31P NMR of electrically stimulated rectus femoris muscle: an in vivo graded exercise model

Mr Spectroscopy in Clinical Research

MR spectroscopy (MRS) offers unique possibilities for non-invasive evaluation of biochemistry in vivo. During recent years there has been a growing body of evidence from clinical research studies on human beings using 31P and 1H MRS. The results indicate that it is possible to evaluate phosphorous energy metabolism, loss of neurones, and lactate production in a large number of brain diseases. Furthermore, 31P and 1H MRS may be particularly clinically useful in evaluation of various disorders in skeletal muscle. In the heart 31P MRS seems at the moment the most suitable for evaluation of global affections of the myocardium. In the liver 31P MRS appears to be rather insensitive and non-specific, but absolute quantification of metabolite concentrations and using metabolic “stress models” may prove useful in the future. The clinical role of MRS in oncology is still unclear, but it may be useful for noninvasive follow-up of treatment.

Taken together, the evidence obtained so far certainly shows some trends for clinical applications of MRS. Methods are now available for the clinical research necessary for establishing routine clinical MRS examinations.

The pH heterogeneity in human calf muscle during neuromuscular electrical stimulation

PURPOSE

The aim of the study was to examine pH heterogeneity during fatigue induced by neuromuscular electrical stimulation (NMES) using phosphorus magnetic resonance spectroscopy (³¹ P-MRS). It is hypothesized that three pH components would occur in the ³¹ P-MRS during fatigue, representing three fiber types.

METHODS

The medial gastrocnemius of eight subjects was stimulated within a 3-Tesla whole body MRI scanner. The maximal force during stimulation (F_stim ) was examined by a pressure sensor. Phosphocreatine (PCr), adenosintriphosphate, inorganic phosphate (Pi), and the corresponding pH were estimated by a nonvolume-selective ³¹ P-MRS using a small loop coil at rest and during fatigue.

RESULTS

During fatigue, F_stim and PCr decreased to 27% and 33% of their initial levels, respectively. In all cases, the Pi peak increased when NMES was started and split into three different peaks. Based on the single Pi peaks during fatigue, an alkaline (6.76 ± 0.08), a medium (6.40 ± 0.06), and an acidic (6.09 ± 0.05) pH component were observed compared to the pH (7.02 ± 0.02) at rest.

CONCLUSION

It is suggested that NMES is able to induce pH heterogeneity in the medial gastrocnemius, and that the single Pi peaks represent the different muscle fiber types of the skeletal muscle. Magn Reson Med 77:2097-2106, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Responders to Wide-Pulse, High-Frequency Neuromuscular Electrical Stimulation Show Reduced Metabolic Demand: A 31P-MRS Study in Humans

Conventional (CONV) neuromuscular electrical stimulation (NMES) (i.e., short pulse duration, low frequencies) induces a higher energetic response as compared to voluntary contractions (VOL). In contrast, wide-pulse, high-frequency (WPHF) NMES might elicit--at least in some subjects (i.e., responders)--a different motor unit recruitment compared to CONV that resembles the physiological muscle activation pattern of VOL. We therefore hypothesized that for these responder subjects, the metabolic demand of WPHF would be lower than CONV and comparable to VOL. 18 healthy subjects performed isometric plantar flexions at 10% of their maximal voluntary contraction force for CONV (25 Hz, 0.05 ms), WPHF (100 Hz, 1 ms) and VOL protocols. For each protocol, force time integral (FTI) was quantified and subjects were classified as responders and non-responders to WPHF based on k-means clustering analysis. Furthermore, a fatigue index based on FTI loss at the end of each protocol compared with the beginning of the protocol was calculated. Phosphocreatine depletion (ΔPCr) was assessed using 31P magnetic resonance spectroscopy. Responders developed four times higher FTI's during WPHF (99 ± 37 × 10(3) N.s) than non-responders (26 ± 12 × 10(3) N.s). For both responders and non-responders, CONV was metabolically more demanding than VOL when ΔPCr was expressed relative to the FTI. Only for the responder group, the ∆PCr/FTI ratio of WPHF (0.74 ± 0.19 M/N.s) was significantly lower compared to CONV (1.48 ± 0.46 M/N.s) but similar to VOL (0.65 ± 0.21 M/N.s). Moreover, the fatigue index was not different between WPHF (-16%) and CONV (-25%) for the responders. WPHF could therefore be considered as the less demanding NMES modality--at least in this subgroup of subjects--by possibly exhibiting a muscle activation pattern similar to VOL contractions.

Effects of stimulation frequency and pulse duration on fatigue and metabolic cost during a single bout of neuromuscular electrical stimulation

We have investigated the effects of stimulation frequency and pulse duration on fatigue and energy metabolism in rat gastrocnemius muscle during a single bout of neuromuscular electrical stimulation (NMES). Electrical pulses were delivered at 100 Hz (1-ms pulse duration) and 20 Hz (5-ms pulse duration) for the high (HF) and low (LF) frequency protocols, respectively. As a standardization procedure, the averaged stimulation intensity, the averaged total charge, the initial peak torque, the duty cycle, the contraction duration and the torque-time integral were similar in both protocols. Fatigue was assessed using two testing trains delivered at a frequency of 100 Hz and 20 Hz before and after each protocol. Metabolic changes were investigated in vivo using 31P-magnetic resonance spectroscopy (31P-MRS) and in vitro in freeze-clamped muscles. Both LF and HF NMES protocols induced the same decrease in testing trains and metabolic changes. We conclude that, under carefully controlled and comparable conditions, the use of low stimulation frequency and long pulse duration do not minimize the occurrence of muscle fatigue or affect the corresponding stimulation-induced metabolic changes so that this combination of stimulation parameters would not be adequate in the context of rehabilitation.

Electrical stimulation as a modality to improve performance of the neuromuscular system

Transcutaneous neuromuscular electrical stimulation (NMES) can modify the order of motor unit recruitment and has a profound influence on the metabolic demand associated with producing a given muscle force. Because of these differences, interventions that combine NMES with voluntary contractions can provide beneficial outcomes for some individuals. The adaptations evoked by NMES are not confined to the activated muscle but also involve neural adaptations through reflex inputs to the spinal cord and supraspinal centers.

A comparison of voluntary and electrically induced contractions by interleaved 1H- and 31P-NMRS in humans

Skeletal muscle voluntary contractions (VC) and electrical stimulations (ES) were compared in eight healthy men. High-energy phosphates and myoglobin oxygenation were simultaneously monitored in the quadriceps by interleaved (1)H- and (31)P-NMR spectroscopy. For the VC protocol, subjects performed five or six bouts of 5 min with a workload increment of 10% of maximal voluntary torque (MVT) at each step. The ES protocol consisted of a 13-min exercise with a load corresponding to 10% MVT. For both protocols, exercise consisted of 6-s isometric contractions and 6-s rest cycles. For an identical mechanical level (10% MVT), ES induced larger changes than VC in the P(i)-to-phosphocreatine ratio [1.38 +/- 1.14 (ES) vs. 0.13 +/- 0.04 (VC)], pH [6.69 +/- 0.11 (ES) vs. 7.04 +/- 0.07 (VC)] and myoglobin desaturation [43 +/- 15.9 (ES) vs. 6.1 +/- 4.6% (VC)]. ES activated the muscle facing the NMR coil to a greater extent than did VCs when evaluated under identical technical conditions. This metabolic pattern can be interpreted in terms of specific temporal and spatial muscle cell recruitment. Furthermore, at identical levels of energy charge, the muscle was more acidotic and cytoplasm appeared more oxygenated during ES than during VC. These results are in accordance with a preferential recruitment of type II fibers and a relative muscle hyperperfusion during ES.

Metabolic determinants of the onset of acidosis in exercising human muscle: a 31P-MRS study

Onset of intracellular acidosis during muscular exercise has been generally attributed to activation or hyperactivation of nonoxidative ATP production but has not been analyzed quantitatively in terms of H(+) balance, i.e., production and removal mechanisms. To address this issue, we have analyzed the relation of intracellular acidosis to H(+) balance during exercise bouts in seven healthy subjects. Each subject performed a 6-min ramp rhythmic exercise (finger flexions) at low frequency (LF, 0.47 Hz), leading to slight acidosis, and at high frequency (HF, 0.85 Hz), inducing a larger acidosis. Metabolic changes were recorded using (31)P-magnetic resonance spectroscopy. Onset of intracellular acidosis was statistically identified after 3 and 4 min of exercise for HF and LF protocols, respectively. A detailed investigation of H(+) balance indicated that, for both protocols, nonoxidative ATP production preceded a change in pH. For HF and LF protocols, H(+) consumption through the creatine kinase equilibrium was constant in the face of increasing H(+) generation and efflux. For both protocols, changes in pH were not recorded as long as sources and sinks for H(+) approximately balanced. In contrast, a significant acidosis occurred after 4 min of LF exercise and 3 min of HF exercise, whereas the rise in H(+) generation exceeded the rise in H(+) efflux at a nearly constant H(+) uptake associated with phosphocreatine breakdown. We have clearly demonstrated that intracellular acidosis in exercising muscle does not occur exclusively as a result of nonoxidative ATP production but, rather, reflects changes in overall H(+) balance.

Effects of muscle activation on fatigue and metabolism in human skeletal muscle

Increasing stimulation frequency has been shown to increase fatigue but not when the changes in force associated with changes in frequency have been controlled. An effect of frequency, independent of force, may be associated with the metabolic cost resulting from the additional activations. Here, two separate experiments were performed on human medial gastrocnemius muscles. The first experiment (n = 8) was designed to test the effect of the number of pulses on fatigue. The declines in force during two repetitive, 150-train stimulation protocols that produced equal initial forces, one using 80-Hz trains and the other using 100-Hz trains, were compared. Despite a difference of 600 pulses (23.5%), the protocols produced similar rates and amounts of fatigue. In the second experiment, designed to test the effect of the number of pulses on the metabolic cost of contraction, 31P-NMR spectra were collected (n = 6) during two ischemic, eight-train stimulation protocols (80- and 100-Hz) that produced comparable forces despite a difference of 320 pulses (24.8%). No differences were found in the changes in P(i) concentration, phosphocreatine concentration, and intracellular pH or in the ATP turnover produced by the two trains. These results suggest that the effect of stimulation frequency on fatigue is related to the force produced, rather than to the number of activations. In addition, within the range of frequencies tested, increasing total activations did not increase metabolic cost.

Spatial distribution of blood flow in electrically stimulated human muscle: a positron emission tomography study

Neuromuscular electrical stimulation (NMES) was studied with positron emission tomography (PET) and H(2)(15)O in the quadriceps muscle of 11 men. The subjects were submitted to simultaneous bilateral isometric contraction (5 s)-rest (5 s) cycles for 12 min, with a workload corresponding to 5% of quadriceps maximal isometric voluntary torque (QMIVT) for one thigh (5%T) and 10% of QMIVT for the other (10%T). Scans were centered at the electrodes and tissue blood flow (TBF) was evaluated in square regions of interest (ROIs) (3.5 cm(2)) in the transverse section (TS) of both thighs. The mean TBF reached 8.9 mL min(-1) 100 g(-1) in the TS of the 5%T and 11.5 mL min(-1) 100 g(-1) in the TS of the 10%T (P > 0.05). A negative linear relationship was found for both thighs between the ROI-electrode distance and the TBF (P </= 0.009). The mean percentage of activated ROIs (TBF > 5 mL min(-1) 100 g(-1)) was lower in the 5%T than in the 10%T (50.6% vs. 62.2%; P = 0.017). With NMES, the pattern of spatial recruitment appears linked to electrode proximity and is spatially extended. These results confirm the utility of combining NMES with voluntary exercise in the treatment of atrophied muscle.

Noninvasive measurement of phosphocreatine recovery kinetics in single human muscles

The rate at which phosphocreatine (PCr) is resynthesized after exercise is related to muscle oxidative capacity (Vmax). With the use of a one-dimensional image-guided, localized nuclear magnetic resonance spectroscopy technique, PCr kinetics were monitored in the medial gastrocnemius of eight healthy subjects after voluntary, short duration, maximal rate exercise. Localized spectra were obtained every 6 s with <5% contamination from nonselected regions. Maximal rate exercise elicited near-maximal to maximal muscle activation, as indicated by the high-PCr hydrolysis rate (2.26 +/- 0.07 mM/s) and extensive PCr depletion. At the end of 9 s of maximal rate exercise, PCr was depleted by 61.4 +/- 2.4% and intracellular pH was 7.04 +/- 0.03. After 9 s of maximal rate exercise, PCr recovered with a rate constant (kPCr) of 1.87 +/- 0.15 min(-1) and a Vmax of 67.2 +/- 6.0 mM/min. Independent of prior activity, aerobic ATP synthesis rates reached 48.6 +/- 4.9 mM/min within 9 s. Extending maximal rate exercise to 30 s resulted in 92.0 +/- 1.2% PCr depletion and an intracellular pH of 6.45 +/- 0.07. The intracellular acidosis separated the direct relationship between kPCr and muscle Vmax but did not affect the initial PCr resynthesis rate.