Measuring the vertical displacement of the center of mass (COM) of the body during walking may provide useful information about the energy required to walk. Four methods of varying complexity to estimate the vertical displacement of the COM were compared in 25 able-bodied, female subjects. The first method, the sacral marker method, utilized an external marker on the sacrum as representative of the COM of the body. The second method, the reconstructed pelvis method, which also utilized a marker over the sacrum, theoretically controlled for pelvic tilt motion. The third method, the segmental analysis method, involved measuring motion of the trunk and limb segments. The fourth method, the forceplate method, involved estimating the COM displacement from ground reaction force measurements. A two-tailed paired t-test within an ANOVA showed no statistically significant difference between the sacral marker and the reconstructed pelvis methods (p = 0.839). There was also no statistically significant difference between the sacral marker and the segmental analysis method (p = 0.119) or between the reconstructed pelvis and the segmental analysis method (p = 0.174). It follows that the first method, which is the most simple, can provide essentially the same estimate of the vertical displacement of the COM as the more complicated second and third measures. The forceplate method produced data with a lower range and a different distribution than the other three methods. There was a statistically significant difference between the forceplate method and the other methods (p < 0.01 for each of the three comparisons). The forceplate method provides information that is statistically significantly different from the results of the kinematic methods. The magnitude of the difference is large enough to be physiologically significant and further studies to define the sources of the differences and the relative validity of the two approaches are warranted.

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A goal of many physiatric interventions is to improve biomechanical walking efficiency. Thus, a tool that helps assess this efficiency, independent of cardiac, pulmonary, psychologic, or other nonbiomechanical factors, would be useful. Currently used methods to measure efficiency, including comfortable walking speed, are not specific to biomechanical variables. A potential tool, the biomechanical efficiency quotient (BEQ), which uses three variables--average stride length, vertical displacement of the trunk during walking, and sacral height during standing--is proposed and preliminarily tested. This quotient is based on Saunders, Inman, and Eberhart's theories and on a prior study in able-bodied subjects. The BEQ was computed in 20 consecutive patients with neurologically based gait disability referred for gait laboratory evaluation who subjectively reported that one or two ankle-foot-orthoses (AFOs) reduced the effort necessary to walk. The quotient was calculated with and without the AFO(s) by dividing the average vertical displacement of the sacrum, which was measured with an optoelectronic system, and by a predicted displacement, which was based on the patient's sacral height and average stride length. The mean BEQ with the AFO(s) (6.3 +/- 4.4) was significantly less than the mean BEQ without the AFO(s) (9.7 +/- 7.1); P = 0.005. Furthermore, the BEQ was less with the AFO(s) compared with trials without the AFO(s) in all subjects. Percent change in BEQ with the AFO(s) (26.8 +/- 19.6) correlated with percent change in comfortable walking velocity (24.8 +/- 31.8), r = 0.73, P<0.001, across all subjects. The BEQ may be useful in specifically assessing the effect on biomechanical efficiency of physiatric interventions, despite variable nonbiomechnical factors. An instrument to measure vertical trunk displacement during walking outside of the gait laboratory would be extremely useful for further necessary longitudinal studies.