Resistive vibration exercise was of no significant influence on the response in popliteal vein diameter, capacitance or compliance to 52 days of bed rest. Although the short-term response (pre-day 25) of capacitance in the exercise group was different from the control group (two-way ANOVA,
P = 0.01), resistive vibration exercise had no significant effect on the long-term response (pre-day 52, 2-way ANOVA,
P = 0.09). This might indicate a difference in time-course of capacitance response to bed rest between the control and exercise group. We hypothesize that resistive vibration exercise might prevent the rapid decline in capacitance in response to bed rest, as observed in the control group. However, in the long-term, this exercise is not sufficient to effectively counteract the decline in capacitance. As only one additional test was performed between the pre- and post-bed rest measurement, we can only speculate about a possible difference in time-course in adaptations of venous capacitance regarding the use of exercise. Considering the total period of bed rest, our data do not support a protective function of resistive vibration exercise in the long term. A recent study focusing on several other venous parameters is also lacking proof for the long-term countermeasure effect of resistive exercise during 90 days of bed rest inactivity (Belin de Chantemele et al.
2004). In both studies, the absence of exercise effect on vein response to bed rest might be ascribed to the lack of muscle mass directly surrounding the vein. On the contrary, resistive vibration exercise was previously shown to be an effective countermeasure against deterioration of the arterial dimension (Bleeker et al.
2005b), muscle mass and muscle function (Akima et al.
2003; Mulder et al.
2006), and tendon function (Kubo et al.
2004). Resistive vibration exercise is also known to increase heart rate and oxygen uptake (Rittweger et al.
2000) and blood flow (Kerschan-Schindl et al.
2001). When applying the exact same stimulus to the arterial and venous system, i.e. resistive vibration exercise, only the arterial system responds in a counteractive and protective manner. This finding corresponds with our previous suggestion that the arterial and venous vascular system require different physical triggers for adaptations to changing conditions [i.e. blood flow or (peak) shear stress versus pressure or blood volume, respectively]. However, the range of vibration frequency of the exercise has to be taken into account. A recent study reported that plantar-based vibration in the regime of 30–60 Hz significantly inhibited the effects of the orthostatic stress of quiet sitting on the cardiovascular system of adult women (Madhavan et al.
2006). Thus, applying resistive vibration exercise with a higher frequency may have led to alternative results.
Due to the relatively small group sample size in this study, statements must be made carefully. Combining the present results of a decreased capacitance and the absence of a resistive vibration exercise effect after 52 days of bed rest, we propose that the bed rest inactivity itself is not the main stimulus for adaptations of the venous vascular system. There ought to be an alternative or additional specific factor during bed rest deconditioning which is responsible for venous changes, e.g. microgravity.
In conclusion, this study demonstrates that 52 days of bed rest deconditioning results in a significantly decreased popliteal vein capacitance, while popliteal vein diameter and compliance did not change. Furthermore, the use of resistive vibration exercise (19–25 Hz) during bed rest deconditioning has no influence, and therefore no protective effect on long-term bed rest responses in venous vascular dimension and function. During bed rest deconditioning, adaptations in the venous vascular system are triggered by different stimuli than in the arterial vascular system.