Impact of Resistance Exercise on Cardiovascular Dynamics.
INTRODUCTIONThe transition from rest to exercise requires adjustments in both respiratory and cardiovascular function in order to meet the increased metabolic demands of the working muscles. The cardiovascular responses that occur during an acute bout of exercise include changes in heart rate (HR), cardiac output (CO), stroke volume (SV), blood pressure (BP), and rate pressure product (RPP) (2,5). Although both aerobic and resistance exercise results in these cardiovascular adjustments, there are some compelling differences that occur that allow both types of exercise to continue.
At the onset of a constant-load exercise, there is a rapid change in central nervous control that causes a rapid increase in HR and contractility of the heart muscle, both of which result in an increase in CO. In addition, the muscle pump during aerobic exercise and an increase in vasoconstrictor tone in non-active tissue results in more blood being returned to the heart (i.e., venous return). This increase in preload on the heart reduces afterload on the heart (due to the local mediated vasodilation at the working muscles), and increases contractility of the left ventricle; all contribute to an increase in SV and, subsequently, an increase in CO (8).
The challenge to the cardiovascular system during aerobic exercise is to respond to the increase volume load placed on the heart. Resistance training, though, poses a different challenge. As opposed to the volume overload placed on the heart, resistance exercise results in an increase pressure load on the heart.
When performing acute resistance exercise, the cardiovascular adaptations are influenced by the force of the muscular contraction and duration of effort (i.e., number of repetitions) (7). During heavy resistance exercise, there is relatively no change in CO and SV compared to resting levels. However, as the load is reduced and more repetitions are performed, CO may respond in a similar way to what is seen during aerobic exercise (14). The SV response is also dependent on intensity and duration of the effort. During heavy resistance exercise the muted response of SV can be explained by the high intraabdominal and intrathoracic pressures generated. This rise in BP results in a reduced venous return to the heart, a subsequent smaller preload on the heart, and a reduced end diastolic volume (EDV) (9). Resistance training also results in an increase in HR (12). Similar to the CO and SV response during resistance exercise, the changes in HR are dependent on the duration and intensity of effort.
Systolic blood pressure values can rise substantially during the performance of resistance exercise (9). During light intensity resistance training there is no change or a slight decrease in DBP (14). However, heavy resistance exercise elicits a dramatic increase in DBP (9). The elevated pressure response can be explained by the actual mechanical pressure of the muscles acting on the blood vessels and the pressor reflex that occurs during static contraction. In addition, the elevated intrathoracic and intraabdominal pressures that occur during the Valsalva maneuver can cause a more dramatic increase in blood pressure (9). However, during light resistance exercise using more repetitions, the increase in blood pressure is minimal and may not be as great as what is seen during aerobic exercise (10).
In summary, the responses of the cardiovascular system during resistance training varies based on the muscle groups involved, the number of sets performed, and the intensity of the exercise. Given the fact that resistance training employs exercises in different body positions, such as upright versus supine and overhead versus below the heart, it is of interest to examine the impact on cardiovascular function. Therefore, the purpose of this investigation was to examine if resistance exercise in various body positions impacts CO, SV, HR, and BP.
METHODS
Subjects
Fourteen (F=8; M=6) apparently healthy volunteers between the ages of 18 and 23 were recruited from the Bloomsburg University Campus (Table 1). A power analysis established that 14 subjects would provide an 80% chance of detecting a correlation coefficient of r = 0.70 between the different cardiovascular variables and body positions as statistically significant at the P=0.05 level.
The Bloomsburg University Institutional Review Board approved the study protocol and methods. Prior to participation, all subjects signed an informed consent document consistent with the Bloomsburg University policy for the protection of human subjects, and answered a physical activity readiness questionnaire (PAR-Q). Subjects were excluded if they resistance trained for three or more days per week for 30 min or more per day within the previous 2 yrs.
PROCEDURES
Testing Sequence
The subjects were asked to arrive well rested with a small meal two or more hours prior to testing and to refrain from the following: (a) over the counter drugs; (b) stimulants such as caffeine; (c) showering or bathing within 4 hrs prior; (d) sauna use; and (e) excessive food or beverage consumption. Also, the subjects were asked to avoid alcohol and heavy exercise at least 24 hrs prior to testing.
All subjects underwent body composition analysis, performed using bioelectrical impedance analysis (BIA) (BC-418, Tanita Corporation of America, Inc., Arlington Heights, IL), and 3 sets of a submaximal resistance training exercise in three different body positions. All testing was completed with a minimum of 24 hrs between test days. During the first visit, which served as a familiarization period, the subjects had their body composition assessed and performed all three exercises in order to determine the appropriate amount of resistance to use during the testing. On the testing day (visit two), each subject performed a resistance exercise in the three body positions while measurements were recorded.
Exercise Tests
The body position testing order was counterbalanced for each subject. Three sets of 8 to 12 reps were performed for each exercise. The resistance selected was determined by the weight each subject could move through the full-range of motion 8 to 12 times, which was terminated by fatigue. The subjects should have rated their last repetitions as "Hard" (i.e., 15) on the Borg RPE scale (6). There was a 2-min rest period between sets and a 15-min rest period between exercises. The body positions included supine (bench press), upright above the heart (seated shoulder press), both of which were performed using the Body Masters BE 218A Multipress machine (Rayne, Louisiana), and upright at heart level (bicep curl) using a Body-Solid GPCB329 Preacher Curl Bench (Forest Park, Illinois). Three sets of 8 to 12 reps were selected to be in accordance with ACSM recommendations for resistance training in apparently healthy adults (1).
Cardiovascular Measurements
The Cheetah Medical, Inc. (Portland, Oregon) noninvasive cardiac output monitoring (NICOM) system utilizing bioreactance was used to measure CO, SV, and HR throughout each exercise session.
Resting measures were obtained with the subject in the particular body position for ~3 min before each exercise. Exercising measurements of CO, SV, and HR were recorded during the last 2 reps of each set for the exercise. Blood pressure was measured on the brachial artery by the auscultatory method using a sphygmomanometer and stethoscope. These measurements were taken at rest immediately upon completion of the exercise set. At the completion of each set the subjects were instructed to keep their hands on the bar in a relaxed state until blood pressure was obtained.
Statistical Analyses
Statistical analyses were performed using the SPSS Version 22.0 (SPSS, Inc., Chicago, IL, USA). All values are expressed as mean [+ or -] standard deviation unless otherwise noted. A two-way analysis of variance (ANOVA) with repeated measures was performed to determine significant differences at the P<0.05 that occurred between the 3 sets and between the three exercise positions and any treatment interactions. The dependent variables included CO, SV, HR, and BP.
RESULTS
Characteristics of the 14 subjects involved in this study are presented in Table 1. The subjects varied greatly in body mass and composition.
Cardiac output (L x [min.sup.-1]) was significantly greater (P<0.05) during the bench press when compared to the bicep curl. Cardiac output was also significantly greater (P<0.05) while performing the shoulder press when compared to the bicep curl. No significant difference was observed in CO between the bench press and shoulder press (Figure 1A). Cardiac output was significantly (P<0.05) greater during each set when compared to the resting values with the highest values during set 3 (Figure 1A).
Stroke volume was significantly (P<0.05) greater during the bench press when compared to the bicep curl and when compared to the shoulder press (Figure 1B). There was also a significant (P<0.05) difference in SV between the shoulder press and bicep curl with shoulder press exercise yielding the greater values (Figure 1B). During the bicep curl and shoulder press, SV was significantly (P<0.05) greater during set 2 and set 3 when compared to the resting values (Figure 1B).
Heart rate was higher while exercising in an upright position than when exercising in the supine position. During the bicep curl, the subjects' HR was significantly greater (P<0.05) than during the bench press. Shoulder press yielded significantly greater (P<0.05) HR values than the bench press exercise. No differences were observed in HR between the two upright exercises (Figure 1C). Heart rate was significantly greater (P<0.05) during each of the three exercise sets when compared to resting values with the greatest values obtained during set 3 (Figure 1C).
The subjects' SBP was significantly greater (P<0.05) during all 3 sets of resistance exercise when compared to the resting values, with the greatest values during set 3 (Figure 2A). A significant difference (P<0.05) in the subjects' DBP was observed during the third set of the bicep curl (Figure 2B). Diastolic BP was significantly greater (P<0.05) when the subjects performed the bicep curl when compared to the bench press. In addition, DBP for set 3 of the bicep curl was significantly greater (P<0.05) than the shoulder press (Figure 2B).
DISCUSSION
It is apparent that the cardiovascular response to resistance training is impacted by a number of factors, including the body position during the exercise and whether the exercise is performed above or below the heart. The purpose of this investigation was to determine the different responses of cardiac output (CO), stroke volume (SV), heart rate (HR), and blood pressure (BP) while performing resistance exercises in various body positions both above and below the heart.
Cardiac output was greater in the supine position and also in the upright above heart level position when compared to the upright at heart level position. A greater CO during supine exercise can be attributed to a significantly greater SV when compared to exercise in the upright position (13). While supine, there is a greater venous return to the heart that results in a higher EDV of the left ventricle. As a result, there is greater contractility of the myocardial tissue and, subsequently, more blood ejected into the aorta per contraction of the heart.
Cardiac output was also higher while exercising in the upright above heart level position when compared to the upright heart level position. Despite the fact that HR was similar in both upright positions, CO was still greater while exercising above the level of the heart. Once again, this can be explained by a higher SV while exercising above the level of the heart. While in this position, gravitational influence causes less pooling of blood in the distal portions of the upper extremities. As a result, there is greater venous return to the heart through the superior vena cava, a higher EDV and contractility of the myocardial tissue, and a larger SV (3,4).
Cardiac output was similar while exercising in the supine position and in the upright above heart level position. These findings can be explained by the differences found between HR and SV while exercising in these positions. Stroke volume was significantly greater during supine exercise. However, HR values were significantly higher during upright above heart level exercise. The trade-off between HR and SV allows for the maintenance of CO.
Stroke volume was found to be higher in the supine position than both upright body positions. While exercising in a supine position there is less pooling of blood in the lower extremities and greater venous return to the heart through the inferior vena cava. This larger venous return results in a larger EDV and preload of the heart (13). According to the Frank-Starling Principle, an increase in preload of the heart will subsequently result in an increased stretch of the myocardial tissue. A greater stretching of the myocardium leads to a greater force of contraction and, therefore, a larger SV. This mechanism could also explain the observed differences in SV between the two upright positions (3,4,12,15).
Exercising in the upright position above the level of the heart produced significantly greater SV values than those obtained while exercising in the upright heart level position. These findings can be attributed to the diminished pooling of blood in the upper extremities while exercising above the level of the heart. As a result, more blood is returned to the heart producing a larger venous return. Once again an increase in venous return brings about a larger EDV, contractility of the left ventricle, and, therefore, an increased SV. The changes in SV can help explain the differences found in HR between the different body positions.
While exercising in the supine position, HR was significantly lower than when exercising in both upright positions. This can be explained by the relationship between CO, SV, and HR. As the metabolic demand of working muscles increases with exercise, CO must also increase in order to supply the working muscles with an adequate amount of oxygen from arterial blood to meet that demand. Cardiac output is the product of HR and SV. Therefore, any increase in CO must be brought about by an increase in HR, SV, or both. Because SV was significantly higher in the supine position when compared to both upright positions, HR must rise to a lesser degree in order to maintain an adequate CO while supine (3,12,15,16).
Exercise performed in the upright heart level position produced similar HR values as when exercising above the level of the heart. There was a need for a greater CO in the upright above heart level position than the upright heart level position. This was achieved by the increase in SV while exercising above the heart, which was great enough to maintain an adequate CO that was needed to supply the working muscles. As a result, any further increase in HR in the upright above heart level position was not necessary.
Although the differences were not significant, SBP was highest while exercising in the upright heart level body position. Diastolic blood pressure was significantly greater in the upright heart level body position when compared to the supine position. Similar DBP values were found between the supine and upright above heart level body positions and also between the two upright positions (11,12,16,17).
Higher blood pressure values (both systolic and diastolic) during upright heart level exercise can be attributed to the exercise chosen for this position. During the seated bicep curl, the contracting bicep muscle increases mechanical pressure within the brachial artery (9). Since blood pressure measurements were taken at the brachial artery, it is believed that this affected both the SBP and the DBP values. This is especially true due to the fact that the pressure measurements were taken immediately upon completion of the exercise sets.
CONCLUSIONS
Performing resistance exercises in various body positions altered the CV response to exercise. These observed differences can be attributed to the relationship between CO, SV, and HR. Overall, CO and SV were highest while exercising in the supine and upright above heart level body positions. Heart rate was greatest while exercising in the upright body position. Systolic blood pressure was similar between all three body positions, and DBP was highest while in the upright heart level position.
The practical implications of this investigation are that resistance training in different positions may impact how some individuals tolerate resistance training. Patients with impaired left ventricular function may have difficulty tolerating the enhanced preload on the heart that occurs during supine exercise. In addition, those suffering with ischemic heart disease may reach their ischemic threshold sooner during upright exercise. Therefore, exercise physiology professionals must exhibit caution when prescribing exercise for those with heart disease and educate patients regarding their tolerance for exercise while in different positions.
Our findings are similar to those reported by previous investigations (12,16). However, it should be noted that most of the previous research conducted has been done using cycle ergometry. There is very limited research that has investigated the CV differences during resistance exercise in different body positions. Further investigations on resistance exercise in various body positions are warranted and should focus on different populations, exercises, and intensities.
ACKNOWLEDGMENTS
We would like to gratefully acknowledge all subjects for their participation in this investigation. This investigation was supported by a Bloomsburg University School of Graduate Studies Graduate Thesis Award (TS).
Address for correspondence: Joseph L. Andreacci, PhD, Department of Exercise Science, Bloomsburg University, Bloomsburg, PA, USA 17815. Email: [email protected]
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Timothy P. Sheehan (1,2), Timothy R. McConnell (1), Joseph L. Andreacci (1)
(1) Department of Exercise Science, Bloomsburg University, Bloomsburg, PA, USA; (2) Department of Medicine, Penn State College of Medicine, Hershey, PA, USA
Table 1. Characteristics for the Study Participants (N=14). Variables Mean [+ or -] SD Range Age (yrs) 20.4 [+ or -] 1.5 18-23 Height (cm) 169.6 [+ or -] 8.4 158.1-182.9 Body Mass (kg) 77.3 [+ or -] 20.1 55.2-130.6 Body Mass Index (kg x [m.sup.2]) 26.9 [+ or -] 6.1 21.2-39.1 Body Fat (%) 27.4 [+ or -] 11.2 10.6-46.6 Fat Mass (kg) 21.9 [+ or -] 12.2 7.5-45.5 Fat-Free Mass (kg) 55.6 [+ or -] 14.2 41.7-90.5 Total Body Water (kg) 40.7 [+ or -] 10.4 30.5-66.3
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| Author: | Sheehan, Timothy P.; Mcconnell, Timothy R.; Andreacci, Joseph L. |
|---|---|
| Publication: | Journal of Exercise Physiology Online |
| Article Type: | Report |
| Date: | Feb 1, 2018 |
| Words: | 3497 |
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