. Only the EMG-to-force model was able to predict MET from knee to hip via hamstrings which took place in the latter part of the swing phase. MET via gastrocnemius from ankle to knee (early stance) and knee to ankle (late stance) was predicted by both models but was not evident in walking. The amount of MET was observed to increase with gait speed but was strongly influenced by modelling assumptions. EMG-to-force model predictions indicated that MET accounted for 6-83% of the work done by the net joint moment at the joint to which the energy flowed. The amount of elastic energy recovered from the SEC of the plantarflexors and knee extensors during locomotion accounted for 17-25% of the work done by the respective net joint moments.
The results of the present study indicate that series elasticity and biarticular muscles play an important functional role in improving the efficiency of lower extremity movements. Elastic energy recovered from the tendon reduces the work to be performed by the energy consuming muscle fibres, while biarticular muscles facilitate the distribution of energy amongst joints in order meet task demands.