The Correlation Between Elbow Flexor And Knee Extensor Steadiness Is Modified By Visual Feedback

Abstract

The neural mechanisms that underlie the correlation in force fluctuations between the elbow flexor (EF) and knee extensor (KE) muscles are unknown. PURPOSE To determine the effect of visual feedback and target force on the correlation between the steadiness of contractions of the EF and KE muscles. METHODS Healthy, minimally active young (N = 9; 23 ± 3 yrs) and old adults (N = 17; 75 ± 7 yrs) underwent assessment of maximum voluntary contraction (MVC) and force fluctuations during submaximal constant-force contractions of the EF and KE muscles. Constant-force contractions were performed in the EF and KE separately (SEP) at target forces of 2.5, 30, and 65% MVC and simultaneously (SIMUL) in the EF and KE at 2.5 and 30% of MVC. The SEP contractions were performed with and without visual feedback of the force (VIS, NOVIS). SIMUL contractions were performed only with NOVIS. DC drift in the force (< 0.05Hz) was removed before measuring the fluctuations. The coefficient of variation (CV) of force was measured similarly for all conditions (SD of force/mean force for the segment). Correlations were calculated between the EF and KE for the CV of force for each target force and condition. RESULTS MVC force was correlated between EF and KE (r = 0.82, P <0.001). The CV of force was 23% lower in the NOVIS condition (P <0.001). SEP condition: The CV of force was positively correlated between EF and KE for the VIS condition at 2.5% (r = 0.46, P = 0.02) and 30% MVC (r = 0.8, P <0.001), with a trend at 65% MVC (r = 0.32, P = 0.1). With NOVIS the correlations persisted strongly at 30 and 65% MVC (0.67 and 0.77, P <0.001), but not at the lowest target force of 2.5% MVC (r = 0.06, P = 0.76). SIMUL condition: The CV of force was strongly correlated between EF and KE at 30% MVC (r = 0.86, P <0.001) but was uncorrelated at the lowest target force of 2.5% MVC (r = 0.02, P = 0.9). CONCLUSION Visual correction of force substantially increases the amplitude of force fluctuations during isometric force-matching tasks with the knee extensors and elbow flexors. The correlation in steadiness between the EF and KE corroborates our previous findings. Removal of visual feedback eliminates the between-muscle correlation only at low muscle forces in both SEP and SIMUL. This finding suggests that the neural factors that underlie the correlation in steadiness between these two muscle groups at low activation levels are influenced by visual feedback. Supported by NIH AG19171 to BLT.