Interrelationships between body fat and skin blood flow and the current required for electrical stimulation of human muscle.

There is variability between individuals in the current needed to elicit a contraction in human muscle with surface electrodes. To understand what might be causing some of this variability, 25 subjects whose average age was 24.4+/-2.3 years, whose height was 165.5+/-9.5 cm, and whose average weight was 70.3+/-21 kg were examined. Electrical stimulation was applied above the motor point of the quadriceps, biceps, and lateral gastrocnemius muscles. To assess body fat, 2D ultrasound was used with a 1cm stand off. Electrical stimulation was applied with sine wave stimulation at 100 micros pulse width and at a frequency of 30 Hz. To alter skin blood flow, aside from the natural difference in skin blood flow at rest, hot packs and cold packs were used for 5 min. The average fat thickness below the quadriceps and gastrocnemius muscles was 0.75+/-0.13 cm and under the biceps was 0.48+/-0.16 cm. Without the use of hot or cold packs, the currents for the quadriceps and gastrocnemius muscles were significantly higher than that of the biceps (p<0.01). While there was some relationship between stimulation current and blood flow without the application of hot or cold packs, when hot packs were applied, skin blood flow increased as did the current required to stimulate muscle to threshold. When cold packs were applied, there was a decrease in the current required to stimulate these muscles. In conclusion, there is a causal relationship between skin blood flow, the thickness of the fat layer below the skin, and the current required to stimulate the muscle.

The influence of local versus global heat on the healing of chronic wounds in patients with diabetes.

BACKGROUND: In a previous study, it was shown that placing a subject with chronic diabetic ulcers in a warm room prior to the use of electrical stimulation dramatically increased the healing rate. However, global heating is impractical in many therapeutic environments, and therefore in the present investigation the effect of global heat versus using a local heat source to warm the wound was investigated. METHODS: Twenty-nine male and female subjects participated in a series of experiments to determine the healing associated with electrical stimulation with the application of local heat through a heat lamp compared to global heating of the subject in a warm room. Treatment consisted of biphasic electrical stimulation at currents at 20 mA for 30 min three times per week for 4 weeks in either a 32 degrees C room or, with the application of local heat, to raise skin temperature to 37 degrees C. Skin blood flow was measured by a laser Doppler imager. RESULTS: Blood flow increased with either local or global heating. During electrical stimulation, blood flow almost doubled on the outside and on the edge of the wound with a smaller increase in the center of the wound. However, the largest increase in blood flow was in the subjects exposed to global heating. Further, healing rates, while insignificant for subjects who did not receive electrical stimulation, showed 74.5 +/- 23.4% healing with global heat and 55.3 +/- 31.1% healing with local heat in 1 month; controls actually had a worsening of their wounds. CONCLUSIONS: The best healing modality was global heat. However, there was still a significant advantage in healing with local heat.

Estimation of the distribution of intramuscular current during electrical stimulation of the quadriceps muscle.

Electrical stimulation is commonly used for strengthening muscle but little evidence exists as to the optimal electrode size, waveform, or frequency to apply. Three male and three female subjects (22-40 years old) were examined during electrical stimulation of the quadriceps muscle. Two self adhesive electrode sizes were examined, 2 cm x 2 cm and 2 cm x 4 cm. Electrical stimulation was applied with square and sine waveforms, currents of 5, 10 and 15 mA, and pulse widths of 100-500 micros above the quadriceps muscle. Frequencies of stimulation were 20, 30, and 50 Hz. Current on the skin above the quadriceps muscle was measured with surface electrodes at five positions and at three positions with needle electrodes in the same muscle. Altering pulse width in the range of 100-500 micros, the frequency over a range of 20-50 Hz, or current from 5 to 15 mA had no effect on current dispersion either in the skin or within muscle. In contrast, the distance separating the electrodes caused large changes in current dispersion on the skin or into muscle. The most significant finding in the present investigation was that, while on the surface of the skin current dispersion was not different between sine and square wave stimulation, significantly more current was transferred deep in the muscle with sine versus square wave stimulation. The use of sine wave stimulation with electrode separation distances of less then 15 cm is recommended for electrical stimulation with a sine wave to achieve deep muscle stimulation.

Isometric endurance, body and skin temperature and limb and skin blood flow during the menstrual cycle.

BACKGROUND: Limb blood flow and skin and deep tissue temperature change during the menstrual cycle. However, the impact of these changes on isometric exercise performance has not been investigated. MATERIAL/METHODS: Isometric endurance was assessed at contraction tensions of 20, 40 and 60% of the maximal voluntary contraction strength (MVC) of the handgrip muscles in 8 women every other day throughout their menstrual cycles (MC). For any MC, contractions were only accomplished at one of the three tensions. For contractions at 40% MVC, an additional two MC were studied during which the circulation to the forearm was occluded during the contractions in one cycle and muscle temperature stabilized and occluded in another cycle. RESULTS: The results of these experiments showed a small variation in endurance during the MC for contractions at 60% MVC. This effect increased at 40% and was greatest for contractions at 20% MVC. When circulation was occluded, the response was changed in both magnitude and timing during the MC showing that there was variation in muscle performance due to the menstrual cycle not due to changes in blood flow. Further, when muscle temperature was stabilized and flow occluded, there was still lower endurance at the end of the Follicular phase showing an effect of the menstrual cycle on endurance absent changes in muscle temperature and blood flow. CONCLUSIONS: The results show that the MC alters isometric endurance due to 1) a cyclic variation in muscle temperature, 2) direct effects of the menstrual cycle on circulation and 3) Direct effects of the MC on muscle.

The interrelationships between electrical stimulation, the environment surrounding the vascular endothelial cells of the skin, and the role of nitric oxide in mediating the blood flow response to electrical stimulation.

BACKGROUND: Electrical stimulation (ES) is a commonly used modality in physical therapy for treating wounds such as diabetic ulcers and pressure sores but the mechanism of its effect on skin blood flow (BF) has not been determined. MATERIAL/METHODS: Ten subjects were examined during ES of the skin above the quadriceps muscle with biphasic sine wave stimulation at a frequency of 30 Hertz, pulse width 250 microseconds. BF was measured between the electrodes with a Laser Doppler Flow meter. In one series of experiments, N-Nitro-L-Arginine Methyl Ester (L-NAME) was infused to determine the effect of blocking nitric oxide production on the blood flow response to ES. In another series, acetylcholine and epinephrine were infused to determine the effects of the initial blood flow prior to stimulation on the magnitude of the BF response to ES. RESULTS: The apparent mechanism of the increase in blood flow during electrical stimulation seems to be mediated by nitric oxide since the increase in blood flow as a result of stimulation was blocked by L-NAME. Modulation of the vasoconstrictor or vasodilator tone of the skin blood vessels altered the magnitude of the BF response to electrical stimulation; epinephrine blocked the response and acetylcholine increased the response. CONCLUSIONS: The BF response to ES is caused by NO and can be modulated by changing the environment of the vascular endothelial cell.

The effect of aging on conductive heat exchange in the skin at two environmental temperatures.

BACKGROUND: Ageing diminishes the blood flow (BF) response of the skin to autonomic stressors. While the diminished response of skin BF to global heating has been well documented, the effect of this reduction in skin BF on the ability of the skin to dissipate heat has not. When heat is added to the skin by the application of hot packs, if heat is not adequately removed, the skin can become dangerously hot and become damaged. The present investigation examined the heat dissipating properties of the skin in older individuals. This study has importance for the therapeutic application of hot packs which might cause burns easier in older people. MATERIAL/METHODS: In the present investigation, 10 younger and 10 older subjects were examined. The average age of the younger group was 25.9+/-3.4 years and the older group was 60 +/- 5.8 years. Heat was applied through a 49 gram brass probe that was heated to 41 degrees C and by a Peltier junction in a cool and warm environment. RESULTS: Skin required about 20 calories of heat to raise skin temperature 1 degrees C the cool room and double this Figure in the warm room. Ageing reduced the caloric requirement to increase skin temperature under both conditions (p < 0.01). CONCLUSIONS: The results of the experiments showed that older individuals had impaired ability of the skin to dissipate heat in both environments. Special precautions should be taken in physical therapy when applying hot packs in older populations.