Microvascular response to exercise varies along the length of the tibialis anterior muscle

Abstract

Microvascular function is an important component in the physiology of muscle. One of the major parameters, blood perfusion, can be measured noninvasively and quantitatively by arterial spin labeling (ASL) MRI. Most studies using ASL in muscle have only reported data from a single slice, thereby assuming that muscle perfusion is homogeneous within muscle, whereas recent literature has reported proximodistal differences in oxidative capacity and perfusion. Here, we acquired pulsed ASL data in 12 healthy volunteers after dorsiflexion exercise in two slices separated distally by 7 cm. We combined this with a Look‐Locker scheme to acquire images at multiple postlabeling delays (PLDs) and with a multiecho readout to measure T2*. This enabled the simultaneous evaluation of quantitative muscle blood flow (MBF), arterial transit time (ATT), and T2* relaxation time in the tibialis anterior muscle during recovery. Using repeated measures analyses of variance we tested the effect of time, slice location, and their interaction on MBF, ATT, and T2*. Our results showed a significant difference as a function of time postexercise for all three parameters (MBF: F = 34.0, p < .0001; T2*: F = 73.7, p < .0001; ATT: F = 13.6, p < .001) and no average differences between slices over the total time postexercise were observed. The interaction effect between time postexercise and slice location was significant for MBF and T2* (F = 5.5, p = 0.02, F = 6.1, p = 0.02, respectively), but not for ATT (F = 2.2, p = .16). The proximal slice showed a higher MBF and a lower ATT than the distal slice during the first 2 min of recovery, and T2* showed a delayed response in the distal slice. These results imply a higher perfusion and faster microvascular response to exercise in the proximal slice, in line with previous literature. Moreover, the differences in ATT indicate that it is difficult to correctly determine perfusion based on a single PLD as is commonly performed in the muscle literature.

Muscle fiber strain rates in the lower leg during ankle dorsi‐/plantarflexion exercise

“…19 MRI has the ability to capture complete muscle volumes of multiple muscles simultaneously, but to date has been primarily focused on specific muscle parts or planes 8,11,20 using 2D implementations to reduce the scan time. However, from studies in healthy muscle tissue, it is known that both architectural characteristics (e.g., pennation angle) and more functional features (e.g.,microvascular function), vary between and within individual muscles, 8,9,13,[21][22][23][24][25][26][27][28][29] making such 2D implementations and focused studies incomplete. At the same time, this also indicates that it is generally impossible to define a MRI plane aligned with the muscle fibers, particularly during exercise when fibers are moving and deforming.…”