Table 1.
Participant Characteristics.
Table 2.
Description of Surgical Procedures.
Fig 1.
The arm was supported in each of three functional postures with an elbow moment transducer affixed to record elbow moments.
The postures included the arm positioned in the horizontal plane (90° shoulder flexion, 45° shoulder internal rotation, 90° elbow flexion) and postures consistent with initiating an overhead reach (110° shoulder flexion, 70° shoulder internal rotation, 120° elbow flexion), and pressure relief (45° shoulder extension, 20° shoulder abduction, 90° elbow flexion). Shoulder and elbow joint angles followed International Society of Biomechanics recommendations [24].
Fig 2.
Cross-section illustration of muscle to demonstrate the recruitment of motor units during electrical stimulation, maximum voluntary effort, and electrical stimulation superimposed on maximum voluntary effort.
Electrical stimulation with the muscle at rest recruits part of the motor pool (light blue fill represents muscle fibers innervated by motor units recruited by electrical stimulation). In nonimpaired muscle, nearly the entire motor pool can be voluntarily recruited at maximum effort (brown fill represents muscle fibers innervated by motor units recruited voluntarily); superposition of electrical stimulation results in additional recruitment of only a few motor units. In muscle with an activation deficit, only a percentage of the motor pool can be recruited during maximum voluntary effort; superposition of electrical stimulation recruits many additional motor units.
Fig 3.
Example isometric moment-matching trials with electrical stimulation superimposed and twitch moments for two representative arms with tendon transfer.
Panels (A) and (B) are Arm 2 with biceps transfer in Table 1; panels (C) and (D) are Arm 8 with deltoid transfer in Table 1). (A) and (C) Example individual trials during which electrical stimulation (applied at time 0 seconds) of the transferred muscle was superimposed on voluntary elbow extension at effort levels of 33%, 66% and 100% of maximum. For each trial, a second stimulus event was delivered at 0% of the maximum moment (black), seven seconds after the first stimulus event, but shown here at time 0 s for comparison. Twitch moments were computed as the difference in the maximum and the pre-stimulus moment within the analysis windows (dashed line). (B) and (D) Twitch moments during elbow extension and at rest corresponding to the example trials shown.
Fig 4.
Linear extrapolation of twitch moment and voluntary pre-stimulus moment data to compute predicted moments for two representative arms with tendon transfer.
(A) Arm 2 with biceps transfer in Table 1. (B) Arm 8 with deltoid transfer in Table 1.
Fig 5.
Differences in voluntary activation and maximum voluntary elbow extensor moments were detected between the biceps transfer and deltoid transfer groups.
Black solid lines indicated significant differences (* indicates p < 0.05, † indicates p < 0.001). Within the deltoid transfer group, voluntary activation differed due to posture (dashed lines and * indicates p < 0.05). Within the biceps transfer group, maximum voluntary moments differed due to posture (dashed lines and † indicates p < 0.001). Error bars are ± one standard error of the means.