Changes in cross-sectional area in human exercising and non-exercising skeletal muscles

This research was performed to study how the cross-sectional area (CSA) changes in the skeletal muscles of exercising (E-leg) and contralateral nonexercising (N-leg) legs and to evaluate to what extent changes in CSA mirror changes in blood flow or extravascular water displacement. Seven healthy volunteers performed plantar flexion exercise at three different exercise intensities for 10 min each. Six plantar flexions followed by a 2-s rest in between allowed repeated measurement of the blood flow to the lower limbs by duplex ultrasonography in the popliteal artery and CSA by magnetic resonance imaging. The CSA was measured using manual planimetry at rest and after 3 and 9 min of the exercise periods. The CSA increased in the E-leg by 4.5% and decreased in the N-leg by -2.4%, from rest to highest exercise intensity. Post-exercise imaging of the E-leg showed a bi-phasic recovery of CSA with a rapid phase followed by a slower phase while the blood flow very rapidly returned almost to basal. The time course of the post-exercise decrease indicated that about 50% of the increase in CSA at the highest exercise intensity might have been a result of extravascular water displacement and 50% of an increase in the vasculature volume related to the flow increase. The CSA reduction in N-leg seems to have been related to vasoconstriction, probably mainly of the capacitance vessels since blood flow was not reduced.