Resistance to blood flow in leg muscles of dog during tetanic isometric contraction

Abstract

The effects of tetanic, isometric leg muscle contraction in anesthetized dogs on the resistance to blood flow was reinvestigated. In 19 of 20 dogs vascular resistance increased during the contraction. There appear to be 2 phases of resistance change ([DELTA]R) as a function of fraction of maximal tension (Tmax) achieved. The 1st phase, occurring between 0 and 40% Tmax, is so small [DELTA]R[less/equal] 20%) and so variable as to be not statistically significant. The 2nd phase, occurring above 40% Tmax, is large and highly significant. [DELTA]R appears to be independent of changes in vascular tone. Passive tension has much less effect than a similar level of active tension. Intravascular wedge pressure during contraction on a clamped circulation was taken as an index of tissue pressure. The changes were small in almost all experiments and therefore deemed insignificant as a factor causing the resistance change. The data are best explained by geometrical changes during contraction causing shearing, twisting, or nipping of interfascicular vessels as Barcroft has suggested. The accompanying paper gives anatomical proof that this is indeed the explanation.