Introduction
Determination of limb blood flow (BF) in relation to exercise may be useful in rehabilitation programs, increasing the general knowledge on oxygen supply, energy metabolism as well as on central and peripheral hemodynamics. Peak leg muscle oxygen uptake has previously been found to be closely related to the diameter of the feeding artery, which may vary in relation to the dependent muscle mass and oxygen need [1]. The time course of BF alterations may be influenced by remodeling of the arterial structure or restricted motor control as seen in musculoskeletal disorders like disuse syndrome and cerebrovascular disorders with hemiplegia [2-5]. Exercise hyperemia with vasodilation is related to intrinsic (endothelial-related factors, autacoid substances, metabolite and myogenic response) as well as extrinsic (autonomic nerve regulation, signal/reflexes with central command and exercise pressor reflexes with mechanical muscle contraction/accumulated metabolite product) regulation, as well as changes in arteriovenous pressure gradient due to the muscle pump. Furthermore, during exercise the increase in limb oxygen uptake (calculated as the product of exercising arterial BF and the arteriovenous oxygen difference to the exercising limb) is directly proportional to the work performed in relation to the interplay between cardiovascular regulation and muscle energy metabolism. Therefore, determination of limb BF response to exercise through physical training may yield information about an integrated circulatory adaptation and strengthening of muscle force at the target of exercise/rehabilitation prescription. Furthermore, the comparison of hemodynamics during transient exercise such as repeated limb muscle contractions between pre- and post-physical therapy may increase our understanding of the peripheral BF adjustment (so called physical training induced- circulatory adaptation). Non-invasive Doppler ultrasound with a high temporal resolution can continuously detect alterations in pulsatile blood velocity profiles as “time and space-averaged and amplitude, signal intensity weighted mean blood velocity” in the conduit artery. The arterial BF can be calculated as the mean blood velocity multiplied by the cross-sectional area in the target artery. Based on this technique, rapid changes in time courses of blood velocity profiles in the conduit artery have been found, with muscle contraction and/or muscle relaxation during exercise (dynamic/static), in different states of muscle contraction time/frequency and workload, and in relation to vasodilatation/vasoconstriction. Furthermore, the determination of a comprehensive exercise BF, for instance in a brachial, femoral or popliteal artery feeding a limb working muscle group can also be performed during muscle contractions such as with the exercise model of forearm handgrip, lower limb knee extensor or plantar flexion exercise. Following our previous reports with the series of investigation for muscle/exercise BF regulation during limb exercise using Doppler ultrasound [6-15], large differences have been observed in the time course of the magnitude of the blood velocity profile during steady-state muscle contraction-relaxation phases and between dynamic and static muscle contraction. This raises the issue how to determine exercise BF optimally during repeated muscle contractions. In general, an optimal/valid BF in a non-exercise limb may exhibit minimum physiological BF variability using samplings of cardiac beat-by-beat cycle (BBcycle). However, during muscle contractions, the muscle contraction-induced blood velocity profile in the working limb muscle may be greatly influenced by the magnitude of intramuscular pressure variation and the superimposed influence of perfusion pressure variation. Thus, for determination of optimal exercise BF we must consider how to treat the minimum physiological variability in exercise BF via muscle contraction-relaxation cycle (CRcycle) and/or cardiac BBcycle. The present commentary visualizes how to determine BF during exercise in relation to CRcycle or BBcycle, in dynamic/isotonic and static/isometric exercise, utilizing the knee extensor model and the Doppler ultrasound technique.

Key words: muscle blood flow fluctuations, dynamic and static muscle contractions, knee extensor exercise, Doppler ultrasound