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The center of foot pressure (CP) motions, representing the net neuromuscular control, was compared to the center of gravity (CG) motions, representing the net performance. The comparison focused on the trajectory path length parameter along the mediolateral and antero-posterior axes because these two variables depend on amplitude versus frequency relationship. This relationship was used to evaluate the CG motions based on the CP motions. Seven subjects stood still on a force plate with eyes open and eyes closed. The results showed that the ratio of (CP – CG)/CP trajectory path length was personal for each subject. These results suggest different levels of passive (ligaments, elastic properties) and active (reflex activity) stiffness. For some subjects, this ratio was significantly lower for the eyes open condition than for the eyes closed condition, indicating a decrease of the active stiffness for the eyes open condition. Therefore, a CG – CP comparative analysis appeared helpful in understanding the control of balance and necessary to quantify the subjects’ net performance.

Past studies have emphasized the beneficial effect of additional visual feedback (VFB) on the capacity of healthy adults to decrease the amplitudes of the center-of-pressure minus center-of-gravity (CP-CG_v) movements. To better assess these capacities, 56 subjects were asked to stand still on a force platform and to use the visual information provided. Dependency coefficients, based on their capacity to lower their CP-CG_v movements and therefore relax their lower limb muscles, as well as parameters aimed at characterizing their postural strategies were measured across VFB conditions including (1) CP displacements in real time (VFB_CP0), (2) CP displacements with a 600-ms delay (VFB_CP600), and (3) CP-CG_v displacements with a 600-ms delay (VFB_CP-CG600). A non-VFB condition (eyes open) was also included. Several linear correlations were used to specify the relation between subjects’ capacity to relax, compared with the VFB_CP0 condition, across the three remaining conditions. The data highlight the complementary nature of the VFB conditions and establish the postural control behaviors necessary to use these VFB protocols efficiently.

The primary purpose of this paper was to compare the effect of reversing the direction of step initiation in Parkinson's disease. Forward (FDS) and backward (BDS) oriented stepping initiation analyses were conducted on combined kinematic and kinetic data recorded on Parkinsonian patients (PD) and healthy age-matched subjects. Two successive phases were examined: a postural phase from T1 (onset of the center of pressure [CP] displacement) to T2 (onset of the malleolus displacement), which was followed by a stepping phase from T2 to T3 (end of the malleolus displacement; i.e., the end of the step). In healthy subjects, the duration of the postural phase remained unchanged regardless of the direction in which the step was initiated. The stepping phase duration and the first step length were reduced in BDS in comparison with FDS. In both tasks, the absolute value of the horizontal force in sagittal plane (Fx) remained unchanged. The maximal velocity of the iliac crest marker (estimated whole body center of gravity [CG]) in the sagittal plane (V_max CG) remained within the same range regardless of direction of stepping. In Parkinsonian patients, the duration of the postural phase was markedly prolonged in both tasks in comparison with healthy subjects. The mean duration of stepping phase was approximately the same as in normal subjects, but the first step length was considerably reduced, as were horizontal force (Fx) and V_max CG. This impairment, which was due to a decrease in the propulsive forces, was significantly more pronounced in BDS that in FDS.

Accurate mass distribution in computational human body models is essential for proper kinematic and kinetic simulations. The purpose of this study was to investigate the mass distribution of a 50th percentile male (M50) full body finite element model (FEM) in the seated position. The FEM was partitioned into 10 segments, using segment planes constructed from bony landmarks per the methods described in previous research studies. Body segment masses and centers of gravity (CGs) of the FEM were compared with values found from these studies, which unlike the present work assumed homogeneous body density. Segment masses compared well to literature while CGs showed an average deviation of 6.0% to 7.0% when normalized by regional characteristic lengths. The discrete mass distribution of the FEM appears to affect the mass and CGs of some segments, particularly those with low-density soft tissues. The locations of the segment CGs are provided in local coordinate systems, thus facilitating comparison with other full body FEMs and human surrogates. The model provides insights into the effects of inhomogeneous mass on the location of body segment CGs.

To assess whether prior stretching of a muscle can induce improved postural control, 15 healthy adults stood still upright with their eyes closed before and after a series of bilateral stretches of the triceps surae muscles. The analysis focused on the center of pressure (CP) and the vertical projection of the center of gravity (CG_v) trajectories and their difference (CP – CG_v). The prolonged stretching induced a forward shift of the mean position of the CG_v. The frequency analysis showed a constancy of the amplitudes of both basic movements whereas an increased mean power frequency was seen for the CP – CG_v movements. A fractional Brownian motion modeling of the trajectories indicates shortest time intervals and lower covered distances by the CG_v before a change in its control occurs along the antero-posterior axis. This reorganization is thought to be a result of improved body movement detection, which allows postural control over the longest time intervals to be triggered more rapidly.

Possibilities and limitations in the biomechanical analysis of countermovement jump performance were examined using force plate data. Four male and 4 female sport students participated in the study. Software designed to test jumping performance was used to evaluate recordings from a force plate and to compute net velocity and net displacement measures for the center of gravity. In parallel, a film analysis incorporating Dempster's center of gravity model was used for a comparison. Validity of the computed kinetic measures was evaluated with a general analysis of the major error sources including the data acquisition and numerical computations. Numerical integration procedures were found to be a reasonable tool for calculating net velocity and net displacement parameters for a more detailed analysis of athletic jumping performance. On the other hand, it appeared that Dempster-like center of gravity models can cause errors that disqualify their use as validation criteria for kinetic parameters.