Maximum muscle force and MUNE were measured to analyze muscle innervation (27). Analyses were conducted bilaterally to compare the effects of the nerve transfer to the intact contralateral control side as a reference. The proximal tendon of the biceps muscle was folded into a loop and attached to a force transducer (BG-1000; Kulite Semiconductor Products, Leonia, NJ). The musculocutaneous nerve was cut, and the biceps’ medial head was denervated to prevent any additional muscular contraction.

A shielded bipolar silver cuff electrode was used to apply electrical stimuli generated by a Grass S88 Stimulator (Grass Instrument Co., Quincy, MA). For the MUNE, the donor ulnar nerve was stimulated on the nerve transfer side, and the original motor branch was stimulated on the contralateral control side using 100-μs pulses at a rate of 0.5 Hz, with manually adjusted amplitude ranging from 0 to 10 V. The stimulation started with low enough amplitude to not elicit any force and was then gradually increased until the first motor unit produced a detectable force. Every time the force increased, the stimulus amplitude was held constant for 8 to 10 stimuli to average the force readings of the recruited motor unit. This procedure was repeated until 10 motor units were identified, and then the average force across the 10 motor units was calculated. For the maximum isometric tetanic force measurement, the nerve innervating the biceps’ lateral head was stimulated for 300 ms at increasing frequencies from 30 to 100 Hz and 2- to 6-V amplitude. The procedure was paused for 2 min after each set of stimuli set at the same amplitude to allow for muscle recovery. The maximum force was divided by the average motor unit force to estimate the total number of motor units. One animal from the 12-week’s group had to be excluded because of premature decease during testing of the control side.

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