Deficit in human muscle strength with cast immobilization: contribution of inorganic phosphate

Metabolic factors have been proposed to explain strength deficits observed in skeletal muscle with immobilization that are not completely accounted for by changes in muscle cross-sectional area (CSA) and neural adaptations. The aim of this study was to quantify changes in the resting inorganic phosphate (P_i) concentration from the medial gastrocnemius muscle during immobilization, reloading and rehabilitation. Additionally, we assessed the contributions of CSA, muscle activation and P_i concentration to plantar flexor torque during rehabilitation following immobilization. Eight persons with a surgically stabilized ankle fracture participated. Subjects were immobilized for 6–8 weeks and subsequently participated in 10 weeks of rehabilitation. Localized ³¹P-Magnetic resonance spectroscopy, magnetic resonance imaging, isometric torque and activation testing were performed on the immobilized and uninvolved limbs. At 6 weeks of immobilization, significant differences were noted between the immobilized and uninvolved limbs for the P_i concentration and the P_i/PCr ratio (P < 0.05). From 6 weeks of immobilization to 3–5 days of reloading, the increase in P_i concentration (15%, P = 0.26) and P_i/PCr (20%, P = 0.29) was not significant. During rehabilitation, the relative contributions of CSA, muscle activation and P_i concentration to plantarflexor torque were 32, 44 and 40%, respectively. Together, CSA, muscle activation and P_i concentration accounted for 76% of the variance in torque (P < 0.01). In summary, our findings suggest that immobilization, independent of reloading, leads to a significant increase in the resting P_i concentration of human skeletal muscle. Additionally, alterations in resting P_i concentration may contribute to strength deficits with immobilization not accounted for by changes in muscle CSA or neural adaptations.