31P MRS evaluation of fatigue in anterior tibial muscle in postpoliomyelitis patients and healthy volunteers

Quantification of skeletal muscle mitochondrial function by 31P magnetic resonance spectroscopy techniques: a quantitative review

Magnetic resonance spectroscopy (MRS) can give information about cellular metabolism in vivo which is difficult to obtain in other ways. In skeletal muscle, non-invasive (31) P MRS measurements of the post-exercise recovery kinetics of pH, [PCr], [Pi] and [ADP] contain valuable information about muscle mitochondrial function and cellular pH homeostasis in vivo, but quantitative interpretation depends on understanding the underlying physiology. Here, by giving examples of the analysis of (31) P MRS recovery data, by some simple computational simulation, and by extensively comparing data from published studies using both (31) P MRS and invasive direct measurements of muscle O2 consumption in a common analytical framework, we consider what can be learnt quantitatively about mitochondrial metabolism in skeletal muscle using MRS-based methodology. We explore some technical and conceptual limitations of current methods, and point out some aspects of the physiology which are still incompletely understood.

Skeletal muscle fatigue

Skeletal muscle fatigue is defined as the fall of force or power in response to contractile activity. Both the mechanisms of fatigue and the modes used to elicit it vary tremendously. Conceptual and technological advances allow the examination of fatigue from the level of the single molecule to the intact organism. Evaluation of muscle fatigue in a wide range of disease states builds on our understanding of basic function by revealing the sources of dysfunction in response to disease.

Use of imaging to assess normal and adaptive muscle function

Physical therapists must be able to determine the activity and passive properties of the musculoskeletal system in order to accurately plan and evaluate therapeutic measures. Discussed in this article are imaging methods that not only allow for the measurement of muscle activity but also allow for the measurement of cellular processes and passive mechanical properties noninvasively and in vivo. The techniques reviewed are T1- and T2-weighted magnetic resonance (MR) imaging, MR spectroscopy, cine-phase-contrast MR imaging, MR elastography, and ultrasonography. At present, many of these approaches are expensive and not readily available in physical therapy clinics but can be found at medical centers. However, there are ways of using these techniques to provide important knowledge about muscle function. This article proposes creative ways in which to use these techniques as evaluative tools.

A gated 31P NMR method for the estimation of phosphocreatine recovery time and contractile ATP cost in human muscle

Muscle phosphocreatine (PCr) recovery time constant (an index of muscle aerobic capacity) and contractile ATP cost were estimated from a gated (31)P NMR protocol which does not require intense, repetitive exercise. Subjects performed 2-s duration, maximum voluntary isometric ankle dorsiflexion contractions at 30-s intervals for 8 min (total 15 contractions), while single-shot (31)P spectra (51.7 MHz, TR 3 s) were acquired from the anterior compartment muscle. Spectra from the sixth through 15th contractions were retrospectively sorted, yielding 10 spectra (each 10 averages) gated to times before and after contraction. There was no significant decrease in muscle pH, allowing the calculation of contractile ATP cost directly from the percentage change in PCr during contraction cycles [8.86 +/- 0.82% (SE, n = 11) of PCr at rest], corresponding to an ATP cost of 1.69 +/- 0.16 mM/s (range 0.99-2.49 mM/s), assuming an 8.2 mM ATP concentration. The time constant for PCr recovery (tau 41.8 +/- 4.2 s, range 22.0-60.8 s) was calculated from tau = -Deltat/ln[D/(D + Q)], where Q is the percentage change in PCr due to contraction, D is the additional steady-state percentage drop in PCr from rest and Deltat is the interval between contractions. In the same subjects, the monoexponential PCr recovery time constant after more intense, repetitive isometric ankle dorsiflexion exercise (30 s at 0.5 Hz, 50% duty cycle) was similar to (36.2 +/- 3.5 s, range 16.5-58.8 s) and well correlated with (r = 0.82) the gated result. In contrast to the gated protocol, muscle pH decreased from 7.01 +/- 0.01 to 6.78 +/- 0.04 during recovery after the repetitive protocol. Hence the gated protocol allows the estimation of muscle ATP cost and PCr recovery without intense exercise or muscle acidification.