Biology subjects

Sergi, F.

Publications and source records attributed to Sergi, F..

3 recordsLinked to original sources

bioRxiv2018

The effect of physiological and measurement noise on the estimate of individual muscle force from indirect measurements of muscle activity

Several forward dynamics estimators have been proposed to quantify individual muscle force using non-invasive measurements of muscle activity. None of them, however, addresses the inaccuracy that arises when measurements are available only from a subset of the muscles involved in the movement under analysis. We present a novel estimator that integrates a forward dynamics estimation approach with knowledge of the optimal contraction strategy to obtain accurate estimates of individual muscle force when measurements of muscle activity are not available for all muscles. A following in-silico characterization showed that when trying to estimate forces form the forearm muscles acting around the wrist joint, our novel estimator is able to decrease the mean estimation bias by about 25% of the true value of muscle force. With a sensitivity analysis, we show that the model-based estimator is robust against physiological variability in muscle co-contraction strategy.

bioengineering↗

bioRxiv2018

The effect of stride length on lower extremity joint kinetics at various gait speeds

Robot-assisted training is a promising tool under development for improving walking function based on repetitive goal-oriented task practice. The challenges in developing the controllers for gait training devices that promote desired changes in gait is complicated by the limited understanding of the human response to robotic input. A possible method of controller formulation can be based on the principle of bio-inspiration, where a robot is controlled to apply the change in joint moment applied by human subjects when they achieve a gait feature of interest. However, it is currently unclear how lower extremity joint moments are modulated by even basic gaitspatio-temporal parameters.\n\nIn this study, we investigated how sagittal plane joint moments are affected by a factorial modulation of two important gait parameters: gait speed and stride length. We present the findings obtained from 20 healthy control subjects walking at various treadmill-imposed speeds and instructed to modulate stride length utilizing real-time visual feedback. Implementing a continuum analysis of inverse-dynamics derived joint moment profiles, we extracted the effects of gait speed and stride length on joint moment throughout the gait cycle. Moreover, we utilized a torque pulse approximation analysis to determine the timing and amplitude of torque pulses that approximate the difference in joint moment profiles between stride length conditions, at all gait speed conditions.\n\nOur results show that gait speed has a significant effect on the moment profiles in all joints considered, while stride length has more localized effects, with the main effect observed on the knee moment during stance, and smaller effects observed for the hip joint moment during swing and ankle moment during the loading response. Moreover, our study demonstrated that trailing limb angle, a parameter of interest in programs targeting propulsion at push-off, was significantly correlated with stride length. As such, our study has generated assistance strategies based on pulses of torque suitable for implementation via a wearable exoskeleton with the objective of modulating stride length, and other correlated variables such as trailing limb angle.

bioengineering↗

bioRxiv2018

Model-based analysis of the stiffness of the wrist joint in active and passive conditions

The control of joint stiffness is a fundamental mechanism used to control human movements. While many studies have observed how stiffness is controlled for tasks involving shoulder and elbow motion, a limited amount of knowledge is available for wrist movements, though the wrist plays a crucial role in fine manipulation.\n\nWe have developed a computational framework based on a realistic musculoskeletal model, which allows to calculate the passive and active components of the wrist joint stiffness. We first used the framework to validate the musculoskeletal model against experimental measurements of the passive wrist joint stiffness, and then to study the contribution of different muscle groups on the passive joint stiffness. We finally used the framework to study the effect of muscle co - contraction on the active joint stiffness.\n\nThe results show that thumb and finger muscles play a crucial role in determining the passive wrist joint stiff - ness: in the neutral posture, the direction of maximum stiffness aligns with the experimental measurements, and the magnitude increases by 113% when they are included. Moreover, the analysis of the controllability of joint stiffness showed that muscle co - contraction positively correlates with the stiffness magnitude and negatively correlates with the variability of the stiffness orientation (p < 0.01 in both cases). Finally, an exhaustive search showed that with appropriate selection of a muscle activation strategy, the joint stiffness orientation can be arbitrarily modulated. This observation suggests the absence of biomechanical constraints on the controllability of the orientation of the wrist joint stiffness.

bioengineering↗