Cardiopulmonary Responses at Various Angles of Cycle Backrest Inclination

Abstract

The purpose of this study was to evaluate cardiopulmonary responses during submaximal cycle exercise at various angles of backrest inclination. Ten healthy Japanese men of mean age 25.9 yrs, height 170.6 cm, and body mass 66.1 kg, performed cycle exercises at a constant workload which reached the anaerobic threshold, at 20 degrees, 40 degrees, and 60 degrees of backrest inclination from the vertical plane, but the angle between the seat and back rest was kept at 110 degrees. The results were as follows: 1) Both cardiac output and stroke volume showed a higher value at the resting control state and during exercise as the backrest angle increased. 2) Oxygen consumption, carbon dioxide output, heart rate, gas exchange ratio, and oxygen pulse were not affected by the angle of backrest inclination. 3) Tidal volume at 20 degrees of backrest inclination was higher than at 60 degrees. 4) No significant differences were found in minute ventilation between each backrest angle. These findings suggest that changes in the backrest angle significantly alter cardiopulmonary parameters at rest and during exercise; in particular, an angle difference of 40 degrees may be enough to alter tidal volume, cardiac output and stroke volume, but not the minute ventilation.

Peak oxygen consumption () and anaerobic threshold (AT) are influenced by exercise position¹⁾. Many researchers have reported comparisons of cardiopulmonary responses during upright and recumbent (supine) exercises^2–8). Generally, it has been reported that work performance in incremental exercise is enhanced in the upright position as compared with the supine position^3–5). In addition, Peak and/or AT were significantly higher in the upright position than in the supine position⁵⁾⁸⁾. The difference in working muscle groups and improvement of the perfusion of the working muscles may be attributed to the enhancement of work performance and increased peak and AT.

A semi-recumbent cycle ergometer has recently been developed⁹⁾¹⁰⁾. A semi-recumbent cycle ergometer reduces the risk of falling and is more comfortable. However, the cardiopulmonary responses during exercise in semi-recumbent positions are still unclear, and there has been no objective data on the cardiopulmonary responses during bicycle exercise at various angles of backrest inclination.

Therefore, the purpose of this study was to determine cardiopulmonary responses to submaximal exercise on a semi-recumbent cycle in various angles of backrest inclination and to compare the cardiopulmonary responses between various angles of backrest inclination.

Methods

Ten healthy Japanese males participated in this study. The mean age, weight, and height (±SD) of the subjects were 25.9 (± 2.0) years, 66.1 (± 4.6) kg, and 170.6 (± 3.7) cm, respectively. None of the subjects had a history of cardiopulmonary diseases and all subjects denied taking medications. Prior to volunteering to the study, the participants signed an informed consent form outlining the possible risks and discomforts associated with the experiment.

Exercise testing was performed using an electromagnetically braked ergometer (Space cycle SSR, Health Systems Co.). The experiment was divided into two parts:

Part 1: The subjects performed a symptom-limited incremental exercise at a 20 degree (°) backrest angle from the vertical plane. The aim was to measure the anaerobic threshold through respiratory gas analysis for the determination of work intensity to be used in Part 2. After a warming-up period of 3 minutes at 20 watts, the work rate increased in a ramp protocol at a rate of 20 watts per minute up to 180 watts. Pedal frequency was maintained near 50 cycles per minute.

The variables of , carbon dioxide output (), and other ventilation data were measured at rest and throughout the exercise period with a respiromonitor RM-300 (Minato Medical Science Co.). The respiromonitor RM-300 contains a microcomputer, a hot wire flow-meter, and a gas analyser. The gas analyser contains a sampling tube, filter, suction pump, oxygen analyser made with a zirconium element, and an infrared carbon dioxide analyser. The respiromonitor RM-300 calculated breath-by-breath and . The system was calibrated carefully as per calibration protocols. Heart rate (HR) was measured from the R-R interval of the electrocardiogram throughout the test (Bioview-G, Nihonkoden San-ei Co.). Peripheral arterial blood pressures (systolic and diastolic) were determined every minute via the mercury column sphygmomanometer. The AT was determined synthetically by gas exchange criteria at the point of non-linear increase in the ventilatory equivalent for oxygen and the V-slope analysis (- plot)¹¹⁾.

Part 2: The workloads of part 2 for each subject were derived from Part 1 and were calculated to be 10 watts less than the workload at the subject's AT in the ramp incremental test. The angle between the seat and back rest was kept at 110°, while all the subjects performed the three exercise tests at a constant workload. The three backrest angles from the vertical were 20°, 40°, 60° (Fig. 1). After warming up at 20 watts for 3 minutes, each subject performed the cycle exercise at a constant workload for 6 minutes in each of the randomised backrest angles. Pedaling frequency was at a rate of 50 per minute and was constant in each position in cadence with an electric metronome. In all the subjects, HR, and other ventilation data were also corrected continuously throughout the exercise session, was done in Part 1. Peripheral arterial blood pressure was determined at a resting control state and at a steady state during exercise at a constant workload. In addition, an impedance cardiograph (NCCOM3-R7, BOMED Co., Ltd.) was used to obtain cardiac output (CO) and stroke volume (SV) at rest and during exercise. Both the CO and the SV were determined to be the average of the 16 heart-beats throughout exercise. The resting and , other ventilation data, HR, O₂ pulse, CO and SV were all determined to be the average of the data obtained the 2 minutes before the start of the exercise. All data during the constant work rate exercise were determined to be the average of the data obtained among 300 and 360 seconds.

Fig. 1.

Diagram illustrating the 3 postures used during testing. The inclination of the backrest was at 20°, 40° or 60° from the upright position, but the angle between the seat and backrest was kept at 110°.

Data Analysis

Data were compared using a paired t-test. A p value <0.05 was considered statistically significant. Values are expressed as the mean ± standard deviation.

Results

Two exercise tests were completed without complications. None of the subjects experienced chest pains or manifested S-T segment depression during exercise.

Table 1 lists the anthropometric data, AT, HR at AT, exercise time at AT, peak , peak HR, and workload for Part 2. The mean values of AT, AT-HR, peak , and peak HR were 19.2 ± 5.6 ml·min⁻¹·kg⁻¹, 115.1 ± 11.7 bpm, 31.7 ± 5.9 ml·min⁻¹·kg⁻¹, and 160.5 ± 11.4 bpm, respectively.

Table 1. Anthropometric data and the results of Part 1.

Subject	Age (yr)	Height (cm)	Weight (kg)	AT (ml/kg/min)	AT HR (bpm)	AT TIME (sec)	peak (ml/kg/min)	peak HR (bpm)	Work Intensity for Part 2 (W)
A	23	167	62	15.7	115	105	35.7	175	50
B	27	173	68	19.9	109	219	35.2	149	90
C	24	178	63	21.5	130	210	32.5	175	90
D	23	175	63	24.8	132	255	38.6	173	110
E	26	167	68	13.0	100	66	32.0	159	50
F	26	167	65	21.3	100	162	39.7	149	70
G	29	173	73	12.7	100	105	24.4	141	50
H	25	168	68	12.5	120	138	26.7	168	70
I	28	170	73	20.1	124	150	32.3	157	70
J	28	168	58	20.3	121	126	30.7	159	70
Mean	25.9	170.6	66.1	19.2	115.1	153.6	31.7	160.5	72
SD	2.0	3.7	4.6	5.6	11.7	55.9	5.9	11.4	6.3

Figure 2 shows the CO and SV at a resting control state and during exercise at a constant workload for each angle of backrest inclination. The CO at a resting control state showed a markedly higher value as the backrest angle increased (20°; 4.5 ± 0.8 l·min⁻¹, 40°; 5.6 ± 0.8 l·min⁻¹, 60°; 6.2 ± 1.6 l·min⁻¹, respectively). The CO during exercise at a constant workload also showed a higher value as the backrest angle increased (20°; 11.3 ± 3.3 l·min⁻¹, 40°; 11.9 ± 2.9 l·min⁻¹, 60°; 12.0 ± 3.1 l·min⁻¹, respectively). The SV at a resting control state also showed a higher value as the backrest angle increased (20°; 62.6 ± 16.2 ml·beat⁻¹, 40°; 77.7 ± 17.1 ml·beat⁻¹, 60°; 87.9 ± 28.6 ml·beat⁻¹, respectively). The SV during exercise at a constant workload also showed a higher value as the backrest angle increased (20°; 96.3 ± 24.5 ml·beat⁻¹, 40°; 102.7 ± 21.0 ml·beat⁻¹, 60°; 107.9 ± 20.7 ml·beat⁻¹, respectively). The CO and SV obtained from the constant workload exercise at the 60° angle were significantly higher than those at the 20° angle.

Fig. 2.

The cardiac output and stroke volume at rest and during exercise at a constant workload.

Table 2 demonstrates , , gas exchange ratio (GER), HR, oxygen pulse (O₂ pulse) at rest and during exercise at a constant workload. No significant differences in , , GER, HR or O₂ pulse were observed at the resting control state, except for the HR between 40° and 60° backrest angle. Moreover, no significant difference in , , GER, HR or O₂ pulse were observed during exercise.

Table 2. Oxygen consumption () and carbon dioxide output (), gas exchange ratio (GER), heart rate (HR), and oxygen pulse (O₂ Pulse) at rest and warming up and during exercise at a constant workload.

		20°	40°	60°
	REST	4.0 ± 0.5	3.9 ± 0.5	4.0 ± 0.6
(ml/kg/min)	EXERCISE	19.0 ± 3.3	18.5 ± 3.7	18.3 ± 3.7
	REST	229.4 ± 35.4	228.5 ± 31.1	225.4 ± 45.5
(ml/min)	EXERCISE	1164.3 ± 225.1	1094.3 ± 245.8	1095.7 ± 218.4
GER	REST	0.87 ± 0.06	0.89 ± 0.09	0.86 ± 0.07
	EXERCISE	0.92 ± 0.06	0.90 ± 0.06	0.91 ± 0.05
HR	REST	75.0 ± 9.3	73.4 ± 10.8*	69.5 ± 10.9
(beats/min)	EXERCISE	114.5 ± 13.1	113.0 ± 14.8	111.3 ± 14.0
O2 pulse	REST	3.6 ± 0.7	3.6 ± 0.7	3.9 ± 0.9
(ml/beat)	EXERCISE	11.0 ± 1.6	10.9 ± 1.7	10.8 ± 1.8

^*40° vs 60°, p<0.05.

The arterial-venous oxygen difference [C(a-v)O₂], calculated by dividing the by the CO, showed a lower value as the backrest angle increased at rest and during exercise (Table 3). At the 20° backrest angle at rest, the mean C(a-v)O₂ was significantly lower than at the 40° (p<0.01) or 60° angles (p<0.05). Similarly, the mean C(a-v) O₂ at the 20° backrest angle during exercise was significantly lower than at the 60° backrest angle (p<0.01). Table 3 also demonstrates systolic blood pressure (SBP) and diastolic blood pressure (DBP) at rest and during exercise. The mean SBP at rest showed a higher value as the backrest angle increased. At the 60° backrest angle at rest, the mean SBP was significantly higher than at the 20° backrest angle (p<0.05). On the other hand, the mean SBP during exercise at a constant workload showed a higher value as the backrest angle decreased. The SBP at the 60° backrest angle during exercise was significantly lower than at the 40° backrest angle (p<0.05) or at the 20° backrest angle (p<0.05). No statistical difference in the diastolic blood pressure at rest and during exercise was observed.

Table 3. Blood pressure (systolic blood pressure, SBP; diastolic blood pressure, DBP) and arteriovenous oxygen difference [C(a-v)O₂] at rest and during exercise at a constant workload.

		20°	40°	60°
SBP	REST	116.6 ± 12.7 b	117.1 ± 39.3	124.2 ± 11.6
DBP		80.2 ± 10.8	78.0 ± 8.3	75.6 ± 13.2
(mmHg)	EXERCISE	162.6 ± 19.8 b	158.6 ± 22.4 d	153.6 ± 19.4
		87.0 ± 10.1	84.6 ± 6.0	80.6 ± 9.3
C(a-v)O2	REST	57.8 ± 10.1 ab	46.1 ± 7.3	45.8 ± 12.9
(vol%)	EXERCISE	119.8 ± 26.5 c	108.1 ± 28.8	106.4 ± 22.5

^a20 vs 40, p<0.01.

^b20 vs 60, p<0.05.

^c20 vs 60, p<0.01.

^d40 vs 60, p<0.05.

Table 4 demonstrates minute ventilation (), tidal volume (TV), and respiratory rate (RR). During exercise at a constant work and at the resting control state, the was not affected by the backrest angle. At rest, the control state TV was also not significantly affected by the backrest angle. During exercise, however, the TV at the 20° backrest angle was higher than that at the 60° (p<0.01) or 40° backrest angles (p<0.01). RR showed higher values as the backrest angle increased, but it was not significant.

Table 4. Minute ventilation (), tidal volume (TV), and respiratory rate (RR) at rest and during exercise at a constant workload.

		20°	40°	60°
	REST	11.5 ± 1.3	11.5 ± 1.9	10.8 ± 2.0
(l/min)	EXERCISE	36.4 ± 5.0	34.9 ± 6.7	34.3 ± 6.6
TV	REST	615.9 ± 85	615.4 ± 65.7	563.9 ± 92.7
(ml)	EXERCISE	1395.9 ± 250.7 ac	1273.4 ± 232.4	1282.6 ± 201.8
RR	REST	19.1 ± 3.2	19.1 ± 3.6	19.4 ± 2.7
(breaths/min)	EXERCISE	26.9 ± 2.6	27.7 ± 2.3	27.3 ± 2.7

^a20 vs 40, p<0.01.

^c20 vs 60, p<0.01.

Discussion

It has previously been reported that cardiovascular responses and exercise performance were affected by body position^2–8). In general, the AT and peak , which are determined from cardiopulmonary exercise testing, were lower in the supine position than in the upright position⁵⁾⁸⁾. Left ventricular end diastolic pressure (LVEDP) and SV in the supine position were higher than those in the upright position at rest and during moderate exercise¹²⁾. The difference in LVEDP and SV are gradually reduced as the work intensity increases¹³⁾. We previously reported cardiopulmonary responses, cardiac output dynamics and hormonal responses during supine and sitting exercises⁸⁾. In our previous study, the cardiac index during the AT level intensity exercise was the same in both the supine and sitting positions. Noradrenaline and angiotenshin II in the supine position were generally lower than in the sitting position. Therefore, we suggested that creating a lower anaerobic threshold in the supine position is due to changes in blood redistribution and the lowered blood flow to working muscles. However, there is the difference in the working muscles used in exercising in the supine position and sitting (upright) position, in addition to the difference in cardiac output and synthetic activity. Therefore, we designed this study so that there was a minimal difference in the working muscles. This was obtained by retaining the angle between the seat and backrest.

The CO and SV at rest and during exercise at a constant workload were significantly higher as the seat angle increased. The CO and SV at the backrest angle of 60° was especially significantly greater, as compared with that at the 20° backrest angle. Cotsamire¹²⁾ and Thadani¹³⁾ reported that the CO and left ventricular end diastolic volume in supine exercise showed a significantly higher value than that in sitting exercise. Rowell¹⁴⁾ reported that the blood flow volume in the lung and the abdominal cavity increases more in the horizontal position than in the vertical position. Moreover, Roddie¹⁵⁾ also reported that blood flow volume in the upper extremity increases in the supine position. Therefore, in this study, the CO during exercise at the 60° backrest angle, the nearest position to the ground, showed the highest value. It would appear that the highest CO and SV at the 60° backrest angle were due to increased venous return and central blood volume.

The C(a-v)O₂, calculated using the Fick equation, showed a lower value as the backrest angle increased, especially, as the C(a-v)O₂ at the 20° backrest angle was significantly lower than that at the 60° backrest angle. This result supported previous reports^2)7)16). We hypothesised that, at the 60° backrest angle which is the nearest position to the ground, the blood supply to the working muscles, including the capillaries, was relatively reduced. Consequently, the C(a-v)O₂ may show a lower value in the 60° recumbent position.

For the results concerned with ventilation, it has been observed that and TV were greater in the sitting position than in the supine position, and the greater observed during sitting was due to the greater TV¹⁷⁾¹⁸⁾. In the supine position, the diaphragm is elevated, and the elevated diaphragm has a lower mechanical advantage¹⁹⁾. In addition, in supine, an increased venous return makes the lung blood volume increase and lung volume decrease¹⁴⁾¹⁹⁾. Increased lung blood volume also causes a decrease in the physiological dead space^19–21). In this study, the TV was significantly higher at the 20° position than at the 60° position during exercise, but no significant differences were found in between each backrest angle. It seems that an angle difference of 40° (60°–20°) backrest angle was insufficient to alter the .

Previous studies have reported that supine cycle training is more effective to improve the exercise capacity than upright training and may also cause greater central circulatory adaptation²²⁾²³⁾. However, when prescribing exercises close to a supine position, we should consider the changes in CO and venous return. Our findings suggest that changes in the backrest angle significantly alter cardiopulmonary parameters at rest and during exercise, in particular, an angle of 40 degrees may be enough to alter TV, CO and SV, but not . Further study to determine the defference of cardiopulmonary responses between standard upright ergometre exercise and recumnent ergometre exercise is necessary before implementation of recumbent exercise in the clinical setting.