Positive and Negative Muscular Work.

Abstract

Summary.: It was the purpose of the present experiments to determine the ratio cost of positive work: cost of megative work under different conditions of work. The cost of the work was expressed as the oxygen uptake per minute in the steady state of work. The work itself consisted in riding a bicycle on the inclined belt of a motordriven treadmill, uphill for positive work, downhill for negative work. The rate of work was expressed in kg/m per minute and defined as the product of the weight of subject plus bicycle and the vertical distance of lifting or lowering this weight per minute. The oxygen uptake per minute (y) plotted against the rate of work (x) was found to lie on two straight lines for the two kinds of work, and the equations for these two lines could be expressed as y = b pos x + a pos for positive work and y = h neg + a neg for negative work. Under the same conditions of speed, saddle height and pedal length a pos and a neg were found to be identical and are assumed to represent the cost of extra work (balancing, postural efforts etc.) plus the resting metabolism. b pos: b neg should consequently represent the ratio cost of positive work: cost of negative work. The ratio b pos: b neg was found to increase from 3.1 ± 0.7 to 6.1 ±w 0.7 when the distance saddle to crank increased from 60 to 90 cm, and the pedal length and velocity of movement were constant. Variation of the pedal length between 8.5 cm and 25.5 cm with constant velocity of muscular movements and saddle height had no statistically significant effect on the ratio, which under these conditions was found to lie between 7.0 and 9.3. With increasing speed of movements the ratio increased from 5.9 to 125, mainly because b neg decreased and approached zero. The results are discussed under consideration of the length‐tension diagrams and the force‐velocity curve of isolated muscle. It is concluded that besides the effects that could be expected from the form of the length‐tension diagrams and from the force‐velocity curve another factor must be responsible for the very low values of b neg and the high value of b pos: br neg. It is tentatively suggested that this factor is the reversal of the chemical processes in excentric muscle contractions previously found by Abbott, Aubert and Hill (1951). The author wishes to express his sincere thanks to Mr. P. Rosenfalck, M. Sc., and to Dr. Steen Knudsen, both of the Neurophysiological Institute, Copenhagen, for their kind help with the statistics in this work and for many suggestive discussions during its progress. Dr. Steen Knudsen designed the filter used as an integrator of the electromyograms in fig. 5.