Foot anatomy specialization for postural sensation and control

Abstract

Anthropological and biomechanical research suggests that the human foot evolved a unique design for propulsion and support. In theory, the arch and toes must play an important role, however, many postural studies tend to focus on the simple hinge action of the ankle joint. To investigate further the role of foot anatomy and sensorimotor control of posture, we quantified the deformation of the foot arch and studied the effects of local perturbations applied to the toes (TOE) or 1st/2nd metatarsals (MT) while standing. In sitting position, loading and lifting a 10-kg weight on the knee respectively lowered and raised the foot arch between 1 and 1.5 mm. Less than 50% of this change could be accounted for by plantar surface skin compression. During quiet standing, the foot arch probe and shin sway revealed a significant correlation, which shows that as the tibia tilts forward, the foot arch flattens and vice versa. During TOE and MT perturbations (a 2- to 6-mm upward shift of an appropriate part of the foot at 2.5 mm/s), electromyogram (EMG) measures of the tibialis anterior and gastrocnemius revealed notable changes, and the root-mean-square (RMS) variability of shin sway increased significantly, these increments being greater in the MT condition. The slow return of RMS to baseline level (>30 s) suggested that a very small perturbation changes the surface reference frame, which then takes time to reestablish. These findings show that rather than serving as a rigid base of support, the foot is compliant, in an active state, and sensitive to minute deformations. In conclusion, the architecture and physiology of the foot appear to contribute to the task of bipedal postural control with great sensitivity.

Fig. 1.

Experimental setup. A: an analog linear displacement transducer measured foot dorsum changes in height during 10-kg loading and unloading of the knee while seated (protocol 1). A force plate measures loading force. B: during quiet standing (protocol 2), an angular potentiometer measures anterior (ANT.)-posterior (POST.; A/P) shin tilt, a force plate measures center of pressure (COP), and the linear transducer measures changes in the height of the foot dorsum. C: the displacement transducer accurately shows no error when measuring fixed surface height (top trace), whereas the optoelectronic motion analysis camera system shows up to 0.2 mm of variability when measuring an earth-fixed ground marker (2nd trace). The camera system (3rd trace) and angular pot (bottom trace) both reliably measured A/P shin tilt, but the former less so because of reduced sensitivity as well as potential skin movement artifacts of the markers at the ankle and knee joints. deg, Degrees. D, top: a top view of foot placement on moveable and stationary surfaces (protocol 3). In the TOE condition (dashed line), only 1st through 3rd phalanges are perturbed. In the metatarsals (MT) condition (solid line), the 1st and 2nd MT and all toes are perturbed by a moveable surface that can move the forefoot upward. A greater portion of the forefoot is positioned on the moveable surface in the MT condition than the TOE condition. Medial view of TOE condition shows placement of the linear transducer on the hallux toenail, which measures the onset of the upward surface displacement (bottom left). Similarly, the linear transducer placed on the foot dorsum over the head of the 1st MT (MT1) measures the onset of the surface displacement in MT conditions (bottom right). In protocol 3, subjects are standing with eyes closed while shin tilt is measured as depicted in B.

Fig. 2.

A: static weight loading of the knee deforms the foot arch as measured by a linear displacement transducer placed on foot dorsum (top trace). A force plate measuring 10-kg weight loading onto the seated subject's knee (bottom trace) shows high correlation with the foot arch deformation. B: the average height change (± SD) across subjects as measured by the linear transducer placed on the head of the MT1 (black) or on the foot dorsum (white) was significantly less at the MT1 compared with the dorsum (P < 0.01) during static loading.

Fig. 3.

A: raw data from 2 different subjects showing that longitudinal foot arch displacement, as measured by linear transducer placed over the foot dorsum (top trace), is highly correlated with A/P shin tilt, as measured by angular potentiometer attached (2nd trace). A/P COP (3rd trace) is also highly correlated with foot arch and shin tilt, but mediolateral (M/L) COP (bottom trace) is not. B: overall subject averages of the trial-by-trial correlation of shin tilt with height displacement of the hallux toenail (gray), head of the MT1 (black), and the foot dorsum (white) all have high negative correlations with shin tilt (left plot). Overall subject averages (± SD) of the height change (peak-to-peak amplitude of vertical oscillations) of the TOE, MT1, and dorsum during eyes-closed quiet standing (right plot) are shown. Corr. coeff., correlation coefficient.

Fig. 4.

Shin tilt changes with surface perturbation. A: dark solid traces show mean shin tilt (solid line) in the posterior direction, which occurs after the upward surface displacement (vertical dashed line). Note that the postural response has a latency often >500 ms after the surface perturbation. The 95% confidence envelopes are shown by light dotted lines. MT condition (left column) and TOE condition (right column) shin tilt responses at 3 displacement amplitudes show increasing response with increasing surface displacement amplitude. MT condition responses are larger than TOE condition responses. All postural responses show a delay. B: group means for each condition shows significantly larger means of absolute shin tilt in MT compared with TOE at all surface displacement amplitudes. Error bars show 95% confidence interval (CI).

Fig. 5.

Electromyogram (EMG) responses to foot perturbation. A: small perturbation (<4 mm) at 10 s (dotted line) lifts MT. EMGs of tibialis anterior (TA) or gastrocnemius (GAST) and A/P shin tilt data from 3 representative subjects reveal that even if the majority of the foot is supported on a stable, fixed surface, a postural response to small toe or MT perturbation occurs. Root-mean-square (RMS) shin tilt postperturbation is greater than quiet stance preperturbation. B: during surface perturbations, subjects either responded with an increase in GAST and decrease in TA muscle activity typically during forward lean (○), an increase in TA and decrease in GAST muscle activity typically during backward lean (□), or no clear change in muscle activity (♦). C: the absolute change in EMG activity (AbsEMG) in TA and GAST was significantly greater in the MT than in the TOE condition.

Fig. 6.

Schematic representation of distributed foot deformations during standing. A: the axes of all the foot joints (except for those of the middle 3 rays) adapted from radiographs of the experimental foot. Original figure by W. G. Wright, Y. P. Ivanenko, and V. S. Gurfinkel, adapted from Hicks (1953). B: simplistic model of the foot (sagittal plane) composed of several parts behaving as an elastic beam.