Functional investigations of exercising muscle: a noninvasive magnetic resonance spectroscopy-magnetic resonance imaging approach

Abstract

Muscle fatigue, which is defined as the decline in muscle performance during exercise, may occur at different sites along the pathway from the central nervous system through to the intramuscular contractile machinery. Historically, both impairment of neuromuscular transmission and peripheral alterations within the muscle have been proposed as causative factors of fatigue development. However, according to more recent studies, muscle energetics play a key role in this process. Intramyoplasmic accumulation of inorganic phosphate (P(i)) and limitation in ATP availability have been frequently evoked as the main mechanisms leading to fatigue. Although attractive, these hypotheses have been elaborated on the basis of experimental results obtained in vitro, and their physiological relevance has never been clearly demonstrated in vivo. In that context, noninvasive methods such as 31-phosphorus magnetic resonance spectroscopy and surface electromyography have been employed to understand both metabolic and electrical aspects of muscle fatigue under physiological conditions. Mapping of muscles activated during exercise is another interesting issue which can be addressed using magnetic resonance imaging (MRI). Exercise-induced T2 changes have been used in order to locate activated muscles and also as a quantitative index of exercise intensity. The main results related to both issues, i.e. the metabolic and electrical aspects of fatigue and the MRI functional investigation of exercising muscle, are discussed in the present review.

Lumbar Muscle Dysfunction During Remission of Unilateral Recurrent Nonspecific Low-back Pain

“…Several authors have investigated the relationship between the muscles' basic characteristics and the concurrent T2 shift: the magnitude of T2 shift was found to correlate inversely with the level of aerobic capacity of a given muscle. 33,[39][40][41] If MF has undergone a glycolytic shift in fiber type percentage (less aerobic capacity) in the recurrent LBP group as suggested by the lower T2 rest in the recurrent LBP group compared with the healthy controls, more metabolically active substances will be produced during MF contraction, contributing to higher T2 shifts on mfMRI. Consequently, when the muscle composition differs at rest, caution is warranted in drawing conclusions on the amount of muscle activity from the magnitude of T2 shift.…”

Use of Imaging to Assess Normal and Adaptive Muscle Function

“…With EMG electrodes, the signal can be used to search for changes in firing frequency and hence fatigue but cannot define changes in the ionic or chemical milieu of the muscle fibers that may contribute to fatigue and help differentiate between central fatigue and peripheral fatigue. 31 P MRS has been successfully used to measure phosphorus-containing metabolites (inorganic phosphates) created by muscle during fatiguing protocols (reviewed by Bendahan et al 56 and Roy 57 ). This type of knowledge can be used to understand the mechanisms of disease and the functional capacity of patients.…”

Rats Bred for Low Aerobic Capacity Become Promptly Fatigued and Have Slow Metabolic Recovery after Stimulated, Maximal Muscle Contractions

“…, PCr and Pi [15], [16]. With these parameters one can estimate the energy production efficiency of the muscle, such as the speed of PCr degradation and resynthesis, provided that the simultaneous force output is recorded [17].…”