The purpose of this study was to compare the magnitude and mechanisms of ankle dorsiflexor muscle fatigue in 20 young (33 +/- 6 yr, mean +/- SD) and 21 older (75 +/- 6 yr) healthy men and women of similar physical activity status. Noninvasive measures of central and peripheral (neuromuscular junction, sarcolemma) muscle activation, muscle contractile function, and intramuscular energy metabolism were made before, during, and after incremental isometric exercise. Older subjects fatigued less than young (P < 0.01); there was no effect of gender on fatigue (P = 0.24). For all subjects combined, fatigue was modestly related to preexercise strength (r = 0.49, P < 0.01). Neither central (central activation ratio) nor peripheral (compound muscle action potential) activation played a significant role in fatigue in any group. During exercise, intracellular concentrations of P(i) and H(2)PO increased more and pH fell more in young compared with older subjects (P < 0.01) and in men compared with women (P < 0.01). These varied metabolic responses to exercise suggest a greater reliance on nonoxidative sources of ATP in young compared with older subjects and in men compared with women. These results suggest that the mechanisms of fatigue vary with age and gender, regardless of whether differences in the magnitude of fatigue are observed.

The purpose of this study was to test the hypotheses that, under isovelocity conditions, older compared with young humans would 1). be slower to reach target velocity and 2). exhibit a downward shift in the torque-velocity and power-velocity relationships in the ankle dorsiflexor and knee extensor muscles. We studied 12 young (26 +/- 5 yr, 6 men/6 women) and 12 older (72 +/- 6 yr, 6 men/6 women) healthy adults during maximal voluntary concentric contractions at preset target velocities (dorsiflexion: 0-240 degrees /s; knee extension: 0-400 degrees /s) using an isokinetic dynamometer. The time to target velocity was longer in older subjects in the dorsiflexors and knee extensors (both P

Transient increases in signal intensity (DeltaSI, peak 2.6 +/- 0.6 %, mean +/- SE, n = 14) were observed in axial, gradient-echo, echo-planar magnetic resonance images acquired at 1.5 T from human anterior tibialis muscle following single, 1 s duration, isometric ankle dorsiflexion contractions. The magnitude of the MRI-measured DeltaSI was not significantly different using TR of 2000 vs 500 ms, or using spin-echo vs gradient-echo echo-planar pulse sequences. However, DeltaSI measured by gradient-echo sequences was significantly greater at 3 vs 1.5 T (3.8 +/- 0.8 vs 1.6 +/- 0.2 %, n = 5). The time course of the transient DeltaSI (peak at 7.9 +/- 0.4 s after each contraction, decay with half-time of 4.6 +/- 0.6 s) was comparable to the time course of the transient increase in relative heme saturation (13 +/- 2 %, n = 5) measured after single contractions in another group of subjects by near-infrared spectroscopy (peak at 9.3 +/- 0.5 s, decay with half-time 6.2 +/- 0.8 s, n = 8). Simulations of intravascular and extravascular blood-oxygenation level-dependent (BOLD) effects in muscle suggested that intravascular BOLD makes a major contribution to the transient changes, although other factors such as increased vascular volume or increased muscle cell T2 may also contribute. The transients can be exploited for muscle functional imaging analogous to BOLD-based brain functional imaging, and might provide an index of peripheral vascular function.

Gifford A, Towse TF, Walker RC, Avison MJ, Welch EB. Characterizing active and inactive brown adipose tissue in adult humans using PET-CT and MR imaging.

Forbes SC, Paganini AT, Slade JM, Towse TF, Meyer RA. Phosphocreatine recovery kinetics following low-and high-intensity exercise in human triceps surae and rat posterior hindlimb muscles. Am J Physiol Regul Integr Comp Physiol 296: R161-R170, 2009. First published October 22, 2008 doi:10.1152/ajpregu.90704.2008.-Previous studies have suggested the recovery of phosphocreatine (PCr) after exercise is at least second-order in some conditions. Possible explanations for higher-order PCr recovery kinetics include heterogeneity of oxidative capacity among skeletal muscle fibers and ATP production via glycolysis contributing to PCr resynthesis. Ten human subjects (28 Ϯ 3 yr; mean Ϯ SE) performed gated plantar flexion exercise bouts consisting of one contraction every 3 s for 90 s (low-intensity) and three contractions every 3 s for 30 s (highintensity). In a parallel gated study, the sciatic nerve of 15 adult male Sprague-Dawley rats was electrically stimulated at 0.75 Hz for 5.7 min (low intensity) or 5 Hz for 2.1 min (high intensity) to produce isometric contractions of the posterior hindlimb muscles. [31 P]-MRS was used to measure relative [PCr] changes, and nonnegative leastsquares analysis was utilized to resolve the number and magnitude of exponential components of PCr recovery. Following low-intensity exercise, PCr recovered in a monoexponential pattern in humans, but a higher-order pattern was typically observed in rats. Following high-intensity exercise, higher-order PCr recovery kinetics were observed in both humans and rats with an initial fast component ( Ͻ 15 s) resolved in the majority of humans (6/10) and rats (5/8). These findings suggest that heterogeneity of oxidative capacity among skeletal muscle fibers contributes to a higher-order pattern of PCr recovery in rat hindlimb muscles but not in human triceps surae muscles. In addition, the observation of a fast component following high-intensity exercise is consistent with the notion that glycolytic ATP production contributes to PCr resynthesis during the initial stage of recovery. oxidative capacity; fiber types; skeletal muscle; magnetic resonance spectroscopy; nonnegative least-squares analysis THE TIME COURSE OF PHOSPHOCREATINE (PCr) recovery following exercise is often characterized by using a monoexponential model (18,21,26,37). A monoexponential pattern is consistent with a first-order metabolic system in which PCr resynthesis is entirely dependent on ATP produced by oxidative phosphorylation (11, 21) with the inverse of the time constant (tau, ) or rate constant (1/) directly proportional to muscle oxidative capacity (19,26). Specifically, the rate constant of PCr resynthesis has been shown to be directly proportional to citrate synthase activity (which reflects mitochondrial content) and inversely proportional to total creatine content (20, 26). Nevertheless, multiexponential PCr recovery kinetics have been commonly reported after high-intensity exercise (1,10,23,31,32,41). Although the factor(s) contributing to this higher-order response have...