The training environment for competitive swimmers, who regularly exercise in water, differs from that for athletes who perform on land in environmental temperature, pressure, and gravity. The effects of gravity in water are reduced in proportion to the depth by action of buoyancy, which relaxes the antigravity muscles in water. Therefore, it is expected that the cardiovascular function of competitive swimmers would be adapted to the hypogravity environment. To our knowledge, however, only a few studies (6) have been conducted comparing the cardiovascular responses in competitive swimmers with those in other athletes from the viewpoint of gravity in exercise-training environments.
By causing changes in intramuscular pressure, the antigravity muscle modulates cardiovascular regulation during upright standing (2,8,18,21). The intramuscular pressure decreases after prolonged bed rest and after prolonged weightlessness during space flight (4,18) because of the decrease in muscle mass. We hypothesized that competitive swimmers who undergo regular exercise training in a hypogravity environment for long periods of time would have less antigravity muscle activity during upright standing compared with athletes who exercise under full terrestrial gravity, for example, track and field (T & F) athletes.
In the present study, we tested this hypothesis by comparing cardiovascular responses in nonendurance competitive swimmers with those in nonendurance T & F athletes during passive head-up tilt with engagement of the antigravity muscles in the lower extremities and during passive head-up suspension without it.
Two series of experiments were conducted. In series A, blood pressure and heart rate during passive tilting (head-up tilt) were compared among T & F athletes, competitive swimmers, and untrained subjects. In series B, during passive tilting with hip suspension so as not to induce antigravity responses in antigravity muscles of the lower extremities (head-up suspension), blood pressure, heart rate, stroke volume, cardiac output, and total peripheral resistance (TPR) were measured in T & F athletes and competitive swimmers. The results of the two head-up conditions were compared.
Thirty-seven healthy male and female subjects aged 18–23 yr (20.2 ± 1.2, mean ± SD) volunteered for this study. The experimental procedures were approved by our institutional committee for the protection of humans in research, and written informed consent was obtained from each subject before participating in the study. Eleven (6 males, 5 females) were nonendurance athletes in T & F. Fifteen (10 males, 5 females) were nonendurance competitive swimmers. On average, they had been practicing their sports for more than 7 yr. A control group was composed of 11 healthy nonathletic and untrained students (4 males, 7 females); they had never done any physical training. All subjects had no medical history of circulatory disease, and their ECG and blood pressure were normal. The physical characteristics of the groups are presented in Table 1.
Preliminary experiments were carried out to familiarize the subjects with the procedures and environments. Sudden noises were avoided during the experiments so as not to activate the sympathetic nervous system. The subjects were instructed not to eat any food for at least 2 h before the experiments. After resting for 10–20 min in a supine position, the subjects repeatedly underwent passive head-up tilt (90° head-up) on a tilt table (UA-450, OG Giken, Okayama, Japan) seven times. The supine and 90° head-up positions were maintained for approximately 60 s. The time required to move from supine to 90° head-up position or from 90° head-up to supine position was approximately 30 s. In this series, blood pressure and heart rate were measured. The room temperature was 23.5 ± 0.6°C (mean ± SD).
Thirteen of the 37 subjects involved in series A participated in series B; six T & F athletes (4 males, 2 females) and seven competitive swimmers (5 males, 2 females). The groups did not differ for resting heart rate, blood pressure, and maximum oxygen uptake estimated from the Åstrand nomogram (3) (P > 0.05, Student’s t-test).
The procedures were the same as for series A, except for the use of a specially constructed tilt table with a pelvic suspension by making subjects sit on a bicycle seat (marked “a” in Fig. 1); the legs of the subject were unsupported in space (head-up suspension). The subjects could not support their weight by the legs, only by the seat. Thus, the subjects could not use the antigravity muscles in the lower extremities while in the suspended position (15,22). However, this method often produced contraction of the back and abdominal muscles due to the instability and discomfort felt around the groin during upright standing (21). To deal with these shortcomings, we made the following improvements: to increase stability, we fastened the subject with a safety belt (11 cm wide; marked “c” in Fig. 1) fixed to the tilt table around the anterior superior iliac spine so lightly as not to disturb venous blood flow, and allowed the subjects to touch the tiptoes on footboards (marked “d” in Fig. 1) at the expense of maximum relaxation of the antigravity muscles. To alleviate the uncomfortable feeling, we conducted passive head-up tilt and suspension for a shorter term (1 min). In this series, stroke volume, cardiac output, and TPR were measured in addition to blood pressure and heart rate.
Measurement of blood pressure and heart rate.
Blood pressure and heart rate were measured using an automated machine (STBP-780, Colin, Aichi, Japan); the systolic blood pressure (SBP) and the diastolic blood pressure (DBP) were determined by synchronizing the sound signals from two microphones in the cuff with the ECG-R wave. The sound signals were analyzed by a detection algorithm. Because it has been reported previously that head-up tilt from supine to upright posture resulted in immediate changes in the heart rate and the blood pressure during first 30 s (immediate response), which were followed by steady changes in both parameters (stabilized response) (23), the blood pressure was measured for about 30 s during the early phase of the stabilized response (30–60 s). During passive tilting, the arm for measurement was rested at the heart level on the half-pipe made of polyvinyl chloride (10 cm in diameter, 15 cm in length; marked “b” in Fig. 1) that was fixed to the tilt table. Mean blood pressure (MBP) was one-third of the pulse pressure plus the DBP. The ECG was recorded from bipolar chest leads. The R-R intervals of ECG were measured, and the instantaneous heart rate was calculated by multiplying the inverse of the R-R intervals (per second) by 60 s. The heart rate corresponding to the blood pressure was expressed as the average of six successive heart beats before the end of blood pressure measurement.
Measurement of stroke volume, cardiac output, and TPR.
Thoracic impedance (ΔZ) was recorded by using an impedance plethysmograph (4134, NEC San-ei, Tokyo, Japan). Two aluminum tape electrodes separated by at least 3 cm were placed around the neck, and two other tape electrodes were placed around the trunk at the level of the xiphisternum and around the abdomen. When the subjects exhaled in the ordinary way for 2 s and then held the breath for 3 s without bearing down at 55–60 s after each tilting position, ΔZ and the first derivative of the impedance wave (d z/d t) were measured. At the same time, the left ventricular ejection time was measured on carotid artery pulses according to our method (7): a pulse transducer (45259, NEC San-ei) fixed on our improved apparatus was placed over the right carotid artery. Stroke volume was calculated according to the method recommended by Kubicek et al. (11). The stroke volume in each position was expressed as the average of three successive heart beats. Cardiac output was calculated by multiplying the stroke volume by the heart rate, and the TPR by dividing the MBP by the cardiac output. The stroke volume, cardiac output, and TPR in standing position were expressed as relative values to those in supine position.
The ECG, ΔZ, and carotid artery pulse were recorded on the data recorder (RD-135, TEAC, Tokyo, Japan). After the experiments, they were reproduced and led to a laboratory-oriented microcomputer (Power Macintosh 8500/150, Apple) and analyzed with software (Chart v3. 6. 8/s, AD Instruments Japan, Aichi, Japan) based on MacLab (4s, AD Instruments, Japan).
For each subject, the data in each position were expressed as the average of data from the seven successive tilts (mean ± SD). The statistical significance of the differences between supine and standing positions, between the groups, and between head-up tilt and head-up suspension were analyzed by using the Student’s t-test. A P value of less than 0.05 was considered significant.
The SBP, DBP, MBP, and heart rate in head-up tilt for three groups of subjects are shown in Table 2. By passive tilting from supine to standing position, the heart rate increased significantly for all subject groups (P < 0.001 for untrained subjects and competitive swimmers; P < 0.01 for T & F athletes). However, the MBP decreased for untrained subjects (P < 0.05) but did not change significantly for T & F athletes or competitive swimmers. Moreover, the SBP decreased significantly for all subject groups (P < 0.05 for untrained subjects; P < 0.01 for T & F athletes and swimmers), and the DBP did not change significantly for untrained subjects or T & F athletes but increased significantly for swimmers (P < 0.01).
Blood pressure and heart rate did not differ significantly in either supine or standing position between the T & F athletes and swimmers (P > 0.05).
The blood pressure and heart rate in head-up tilt or head-up suspension for both groups of athletes are shown in Table 3. In the head-up tilt (Table 3A), the changes in the blood pressure and heart rate, recollected in this series, showed the same tendencies as in series A for both types of athlete. In both types of athlete, the head-up suspension (Table 3B) significantly decreased the SBP (P < 0.05 for T & F athletes; P < 0.01 for swimmers) and significantly increased the heart rate (P < 0.01 for T & F athletes; P < 0.001 for swimmers) as with tilt, though it significantly decreased the MBP (P < 0.05). The DBP did not change in either group (P > 0.05). The blood pressure and heart rate did not differ significantly between T & F athletes and swimmers in either supine or standing position (P > 0.05).
The relative values of stroke volume, cardiac output, and TPR in head-up tilt or head-up suspension for both groups of athletes are shown in Figure 2. The ratio of decrease in stroke volume and cardiac output was significantly larger in swimmers (−50.4 ± 10.2% for stroke volume; −36.7 ± 13.2% for cardiac output) during head-up tilt than in T & F athletes (−36.5 ± 10.3% for stroke volume; −15.6 ± 9.8% for cardiac output) (P < 0.05 for stroke volume; P < 0.01 for cardiac output), but a significant difference was not noted during head-up suspension (P > 0.05). The ratio of decrease in stroke volume and cardiac output was significantly larger during head-up suspension (−49.3 ± 13.0% for stroke volume; −35.3 ± 12.7% for cardiac output) in T & F athletes than during head-up tilt (P < 0.05 for stroke volume; P < 0.01 for cardiac output), but no significant difference was noted between the two conditions in swimmers (P > 0.05). The ratio of increase in TPR was significantly larger in swimmers (66.7 ± 39.6%) than in T & F athletes (19.1 ± 10.2%) in head-up tilt (P < 0.05), but no significant difference was noted during head-up suspension (P > 0.05). The ratio of increase in TPR was significantly larger in T & F athletes during head-up suspension (51.7 ± 34.4%) than during head-up tilt (P < 0.05), but no significant difference was noted between the two conditions in swimmers (P > 0.05).
The passive tilting from the supine to a standing position results in a transient fall of blood pressure, followed by an increase in the heart rate and peripheral vascular resistance (9,16) through the baroreceptor reflex (10,18,20,23). In the present study, the short-term (1 min) passive head-up tilt resulted in significant increases of the heart rate in all groups of subjects, agreeing with previous reports (10,18,20,23). However, the responses of the MBP in untrained subjects differed from those in both groups of athletes; the MBP significantly decreased in untrained subjects but not in either group of athletes. This finding suggests that in untrained subjects the MBP did not fully restore through the arterial baroreflex within 1 min, but it did in both types of athletes, which may reflect increased orthostatic tolerance for nonendurance athletes (5,12,17).
Moreover, the MBP was maintained with different manners in the athletes. Despite the significant decrease of SBP in both athlete groups, the DBP significantly increased in swimmers but did not change in T & F athletes. Even during quiet upright standing with all movement consciously stopped, rhythmic changes in the activity, i.e., intramuscular pressure, of the antigravity muscles decrease venous transmural pressure (2,8,18). The intramuscular pressure decreases after prolonged bed rest and after prolonged weightlessness during space flight (4,18). It might be assumed that swimmers who undergo regular and prolonged training in a hypogravity environment have smaller intramuscular pressure during upright standing than T & F athletes, who train under terrestrial gravity, so that swimmers have a smaller venous return than do T & F athletes. In fact, the ratio of decrease in stroke volume during head-up tilt was significantly larger in swimmers than in T & F athletes. Thus, it is possible that the baroreceptor reflex (2,18) is required to increase not only the heart rate but also the DBP (or TPR) through vasoconstriction (9,16) in swimmers to maintain the MBP.
In head-up suspension, although the SBP decreased and the heart rate increased significantly in both types of athletes as in the head-up tilt, the MBP decreased significantly without a significant increase in the DBP. Moreover, the ratio of decrease in cardiac output in both athletes was larger during head-up suspension than during head-up tilt, though they did not differ significantly in swimmers. It is possible that the decrease in venous return was larger and that the muscle mechanoreflex and chemoreflex (13,14,19) in antigravity muscles of the lower extremities were smaller during head-up suspension than during head-up tilt, so that the MBP could not remain stable. This finding is in agreement with previous studies in which cardiac output and MBP decreased through the arterial baroreflex when the lower extremities of the subjects were exposed to negative pressure of more than 40 mm Hg (1,4,18).
The change of stroke volume, cardiac output, and TPR during head-up suspension did not differ significantly between the two types of athlete. This finding suggests that the cardiovascular responses to head-up suspension, which does not induce activity of antigravity muscles in the lower extremities, are similar in both types of athlete. However, the ratio of change in these parameters in T & F athletes was significantly larger during head-up suspension than during head-up tilt, whereas no significant differences were noted in swimmers. These findings suggest that during head-up tilt, antigravity muscles were able to prevent the excessive decrease in venous return to maintain the MBP in T & F athletes but not in swimmers.
Because we did not measure stroke volume, cardiac output, or TPR in untrained subjects, we could not compare the ratio of changes in these parameters with those in trained athletes. In future studies, such comparisons should clarify the mechanism of the development of cardiovascular regulation during upright standing for training on land and in water.
In conclusion, the present study indicates that the action of the antigravity muscles on cardiovascular regulation during upright standing is smaller in competitive swimmers than in track and field athletes.
We are indebted to Prof. Hisashi Ogawa for his advice and editorial help, and to Dr. Alan Rosen for his editing of the English used in this manuscript. We also thank Miss Yuriko Matsumura and our laboratory students for excellent technical assistance.
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Keywords:©2004The American College of Sports Medicine
GRAVITY; HEAD-UP SUSPENSION; INTRAMUSCULAR PRESSURE; NONENDURANCE ATHLETES