RELATIONSHIP BETWEEN MUSCLE ACTIVATION AND PINCH STRENGTH AFTER TENDON TRANSFER IN SPINAL CORD INJURY

Abstract

Purpose/Hypothesis: Surgical transfers of the tendon of the brachioradialis muscle (Br), an elbow flexor, to the tendon of the paralyzed flexor pollicis longus (FPL) muscle can restore active pinch strength in tetraplegia. Our previous work reported the transferred Br had significantly lower activation in pinch compared to elbow flexion, even with elbow support. This may be due to inefective postoperative muscle re-education or voluntary reduction in activation to compensate for weakened elbow extensors. The purpose of this study was to determine if postoperative Br activation and non-operated biceps (Bi) and triceps (Tr) muscles were correlated with the production of pinch force, indicating reeducation of the Br in its functional role after transfer. Number of Subjects: Ten individuals (11 arms) with cervical level spinal cord injuries and previous transfer of the Br to FPL were studied with a mean follow-up time of 8 years. Materials/Methods: Muscle activation (EMG) of Br was recorded with fine-wire electrodes as each subject produced their maximum effort pinch force. Surface electrodes recorded EMG from non-transferred Bi and Tr muscles. All trials were done in 60° of elbow fexion. Subjects were tested with the arm self-supported (SS) and externally supported (ES) by limiting elbow motion with padded stops to stabilize the elbow. A force sensor mounted on a custom grip recorded pinch force. EMG signals recorded during pinch were normalized to a maximum voluntary contraction. Linear regression analysis was used to compare EMG (mV) and pinch force (N) during the time period when subjects were actively increasing their pinch force. Coefficients of determination (r2) were calculated and tested for significance (p≤.05). Wilcoxon signed rank tests compared normalized EMG and pinch force in both conditions of support (p≤.05). Results: Activation of the transferred Br and the non-transferred Bi and Tr muscles were significantly correlated with pinch force production (p≤.001). On average, 80% of the variance in pinch force could be explained by Br and Bi activation and 68% of variance by the Tr activity. Significant increases in both normalized Br EMG and pinch force (p≤.05) were observed with external elbow support. In the SS condition the average r2 for Br=0.74, Bi=0.85, and Tr=0.63. In the ES condition the r2 for Br=0.86, Bi=0.78, and Tr=0.73. Conclusions: The strength of the association between EMG and pinch force indicates the subjects in this study were able to re-train the Br to be active during lateral pinch. The relationship between pinch force and the proximal musculature demonstrates the ability to control elbow position is a factor in maximizing postoperative pinch force magnitude. The increase in Br activation and pinch force with elbow support is consistent with previous findings. Clinical Relevance: The significant relationship between Br EMG and pinch force suggests that improving postoperative Br activation will result in higher pinch force levels. Proximal musculature (biceps and triceps) should be included in the postoperative rehabilitation program.