Purpose Tendon transfer is often used to restore key pinch after cervical spinal cord injury. Current postoperative recommendations include elbow immobilization in a flexed position to protect the brachioradialis–flexor pollicis longus (BR-FPL) repair. The purpose of this study was to measure the BR-FPL tendon tension across a range of wrist and elbow joint angles to determine whether joint motion could cause repair rupture. Methods We performed BR-to-FPL tendon transfers on fresh-frozen cadaveric arms (n = 8) and instrumented the BR-FPL tendon with a buckle transducer. Arms were ranged at 4 wrist angles from 45° of flexion to 45° of extension and 8 elbow angles from 90° of flexion to full extension, measuring tension across the BR-FPL repair at each angle. Subsequently, the BR-FPL tendon constructs were removed and elongated to failure. Results Over a wide wrist and elbow range of motion, BR-FPL tendon tension was under 20 N. Two-way analysis of variance with repeated measures revealed a significant effect of wrist joint angle (p<.001) and elbow joint angle (p<.001) with significant interaction between elbow and joint angles (p<.001). Because the failure load of the repair site was 203 ± 19 N, over 10 times the loads that would be expected to occur at the repair site, our results demonstrate that the repair has a safety factor of at least 10. Conclusions Our tendon force measurements support the assertion that the elbow joint need not be immobilized when the BR is used as a donor muscle in tendon transfer to the FPL. This is based on the fact that maximum passive tendon tension was only about 20 N in our cadaveric model and the failure strength of this specific repair was over 200 N. We suggest that it is possible to consider performing multiple tendon transfers in a single stage, avoiding immobilization, which may adversely affect functional recovery. These results must be qualified by the fact that issues unique to living tissues such as postoperative edema and tendon gliding cannot be accounted for by this cadaveric model.