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

Abstract

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 (Pi) concentration from the medial gastrocnemius muscle during immobilization, reloading and rehabilitation. Additionally, we assessed the contributions of CSA, muscle activation and Pi 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 (31)P-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 Pi concentration and the Pi/PCr ratio (P < 0.05). From 6 weeks of immobilization to 3-5 days of reloading, the increase in Pi concentration (15%, P = 0.26) and Pi/PCr (20%, P = 0.29) was not significant. During rehabilitation, the relative contributions of CSA, muscle activation and Pi concentration to plantarflexor torque were 32, 44 and 40%, respectively. Together, CSA, muscle activation and Pi 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 Pi concentration of human skeletal muscle. Additionally, alterations in resting Pi concentration may contribute to strength deficits with immobilization not accounted for by changes in muscle CSA or neural adaptations.

The influence of training status on the drop in muscle strength after acute exercise

“…While exercise causes fatigue (Westerblad and Allen 2002), immobilization leads to weakness of the tissue (Christensen et al 2008;Berg and Tesch 1996;Pathare et al 2006). Fatigue has been deWned as any exerciseinduced reduction in the maximal power capacity to generate force or power output (Vollestad 1997), whereas weakness describes a maintained low maximal force, which persists over longer periods and appears independent of exercise (Vollestad 1997).…”

Two-weeks of elbow immobilization affects torque production but does not change muscle activation

“…They attributed their results to changes in the ability of calcium release from the sarcoplasmic reticulum or to a reduction in the number of cross-bridges, or to some change in the contractile apparatus that might be compatible with the absence of changes in the EMG signals. An attempt to explain this reduction in muscle strength was made by Pathare et al 35 , who found increased levels of inorganic phosphate in the muscle after immobilization, which would inhibit muscle contraction by preventing the connection of the myosin heads with the actin filaments. However, they do not explain how this increase in muscle inorganic phosphate is responsible for the inhibition of the contractile apparatus.…”

Magnetic Resonance Imaging: The Underlying Principles

“…Given the large abundance of water in the human body, clinical MRI typically utilizes the hydrogen nuclei (also known as protons) in free and macromolecular-bound water. Sodium and phosphorus are also of interest in musculoskeletal imaging and can be used to noninvasively quantify total sodium changes during exercise and disease states, 9 examine muscle metabolism during exercise or disuse, 26,27,29 and monitor pH, 22 among other applications. However, these nuclei are less abundant and need larger field strengths (3.0-7.0 T) to have a sufficient signal-to-noise ratio before strong and meaningful clinical conclusions can be made.…”

“…The ability to monitor in vivo spectral changes has a range of clinical applications for the musculoskeletal system, including monitoring intracellular and extracellular fat changes, 31 exercise-induced changes in metabolites, 14 intracellular pH abnormalities in chronic fatigue syndrome, 22 and examining muscular response to neurological insult and/or disuse through intramyocellular lipid content 33 or changes in inorganic phosphate. 26,27 SPATIAL ENCODING I n addition to chemical information, the signal can be encoded with spatial position and used to obtain an image. This is done by applying additional magnetic field gradients with a predictable change in field strength along a particular axis.…”