Feedforward ankle strategy of balance during quiet stance in adults

Abstract

1. We studied quiet stance investigating strategies for maintaining balance. Normal subjects stood with natural stance and with feet together, with eyes open or closed. Kinematic, kinetic and EMG data were evaluated and cross-correlated. 2. Cross-correlation analysis revealed a high, positive, zero-phased correlation between anteroposterior motions of the centre of gravity (COG) and centre of pressure (COP), head and COG, and between linear motions of the shoulder and knee in both sagittal and frontal planes. There was a moderate, negative, zero-phased correlation between the anteroposterior motion of COP and ankle angular motion. 3. Narrow stance width increased ankle angular motion, hip angular motion, mediolateral sway of the COG, and the correlation between linear motions of the shoulder and knee in the frontal plane. Correlations between COG and COP and linear motions of the shoulder and knee in the sagittal plane were decreased. The correlation between the hip angular sway in the sagittal and frontal planes was dependent on interaction between support and vision. 4. Low, significant positive correlations with time lags of the maximum of cross-correlation of 250-300 ms were found between the EMG activity of the lateral gastrocnemius muscle and anteroposterior motions of the COG and COP during normal stance. Narrow stance width decreased both correlations whereas absence of vision increased the correlation with COP. 5. Ankle mechanisms dominate during normal stance especially in the sagittal plane. Narrow stance width decreased the role of the ankle and increased the role of hip mechanisms in the sagittal plane, while in the frontal plane both increased. 6. The modulation pattern of the lateral gastrocnemius muscle suggests a central program of control of the ankle joint stiffness working to predict the loading pattern.

Figure 1. Postural alignment during standing with normal stance width and eyes open (EO), standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR)

Top, group means ± standard error of mean (s.e.m.) (n = 7) of the ankle and hip angles (all in deg) in the sagittal plane and hip angle in the frontal plane during four conditions. Bottom, group means ±s.e.m. of the distances of the right centre of gravity (COG) to the mid-ankle point in the sagittal and frontal planes during the four conditions.

Figure 2

Centre of gravity (COG) motion in the sagittal and frontal planes during standing with normal stance width and eyes open (EO); standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR)Left, group means ±s.e.m. (n = 7) of the s.d. of the COG motion during the four conditions. Right, group means ±s.e.m. (n = 7) of the range of the COG motion during four conditions.

Figure 3. Angular motion of the ankle and hip angles in the sagittal and frontal planes during standing with normal stance width and eyes open (EO); standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR)

Left, group means ±s.e.m. (n = 7) of the s.d. of the angular motion during four conditions. Right, group means ±s.e.m. (n = 7) of the range of the angular motion during four conditions.

Figure 4. The correlation coefficient (r) that is a maximum or a minimum of the cross-correlation function aggregated for the whole group during standing with normal stance width and eyes open (EO); standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR)

Left, r of cross-correlation between anteroposterior motion of centre of gravity and centre of pressure (GYPY);r of cross-correlation between the anteroposterior motion of the right knee and shoulder points (Knee/Shoulder (Y)); r of cross-correlation between the anteroposterior motion of the right head and COG (HEGY);r of cross-correlation between the anteroposterior motion of the right hip point and angular motion of the right hip angle in the sagittal plane (Hip pt (Y)/Hip A (Sag)). Right, r of the cross-correlation between the medio-lateral (ML) motion of the right knee and shoulder points (Knee/Shoulder (X)); r of the cross-correlation between the ML motion of the right hip point and angular motion of the right hip angle in the frontal plane (Hip pt (X)/Hip A (Fron)). Lines were drawn indicating an r value of ±0.3 at which r is different from 0 with P < 0.001.

Figure 5. The correlation coefficient (r) that is the maximum of the cross-correlation function aggregated for the whole group during standing with normal stance width and eyes open (EO); standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR)

Left, r of the cross-correlation between the anteroposterior motion of centre of pressure and ankle angle angular motion (COPY/Ankle A); r of the cross-correlation between the anteroposterior motion of the centre of pressure and knee angle angular motion in the sagittal plane (COPY/Knee A); anteroposterior motion of the centre of pressure and hip angle angular motion in the sagittal plane (COPY/Hip A). Right, r of the cross-correlation between right hip angle angular motion in the sagittal and frontal planes (real and absolute values) (Hip A (Sag)/Hip A (Fron)). Lines were drawn indicating r value of ±0.3 at which r is different from 0 (P < 0.001).

Figure 6

Left, maximum of the cross-correlation function (r) aggregated for the whole group: between the right lateral gastrocnemius muscle activity and the anteroposterior motion of the centre of gravity (GA/COGY); between the right lateral gastrocnemius muscle activity and the anteroposterior motion of the centre of pressure (GA/COPY); between the right lateral gastrocnemius muscle activity and ankle angle angular motion (GA/Ankle A) during standing with normal stance width and eyes open (EO); standing with normal stance width and eyes closed (EC); standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR). Right, group means ±s.e.m. (n = 7) of the lag of the maximum of cross-correlation function GA/COGY and GA/COPY during the four conditions studied.

Figure 7

A, data are shown for the epoch from 20th to 28th second of a single trial in one subject during standing with normal stance width and eyes closed (EC). From bottom to top: lateral gastrocnemius muscle activity (GA), anteroposterior motion of the centre of gravity and anteroposterior motion of the centre of pressure (bold line) (COGY & COPY), ankle angle angular motion (Ankle A), knee angle angular motion in sagittal plane (Knee A), hip angle angular motion in sagittal plane (Hip A (Sag)), and hip angle angular motion in frontal plane (Hip A (Fron)). a.u., arbitrary unit. B, the cross-correlation function aggregated for the whole group between the right lateral gastrocnemius muscle activity and the anteroposterior motion of the centre of pressure (GA vs. COPY): top, during standing with normal stance width and eyes open (EO) and standing with normal stance width and eyes closed (EC) (bold line); bottom, during standing with narrow stance width (Romberg stance) and eyes open (EOR); and standing with narrow stance width and eyes closed (ECR) (bold line). C, the cross-correlation functions for the epoch from 16th to 32nd second of the same trial (and containing the same data) shown in A: between the right lateral gastrocnemius muscle activity and the anteroposterior motion of the centre of gravity (GA vs. COGY) and between anteroposterior motion of centre of gravity and centre of pressure (GAGY vs. COPY) (bold line).