Milad G. Ishac

Trajectory of the body COG and COP during initiation and termination of gait

Abstract Five initiation and five termination plus three steady-state walking trials were collected for each of four subjects using three videos cameras and three force platforms. Data were analysed using a 13-segment three-dimensional biomechanical model. During initiation 90% of steady-state velo-city was achieved during the first step and 100% by the second step. During termination, 10% of the velocity was reduced in the first step and 90% in the last step. The interaction between the centre of mass (COM) and centre of pressure (COP) is tightly regulated to control the trajectory of the COM and thereby control total body balance. Coarse control of this balance is achieved by foot placement, with fine control during weight bearing by the ankle musculature.

Motor mechanisms of balance during quiet standing

The purpose of this paper is to highlight the motor mechanisms involved in balance as the human, as a biped, continuously defends against gravitational and internal forces to maintain a safe posture. The search for these mechanisms needs precise and valid 3D measurements including both limbs plus valid biomechanical models. The literature shows the need for two force platforms to separate the mechanisms at the ankle and hip (load/unload mechanism). Also, precise measures ( approximately 0.03 mm) of markers on a multi-segment 3D bilateral model are required to record the minute trajectories of all segments and joints. The controlled variable, center-of-mass, is seen to be virtually in phase with the controlling variable, the center-of-pressure, which suggests a 0th order system where a simple series elastic spring could maintain balance. The first model involves a mass/spring/damper of medial/lateral balance: the stiffness was varied with stance width and the predicted sway from a spring controlled inverted pendulum closely matched the experimentally measured stiffness and sway. The second was a non-linear model of the plantarflexor series elastic elements which resulted in three closely validated predictions of anterior/posterior balance: the locus of the gravitational load line, the predicted ankle moment and the ankle stiffness at the operating point.

Altered kinetic strategy for the control of swing limb elevation over obstacles in unilateral below-knee amputee gait

Our goal was to document the kinetic strategies for obstacle avoidance in below-knee amputees. Kinematic data were collected as unilateral below-knee traumatic amputees stepped over obstacles of various heights in the walking path. Inverse dynamics were employed to calculate power profiles and work during the limb-elevation and limb-lowering phases. Limb elevation was achieved by employing a different strategy of intra-limb interaction for elevation of the prosthetic limb than for the sound limb, which was similar to that seen in healthy adult non-amputees. As obstacle height increased, prosthetic side knee flexion was increased by modulating the work done at the hip, and not the knee, as seen on the sound side. Although the strength of the muscles about the residual knee was preserved, the range of motion of that knee had previously been found to be somewhat limited. Perhaps more importantly, potential instability of the interface between the stump and the prosthetic socket, and associated discomfort at the stump could explain the altered limb-elevation strategy. Interestingly, the limb-lowering strategy seen in the sound limb and in non-amputees already features modulation of rotational and translational work at the hip, so an alternate strategy was not required. Thus, following a major insult to the sensory and neuromuscular system, the CNS is able to update the internal model of the locomotor apparatus as the individual uses the new limb in a variety of movements, and modify control strategies as appropriate.

Kinematic patterns of participants with a below-knee prosthesis stepping over obstacles of various heights during locomotion

Abstract The focus of this paper was to examine the lead limb preference and the kinematic patterns of the lead limb of participants with a unilateral below-knee prosthesis when stepping over obstacles of different heights. Firstly, ten unilateral below-knee amputees stepped over obstacles of three different heights placed in their walking path to determine if they had a lead limb preference. Five of the ten participants demonstrated a sound limb lead preference, two participants showed a prosthetic limb lead preference and three showed no preference at all. Seven of these participants subsequently stepped over obstacles of six different heights leading with their sound or prosthetic limb as observed in the first experiment, while whole body kinematic data were collected. Relative joint angles of the stance and swing limb were calculated when the swing limb was over the obstacle. Swing hip elevation and hip and knee flexion increased as functions of obstacle height. Stance limb hip flexion, knee flexion and (on the sound side) ankle plantarflexion increased slightly with increasing obstacle height, but stance limb hip elevation did not. Therefore, it appears that these stance limb modulations served to position the pelvis further back from the obstacle as obstacle height increased. The posterior shell of the prosthetic socket limited residual limb swing knee flexion, and the increased ankle dorsiflexion seen on the lead sound side was not present on the lead prosthetic side. These limitations were associated with increased swing prosthetic foot angle and increased stance ankle plantarflexion. These results provide insights into the adaptability of the locomotor system, and have implications for lower extremity prosthetic design and amputee rehabilitation.

Wittenburg's formulation of multibody dynamics equations from a graph-theoretic perspective

Abstract In the past 20 years, many multibody or “self-formulating” computer programs have been developed for the simulation of dynamic mechanical systems. These programs are based on algorithms that enable the computer to formulate, automatically, the equations of motion for a specified system, given only a description of the system as input; numerical methods can be applied to the resulting differential-algebraic equations in order to determine the time response of the system. Whether explicitly or implicitly, all of these multibody algorithms use concepts from linear graph theory in some form or another. The “vector-network technique” represents a direct application of graph-theoretic methods to dynamic mechanical systems. In contrast, Wittenburgs well-known formalism for creating the equations of motion appears, at first glance, to use linear graph theory for the sole purpose of representing the system topology. The goal of this paper is to examine in detail the relationship between these two methodologies. In particular, the authors will establish the equivalence of the equations derived using Wittenburgs formulation, and those arising out of a formal graph-theoretic approach.

A technique to analyse the kinetics and energetics of cane-assisted gait.

UNLABELLED The purpose of this study was to develop a biomechanical technique to analyse the kinetics of cane-assisted gait. Biomechanical measures such as ground reaction forces (force platforms), cane reaction forces, and kinematics have been routinely measured. However, a full kinetic analysis of both the lower limb and the cane-assisted limb has not been reported: joint reaction forces, moments of force and mechanical powers. Such estimates give the researcher and clinician insight into the levels at each of the joints and the kinetics of the muscles responsible for the altered locomotion and stability. Standard inverse dynamics techniques were employed using a three-dimensional force transducer in the tip of the cane and as the subject walked over a force platform while his movement was recorded on video. Special problems existed when both the cane and foot bore weight on the force platform; the resultant indeterminacy problem was resolved so that independent solutions could be applied to both the lower and upper limbs. RELEVANCE A full kinetic and energetic biomechanical analysis is needed to identify motor pattern changes at each joint resulting from the use of the cane. Such information will be useful in pinpointing not only motor pattern changes at an affected joint but also adaptive motor pattern changes at other joints. Also, the contribution of the muscles at the wrist, elbow, and shoulder becomes evident not only to the stability of the gait but also to the energetics of forward propulsion.