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Post Exercise Cutaneous Hypermia as a Result of Local Exercise of an Extremity

This study was done at:

The Creighton Diabetes Center
601 North 30th Street
Omaha, Nebraska 68131
by
M. S. Rendell*, S.S. Green, A. Catania, J. Oliveto, J. Wells, E.J. Banset, H. Wang

RUNNING TITLE: POST EXERCISE HYPEREMIA

KEY WORDS: LASER DOPPLER, MICROVASCULAR BLOOD FLOW,
NEUROMUSCULAR ELECTRONIC STIMULATION

*To whom page proofs and reprint requests should be addressed

Copyright to Tom Oliver, 1996

SUMMARY

Large changes in skin blood flow occur after exercise. Most studies have concentrated on the systemic effects of vigorous exercise on skin blood flow. We were interested in the post-exercise response in the neighborhood of focal exercise. We used a painless neuromuscular electronic stimulator to exercise the muscles of the forearm, producing flexion of the fingers. There was no change in blood pressure and only a small increase in heart rate during this exercise. We measured blood flow during a five minute pre-exercise period and a five minute post-exercise period at the forearm, at the dorsum of the index finger, and on the pad of the index finger. We also measured values on the contralateral non-exercised extremity during exercise as well as during matched time periods in control experiments with no exercise. Exercise did elicit an increased blood flow in the post-exercise period at all three sites both as compared to the no exercise and to the contralateral extremity control periods. For example, the increase in blood flow at the finger dorsum was 2.1 + 0.1 ml/min/100 gm after exercise compared to -0.08 + 0.09 ml/min/100 gm during the control experiment and 0.1 + 0.1 ml/min/100 gm on the contralateral arm (all p<0.01). The local application of heat at the site of blood flow monitoring produced a substantial increase in the post-exercise response at the two finger locations (27.4 + 0.4 ml/min/100 gm at the finger dorsum), but not at the arm.

This is the first demonstration that highly focal exercise, unaccompanied by a systemic hemodynamic response, can elicit a post-exercise cutaneous hyperemia. Local heating produced a large synergistic increase in the post-exercise hyperemia at sites with arteriovenous microvascular perfusion but not at sites with primarily nutritive perfusion. These findings show that local vasoregulatory changes occur in response to exercise, even in the absence of whole body hemodynamic and thermal change.

The recent development of painless neuromuscular electronic stimulators makes it possible to induce intense focal exercise of a single muscle or muscle group. With the use of such a device to create vigorous local exercise, we felt that it would be possible to avoid significant systemic cardiovascular and thermogenic stimulation. Our goal was to assess post-exercise cutaneous hyperemia in the absence of substantial hemodynamic changes.

We used laser Doppler techniques to measure skin blood flow before and after vigorous forearm exercise. In accordance with the work of previous investigators, we measured blood flow on the skin surface of the forearm. However, we also performed measurements on the finger. The skin of the forearm is perfused primarily by small nutritive (NUTR) capillaries. In contrast, areas such as the tips of the fingers and toes have a high density of large diameter arteriovenous anastomoses (AV). There is a substantial difference in the microcirculatory response mechanisms at AV areas. Postural changes elicit a much more marked response at AV than at NUTR sites (Rendell et al, 1992). We have shown that the vasodilatory effect of local heating is greater at AV but not at NUTR areas (Rendell et al, 1993a). Significant hypertension affects flow at AV but not at NUTR areas (Rendell et al, 1993b). The effects of polycythemia on blood flow are expressed AV much more than at NUTR sites (Rendell et al, 1995). In essence, upstream flow changes in the arterial system affect AV sites more than NUTR sites. Therefore, we wanted to contrast post exercise responses at the finger with those at the forearm. In order to evaluate the role of thermal effects in the post-exercise hyperemia, we used local heating to stimulate maximal vasodilation.

Exercise Procedures: Testing took place in a room controlled at an ambient temperature of 24° C. The muscles of the forearm were electrically stimulated using a NEURO CARE™ 1000, neuromuscular electronic stimulator, generously provided by Tom Oliver/EMS Northwest and distributed through Electronic Medical Systems, Inc. (Vancouver, WA). The device emits a biphasic pyramidal stimulus at a frequency of 47 Hz. The stimulus is delivered via pulse train, on for 1.5 seconds and off for 1.8 seconds. Two 70 cm2 TENS electrodes were placed on the flexor and extensor surfaces of the upper forearm. After a pre-exercise baseline period of 5 minutes, we began exercising the limb by delivery of an initial low level stimulus which was progressively incremented to 1.5 mlAmps, the maximal output of the device. Although there was contraction of both the muscles of extension and contraction, the predominant effect was one of flexion of the fingers. Despite the involuntary nature of the rhythmic muscle contractions, the subjects experienced no discomfort. The electric discharge produced a buzzing or tingling sensation which was not considered painful. Therefore, we were able to maintain exercise for a prolonged time. At the maximal level, exercise was continued for a ten minute period, followed by a one minute rest period, and then resumed for another ten minute period. The force generated by the exercise was quantitated by a strain gauge attached to the distal interphalangeal joints of the second and third fingers. This gauge measured the force of the flexion of the fingers at these distal joints. We measured the skin blood flow response for a five minute period following the conclusion of exercise. We determined systolic blood pressure, diastolic blood pressure, and pulse before, during, and after exercise. Flow values were acquired at 0.5 second intervals using PROCOMM, a data acquisition program, and stored to hard disk on an IBM 386 PC. Data manipulation and analyses were then carried out using Microsoft EXCEL.

Experiments were performed with measurement of skin blood flow at basal skin temperature, as well as with heating the laser Doppler probe to 44° C, a temperature which is warm enough to elicit maximal local vasodilatation, but not so hot as to burn the skin. We performed measurements on the forearm distal to the location of the electrode pads, the dorsum of the index finger, and the plantar surface (pad) of the index finger. We furthermore performed measurements on the opposite, non-exercising extremity. Measurements at these sites were carried out during separate exercise experiments performed in identical fashion on each subject. In addition, we performed measurements at all sites during control experiments. These control experiments were performed in the same way as the exercise experiments, with the sole difference being the lack of neuromuscular electronic stimulation. Thus, each subject was tested with and without exercise at three sites on the exercised extremity, and also on the contralateral arm during exercise. Measurements were performed at basal skin temperature and at 44° C.

Statistical Analysis:
Comparisons were made point to point for the various experimental conditions using weighted analysis of variance techniques. Baseline pre-exercise values were contrasted to post-exercise values by matching time points pre and post-exercise. Similarly control experiments with no exercise were contrasted to experiments with exercise by matching equivalent time points for the baseline and post-exercise periods. Skin blood flow values are pulsatile due to cardiac action. The standard deviation of the pulsation was calculated as a measure of the degree of pulsatile excursion. The variance of the pulsatile activities were compared using an F test. Values were five to ten fold greater than values obtained at basal skin temperature.

DISCUSSION

The advent of painless neuromuscular electronic stimulators makes it possible to carry out highly localized exercise. We exercised the muscles of the forearm, resulting in vigorous, repetitive flexion of the fingers. We measured the force generated by the second and third fingers alone, although all fingers participated in the strong flexion. Despite the substantial effort demonstrated, the systemic effect of this focal exercise was very small. There was only a small increase in pulse rate and no change in blood pressure. We were able to use the contralateral non-exercising arm to observe and control for the very small systematically induced changes. Thus, we were able to assess the effects of focal exercise alone on post-exercise hyperemia without confounding by systemic hemodynamic changes. In addition, by performing measurements with the probe head heated to 44° C, we were able to distinguish thermal effects on post-exercise hyperemia from the effects of focal exercise. Therefore, we isolated the local effects of exercise, differentiating the purely exercise induced changes from those caused by the systemic hemodynamic and thermogenic effects which have been a part of all previous studies. Our results demonstrate that local exercise does produce a modest post-exercise increase in cutaneous blood flow. This increase can be demonstrated not only in the forearm area in proximity to the exercising muscle, but also at the finger in the arterial distribution downstream from the exercising muscle. The cutaneous microvascular perfusion of the arm differs temperature. It is, in fact, the highly localized nature of the phenomena which we observed which is the most intriguing feature of our results. Although the exercise produced by the NMES device is quite vigorous, it nevertheless constitutes exercise on a very small scale. Other investigators have used isometric exercise, although low intensity, does produce significant changes in blood pressure. Unlike isometric exercise, our focal exercise, although quite intense, does not affect blood pressure. Yet, despite the local nature of the forearm exercise, the skin blood flow changes are notable both locally and in the arterial perfusion area of the finger, downstream from the forearm. Our results tend to support the concept of a regulatory redistribution of blood flow from muscle to skin following active exercise. It has been shown that intense leg exercise results in an increase in forearm skin blood flow. There is a further increase in skin blood flow following exercise (Blair et al, 1961; Zelis etal, 1969). Our findings would appear to substantiate such a redistribution of blood flow and further suggest that the change is a primary phenomenon, rather than the result of substantial cardiovascular alterations.

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