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Changes in inorganic phosphate and force production in human skeletal muscle after cast immobilization

Departments of ¹Physical Therapy and ²Physiology and Functional Genomics, University of Florida, Gainesville, Florida; Departments of ³Physiology, ⁴Orthopedic Surgery, and ⁵Rehabilitation Medicine, University of Pennsylvania, Philadelphia, Pennsylvania; and ⁶Department of Orthopedics and Rehabilitation, University of Florida, Gainesville, Florida

Submitted 16 June 2004 ; accepted in final form 25 August 2004

Cast immobilization is associated with decreases in muscle contractile area, specific force, and functional ability. The pathophysiological processes underlying the loss of specific force production as well as the role of metabolic alterations are not well understood. The aim of this study was to quantify changes in the resting energy-rich phosphate content and specific force production after immobilization. ³¹P-magnetic resonance spectroscopy, three-dimensional magnetic resonance imaging, and isometric strength testing were performed in healthy subjects and patients with an ankle fracture after 7 wk of immobilization and during rehabilitation. Muscle biopsies were obtained in a subset of patients. After immobilization, there was a significant decrease in the specific plantar flexor torque and a significant increase in the inorganic phosphate (P_i) concentration (P < 0.001) and the P_i-to-phosphocreatine (PCr) ratio (P < 0.001). No significant change in the PCr content or basal pH was noted. During rehabilitation, both the P_i content and the P_i-to-PCr ratio decreased and specific torque increased, approaching control values after 10 wk of rehabilitation. Regression analysis showed an inverse relationship between the in vivo P_i concentration and specific torque (r = 0.65, P < 0.01). In vitro force mechanics performed on skinned human muscle fibers demonstrated that varying the P_i levels within the ranges observed across individuals in vivo (4–10 mM) changed force production by 16%. In summary, our findings clearly depict a change in the resting energy-rich phosphate content of skeletal muscle with immobilization, which may negatively impact its force generation.

magnetic resonance spectroscopy; muscle strength; skinned fibers

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