Suppression of cardiocirculatory responses to orthostatic stress by passive walking-like leg movement in healthy young men

Abstract

Background: Although passive walking-like leg movement in the standing posture (PWM) has been used in the clinical field, the safety of PWM has not been fully determined despite the risks of orthostatic intolerance due to standing posture. The aim of the present study was to examine cardiocirculatory response during PWM in healthy young men.

Methods: The subjects (n = 13) spent 5 min in a sitting position and then 5 min in a quiet standing position to determine baseline levels. Thereafter, they underwent 25-min rhythmic PWM at 1 Hz while standing. In another bout, subjects experienced the same protocol except that they underwent 25-min quiet standing (QS) instead of 25-min PWM. Two subjects dropped out of the 25-min QS due to feeling of discomfort. Thus, data obtained in the remaining eleven subjects are presented.

Results: In the PWM trial, systolic arterial blood pressure (SAP) decreased from 112 ± 8 mmHg during the sitting baseline period to 107 ± 8 mmHg during the standing baseline period (p <0.05), while heart rate (HR) increased from 73 ± 9 bpm during the sitting baseline period to 84 ± 10 bpm during the standing baseline period (p <0.001). After the imposition of PWM, SAP increased from 107 ± 8 mmHg in the standing baseline period to 120 ± 6 mmHg (p <0.001), while HR decreased from 84 ± 10 bpm in the standing baseline period to 76 ± 9 bpm (p <0.05). In the QS trial, SAP, which had decreased during the standing baseline period compared to that during the sitting baseline period, remained lowered during the 25-min QS period, while HR, which had increased during the standing baseline period compared to that during the sitting baseline period, remained elevated during the 25-min QS period. In both bouts, HR showed almost mirror-image changes in the high-frequency component of HR variability, suggesting that the changes in HR were due to change in parasympathetic activation. Double product (HR × SAP), as a predictor of myocardial oxygen consumption, during the 25-min QS period tended to increase with time, but double product remained almost constant during the 25-min PWM period.

Conclusions: The results suggest that PWM is effective for suppressing cardiocirculatory responses to orthostatic stress.

Figure 1

Experimental protocol. Subjects spent 5 minutes in a sitting position and this was followed by a 5-minute quiet standing period with a 1-minute transition period for postural change between sitting and standing. Thereafter, the subjects underwent 25-minute rhythmic PWM or 25-minute quiet standing. A commercially available device was used for PWM and quiet standing. The device enables subjects to change their posture from sitting to standing by pulling a built-in hydraulic lever. Standing posture is stabilized by fixing the trunk, pelvis and knees using front, lateral and back trunk pads, lateral pelvic pads, and kneepads. Bilateral handles located in front of the trunk are linked to the footplates, thus allowing one leg to move forward while the other moves back by pushing and pulling the handles alternately. PWM: passive walking-like leg movement.

Figure 2

Changes in systolic, diastolic and mean arterial blood pressure. PWM: passive walking-like leg movement. QS: quiet standing. *P < 0.05, **P < 0.01, ***P < 0.001 compared to the sitting baseline level; ^#P < 0.05, ^##P < 0.01, ^###P <0.001 compared to the quiet standing baseline level; †P < 0.05, †† P < 0.01, †††P < 0.001 for comparison between values in the PWM and QS trials.

Figure 3

Changes in heart rate, the ratio of low-frequency component to high-frequency component of the heart rate variability spectrum (LF/HF), and high-frequency component of the heart rate variability spectrum normalized by total power (HF_norm). PWM: passive walking-like leg movement. QS: quiet standing. *P < 0.05, **P < 0.01, ***P < 0.001 compared to the sitting baseline level; ^#P < 0.05, ^##P < 0.01, ^###P <0.001 compared to the quiet standing baseline level; †P < 0.05, †† P < 0.01, †††P < 0.001 for comparison between values in the PWM and QS trials.

Figure 4

Change in double product. PWM: passive walking-like leg movement. QS: quiet standing. *P < 0.05, **P < 0.01, ***P < 0.001 compared to the sitting baseline level; ^#P < 0.05, ^##P < 0.01, ^###P <0.001 compared to the quiet standing baseline level; †P < 0.05, †† P < 0.01, †††P < 0.001 for comparison between values in the PWM and QS trials.