Experimental motion capture studies have commonly considered the foot as a single rigid body. However, the presence of internal joints as well as soft-tissue interactions inside the foot demonstrate that the foot is in fact not rigid. Various methods have been applied to study the deviations of the foot from rigid body mechanics, such as developing multi-segment foot models or employing compensation strategies for the rigid foot model. However, no study has compared these compensation strategies with multi-segment models in any movement such as gait or jumping. This study compared two main compensation strategies (distal foot power and power balance technique) as well as a two-segment foot model to study the power and work of the foot and the ankle in the takeoff phase of the standing vertical jump. Physically active participants (ages 20 – 26 years) performed several standing vertical jumps from a specific starting position spanning two adjacent force platforms such that the ground reaction forces (GRF) acting on the foot were divided at the metatarsophalangeal (MTP) joints.
The results of the study showed that the three methods for calculating work internal to the foot were significantly different from each other. Distal foot work was – 4.0 ± 1.0 J, foot work from power balance was 1.8 ± 1.1 J, and MTP joint work was 5.1 ± 0.5 J. Distal foot power showed a power absorption peak up to 111 W at around 0.1 seconds before takeoff, immediately followed by a peak power generation of 102 W. Foot power imbalance followed a similar pattern with a 31 W power absorption peak immediately followed by peak power generation of 135 W. There was minimal power absorbed at the MTP joints (a minor 7 W peak absorption) shortly after movement initiation, but apart from that, the MTP joints only generated power reaching a peak of 127 W. The results for ankle power and work did not show clinically significant difference between using the rigid foot model (58.3 ± 3.1 J) and multi-segment foot model (59.9 ± 3.4 J), even with the substantial power generation at the MTP joints. The likely cause for the similar ankle work values was that the anatomical reference frames for the entire rigid foot and for the rear foot were defined using the same markers (on the calcaneus, first and fifth metatarsal head). When the marker set defining the anatomical reference frame of the rear-foot were changed to ones on the calcaneus, navicular, and cuboid bones, the rigid foot model overestimated the ankle power in comparison to the multi-segment foot. The possible reason for this was that the new markers were all on the more rigid hind-foot, which led to the rear-foot being modeled as equivalent to a more rigid hind-foot and a massless midfoot.
The results suggest that MTP joints are only one source of the foot power and that comparison between distal foot power and power balance technique should be further explored in jumping and other movements. Improvements in the understanding of foot and ankle mechanics in standing vertical jump might also be obtained by implementing a foot model with more than two segments. This would require a better way of distributing the ground reaction kinetics between the foot segments because the adjacent force platform method would not be practicable.
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