Alan Hayes

Background Muscle mass and function decline in middle age, ultimately resulting in sarcopenia in the elderly and poor health outcomes, reducing quality of life. There is a lack of cost‐ and time‐effective murine models that recapitulate the physiological changes associated with muscle mass decline to study possible interventions to delay sarcopenia. We aimed to evaluate the effectiveness of combining orchiectomy (ORC) surgery to simulate age‐related androgen decline and hindlimb immobilization (IM) in inducing age‐related skeletal muscle changes. Methods Four‐month‐old male C57BL/6J mice ( n = 10) were subjected to ORC, followed by IM (right hindlimb casting) for 14 days. Upon completion of the casting period, ex vivo muscle contractile function, histology, and various mitochondrial markers were assessed, and results were compared with age‐matched controls (CON; n = 8) and middle‐aged (MA; 12 ± 1 months, n = 9) animals. Results IM combined with ORC induced a 30%–40% decrease in muscle mass across multiple hindlimb muscles ( P < 0.0001), with the magnitude of muscle loss comparable with the MA group when corrected for body weight ( P < 0.0001). In the IM limb of ORC mice, soleus muscle force significantly decreased when compared with the contralateral limb ( P < 0.05) and aged‐matched CON group ( P < 0.05). The decrements in muscle force and mass present in the IM limb of ORC mice were accompanied by a 70% reduction in the expression of the muscle structural protein dystrophin and various mitochondrial markers, including cytochrome C (−55%), peroxisome proliferator‐activated receptor gamma co‐activator 1‐beta (PGC1‐β) (−49%), and cytochrome oxidase IV (COX‐IV) (−73%) when compared with CON animals ( P < 0.001). Lastly, our model also demonstrated specific fibre‐type shifts in fast‐ and slow‐twitch muscles, which mimicked changes in the MA group. Conclusions Applying treatments during IM could target acute muscle atrophy in MA adults, while applying them following cast removal in a low‐testosterone environment could represent a window for rehabilitation therapeutics.