Balance and stability.

Summary.-An examination of balance and stability indicates that the functional aspects of stability may mediate against optimal balance development. Balance is the ability to maintain functional equilibrium whereas stability refers to the structural components associated with equilibrium. In general, structural components such as a larger base of support or a lower center of gravity are conducive to greater stability and, hence. should enhance potential functional equilibrium, or balance. After resting postural sway during 3 min. of standing, Hellebrandt and Braun (1939) reported a mean area of maximum sway of 1.06 cm.' for 66 females which was 3.28% of the average base of 433.8 cm.' For males, the sway was greater in both absolute and relative terms as the 1.59 cm.' of mean maximum sway area comprised 4.4% of 462.1 cm.5verage base indicating the greater stability of the female physique as a component of functional equilibrium during standing. Studies of balance rend to classify test items in terms of static and dynamic balance rather than analysis on the basis of structural and functional components associated with equilibrium. In these instances, little or no relationship is reported between static and dynamic balance (Bass, 1939; Drowatsky & Zuccato, 1967). T he relationship between balance and stability may be considered as paradoxical in that greater stability has the potential for enhancing balance performance but may, because of its lesser demands upon the balancing assemblies, actually acr as a deterrent to the optimal development of sensorymotor systems associated with functional equilibrium Bachman's study (1961) of static balance as measured by the stabilometer reports poorer initial trial scores for females from 6 to 25 yr. but after 9 yr. of age the amount of learning for the females over the 10 trials exceeded that of the males and final scores were better for females from 16 through 25 yr. However, for the dynamic test of ladder climbing the males had better scores for all 10 trials at all age levels. The greater stability of physique may partially account for the greater learning gains for the females on the static balance test whereas such stability would not appear to be advantageous for dynamic balance. Girls are reported to be slightly superior or equal to boys in rail walking from 5 through 11 yr. (Keogh, 1965) but boys are significantly superior in performance by 12 yr. of age (Goetzinger. 1961). A comparison of dynamic balance performance with the structural changes occurring during the growth years poinrs out the paradoxical nature of balance and stability. Ratios of sitting height to stature from 1 to 18 yr. indicate that the relative leg length of girls tends to be slightly higher or equal to that of boys until 11 yr. of age when the leg length of boys exceeds that of girls. Moreover, after 7 yr. of age the shoulder width of boys becomes relatively greater than that of girls (Bayer & Bayley, 1959). Longer leg length and a greater shoulder-hip ratio are associated with a higher center of gravity and, hence, less stability. Developmental trends appear to indicate an inverse relationship between structural stability and balance development. Held (1965) has shown that the sensory-motor system is dependent upon the level of sensory input for its functional development. Individuals who have a higher center of graviry and less stability will tend to place greater demands upon the sensory-motor systems associated with the development of functional equilibrium than will individuals