Bilateral Force Deficit in Proximal Effectors Versus Distal Effectors in Lower Extremities.

Purpose: Bilateral force deficit occurs when the maximal generated force during simultaneous bilateral muscle contractions is lower than the sum of forces generated unilaterally. Neural inhibition is stated as the main source for bilateral force deficit. Based on differences in bilateral neural organization, there might be a pronounced neural inhibition for proximal compared to distal effectors. The aim of the present experiment was to evaluate potential differences in bilateral force deficit in proximal compared to distal effectors in lower extremities. Methods: Fifteen young adults performed single-joint maximal voluntary contractions in isometric dorsiflexion of ankle (distal) and knee (proximal) extension unilaterally and bilaterally. Results: Results showed a significant absolute bilateral force deficit for both proximal (123.46 ± 59.51 N) and distal effectors (33.00 ± 35.60 N). Interestingly, the relative bilateral force deficit for knee extension was significantly larger compared to dorsiflexion of ankle, 19.98 ± 10.04% and 10.27 ± 9.57%, respectively. Our results indicate a significantly higher bilateral force deficit for proximal effectors compared to distal effectors. Conclusion: Plausible explanations are related to neuroanatomical and neurophysiological differences between proximal effectors and distal effectors where proximal muscles have a higher potential for bilateral communication compared to distal muscles. In addition, higher forces produced with proximal effectors could cause a higher perceived exertion and cause a more pronounced bilateral force deficit to proximal effectors.

Comparison of bilateral force deficit in proximal and distal joints in upper extremities.

Bilateral force deficit refers to the phenomenon that maximal generated force during simultaneous bilateral muscle contractions is lower than the sum of forces generated unilaterally. Based on the notion that neural inhibition is the main source for bilateral force deficit and existing differences in neural inhibiting interhemispheric organization of proximal and distal muscles, we expected differences in bilateral deficit in proximal and distal joints. The aim of the current behavioral experiment was to compare bilateral force deficit in proximal compared to distal upper extremity joints. Ten young adults performed single-joint maximal voluntary contractions in isometric flexions of the shoulder and index finger unilaterally and bilaterally. The results showed a significant absolute bilateral force deficit for both proximal (140.01 ± 86.99 N) and distal muscles (4.64 ± 4.86 N). More importantly, relative bilateral force deficit for shoulder flexion was significantly larger than for index finger flexion, -20.51 ± 7.8% and -5.07 ± 3.84% respectively. The hypothesis of a more pronounced bilateral force deficit for proximal compared to distal muscles was confirmed in our results. Thus, our findings, in combination with the neuroanatomical differences for proximal and distal muscles, make it worthwhile to further explore the hypothesis that the commissural fibers provide differences in interhemispheric inhibitory interactions during bimanual actions for proximal and distal muscles.