Six degree-of-freedom analysis of hip, knee, ankle and foot provides updated understanding of biomechanical work during human walking

Abstract

Measuring biomechanical work performed by humans and other animals is critical for understanding muscle-tendon function, jointspecific contributions and energy-saving mechanisms during locomotion. Inverse dynamics is often employed to estimate jointlevel contributions, and deformable body estimates can be used to study work performed by the foot. We recently discovered that these commonly used experimental estimates fail to explain whole-body energy changes observed during human walking. By re-analyzing previously published data, we found that about 25% (8 J) of total positive energy changes of/about the body's center-of-mass and >30% of the energy changes during the Push-off phase of walking were not explained by conventional joint-and segment-level work estimates, exposing a gap in our fundamental understanding of work production during gait. Here, we present a novel Energy-Accounting analysis that integrates various empirical measures of work and energy to elucidate the source of unexplained biomechanical work. We discovered that by extending conventional 3 degree-of-freedom (DOF) inverse dynamics (estimating rotational work about joints) to 6DOF (rotational and translational) analysis of the hip, knee, ankle and foot, we could fully explain the missing positive work. This revealed that Push-off work performed about the hip may be >50% greater than conventionally estimated (9.3 versus 6.0 J, P=0.0002, at 1.4 m s −1). Our findings demonstrate that 6DOF analysis (of hipknee-ankle-foot) better captures energy changes of the body than more conventional 3DOF estimates. These findings refine our fundamental understanding of how work is distributed within the body, which has implications for assistive technology, biomechanical simulations and potentially clinical treatment.

A unified perspective on ankle push-off in human walking

“…ipsilateral hip and knee, contralateral hip, knee and ankle, trunk and arm joints) and segments (e.g. feet) are far too small to explain this observed magnitude of COM pushoff work (Zelik et al, 2015a). The ankle and COM push-off magnitudes are observed to scale together with increasing speed (e.g.…”

“…We can also sum COM and Peripheral rates of energy change together before integrating, and we refer to the resultant time integral as (capitalized) 'Total' energy change (Zelik et al, 2015a). Standard gait analysis measures can be analyzed to provide reasonable estimates of COM (Cavagna et al, 1963;Donelan et al, 2002b), Peripheral (Cavagna et al, 1964(Cavagna et al, , 1977Willems et al, 1995) and Total mechanical energy changes Zelik et al, 2015a) during various locomotor activities. When studying walking, it is preferable to parse out contributions from the leading and push-off limbs, which oppose each other during double support (Donelan et al, 2002b).…”

“…When studying walking, it is preferable to parse out contributions from the leading and push-off limbs, which oppose each other during double support (Donelan et al, 2002b). However, to discuss individual limb contributions it is necessary to momentarily shift our language to COM work (or its derivative, COM power), as this can be estimated empirically for each limb, from the limb's ground reaction force (Donelan et al, 2002b;Zelik et al, 2015a). Individual limb COM work reflects the mechanical work that would be performed by the ground reaction force under one foot if it were applied directly to the COM as it moves; it is used as an estimate of work contributions from the entire limb.…”

“…Hip muscles (iliopsoas and others) contribute a lesser amount of positive power to this step-to-step transition, whereas the knee and foot are estimated to perform net negative work during this phase of the gait cycle (Zelik et al, 2015a). Various measurements and analyses provide evidence that a burst of positive power is performed about the ankle at the end of stance phase in walking, often termed 'push-off' (Fig.…”

The critical phase for visual control of human walking over complex terrain

“…To counteract this loss of energy, the trailing leg applies a pushoff force beginning toward the end of the single support phase and continuing through the double-support phase (13,14). The vertical component of the push-off force from the trailing leg sums with the vertical component of the force from the leading leg to direct the COM into the upward trajectory that it will need for the next step (Fig.…”