We assessed the length-tension relationship of the posterior deltoid to triceps transfer in 8 tetraplegics (n = 11 transfers) and compared the results to the length-tension relationship of the normal triceps measured in a check sample composed of 9 able bodied, right handed women. We designed a device to lock the arm and forearm and used a force transducer to assess the torque output isometrically. The muscle was tested at 6 different lengths (130, 110, 90, 70, 45 and 0 degree of elbow flexion) with the shoulder abducted at 90 degrees. As expected, the transfer behaved differently from the normal triceps. The mean maximum torque recorded was 7.8 Nm in patients while it was 27 Nm in the check sample. When compared, the absolute values (ie values expressed with a dimension of torque) were significantly different between groups (0.00001 < p < 0.002). The expression of this relation (ie the relative values expressed as percentage of maximum values) revealed significant statistical differences (p < 0.002) at 90 and 70 degrees of elbow flexion; the peak torque was recorded at 130 degrees in patients while it was recorded at 110 degrees in the check sample, with a plateau between 110 degrees and 70 degrees. On the other hand, if the length-tension relationship was fairly similar among subjects of the check sample, it exhibited tremendous differences among patients; it seemed that initial tension given by the surgeon represented a variable difficult to control without a device dedicated to that task.