Cardiovascular responses to leg muscle loading during head-down tilt at rest and after dynamic exercises

Figures

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  Figure 1. Upright and head-down tilt postures, and schematic representation of the robotic leg-press device. During upright posture (a) blood tends to shift towards the lower part of the body under the effect of gravity. This displacement of blood is regulated by physiological mechanisms that accomplish hemodynamic homeostasis, which are modulated by the activity of leg muscles engaged in maintaining the upright position. During head-down tilt (b) gravity leads to a displacement of blood towards the upper part of the body, and since there is no weight bearing, leg muscles are inactive. The robotic device MARCOS (c) allowed us to apply leg muscle loading and perform leg-press exercises during HDT. The device provides simulated ground reaction force by means of pneumatic actuators that oppose leg extension movements, and maintains the desired force using force feedback loops that receive force measurements from the foot pedals. Additional details on this device are described in Sec. Methods.

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  Figure 2. Cardiovascular signals at rest for a representative subject. During head-down tilt without leg muscle loading (HDT-noML), the overall BP trace (i.e. systolic and diastolic) was lower than during both orthostatic stress (ORT) and head-down tilt with muscle loading (HDT-ML). In the HDT posture, pulse BP was slightly higher and heart rate was lower than during ORT, and they were minimally affected by leg muscle loading.

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  Figure 3. Cardiovascular signals at rest for all subjects. During head-down tilt without muscle loading (HDT-noML), systolic BP, diastolic BP, mean arterial pressure and heart-rate were lower than during orthostatic stress (ORT). During head-down tilt with muscle loading (HDT-ML), systolic BP, diastolic BP and mean arterial pressure were higher than during HDT-noML, while pulse BP and heart rate were not different from HDT-noML. Note that systolic BP during HDT-ML was not different from ORT. Data are presented as mean ± s.d. across subjects. N = 16, 17, 16 subjects contributed to the bars ORT, HDT-noML, and HDT-ML respectively for all signals (see Sec. Methods). ***p < 0.001, **p < 0.01, *p < 0.05.

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  Figure 4. Cardiovascular signals before, during and after dynamic leg-press exercises for one subject. The trends of the signals obtained with leg muscle loading (HDT-ML) are qualitatively similar to those obtained without muscle loading (HDT-noML). All the signals exhibit very clear transient dynamics shortly after the exercise bout, and then they stabilize at later time points. Gray band: preparation to exercise (i.e. the subject waits with their legs flexed) and exercise bout. Yellow bands: baseline (BAS), early recovery (Rearly, i.e. first 90 seconds after the end of the exercise bout), late recovery (Rlate, i.e. 3.5 to 5 minutes after the end of the exercise bout). During baseline and recovery the subject keeps their legs extended. See Sec. Methods for details on the protocol.

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  Figure 5. Cardiovascular responses during early recovery with and without leg muscle loading for all subjects. Muscle loading resulted in significantly lower pulse BP peaks and heart rate maxima, and did not affect systolic, diastolic and mean arterial pressure. Data are presented as mean ± s.d. across subjects. N = 17 and 16 subjects contributed to the bars HDT-noML and HDT-ML respectively (for all signals). *p < 0.05.

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  Figure 6. Cardiovascular signals before leg-press exercises and during late recovery with and without leg muscle loading for all subjects. At the end of the recovery phase (Rlate), the mean values of all signals returned to values that were similar to those observed before the exercise bouts (baseline, BAS), both with and without leg muscle loading (HDT-ML and HDT-noML respectively). Data are presented as mean ± s.d. across subjects. N = 17, 17, 16 and 16 subjects contributed to the bars BAS (HDT-noML), Rlate (HDT-noML), BAS (HDT-ML), Rlate (HDT-ML) respectively.