Sensitivity of muscle proton spin-spin relaxation time as an index of muscle activation

G. Yue, A. L. Alexander, D. H. Laidlaw, A. F. Gmitro, E. C. Unger, R. M. Enoka

Research output: Contribution to journalArticlepeer-review

72 Scopus citations


The purpose of this study was to determine the minimum number of contractions that are needed to detect an increase in the muscle proton spin- spin relaxation time (T2) at a given exercise intensity. Five healthy human subjects performed five sets of an exercise that included concentric and eccentric contractions of the elbow-flexor muscles with loads that were 25 or 80% of maximum. With the 80% load, the five sets involved 1, 2, 5, 10, or 20 repetitions of the exercise; with the 25% load the five sets were 2, 5, 10, 20, or 40 repetitions. The upper arm of each subject was imaged before and immediately after each set of the exercise. Spin-echo images (repetition time/echo time = 2,000 ms/30, 60, 90, and 120 ms) were collected using an extremity coil, and T2 values were calculated. The signal intensity was measured from the elbow-flexor and -extensor muscles and from the bone marrow of the humerus. With the 80% load, T2 increased in the short head of the biceps brachii after two repetitions of the elbow exercise and after five repetitions in the brachialis and the long head of the biceps brachii. With the 25% load, T2 became longer after five repetitions of the exercise for the short head of the biceps brachii and after 10 repetitions for the brachialis and the long head of the biceps brachii. T2 varied linearly with the number of contraction repetitions for each of the elbow-flexor muscles at either load (r2 ≥ 0.97, P < 0.001). The resting T2 value was not uniformly distributed among the elbow-flexor and -extensor muscles, and the T2 after a given set of the exercise was different among the three heads of the triceps brachii. These findings suggest that 1) the sensitivity of T2 for detecting muscle activity depends on the contraction intensity, 2) within a specified range T2 varies linearly with work performed by the muscle, and 3) recruitment of the muscle fibers is different for a given task among the compartments within the flexor or extensor muscles of the elbow joint.

Original languageEnglish (US)
Pages (from-to)84-92
Number of pages9
JournalJournal of Applied Physiology
Issue number1
StatePublished - 1994


  • activation pattern
  • bone marrow
  • contraction intensities
  • elbow-flexor and -extensor muscles
  • magnetic resonance imaging

ASJC Scopus subject areas

  • Physiology
  • Physiology (medical)


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