title: Changes in muscle proton transverse relaxation times and acidosis during exercise and recovery.
abstract: We studied changes in muscle proton (1H) transverse relaxation times (T2) by magnetic resonance imaging during exercise and compared these changes with alterations in muscle metabolism measured by phosphorus-31 magnetic resonance spectroscopy (31P-MRS). Eleven subjects completed two trials of intermittent incremental forearm wrist flexion exercise requiring 30 contractions/min for 5 min, 7 min of recovery between stages, and 5-N load increments/stage. Between stages of the first trial, T2 images of muscle 1H were obtained. Muscle T2 increased from 27.3 +/- 1.1 (SD) ms at rest to 35.8 +/- 3.6 ms after volitional fatigue (P < 0.05), whereas less active wrist extensor muscle T2 remained unchanged (26.8 +/- 0.9 to 28.8 +/- 1.6 ms; P > 0.05). After localizing the predominant muscle recruited from the T2 images, subjects completed an identical trial at least 1 wk later but involving surface coil 31P-MRS of the T2-enhanced muscle to measure the H+ concentration ([H+]). Intramuscular [H+] of T2-enhancing muscle increased from 1.1 +/- 0.1 x 10(-7) M at rest to 4.1 +/- 2.0 x 10(-7) M after volitional fatigue. Both muscle T2 and intramuscular [H+] increased in a bimodal manner, with T2 increasing before muscle [H+] (P < 0.05). The correlation coefficient between the percent change in T2 and muscle [H+] during exercise was +0.74 (range 0.48-0.98; P < 0.05) and +0.47 during recovery. After 12 min of recovery, muscle [H+] decreased to 1.4 +/- 0.3 x 10(-7) M (P < 0.05), and T2 remained close to postexercise values (32.2 +/- 3.1 ms, P > 0.05). The data indicate that 1) the T2 increases during increases in exercise intensity are nonlinear, 2) the T2 increases during exercise are significantly correlated with increases in [H+], and 3) the slow recovery of T2 compared with [H+] indicates that [H+] has a minor contribution to the recovery in T2.
authors.affiliation: Institute for Basic and Applied Medical Research, Lovelace Institutes, Albuquerque, New Mexico 87108, USA.
authors.forename: H A
authors.initials: HA
authors.lastname: Cheng
authors.forename: R A
authors.initials: RA
authors.lastname: Robergs
authors.forename: J P
authors.initials: JP
authors.lastname: Letellier
authors.forename: A
authors.initials: A
authors.lastname: Caprihan
authors.forename: M V
authors.initials: MV
authors.lastname: Icenogle
authors.forename: L J
authors.initials: LJ
authors.lastname: Haseler
mesh_terms: D000138:Acidosis / Q000378:metabolism*; D000328:Adult; D015444:Exercise / Q000502:physiology*; D005260:Female; D005542:Forearm / Q000033:anatomy & histology / Q000502:physiology; D005583:Fourier Analysis; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D009119:Muscle Contraction / Q000502:physiology; D018482:Muscle, Skeletal / Q000033:anatomy & histology / Q000378:metabolism* / Q000502:physiology*