Effect of physical activity on MRI-measured blood oxygen level-dependent transients in skeletal muscle after brief contractions

Abstract

The signal intensity (SI) in gradient-echo, echo-planar magnetic resonance images (repetition time/echo time = 1,000/40) of anterior tibialis muscle in active [estimated energy expenditure 42.4 +/- 3.7 (SD), n = 8] vs. sedentary (32.3 +/- 0.6 kcal.kg(-1).day(-1), n = 8) young adult (18-34 yr old) human subjects was measured after single, 1-s-duration maximum voluntary ankle dorsiflexion contractions. There was no difference between groups in anterior tibial muscle cross-sectional area or peak force. In both groups there was a transient increase in anterior tibialis muscle SI, which peaked 5-7 s after the end of each contraction. The magnitude of the SI transient was over threefold greater [5.5 +/- 1.0 (SE) vs. 1.5 +/- 0.4%] and persisted twice as long (half-recovery time 5.4 +/- 0.4 vs. 2.7 +/- 0.3 s) in the active subjects. In the same subjects, blood flow in popliteal, anterior tibial, and posterior tibial arteries was measured by cardiac-gated CINE magnetic resonance angiography before and after 2 min of dynamic, repetitive ankle dorsiflexion exercise. There was no difference between groups in resting or postexercise flow in anterior tibial artery, although popliteal and posterior tibial artery flow after exercise tended to be greater in the active group. The results indicate that transient hyperemia and oxygenation in muscle after single contractions are enhanced by chronic physical activity to a greater extent than peak muscle blood flow.

Fig. 1.

Representative anatomical (top: fast spin echo, repetition time/echo time (TR/TE) = 1,500/24) and echo-planar (bottom: TR/TE = 1,000/40) images from an active (left) and sedentary (right) subject. Voxels marked in green on the echo-planar images surround the regions of interest (ROI) from which the time courses of signal intensity (SI) change were obtained.

Fig. 2.

Representative time course of SI changes in anterior tibialis (top) and posterior muscle (2nd panel) during single-contraction protocol. Active (left) and sedentary (right) subjects are the same as in Fig. 1. Spikes coincident with the contractions (force, 3rd panel) are due to changes in signal saturation when the muscles move in the imaged slice during contraction and relaxation (20). Bottom shows the response for each subject averaged over the 7 contractions.

Fig. 3.

Example magnitude images from the cardiac-gated phase-contrast flow study, illustrating location at which flow measurements were made in popliteal (top) and tibial (bottom) arteries before (left) and after (right) repetitive exercise. These images are from peak systole in the same active subject shown in Figs. 1 and 2. Ant, anterior; Post, posterior.

Fig. 4.

Example cardiac-gated flow waveforms from anterior tibial (top), posterior tibial (middle), and popliteal (bottom) arteries before (◯) and after (●) repetitive ankle dorsiflexion exercise. Results from the same subject as Fig. 3.

Fig. 5.

Mean flow (±SE) in anterior tibial (top), posterior tibial (middle), and popliteal (bottom) arteries in active (●) and sedentary (◯) subjects before and after 2-min dynamic, repetitive ankle dorsiflexion exercise. Measurements after exercise are on average slower in the active compared with the sedentary group due to the slower heart rate in the active group [52.5 ± 2.5 (SE) vs. 69.5 ± 4.2 beats/min]. Flow immediately after exercise (plotted at time zero) was computed for each individual measurement assuming exponential flow recovery to the preexercise flow level.