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

Abstract

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 > or = 0.97, P < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Muscle Functional MRI as an Imaging Tool to Evaluate Muscle Activity

“…1,29,33,35,38,61 Studies have shown that T2 shifts are quantitatively dependent on the intensity of skeletal muscle activation when exercises are performed over a wide range of intensities, 1,29,35,38 supporting a linear relationship between T2 times and exercise intensity. Fisher et al 29 demonstrated that increases in T2 values of the human tibialis anterior were linearly related to the forces generated during exercise (r = 0.87), whereas Jenner et al 35 demonstrated a similar correlation when exercise intensity was altered by increasing the rate of contractions at a constant target force (r = 0.64, P<.01).…”

“…Although there is a fast decay of T2, full recovery of muscle T2 is much slower, as T2 generally remains elevated for approximately 30 minutes following exercise. 29,61 If the effect of different exercises on muscle activity is to be evaluated, it is recommended to permit at least 45 minutes of rest between exercise sets, as this allows full recovery of any established T2 shifts. 15 Different sequences can be used, of which multi-spin echo sequences are mostly applied.…”

“…54 A sensitivity study revealed that changes in T2 times for the elbow flexor muscles can be detected with as few as 2 repetitions (1 repetition being a 1-second concentric contraction and a 1-second eccentric contraction) when performed at a high intensity (80% maximum voluntary contraction [MVC]). 15,54,61 Lower intensity exercises (25% MVC) may require up to 5 contractions before changes in T2 times can be appreciated. 61 In conclusion, while both mfMRI and EMG can be used independently to assess muscle activity relevant to research and clinical practice, their utility may depend on the nature of the muscular activity of interest.…”

“…15,54,61 Lower intensity exercises (25% MVC) may require up to 5 contractions before changes in T2 times can be appreciated. 61 In conclusion, while both mfMRI and EMG can be used independently to assess muscle activity relevant to research and clinical practice, their utility may depend on the nature of the muscular activity of interest. It is possible that combining both techniques will provide additional information when evaluating overall muscle function.…”

Intracellular acidification and volume increases explain R2 decreases in exercising muscle

“…The magnetization attributable to the intracellular space (M 0i ) was used to indicate the V i Ј and the relationship between R 2i and the percent of initial M 0i was modeled using linear regression. pH i was altered by exposing four pairs of sartorii to Ringer's solutions modified by replacing 10, 20, 30, or 40 mmol ⅐ l -1 NaCl with an equivalent amount of NH 4 Cl for 3 h. The NH 3 was then washed out by rinsing with Ringer's solution every 15 min for 3 h. pH i and T 2 measurements were then made as described below, except that for the water-suppressed 1 H spectra, the number of averages (N AVG ) ϭ 384, the echo time (TE) ϭ 4 ms, and the repetition time (TR) ϭ 2 s. The data were fit using linear regression.…”