Greg D. Marsh

Multicomponent T2 relaxation of in vivo skeletal muscle

In vivo spin‐spin (T2) relaxation measurements were acquired from the flexor digitorum profundus (FDP) of 13 subjects. A standard imaging T2 measurement technique [number of points (N) = 6, TE = 18 msec, signal‐to‐noise ratio (SNR) ≅ 300] yielded a single T2 value of 31 msec. A novel technique, projection presaturation combined with a CPMG sequence, was used to acquire data (N = 1000, TE = 1.2 msec, SNR 3500) from a cylindrical voxel (2 cm diameter, 5 cm length) within the FDP. All 13 subjects had at least four T2 components, at <5, 21 ± 4, 39 ± 6, and 114 ± 31 msec, with fractional areas of 11 ± 2, 28 ± 15, 46 ± 12, and 11 ± 5% respectively. The shortest and longest components have been observed in ex vivo muscle studies, probably corresponding to water associated with macromolecules and extracellular water, respectively. The middle T2 components are suggestive of an organization of in vivo intracellular water. Magn Reson Med 42:150–157, 1999.

Muscle fiber number in the biceps brachii muscle of young and old men

We have compared the number of muscle fibers in the biceps brachii muscle (BB) of six old men (82.3 ± 4.3 years) and six young men (21.2 ± 1.9 years). Muscle fiber number was estimated by dividing the maximal area of the BB, determined with magnetic resonance imaging, by the mean fiber area of the BB determined in a muscle biopsy. The percentage of type II fibers in the BB (∼60%) and the type I fiber area were not different between the groups. The BB area (−26%), type II fiber area (−24%), mean fiber area (−20%), and maximal voluntary contraction strength (MVC) of the elbow flexor muscles (−27%) were lower in the old than young group. However, the estimated number of muscle fibers was not significantly different between the young (253,000) and old (234,000) men. Consequently, the smaller BB area of the old men could be explained primarily by a smaller type II fiber size. These findings suggest that old age is not associated with a reduced number of muscle fibers in the BB. The relative contribution of a reduction in fiber number to age‐related muscle atrophy may be muscle‐dependent. Muscle Nerve 28: 62–68, 2003

The effects of age on kinetics of oxygen uptake and phosphocreatine in humans during exercise

We compared the kinetics of oxygen uptake (VO2) and phosphocreatine (PCr) during the adjustment to and recovery from plantar flexion exercise in moderately active older (n = 10, 66.9 years) and younger (n = 10, 27.5 years) individuals. VO2 kinetics were similar in the two groups, with time constants (tau) averaging 46.3 +/− 10.2 s (younger, on‐transient), 38.1 +/− 14.4 s (younger, off‐transient), 46.3 +/− 17.8 (older, on‐transient) and 40.7 +/− 19.2 s (older, off‐transient). These were similar to corresponding PCr kinetics, measured by 31P nuclear magnetic resonance spectroscopy, which averaged 50.6 +/− 24.0 s (younger, on‐transient), 42.0 +/− 16.1 s (younger, off‐transient), 39.8 +/− 22.0 s (older, on‐transient) and 37.6 +/− 21.6 s (older, off‐transient). On‐transient tau values for VO2 and PCr were correlated, for combined groups (r = 0.53; P = 0.015). We conclude that: (1) VO2 and PCr kinetics during exercise of a muscle group accustomed to daily activity are not compromised in physically active older humans, and (2) PCr kinetics reflect the kinetics of muscle O2 consumption, and are expressed at the lung (VO2 kinetics) after a transit delay.

Muscle metabolism and acid-base status during exercise in forearm work-related myalgia measured with 31P-MRS

In this study, we examined muscle metabolic and acid-base status during incremental wrist extension exercise in the forearm of individuals with work-related myalgia (WRM). Eighteen women employed in full-time occupations involving repetitive forearm labor were recruited in this cross-sectional study. Nine of these women were diagnosed with WRM, while the other nine had no previous WRM history and were used as age-matched controls (Con). Phosphorus-31 magnetic resonance spectroscopy ((31)P-MRS) was used to noninvasively monitor the intracellular concentrations of phosphocreatine ([PCr]) and inorganic phosphate ([P(i)]) as well as intracellular pH (pH(i)) status during exercise in WRM and Con. We observed a 38% decreased work capacity in WRM compared with Con [0.18 W (SD 0.03) vs. 0.28 W (SD 0.10); P = 0.007]. Piecewise linear regression of the incremental exercise data revealed that the onset of a faster decrease in pH(i) (i.e., the pH threshold, pHT) and the onset of a faster increase in log([P(i)]/[PCr]) (i.e., the phosphorylation threshold, PT) occurred at a 14% relatively lower power output in WRM [pHT: 45.2% (SD 5.3) vs. 59.0% (SD 4.6), P < 0.001; PT: 44.8% (SD 4.3) vs. 57.8% (SD 3.1), P < 0.001; % of peak power output, Con vs. WRM, respectively]. Monoexponential modeling of the kinetics of [PCr] and pH(i) recovery following exercise demonstrated a slower (P = 0.005) time constant (tau) for [PCr] in WRM [113 s (SD 25)] vs. Con [77 s (SD 23)] and a slower (P = 0.007) tau for pH(i) in WRM [370 s (SD 178)] vs. Con [179 s (SD 52)]. In conclusion, our results suggest that WRM is associated with an increased reliance on nonoxidative metabolism. Possible mechanisms include a reduction in local muscle blood flow and perfusion, an increased ATP cost of force production, or both.

Changes in Human Muscle Transverse Relaxation Following Short‐Term Creatine Supplementation

The rapid increase in body mass that often occurs following creatine (Cr) supplementation is believed to be due to intracellular water retention. The purpose of this study was to determine whether Cr consumption alters the magnetic resonance (MR) transverse relaxation (T2) distribution of skeletal muscle. Transverse relaxation can be used to model water compartments within a cell or tissue. In this double‐blind study, subjects were asked to supplement their normal diet with creatine monohydrate (20 g day−1 for 5 days) mixed with a grape drink (Creatine group, n= 7), or the grape drink alone (Placebo group, n= 8). Phosphorous MR spectroscopy was used to determine the effectiveness of the supplementation protocol. Subjects that responded to the Cr supplementation (i.e. showed a > 5% increase in the ratio of the levels of phosphocreatine (PCr) and ATP) were placed in the Creatine group. Both proton MR imaging and spectroscopy were used to acquire T2 data, at 1.89 T, from the flexor digitorum profundus muscle of each subject before and after supplementation. Following the supplementation period, the Creatine group showed a gain in body mass (1.2 ± 0.8 kg, P < 0.05, mean ± S.D.), and an increase in PCr/ATP ratio (23.8 ± 16.4%, P < 0.001). Neither group showed any changes in intracellular pH or T2 calculated from MR images. However, the spectroscopy data revealed at least three components (> 5 ms) at approximately 20, 40 and 125 ms in both groups. Only in the Creatine group was there an increase in the apparent proton concentration of the two shorter components combined (+5.0 ± 4.7%, P < 0.05). According to the cellular water compartment model, the changes observed in the shorter T2 components are consistent with an increase in intracellular water.

Contractile properties, fatigue and recovery are not influenced by short-term creatine supplementation in human muscle

There have been several studies on the effect of short‐term creatine (Cr) supplementation on exercise performance, but none have investigated both voluntary and stimulated muscle contractions in the same experiment. Fourteen moderately active young men (19‐28 years) were randomly assigned, in a double blind manner, to either a creatine (Cr) or placebo (P) group. The subjects supplemented their regular diet 4 times a day for 5 days with either 5 g Cr + 5 g maltodextrin (Cr group), or 5 g maltodextrin (P group). Isometric maximal voluntary contraction (MVC), muscle activation, as assessed using the modified twitch interpolation technique, electrically stimulated contractile properties, electromyography (EMG), endurance time and recovery from fatigue were measured in the elbow flexors. The fatigue protocol involved both voluntary and stimulated contractions. Following supplementation there was a significant weight gain in the Cr group (1.0 kg), whereas the P group did not change. For each group, pre‐supplementation measures were not significantly different from post‐supplementation for MVC, twitch and tetanic tensions at rest, time to peak tension, half‐relaxation time and contraction duration. Prior to Cr supplementation time to fatigue was 10 ± 4 min (mean ± S.E.M.) for both groups, and following supplementation there was a non‐significant increase of 1 min in each group. MVC force, muscle activation, EMG, stimulated tensions and durations were similar for the Cr and P groups over the course of the fatigue protocol and did not change after supplementation. Furthermore, recovery of MVC, stimulated tensions and contractile speeds did not differ as a result of Cr supplementation. These results indicate that short‐term Cr supplementation does not influence isometric elbow flexion force, muscle activation, stimulated contractile properties, or delay time to fatigue or improve recovery.

Correction to: Physiological resolution of periodic breath holding during heavy-intensity Fartlek exercise

The original version of this article unfortunately contained a mistake.