Oron Levin

Muscle fatigue in interrupted stimulation: Effect of partial recovery on force and EMG dynamics.

Muscle fatigue is a major problem in functional electrical stimulation (FES); the understanding of fatigue and recovery processes is thus of great interest. In interrupted stimulation, fatigue and recovery occur in sequence, and the history-dependency of the muscles response to FES becomes significant. In this work, the force and electromyographical (EMG) fatigue characteristics of FES-activated paralysed muscles were studied, both in the initially unfatigued state (primary fatigue) and in the reactivated state, after rest periods of prescribed durations (post-recovery fatigue). Because the data were collected over weeks, longitudinal studies were also made to account for long-term training effects of the muscle. Mechanical and myoelectric profiles, the latter derived from the M-wave, were obtained from the right quadriceps of two paraplegic subjects under isometric stimulation. Force was found to correlate highly with peak-to-peak amplitude of the EMG M-wave. Training did not affect this correlation, but as the recovery duration increased, the force-EMG curves became less concave. Training was found to increase the muscle force and EMG peak-to-peak amplitude, as well as the residual force achieved, but it had no noticeable effects on the M-wave duration parameters. Both the force and EMG parameters demonstrated substantial recovery within the first 3 min of rest, and exhibited a consistent tendency to level off for higher periods of rest. After comparing this finding to those expected from previous metabolic models, it was concluded from the subjects studied and model developed that, in addition to metabolic factors, electrolytic factors may be significant in governing the dynamics of fatigue and recovery.

An iterative model for estimation of the trajectory of center of gravity from bilateral reactive force measurements in standing sway

Abstract The trajectory of the human bodys center of gravity (CG) during motion is usually determined from measurements of whole-body kinematics. Yet another method for estimating the CG trajectory is based on force-plate data of reactive forces and centers of pressure and on applying the equations of motion. However, using this latter method requires additional information on the rate of change of the angular momentum (Ḣ) of the body about its CG. In this study we present a model for the estimation of the CG trajectory during standing sway, based on the reactive forces and centers of pressures as measured bilaterally, i.e. from two force platforms. An iterative algorithm was developed to evaluate Ḣ and accordingly correct the estimated CG trajectory. A three-dimensional, four-joint, five-segment model of the human body was used to describe postural standing sway dynamics. In the first iteration, the kinematics of the segments was calculated from the CG trajectory obtained for Ḣ = 0. From the second iteration, the estimated values of Ḣ were used to update the CG trajectory. The above method is illustrated on a group of 11 able-bodied subjects and the results obtained indicate that the CG trajectory is negligibly affected by the Ḣ contribution. The method is further implemented on two subjects with musculo-skeletal pathologies to illustrate the model behavior and resulting CG trajectory in cases where asymmetry during standing can be expected.

EMG and metabolite-based prediction of force in paralyzed quadriceps muscle under interrupted stimulation

A major issue associated with functional electrical stimulation (FES) of a paralyzed limb is the decay with time of the muscle force as a result of fatigue. A possible means to reduce fatigue during FES is by using interrupted stimulation, in which fatigue and recovery occur in sequence. In this study, we present a model which enables us to evaluate the temporal force generation capacity within the electrically activated muscle during first stimulation fatigue, i.e., when the muscle is activated from unfatigued initial conditions, and during postrest stimulation, i.e., after different given rest durations. The force history of the muscle is determined by the activation as derived from actually measured electromyogram (EMG) data, and by the metabolic fatigue function expressing the temporal changes of muscle metabolites, from existing data acquired by in vivo 31P MR spectroscopy in terms of the inorganic phosphorus variables, Pi or H2PO4-, and by the intracellular pH. The model was solved for supra-maximal stimulation in isometric contractions separated by rest periods, and compared to experimentally obtained measurements. EMG data were fundamental for prediction of the ascending force during its posttetanic response. On the other hand, prediction of the decaying phase of the force was possible only by means of the metabolite-based fatigue function. The prediction capability of the model was assessed by means of the error between predicted and measured force profiles. The predicted force obtained from the model in first stimulation fatigue fits well with the experimental one. In postrest stimulation fatigue, the different metabolites provided different prediction capabilities of the force, depending on the duration of the rest period. Following rest duration of 1 min, Pi provided the best prediction of force; H2PO4- extended the prediction capacity of the model to up to 6 min and pH provided a reliable prediction for rest durations longer than 12 min. The results presented shed light on the roles of EMG and of metabolites in prediction of the force history of a paralyzed muscle under conditions where fatigue and recovery occur in sequence.

Standing sway: iterative estimation of the kinematics and dynamics of the lower extremities from force-plate measurements

Abstract In this study, a model for the estimation of the dynamics of the lower extremities in standing sway from force plate data only is presented. A three-dimensional, five-segment, four-joint model of the human body was used to describe postural standing sway dynamics. Force-plate data of the reactive forces and centers of pressure were measured bilaterally. By applying the equations of motion to these data, the transversal trajectory of the center of gravity (CG) of the body was resolved in the sagittal and coronal planes. An inverse kinematics algorithm was used to evaluate the kinematics of the body segments. The dynamics of the segments was then resolved by using the Newton-Euler equations, and the models estimated dynamic quantities of the distal segments were compared with those actually measured. Differences between model and measured dynamics were calculated and minimized, using an iterative algorithm to re-estimate joint positioning and anthropometric properties. The above method was tested with a group of 11 able-bodied subjects, and the results indicated that the relative errors obtained in the final iteration were of the same order of magnitude as those reported for closed loop problems involved in direct kinematics measurements of human gait.

Muscle Strength and Geometrical Changes in A Paralysed Muscle Following FES

Abstract This study proposes the application of a strengthening index to quantify the effect of training, by functional electrical stimulation (FES), on the force capacity of the quadriceps in spinal cord injury (SCI) subjects. The index is based on evaluating the average muscle force per unit area. This measure is shown to express the overall increase in the muscle force capacity, while accounting for the changes taking place in muscle geometry. The proposed index is demonstrated on one subject with SCI, on whom a longitudinal follow-up was conducted. The measurements included the knee extension torque, from which the force in the quadriceps muscle was evaluated. Additionally, in vivo magnetic resonance imaging of the thigh was used to obtain the muscle anthropometry. In the training period followed-up in this study, the average force per unit area was found to increase from 27 N/cm 2 in the pretrained muscle to 40 N/cm 2 after eight weeks of training by FES. The major increase in the physiological cross-sectional area of the muscle took place during the first four-week period; 12% of the total 13.5%. Conversely, only a minor change in the average force per unit area of the muscle was observed during the first four weeks of training (28 N/cm 2 at the end of the fourth week). Thus, the major increase (43%) in the ratio of peak force to muscle physiological cross-sectional area was observed during the second four-week period of training. This latter response is attributed to neural adaptation of the axons and neuromuscular junction rather than to an increase in the muscle fibre specific tension.