Muscle contraction factors that affect

Stimulus trains of varying lengths (0.5-1.2 s) and frequencies of 10-200 pps can provide additional information. As stimulus frequency is increased, there is greater tetanic fusion and higher peak forces are produced. A force-frequency curve (F-F) can also be generated to evaluate factors that affect muscle contractile speed (time-to-peak, half-relaxation). If, for example, disease processes lead to slower contraction/relaxation time, more tetanization occurs at lower frequencies and the shape of the F-F curve will be shifted compared to normal muscle. 

In the most recent work, Dawson et al (1978) have extended their muscle NMR studies to investigate the biochemical factors affecting muscular fatigue. The substances most directly involved in the transduction of chemical free energy into mechanical work in contracting muscle are ATP, ADP, Pi, H, and Mg all of these substances actually take part in the actomyosin-ATPase reactions which produce contraction. PCr is also involved in normal contractions of vertebrate muscles because the ATP that is hydrolyzed is rapidly rephosphorylated at the expense of PCr by the enzyme creatine phosphotransferase. Since P NMR can monitor all of these substances simultaneously, either directly or indirectly, it becomes possible to relate changes in them to concurrent changes in the mechanical performance of muscles. 

Physical factors include mechanical stresses and temperature. In normal tissues, convection is driven by the pressure difference between blood and lymph vessels. In muscles, convection can also occur due to tissue contraction/relaxation. In solid tumors, there are no functional lymphatics and the IFP is elevated uniformly in the center. Thus, interstitial convection is negligible. In addition to the fluid pressure, drug transport may depend on solid stresses that can be elevated in regions with rapidly proliferating tumor cells, "" causing collapse of tumor microvessels and an increase in vascular resistance to convection in the blood. Temperature change can directly affect the diffusion coefficient and the hydraulic conductivity since both parameters are inversely proportional to the viscosity of interstitial fluid that decreases with increasing temperature. The temperature in tissues is stable and close to 37°C under normal conditions... 

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