Forelimb unloading impairs glenohumeral muscle development in growing rats

Abstract

1Proper joint loading is essential for healthy musculoskeletal development. Many pediatric 2 neuromuscular disorders cause irreversible muscle impairments resulting from both physiological 3 changes and mechanical unloading of the joint. While previous studies have examined the effects 4 of hindlimb unloading on musculoskeletal development in the lower limb, none have examined 5 solely forelimb unloading. Thus, a large deficit in knowledge of the effect of upper limb unloading 6 exists and must be addressed in order to better understand how the glenohumeral joint adapts 7 during development. Two forelimb unloading models were developed to study the effects of 8 varying degrees of unloading on the glenohumeral joint in growing rats: forelimb suspension (n=6, 9 intervention 21 days post-natal) with complete unloading of both limbs via a novel suspension 10 system and forearm amputation (n=8, intervention 3-6 days post-natal) with decreased loading and 11 limb use in one limb after below-elbow amputation. After 8 weeks of unloading, changes in muscle 12 architecture and composition were examined in ten muscles surrounding the shoulder. Results 13 were compared to control rats from a previous study (n=8). Both methods of altered loading 14 significantly affected muscle mass, sarcomere length, and optimal muscle length compared to 15 control rats, with the biceps long head and triceps long head observing the most marked 16 differences. Forearm amputation also significantly affected muscle mass, sarcomere length, and 17 optimal muscle length in the affected limb relative to the contralateral limb. Muscle composition, 18 assessed by collagen content, remained unchanged in all groups. This study demonstrated that 19 forearm amputation, which was administered closer to birth, had greater effects on muscle than 20 forelimb suspension, which was administered a few weeks later than amputation. 21 22 23Mechanical loading is critically important for healthy musculoskeletal development 1,2 and 24 maintenance 3,4 . In adult murine models, unloading via hindlimb suspension, microgravity during 25 spaceflight, and muscle paralysis causes changes in muscle architecture. For example, unloading 26 in adult murine animals caused substantial reductions of 41-66% in skeletal muscle size, mass, and 27 strength 6,8,9 , as well as up to 13% longer sarcomere lengths 8 . However, muscle composition 28 measured by collagen content was unaffected by unloading in adult rodents 7,11 . In growing 29 animals, unloading is particularly impactful, causing irreversible musculoskeletal changes 5,12,56 , 30 including altered joint morphology 12 , which influences surrounding muscle, and decreased muscle 31 mass by over 3 to 5-fold 5,56 . However, the effects of unloading on other muscle architecture metrics 32 (e.g., sarcomere length, optimal muscle length) and muscle composition (e.g., collagen content) 33 have not previously been examined in growing animals. 34Unloading models have traditionally focused on the hindlimbs 15 , resulting i...