Modeling the neuro-mechanics of human balance when recovering from a fall: a continuous-time approach.

BACKGROUND: Balance control deteriorates with age and nearly 30% of the elderly population in the United States reports stability problems. Postural stability is an integral task to daily living reliant upon the control of the ankle and hip. To this end, the estimation of joint parameters can be a useful tool when analyzing compensatory actions aimed at maintaining postural stability. METHODS: Using an analytical approach, this study expands on previous work and analyzes a two degrees of freedom human model. The first two modes of vibration of the system are represented by the neuro-mechanical parameters of a second-order, time-varying Kelvin-Voigt model actuated at the ankle and hip. The model is tested using a custom double inverted pendulum and healthy volunteers who were subjected to a positional step-like perturbation during quiet standing. An in silico sensitivity analysis of the influence of inertial parameters was also performed. RESULTS: The proposed method is able to correctly identify the time-varying visco-elastic parameters of of a double inverted pendulum. We show that that the parameter estimation method can be applied to standing humans. These results appear to identify a subject-independent strategy to control quiet standing that combines both the modulation of stiffness, and the use of an intermittent control. CONCLUSIONS: This paper presents the analysis of the non-linear system of differential equations representing the control of lumped muscle-tendon units. It utilizes motion capture measurements to obtain the estimates of the system's control parameters by constructing a simple time-dependent regressor for estimating the time-varying parameters of the control with a single perturbation. This work is a step forward into the understanding of the neuro-mechanical control parameters of human recovering from a fall. In previous literature, the analysis is either restricted to the first vibrational mode of an inverted-pendulum model or assumed to be time-invariant. The proposed method allows for the analysis of hip related movement for stability control and highlights the importance of core training.

Leg-ligament-thigh-trunk Dynamic Model to Describe Posture Recovery after Double-leg Landing Task.

One of the most common injuries in athletes is that of the Anterior Cruciate Ligament (ACL). This type of injury is commonly analyzed by observing the dynamics of the body in the sagittal plane. ACL injury can be indicated by a the small knee flexion angle and a small angular position of the trunk at start of leg-landing task. In this article a 4 Degrees of Freedom (DOF) dynamic model of the human body restricted to the sagittal plane is presented. The model represents the movement of the legs, an equivalent ligament between the tibia and femur, thighs and trunk. It is used to represent the recovery of vertical posture during a double leg landing task. Initial conditions in velocity are calculated as those resulting from a free fall from a height H. The results obtained from the simulation were satisfactory since the recovery of the vertical posture is achieved and it is possible to approximate the deformation suffered by the equivalent ligament. In conclusion, this model can be very useful in determining the behavior of the ligament and eventually, the possibility of injury after a double-leg landing task.

Experimental Estimation of a Second Order, Double Inverted Pendulum Parameters for the study of Human Balancing.

Balance control deteriorates naturally with age and is reliant upon the control of the ankle and hip joints. To this end, the estimation of ankle and hip parameters in quiet standing can be a useful tool when analyzing compensatory actions aimed at maintaining postural stability. This work presents an experimental study of a theoretical approach built upon previous results where the physiological parameters a second-order time-varying Kelvin-Voigt model are estimated for the actuation of the ankle and hip. These estimates are obtained using a double inverted pendulum based model subject to a step-like perturbation. Making use of RGB camera data to obtain the estimates of the system's visco-elastic parameters, the approach employed is capable of estimating the time-varying values for the body's control parameters. This work presents the first results of the method demonstrating the viability of a low-cost technique for regular testing of subjects with a high fall risk.