Surgical procedure

Anesthesia was induced by administering ketamine (100 mg/kg body weight) and xylazine (5 mg/kg body weight) intraperitoneally and then maintained with inhalation of 1.5% isoflurane via an endotracheal tube. Analgesia was achieved by subcutaneous injection of piritramide (0.3 mg/kg body weight). The rats were positioned in a supine position with both arms fixed at 90° abduction. An approximately 2.5 cm long incision was made from the acromion to the medial epicondyle (Supplemental Fig. ^S4A). Subsequently, the LH was microscopically dissected, and its fascia was removed (Supplemental Fig. ^S4B). The biceps’ two heads were gently separated to expose the LHMB (Supplemental Fig. ^S4C), which was subsequently dissected proximal and transected shortly before its origin from the musculocutaneous nerve. Next, the UN was located proximal to the cubital tunnel just medial of the medial intermuscular septum. The UN’s position beneath the triceps’ medial head can be identified by its accompanying vein (Supplemental Fig. ^S4D). The UN was exposed through a small window in the muscle and dissected both proximally and distally before transecting it just proximal to the cubital tunnel. Afterwards, it was identified proximally in the medial bicipital groove (Supplemental Fig. ^S4E). After dissection and distal mobilization, the UN was easily pulled out underneath the triceps brachii and then transferred directly to the motor entry point of the LH with one 10-0 nylon (Ethicon, USA) single interrupted suture (Supplemental Fig. ^S4F). In the control group, the skin was closed by 6-0 absorbable dermal and simple interrupted sutures, thus finishing the procedure at this point. In the groups using a skin graft, the operating site was covered with a wet swab followed by harvesting of the glabrous dermal skin graft from the ipsilateral hindlimb. The graft was marked between the walking pads (Supplemental Fig. ^S5A). The area was then subcutaneously injected with saline to increase the skin’s tension and was carefully deepithelized using a standard 15-blade. The marked borders were incised, and the proximal end was grasped with a tweezer to separate the graft from the plantar fascia. The defect was primarily closed with 6-0 absorbable inverted simple interrupted sutures (Supplemental Fig. ^{S5B, C}), due to the thin plantar skin and to minimize irritation caused by knots while walking. Subsequently, the graft was defatted, placed on the LH’s surface with the hypodermis facing the muscle, and fixed epimysially using 10-0 nylon simple interrupted sutures (Fig. 1). Lastly, the skin was closed as described before. After the follow-up period, rats were deeply anesthetized, and native, contralateral UN and LHMB samples were harvested from the contralateral extremity. This was followed by intracardial perfusion with 300 ml 0.9% sodium chloride solution and 400 ml 4% paraformaldehyde (PFA) and harvesting of the muscle-skin samples.