Electrical stimulation of unloaded muscles causes cardiovascular exercise by increasing oxygen demand

Abstract

Background

The development of new strategies to encourage increased levels of physical activity can help to reduce the incidence of cardiovascular disease. A new system of electrical muscle stimulation (EMS) has been developed that attempts to cause an increase in energy expenditure by mimicking the action of shivering in the body. The purpose of this study was to show that this form of EMS is capable of eliciting a cardiovascular exercise response in healthy adults.

Design

An observational study.

Methods

Ten healthy volunteers completed a maximal treadmill test and four EMS sessions using a hand-held EMS device that delivered current to the body via five silicone rubber electrodes on each leg. At each session subjects completed 3 min stimulation at each of four stimulation outputs (10, 20, 30 and 40% of maximum output) while cardiopulmonary gas exchange and heart rate (HR) were measured. Physiological responses at increasing levels of stimulation were evaluated.

Results

Average (±SD) HR and oxygen consumption (VO₂) levels of 67 ± 11 bpm and 4.7 ± 1.2 ml/kg per min at rest, respectively, were increased to 186 ± 10 bpm and 44.9 ± 9.8 ml/kg per min at peak exercise intensity on treadmill testing. The electrical stimulation was generally well tolerated by the subjects. Subjects demonstrated statistically significant increases in all physiological variables measured with successive increases in stimulation intensity. Peak HR and VO₂ at 40% stimulation intensity were 101 ±12 bpm and 14.9 ± 4.3 ml/kg per min, respectively.

Conclusions

These results demonstrate that this form of EMS is capable of producing a physiological response consistent with cardiovascular exercise at mild to moderate intensities. It achieves this without producing gross movement of the limbs or loading of the joints. This EMS-induced cardiovascular exercise response could be used to promote increased levels of physical activity in populations unable to participate in voluntary exercise.

Keywords

electrical muscle stimulation energy expenditure oxygen uptake

Introduction

Reduced physical fitness and a sedentary lifestyle are major risk factors for the development of cardiovascular disease. Protection against cardiovascular disease can be achieved by engaging in physical activity/cardiovascular exercise [1]. This involves body movement produced by repetitive muscle contraction that substantially increases energy expenditure [2]. Unfortunately, those who most need to exercise are often unable to do so because of their underlying disease. This includes individuals with spinal cord injury [3], and to a lesser extent, obesity [4].

A solution to this problem is to use electrical muscle stimulation (EMS) to elicit a cardiovascular exercise response through the facilitation of repetitive contractions of large muscle groups. However, conventional EMS-induced tetanic muscle contractions have a minimal effect on energy consumption. Eijsbouts and co-workers [5] demonstrated that bilateral stimulation of the quadriceps, hamstrings, gastrocnemius and tibialis anterior muscle groups at maximally tolerated intensity could produce an increase in oxygen consumption (VO₂)of approximately 0.11/min in healthy adults at rest and during arm-cranking exercise. Such levels of VO₂,in isolation, are unlikely to result in therapeutic benefit. Other efforts have been targeted at investigating the effects of EMS-induced leg cycling exercise (EMS-LCE) in healthy and spinal cord-injured (SCI) individuals. In EMS-LCE, EMS-induced muscle contractions produce movement that dissipates energy through an external apparatus, a cycle ergometer. EMS-LCE can result in VO₂ levels of the order of 0.6-0.8 l/min in SCI individuals [6,7]. Training with EMS-LCE can also result in improvements of 10-35% in aerobic capacity (peak VO₂) in SCI individuals [8–11].

Muscles may be viewed as energy conversion elements, which convert stored energy to mechanical work [12]. The aerobic energy conversion process requires oxygen to be delivered to the muscle fibres. As with any thermodynamic conversion process, some of the energy is dissipated in the form of heat, and is not converted to mechanical energy. The mechanical energy produced by the muscles is transmitted to the external environment through the skeletal system, and is dissipated in the form of work as locomotion or mechanical energy in an external device [13]. Consequently, stimulating a cardiovascular response by increasing oxygen demand is normally associated with the dissipation of energy into the external environment in the form of work. For those individuals whose movement is restricted or who are unable to bear weight on their joints, it is desirable to reduce the efficiency of the energy conversion process, such that the maximum energy is consumed for the minimum mechanical work created.

We have developed a novel EMS system capable of eliciting a cardiovascular exercise response with minimal gross movement or loading of the limbs or joints. The pattern of EMS used in this investigation was modelled on shivering, which is the natural process for generating heat when the body temperature falls. Shivering generates heat with no external work [13] through rhythmical muscle contractions, occurring at a rate of approximately 4-8 Hz. We attempted to mimic this pattern of muscle activity by using EMS to elicit cardiovascular exercise via rapid, short-duration, rhythmical contractions in the large lower extremity muscle groups. The purpose of this study was to show that this form of EMS is capable of eliciting a cardiovascular exercise response in healthy adults by causing a substantial increase in energy expenditure.

Methods

Ten healthy volunteers were recruited from the staff working at Castle Hill Hospital, Hull, UK. The Hospital Ethics Committee approved the study, and written informed consent was obtained in all cases. The 10 subjects included eight men and two women with a mean age of 37.6 ± 6.8 years (range 31-49) and an average weight and body mass index (BMI) of 76.3 ± 14.1 kg and 24.7 ± 3.5 kg/m², respectively. Before participating in the study, each individual had a complete physical examination to ensure that they were fit to undertake physical activity. Physical examination was completely normal in all individuals.

A specially designed hand-held muscle stimulator (BioMedical Research Ltd., Galway, Ireland) powered by a 9 V battery was used in this investigation. The stimulator current waveform was designed to produce rhythmical contractions in the lower extremity muscle groups occurring at a frequency of 4 Hz. The maximum peak output pulse current used in the present study was 120 mA, which was 40% of the maximum rated output. Impulses were delivered through five silicon rubber electrodes on each leg (area per leg 600 cm²)as illustrated in Figure 1. These were applied to the body via a pair of tight-fitting shorts, which extended to the knee. The quadriceps, hamstrings and gluteal muscles were stimulated as the subjects lay supine for all stimulation sessions.

Each subject underwent a maximal cardiopulmonary exercise treadmill test with metabolic gas exchange using the modified Bruce protocol before his or her first stimulation session. Stimulation sessions consisted of 3 min rest in supine lying, followed by 12 min EMS, during which the stimulation intensity was increased every 3 min by increments of 10% up to a limit of 40% of maximum stimulator output. Each subject completed four stimulation sessions and was free to end a session prematurely if they felt uncomfortable for any reason. Eight of the 10 subjects completed all four stimulations sessions, whereas two subjects completed all sessions at 10, 20 and 30% stimulation intensity but did not complete all sessions at 40% intensity; however, these two subjects did reach 40% stimulation intensity during at least one session. Cardiopulmonary gas exchange was simultaneously assessed using a facemask and gas analysis system (Oxycon Delta; Erich Jaeger GmBH, Germany) to assess VO₂, carbon dioxide consumption (VCO₂), and minute ventilation (VE). Continuous heart rate (HR) and electrocardiographic monitoring was carried out during every EMS session.

Data analysis

The data were analysed in two ways: The magnitude of the physiological response to EMS-induced exercise was assessed using the average of the peak HR (bpm) and VO₂ (ml/kg per min) at 40% stimulation intensity during repeated sessions. Subjects' peak HR, peak VO₂, heart rate reserve (HRR; maximum resting HR), and oxygen consumption reserve (VO₂R; maximum resting VO₂) were calculated from their treadmill test data. The level of increases in HR and VO₂ at 40% stimulation intensity relative to resting levels were then calculated and expressed as a percentage of HRR and VO₂R, respectively, for each subject to calculate the exercise training intensity they achieved using EMS. Peak energy expenditure (kcal/h) at 40% stimulation intensity was also calculated for each subject to identify calorie consumption.

Fig. 1

Location of stimulating electrodes.

The dose-response relationship associated with this form of EMS was analysed by measuring physiological responses at increasing intensities. Peak VO₂, VCO₂,VE and HR values were recorded at rest and at each stimulation intensity level at each EMS session. Increases in each variable at 10, 20, 30 and 40% of maximum stimulation output compared with resting values were then calculated for each subject, and group mean (± SD) increases were calculated. Increases above resting values were expressed in absolute and relative terms. Differences in physiological responses between the different levels of stimulation were assessed using repeated measures analysis of variance with post hoc comparisons between levels using paired t-tests. The level of significance was set at the 99% confidence level with a Bonferroni-adjusted P value of less than 0.0008.

Results

Subjects' resting HR and VO₂ plus peak HR and VO₂ from their treadmill test and at 40% stimulation are shown in Table 1. The physiological response to EMS-induced exercise varied between subjects. Group average peak HR and VO₂ were 101 ± 12 bpm and 14.9 ± 4.3 ml/kg per min, respectively, at 40% stimulation intensity. Training intensities experienced by the subjects were similar whether the HRR or VO₂R method was used as the basis for calculation (r = 0.6), and also varied between subjects. Training intensities ranged from 20 to 47% when the HRR method was used and from 16 to 50% when the VO₂R method was used to calculate the training intensity. Group mean peak energy expenditure at 40% stimulation was 351 ± 82 kcal/h.

Increases in VO₂, VCO₂, VE and HR with successive increases in stimulation intensity are illustrated in Figure 2. Subjects exhibited increases in all physiological variables during stimulation compared with resting values. VO₂, VCO₂, VE and HR were all significantly increased at 10% stimulation intensity when compared with resting values (P < 0.01). Group mean values for these variables continued to increase significantly with each increase in stimulation level up to 40% of stimulation output (P < 0.01). At 40% stimulation intensity VO₂ and VCO₂ increased by 0.7 ± 0.2 and 0.6 ± 0.2 l/min, respectively, compared with resting levels. VE increased by 13.8 ± 10.8 l/min and HR increased by 30.1 ± 11.1 bpm above resting values.

Table 1

Physiological responses to 40% stimulation intensity

Subject	Heart rate				Oxygen uptake				Peak EMS energy expenditure (kcal/h)
	Resting HR (bpm)	Max HR (bpm)	Peak EMS HR (bpm)	Training intensity (%)	Resting VO₂ (ml/min per kg)	Peak VO₂ (ml/min per kg)	Peak EMS VO₂ (ml/min per kg)	Training intensity (%)
1	60	167	85	23	4.4	47.0	11.1	16	299.7
2	67	179	108	37	6.1	52.9	19.1	28	425.1
3	55	195	120	47	3.0	43.6	23.2	50	466.2
4	57	179	93	30	4.4	37.9	12.5	24	391.5
5	77	195	110	28	2.9	24.5	8.8	27	227.1
6	68	189	92	20	5.1	47.1	13.4	20	327.6
7	92	191	112	21	4.4	36.5	17.2	40	386.7
8	61	177	85	21	4.5	50.0	12.4	17	233.7
9	68	195	100	25	5.3	50.8	14.4	20	326.4
10	64	190	108	35	6.9	58.9	17.1	20	428.7
Mean	67	186	101	29	4.7	44.9	14.9	26	351.3
SD	11	10	12	9	1.2	9.8	4.3	11	82.1

EMS, Electrical muscle stimulation; HR, heart rate; VO₂, oxygen consumption. Max HR and VO₂, peak HR and VO₂ achieved during treadmill testing using modified Bruce protocol. Peak EMS HR, VO₂, energy expenditure = peak HR, VO₂, energy expenditure at 40% stimulation.

Fig. 2

Increases in physiological responses with successive increases in stimulation intensity. Values are group mean responses from all sessions completed at each stimulation level (±SD). Differences between group mean responses at different stimulation intensities are statistically significant for all variables (P < 0.01). HR, Heart rate; VE, minute ventilation; VCO₂, consumption of carbon dioxide; VO₂, consumption of oxygen.

Discussion

The results from this investigation demonstrate that this novel application of EMS can elicit a cardiovascular exercise response. That is, it can produce a substantial increase in energy expenditure via repetitive muscle contractions. Physiological responses to this EMS-induced exercise were consistent, with a light to moderate exercise intensity; peak EMS-induced exercise intensities ranged from 2.5 to 7.7 metabolic equivalents, and the group mean training intensity was approximately 26-29%. Mean levels of energy expenditure (351 ± 82 kcal/h) were similar to levels expected for activities such as walking at 3-3.5 mph; calculated on the basis of a mean body weight of 76 kg [13]. At this level of energy expenditure, subjects would have to use the device for an approximate cumulative total of 2 h per week to have performed the minimum quantity of exercise recommended for health benefit by the American Heart Association (700 + kcal/week) [14]. However, it is unlikely that repeated exposure to this exercise intensity would be sufficient to result in increases in cardiovascular fitness (peak aerobic capacity), as a training intensity of 40% of HRR or VO₂R is regarded as the threshold level for increases in VO_2max [15].

It is important to note that the subjects in the present investigation only used the stimulator at 40% of its maximum capacity. This limit was introduced as this was the first study of the effects of this system of EMS in healthy adults, and it was necessary to investigate safety and tolerability at submaximal stimulation intensities at this stage of the investigation. Physiological variables observed in the study indicated that none of the subjects experienced an adverse cardiovascular reaction to the EMS-induced exercise. Physiological responses were consistent with those to be expected during voluntary exercise, suggesting that this form of EMS can elicit an exercise response in a safe manner. The stimulation was generally well tolerated by the subjects, with only two subjects reporting it sufficiently uncomfortable to prevent them from reaching a stimulation output of 40% at all sessions. A further two subjects reported that they found the stimulation produced minimal discomfort, whereas the remaining six felt moderate discomfort at 40% output. For most subjects, discomfort levels declined at successive stimulation sessions. This experience is in accordance with our previous experience of EMS, in that subjects usually report greatly varying feelings on its comfort/discomfort, but relative comfort levels improve with use.

We observed statistically significant increases in all physiological variables measured with successive increases in stimulator output. This demonstrates the presence of a clear dose-response relationship with this form of EMS-induced exercise. This indicates that the use of the stimulator at higher intensities is likely to result in a greater cardiovascular response. We have observed higher training intensities at higher stimulation outputs in the course of pilot testing of this form of EMS.

These are important findings, as we have demonstrated that EMS-induced muscle contractions can produce substantial increases in energy expenditure with minimal gross movement of the limbs or loading of the joints. VO₂ increased by 0.7 ± 0.2 l/min above resting VO₂. Eijsbouts et al. [5] had previously demonstrated VO₂ increases of the order of 0.1 l/min in healthy adults using EMS at maximally tolerated intensities. VO₂ increases observed in this study are similar in magnitude to those observed previously during LCE produced by EMS in SCI individuals [6,7].

The results observed in this first investigation into this form of EMS are encouraging. This is the first study to show that unloaded EMS-induced muscle contractions can produce a significant increase in cardiovascular exercise intensity and energy expenditure with a clear dose-response relationship. By improving comfort levels associated with this form of stimulation, it should be possible to elicit cardiovascular responses of greater magnitude and promote increases in physical fitness.

In the future, this form of stimulation may prove beneficial for those populations who experience difficulty with the voluntary performance of loaded physical activity, for example, obese patients and patients with spinal cord injury; patient groups who possess an increased risk of developing cardiovascular disease. It could be argued that there are potential difficulties to overcome if this form of stimulation is to prove beneficial in these patient groups. SCI patients may possess a reduced muscle mass, and therefore may not demonstrate sufficient hemodynamic response to this form of stimulation. On the other hand, these patients are also likely to be able to tolerate very high stimulation intensities, and previous investigators have demonstrated that electrical stimulation can be used to promote LCE in SCI patients to good effect [11]. Excessive cutaneous fat in obese patients will result in increased resistance to the transfer of electrical current to neural tissue, and may cause increases in the level of discomfort associated with muscle stimulation. However, electrical stimulation of muscle is routinely used by physiotherapists for patients with varying levels of body fat percentages with good effect. Obese patients may also benefit from prolonged stimulation at lower intensities using larger electrodes (resulting in lower current densities), in order to produce the cumulative energy expenditure required for weight loss. In future studies, we plan to investigate the effects of a period of training using this form of EMS in healthy adults and in patient groups such as those mentioned above.

References

1 Fletcher

Balady

Blair

. Statement on exercise: benefits and recommendations for physical activity programmes for all Americans. Circulation 1996; 94:857–862.

2 US Centers for Disease Control and Prevention Physical activity and health: a report of the surgeon general. Atlanta: US Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion; 1996.

3 Petrofsky

Stacy

. The effect of training on endurance and the cardiovascular responses of individuals with paraplegia during dynamic exercise induced by functional electrical stimulation. Eur J Appl Physiol 1992; 64:487–492.

4 Ball

Crawford

Owen

. Too fat to exercise? Obesity as a barrier to physical activity. Aust NZ J Public Health 2000; 24: 331–333.

5 Eijsbouts

Hopman

MTE

Skinner

. Effect of electrical stimulation of leg muscles on physiological responses during arm-cranking exercise in healthy men. Eur J Appl Physiol 1997; 75:177–181.

6 Hooker

Figoni

Glaser

Rodgers

Ezenwa

Faghri

. Physiologic responses to prolonged electrically stimulated leg-cycle exercise in the spinal cord injured. Arch Phys Med Rehabil 1990; 71:863–869.

7 Raymond

Davis

Van der Plas

. Cardiovascular responses during submaximal electrical stimulation induced leg cycling in individuals with paraplegia. Clin Physiol Funct Imaging 2002; 22:92–98.

8 Pollack

Axen

Spielholz

Levin

Haas

Ragnarsson

. Aerobic training effects of electrically induced lower extremity exercise in spinal cord injured people. Arch Phys Med Rehabil 1989; 70:14–21.

9 Goss

McDermott

Robertson

. Changes in peak oxygen uptake following computerised functional electrical stimulation in spinal cord injured. Res Q Exerc Sport 1992; 63:76–79.

10.

10 Hooker

Figoni

Rodgers

Glaser

Matthews

Suryaprasad

Gupta

. Physiologic effects of electrical stimulation leg cycle exercise training in spinal cord injured persons. Arch Phys Med Rehabil 1992; 73:470–476.

11.

11 Mutton

Scremin

Barstow

Scott

Kunkel

Cagle

. Physiologic responses during functional electrical stimulation leg cycling and hybrid exercise in spinal cord injured subjects. Arch Phys Med Rehabil 1997; 78:712–718.

12.

12 Brooks

Fahey

White

Baldwin

. Exercise physiology: human bioenergetics and its applications. 3rd ed. London: Mayfield Publishing Company; 2000.

13.

13 McArdle

Katch

. Exercise physiology: energy, nutrition, and human performance. London: Lippincott Williams and Wilkins; 2001.

14.

14 Fletcher

Balady

Froelicher

Hartley

Haskell

Pollock

. Exercise standards – a statement for healthcare professionals from the American Heart Association. Circulation 1995; 91:580–615.

15.

15 Pollock

Gaesser

Butcher

Despres

Dishman

Franklin

Ewing-Garber

. The recommended quantity and quality of exercise for developing and maintaining cardiorespiratory and muscular fitness and flexibility in healthy adults. Med Sci Sports Exerc 1998; 30:975–991.