Mikrozirkulation und lokale statische Muskelausdauer - Analyse  positionsbedingter Effekte auf die Rohmert´sche Kurve

Heinzl, Lena

Thesis

Open Access

Mikrozirkulation und lokale statische Muskelausdauer - Analyse positionsbedingter Effekte auf die Rohmert´sche Kurve

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Date

2025-11-26

Authors

Heinzl, Lena

Abstract

Background: The Rohmert curve describes local static muscular endurance as the relationship between maximum holding time (HZmax) and the percentage of maximal voluntary contraction (MVC). It is largely considered to be independent of external influencing factors. This study questions that assumption and investigates whether a reduced local hydrostatic pressure—caused by an arm position 27.5 cm above heart level—reduces local static muscular endurance compared to positions at or below heart level. Possible mechanisms include a reduced local arteriovenous pressure difference and facilitated microvascular compression of the vein in the upper compared to the middle and lower positions, both of which could restrict blood flow during isometric contraction. Such an effect, not previously studied, was expected especially in the load range up to 40% MVC, where increased muscular blood flow is still possible despite elevated intramuscular pressure, and particularly in the range below 20% MVC, where no compression-induced relative ischemia is assumed. From 20–40% MVC and above, compression-induced relative ischemia is suspected according to the literature. Additionally, the influence of arm position on post-exercise blood flow was examined. Method: Fourteen healthy men (aged 20–31, BMI 21–28.5) performed bilateral isometric handgrip tests to determine HZmax at 15%, 20%, 30%, 40%, and 70% MVC in three forearm positions: +27.5 cm (above), 0.0 cm (at), and −27.5 cm (below) atrial level (3rd intercostal space, parasternal). HZmax was determined using a handgrip dynamometer in a horizontal, pronated forearm position. Blood flow was measured at the point of maximal forearm circumference using venous occlusion plethysmography. The five load levels and three working positions were randomized. The sequence of left and right side tests varied among participants. Results: Averaged across both forearms, HZmax at 15% MVC was significantly lower in the upper position by 14.4% and 18.1% compared to the middle and lower positions, respectively. At the same time, blood flow was significantly reduced in the upper position (by 21.8% and 28.6%, respectively). At 20% MVC, a similar trend was observed, though without statistical significance. In addition to the hydrostatically reduced perfusion pressure, increased vascular resistance likely contributed to the restricted blood flow in the upper position. During exertion, resistance remained elevated in the upper position (27.16 Pa·s·m⁻³) compared to pre-exercise levels, whereas it decreased by 31% and 48% in the middle and lower positions, respectively. At loads above 20% MVC, position-dependent differences in holding time and blood flow disappeared. This suggests that increasing tissue pressure generally limits blood flow, overriding hydrostatic effects. Post-exercise blood flow analysis showed that during the first minute after exertion, blood flow in the upper position was up to 24% lower than in the middle position. In the lower position, blood flow was up to 32% lower than in the middle position. It took about two minutes for blood flow in the lower position to reach the level of the middle position, whereas it remained persistently reduced in the upper position. Discussion: During static exertion, a hydrostatically disadvantageous working position above heart level significantly reduces maximal blood flow and HZmax at load levels below 20% MVC. Thus, the Rohmert curve does not represent a universally valid graph in this range. Two factors likely limit blood flow: 1. Reduced perfusion pressure: Due to elevation, local hydrostatic vascular pressure drops—more so on the arterial than on the venous side, as venous pressure cannot fall much below 0 mmHg. Consequently, the effective perfusion pressure difference is reduced. 2. Increased local vascular resistance: The decreased venous pressure in the elevated arm position (which already leads to venous collapse at rest) facilitates compression of the veins and postcapillary microcirculation due to increased muscular pressure in the upper position compared to the lower and middle positions. From a load level of around 20–40% MVC onward, blood flow is increasingly limited by intramuscular pressure in all arm positions, which levels out any position-dependent differences. Recovery phase also appears to be affected by arm position: The middle position allowed the most efficient post-exercise blood flow increase. However, since the load and recovery positions did not match, causal conclusions must be drawn with caution.