HRV and Type 2 Diabetes
23
Journal of Exercise Physiologyonline
(JEPonline)
Volume 12 Number 4 August 2009
Managing Editor
Tommy Boone, PhD, MPH
Editor-in-Chief
Jon K. Linderman, PhD
Review Board
Todd Astorino, PhD
Julien Baker, PhD
Tommy Boone, PhD
Larry Birnbaum, PhD
Eric Goulet, PhD
Robert Gotshall, PhD
M. Knight-Maloney, PhD
Len Kravitz, PhD
James Laskin, PhD
Derek Marks, PhD
Cristine Mermier, PhD
Chantal Vella, PhD
Ben Zhou, PhD
Official
Research Journal of
the American Society of
Exercise Physiologists
(ASEP)
ISSN 1097-975
Exercise and Health
Effect of High Intensity Interval Training on Heart Rate Variability in
Individuals with Type 2 Diabetes
KOULLA M. PARPA, MARCOS A. MICHAELIDES, BARRY S. BROWN
Human Performance Laboratory/Department of Health Science, Kinesiology,
Recreation and Dance, University of Arkansas, Fayetteville, USA
ABSTRACT
Parpa KM, Michaelides MA, Brown BS. Effect of High Intensity Interval
Training on Heart Rate Variability in Individuals with Type 2 Diabetes.
JEPonline 2009;12 (4): 23-29. The purpose of the study was to examine
the effect of high intensity interval training (HIIT) on cardiovascular
autonomic function as determined by HRV, in individuals with diabetes.
Fourteen sedentary individuals (9 females, 5 males, age: 57±6.7 years,
weight: 94.3 ± 23.8 kg, height: 170.5 ±8.5 cm) met all inclusion criteria
for the study. Resting electrocardiogram (ECG) was recorded at
baseline and 12-weeks after training. HRV was assessed manually from
calculation of the mean R-R interval and its standard deviation
measured on a 5-min ECG. Participants followed a 12-week HIIT on a
treadmill consisting of four ~30-min sessions per week. A HIIT session
involved a 3- minute warm-up period, six short (two minutes) maximumintensity (80-90% heart rate max) efforts separated by six moderate
intensity (50-60% heart rate max) recovery intervals (two minutes) and a
3-minute cool down period. Results demonstrated a statistically
significant difference in HRV pre (HRV: 52.80 ± 8.5 ms) compared to
post training (HRV: 62.60 ± 11.00 ms), t(13) = -7.46, p = 0.0001. In
addition, systolic blood pressure (SBP), diastolic blood pressure (DBP),
resting heart rate (RHR), fasting glucose values (FG) and body weight
were significantly lower following 12 weeks of training. The beneficial
effect on autonomic regulation as a result of exercise training may have
clinical importance in preventing adverse cardiovascular events in
individuals with diabetes.
Key Words: Autonomic Function, EKG, Disease, Activity.
HRV and Type 2 Diabetes
24
INTRODUCTION
Type 2 diabetes is a serious costly disease with several complications and premature mortality (1).
More than 65 percent of deaths in individuals with diabetes are attributed to heart and vascular
disease (1). In studying the effects of exercise on type 2 diabetes, many studies have reported
positive effects of aerobic exercise training (2) as well as strength training (3) on muscle quality,
whole body insulin sensitivity, body composition (4) and blood pressure (5). In addition, it has been
demonstrated that 30 minutes of moderate intensity continuous cycling or treadmill exercise on most
days of the week can significantly increase heart rate variability (HRV) in individuals with type 2
diabetes (6).
Consistent and extensive data have indicated that reduced cardiac autonomic control as determined
by the amount of short and long term variability in heart rate is an independent risk factor for coronary
heart disease and all cause mortality (7). Research have demonstrated that the relative risk of
mortality is 5.3 times higher when HRV is less than 50 ms compared to when HRV is greater than
100 ms in individuals with acute myocardial infarction (8). Also, it has been reported that HRV is lower
in individuals with hypertension (9) dyslipidemia and type 2 diabetes (10). Lower HRV in individuals
with diabetes has been significantly related to the development of coronary heart disease
independent of the duration and severity of the glucose metabolism impairment (11). The
Atherosclerosis Risk in Communities (ARIC) study, which investigated the consequence of diabetes
and pre-diabetic metabolic impairments on a 9-year change on HRV, demonstrated that cardiac
autonomic impairment was present in early stages of diabetes. In addition, autonomic cardiac function
was progressively worsened in individuals with type 2 diabetes.
Despite the documented evidence (6) of the benefits of continuous cycling or treadmill training on
autonomic nervous system balance, a recent study demonstrated that only 38% of adults with
diabetes were engaged in moderate to vigorous activities three or more times a week (12). Among
the most common barriers to physical activity were the perceived difficulty when exercising and
feelings of tiredness (12). It would appear there is a need for alternate forms of exercise training that
produce similar improvements to continuous exercise and could be beneficial in improving
cardiovascular autonomic function in individuals with diabetes.
A possible alternative is the use of interval training. Previous studies have indicated that HIIT is
superior to moderate intensity continuous training in improving endothelial function, aerobic capacity,
and quality of life in patients with post infarction heart failure (13). Moreover, it has been
demonstrated that HIIT is superior to moderate intensity training in reversing risk factors of the
metabolic syndrome (14). Even though previous studies have demonstrated that continuous aerobic
exercise training can result in significant improvement in HRV in patients after coronary angioplasty
(15), individuals with coronary artery disease (16) and those on hemodialysis (17), no studies have
been published that assess the effects of HIIT on HRV in individuals with type 2 diabetes. Therefore,
the significance of this study is to add to the limited body of scientific knowledge regarding the effect
of HIIT on cardiovascular autonomic function in individuals with diabetes. It is hypothesized that mean
HRV will be significantly greater post- compared to pre training. In addition, systolic blood pressure
(SBP), diastolic blood pressure (DBP), resting heart rate (RHR), fasting glucose (FG) and body
weight are expected to be significantly lower post- compared to pre training.
METHODS
Subjects
Approval of the study protocol by the University of Arkansas Institutional Review Board for Human
Subjects was secured prior to the beginning of the study. Participants for the study were 19
HRV and Type 2 Diabetes
25
individuals with type 2 diabetes. Five participants dropped out early in the study due to personal
reasons. Therefore, the results presented are from 14 sedentary individuals (9 females, 5 males, age:
57±6.7 years, weight: 94.3 ± 23.8 kg, height: 170.5 ±8.5 cm) who completed the 12 week
intervention. Inclusion criteria involved: A written approval from their physician to participate in the
exercise portion of the study, no more than 2 risk factors (in addition to diabetes) on the Physical
Activity Readiness Questionnaire, at least moderate glycemic control, and having been diagnosed
with diabetes for at least 3 years. Considering co-morbidity, six participants had both type 2 diabetes
and hypertension. Five women and one man were taking calcium channel blockers for the
management of hypertension. Nine participants were taking insulin for the management of diabetes.
Furthermore, participants were asked if they currently participated in planned exercise (defined as
participation in physical activities for 30 minutes on three or more days of the week). One man and
two women reported participation in physical activities. Eleven participants denied participation in
physical activities. Individuals who smoked, consumed more than 4 alcoholic beverages a day, and
had vascular or cardiovascular complications; liver or renal impairment, life threatening diseases, or
orthopedic problems were excluded from the study.
Procedures
Each participant completed the Physical Activity Readiness Questionnaire and a survey to report
exercise history and social behaviors, such as smoking and alcohol consumption. Qualified
participants were scheduled for an initial assessment, which included a 5-minute resting ECG
recording, measurements for body weight, body height, FG, SBP, DBP and RHR.The aforementioned
measurements were taken at baseline and at the end of the study. Body height (cm) and body weight
(kg) were measured by standard methods (18). After a 5-min rest, participants had their sitting blood
pressure and RHR measured. Blood pressure was measured using an adult size manual
sphygmomanometer cuff placed on the left arm. Finger prick capillary blood samples (approximately
15 µL from each participant) were obtained after fasting overnight using BD Ultra Fine TM 30-Gauge
Lancets (Franklin Lakes, NJ, USA). The CardioChekTM (Polymer Technology Systems Inc.,
Indianapolis, IN, USA) device was used to analyze the samples.
Participants were instructed to abstain from alcohol, caffeine and physical activity 24 hours before the
ECG testing. Measurement of HRV was performed using the time domain analysis which is a general
measure of autonomic nervous system balance. Even though there are six commonly used time
domain measurements (18) for the purpose of this study HRV was assessed manually from
calculation of the mean R-R interval and its standard deviation. Prior to testing, participants
comfortably lay in a supine position for 10 min in a quiet, semi dark room. Resting supine 5 min beatto-beat heart rate was collected between 6:00 am and 10:00 am. Lead II was selected to assess HRV
before and after 12-weeks of training. ECG data was gathered on a Quinton Q4500 machine (Quinton
Q4500 ®, Bothell, WA, USA) (19). Previous research has demonstrated that HRV measures obtained
from a 5-min sample period using time domain analysis are highly stable over time (r=0.9) (20).
Experimental Protocol
One week prior to the beginning of the intervention, participants attended a familiarization session
during which maximal heart rate (HR max) was estimated and exercise intensity was determined.
The HR max was estimated using the formula 208 – (0.7 x age) (21). The target heart rate (THR) was
calculated using the following formula: THR = [(HRmax − HRrest) × %Intensity)] + HRrest
Patients followed a 12-week HIIT program on a treadmill consisting of four ~30 min sessions per
week. A HIIT session involved a 3-minute warm-up period, six short (2 minutes) maximum-intensity
(80-90% HR max) efforts separated by six moderate intensity (50-60% HR max) recovery intervals (2
minutes), and a 3-minute cool down period. Exercise intensity was modified until the THR was
HRV and Type 2 Diabetes
26
reached. Inclination was kept at 1%. The workout was supervised and continuously monitored with a
heart rate monitor (Omron HR100 ®). In addition, blood pressure was monitored before, during and
after the workout sessions. Exercise intensity was lowered in cases where the blood pressure
became elevated to 220/110 mmHg or higher, and was only resumed if blood pressure dropped
below this value. In participants undergoing insulin therapy, the insulin dose was reduced before
exercise. If hypoglycemia occurred during exercise, glucose dissolved in lukewarm water was taken.
Participants received no dietary intervention; however, they were instructed not to change their eating
habits during the course of the study. Furthermore, participants were encouraged to have a day off
after 2 consecutive workouts.
Statistical Analyses
The SAS statistical software (version SAS 8.2) was used for the statistical analysis. Normality and
homogeneity of variance assumptions were assessed. Paired samples t-test was used to analyze
differences in HRV before and after the 12 weeks of training. Paired sample t-test was also used to
determine differences in RHR, SBP, DBP, FG and body weight before and after 12-weeks of training.
The level of significance was set at p = .05.
RESULTS
Participants performed 94 ± 3 % of the scheduled exercise sessions and no exercise- related injuries
were reported. One incidence of hypoglycemia that occurred immediately post exercise was resolved
with administration of glucose dissolved in lukewarm water. Two participants had an exercise session
terminated due to elevated blood pressure.
Table 1. Physical Characteristics of individuals with type 2
diabetes before and following 12 weeks of training.
Characteristics
Pre
Post
Age(years)
57  6.7
57  6.7
HRV (ms)
52.70  8.50
62.60  11.00*
RHR(bpm)
76  9
69  9*
Body Weight (kg)
94.31  23.8
90.47  23.43*
Resting SBP (mmHg)
134  11
127  9*
Resting DBP (mmHg)
84  6
80.00  5*
Values are mean ± SD; Subjects (N=14); *p<0.05. Note: HRV:
Heart Rate Variability, FG: Fasting Glucose; RHR: Resting Heart
Rate; SBP: Systolic Blood Pressure, DBP: Diastolic Blood
Pressure.
Based on Shapiro-Wilk test (p >
0.05) the normality assumption was
met for all variables. Brown and
Forsythe's test was used to test the
homogeneity
of
variance.
Homogeneity of variance assumption
was met for HRV measures (p >
0.05). Table 1 demonstrates the
physical characteristics of the
participants pre- and post training.
HRV was significantly greater
following 12-weeks of training (p
<0.05). Mean SBP was significantly
lower after 12-weeks of training, (p <
0.05). Similar results were identified for changes in DBP (p <0.05). Also, body weight was
significantly lower post- compared to pre-training (p <0.05). Finally, RHR (p <0.05) and FG (p <0.05)
values were significantly lower post- compared to pre training.
DISCUSSION
The goal of this project was to address the effect of HIIT on cardiovascular autonomic function as
determined by HRV, in individuals with diabetes. Previous research has indicated that continuous
aerobic training can result in significant improvements in HRV in individuals with diabetes (6, 22).
Bhagyalakshmi and co-workers demonstrated a significant increase in HRV with increasing duration
of exercise whereas HRV decreased in the control group. The investigators concluded that 30
HRV and Type 2 Diabetes
27
minutes of moderate intensity continuous cycling or treadmill exercise on most days of the week can
significantly increase HRV in individuals with type 2 diabetes (6). Similarly, Pagkalos and colleagues
(22) demonstrated that continuous treadmill exercise 3 times a week for six months (at 70% to 85% of
heart rate reserve) can result in significant increase in HRV. The current study demonstrated that 12
weeks of HIIT can cause significant improvements in HRV suggesting that HIIT may be as effective
as continuous aerobic exercise in improving cardiac autonomic function. Future research is warranted
to examine whether individuals with diabetes find this training less monotonous, more motivating and
enjoyable than continuous aerobic exercise.
Since both RHR and HRV depend on the autonomic nervous system the two variables are not
independent. Research indicated that RHR is a strong predictor of total mortality and mortality from a
number of causes including diabetes (23, 24).The results of the current study indicated a significant
reduction in RHR. The observed mean difference was 7 bpm. Similarly, Pagkalos and co-workers
(22) demonstrated that RHR was reduced by 12.8% and 8% in individuals with diabetes with CAN
and those without CAN, respectively. Thus, HIIT could be as effective as continuous exercise training
in reducing RHR in individuals with diabetes.
Previous research has indicated that continuous aerobic exercise training can result in reductions in
blood pressure (25,26). Figueroa and colleagues (25) demonstrated that 16 weeks of moderate
intensity continuous aerobic exercise resulted in 8% (p < 0.001) decrease in resting SBP in obese
individuals with type 2 diabetes. Similarly, Hotta and colleagues (26) demonstrated that mean resting
SBP decreased from 128.4 to 106.4 mmHg (p < 0.01) while DBP decreased from 78.2 to 66.0 mmHg
(P < 0.01) in individuals with diabetes that followed a continuous aerobic exercise program for 3
weeks along with a low calorie diet. Similar to continuous aerobic exercise, interval training has been
shown to reduce SBP and DBP in middle age and older individuals (27) and in individuals with
metabolic syndrome (14). Similarly, the current study demonstrated that HIIT could be effective in
reducing SBP and DBP in individuals with diabetes.
Limitations
For the purpose of this study HRV was assessed manually from the calculation of mean R-R interval
and its standard deviation measured on the 5-minute ECG recording. The results of the current study
should be viewed with caution since only two of the time domain measures were used for the
determination of HRV. Moreover, both frequency and time domain measures should be used for a
more accurate measurement of HRV.
CONCLUSION
The beneficial effect on autonomic regulation as a result of exercise training may have clinical
importance in preventing adverse cardiovascular events in individuals with diabetes. Exercise and
health care professionals could use this form of training as an alternative to continuous aerobic
exercise to enhance participation and effectively manage diabetic complications. More data utilizing a
larger sample of individuals with type 2 diabetes is needed to confirm the beneficial effects of HIIT on
cardiovascular autonomic function.
Address for correspondence: Koulla Parpa, Ph.D., Department of Health, Kinesiology, Recreation
and Dance, University of Arkansas, Fayetteville, USA
Email:koullap@hotmail.com, kparpa@uark.edu.
HRV and Type 2 Diabetes
28
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