The relationship between visual steadiness and force steadiness in young and old adults

Old adults exhibit a decrease in muscle force steadiness and visual capability compared to young adults. Many studies investigating force steadiness have used a visual target as the stimulus for modulating muscle force. Since vision is used in most investigations of muscle force control with age reduced muscle force control in older adults might be partially related to or explained by altered visual capacity. The purpose of this study was to compare the relationships between eye movement as a component of visual steadiness and quadriceps muscle force steadiness in young and old adults during isometric quadriceps contractions of constant and varying forces. 19 healthy young adults (20.7±1.82yrs) and 18 healthy old adults (71.6±3.01yrs) participated in this study after providing written informed consent. Isometric quadriceps torque data were collected using an isokinetic dynamometer. Horizontal and vertical eye movement data were collected using an eye-tracking system. The tasks consisted of three vision only tasks three vision and force tasks at a relative value of 40%MVC and three at an absolute value of 54Nm. There were no significant differences between age groups for force steadiness in the absolute condition (young 0.76±0.25Nm old 0.84±0.29Nm; p=0.19). Contrary to our expectations old adults showed less force variability (0.79±0.36Nm) than young adults (1.16±0.44Nm) in the relative condition (p<0.05). The static vision-only condition did not show a significant difference between the two groups for the horizontal (p=0.08) or vertical (p=0.28) visual components. The remaining two vision-only conditions did not show a significant difference between young and old adults for the vertical vision component p=0.34 and p=0.47 respectively. A significant difference was observed in the horizontal component for the two conditions. Old adults showed decreased horizontal visual steadiness compared to young (p<0.05). Correlations performed between visual steadiness and muscle force steadiness showed a statistically significant relationship for only one relative vision and force condition (r=0.471) and failed to show statistical significance for any of the remaining conditions using the following critical values for a two-tailed test at p<0.05: young adults (df=17) =0.456 old adults (df=16) =0.468. The absence of a statistically significant difference in force steadiness between young and old adults is indicative of an extremely healthy mobile and capable old adult subject pool. The only differences between the two groups were age and maximal strength (young 209±68.44Nm old 145±51.5Nm). We were not able to identify any physiological relationship between muscle force steadiness and eye movement as a component of visual steadiness. It is possible that the relationship between force steadiness and visual feedback identified in previous research is due to decrements in visual processing capabilities and not due to a decline in visual steadiness. Regardless present data support observations that reduced muscle force steadiness with age may be due to reduced neuromuscular capacity and not visual capability.