In five healthy males sustained isometric torques during elbow flexion, knee extension, and plantar flexion correlated positively with intramuscular tissue pressure (MTP) in the range 0-80% of the maximal voluntary contraction (MVC). During passive compression of the muscle at rest 133-Xenon muscle clearance stopped when MTP reached diastolic arterial pressure (DAP) indicating that the muscle vascular bed was occluded. However, during sustained contraction this relation between DAP, flow and MTP was not seen. In two cases 133-Xenon clearance from M. soleus did not stop in spite of an 80% maximal contraction and MTP stayed below DAP. In other cases MTP would reach as high as 240 mm Hg before clearance was zero. In the deeper parts of the muscles MTP during contraction was increased in relation to the more superficial parts. The means values for the % MVC that would stop MBF varied between 50 and 64% MVC for the investigated muscles. Mean rectified EMG (MEMG) showed a high correlation to MTP during sustained exhaustive contractions: When MEMG was kept constant MTP also remained constant while the exerted force decreased; when force was kept constant both MEMG and MTP increased in parallel. This demonstrated that muscle tissue compliance is decreasing during fatigue. Muscle ischemia occurring during sustained isometric contractions is partly due to the developed MTP, where especially the MTP around the veins in the deeper parts of the muscle can be considered of importance. However, ischemia is also affected by muscle fiber texture and anatomical distorsion of tissues.

The question, if muscles can absorb and temporarily store mechanical energy in the form of elastic energy for later re‐use, was studied by having subjects perform maximal vertical jumps on a registering force‐platform. The jumps were performed 1) from a semi‐squatting position, 2) after a natural counter‐movement from a standing position, or 3) in continuation of jumps down from heights of 0.23, 0.40, or 0.69 m. The heights of the jumps were calculated from the registered flight times. The maximum energy level, Eneg, of the jumpers prior to the upward movement in the jump, was considered to be zero in condition 1. In condition 2 it was calculated from the force‐time record of the force‐platform; and in condition 3 it was calculated from the height of the downward jump and the weight of the subject. The maximum energy level after take‐off, Ep0s, was calculated from the height of the jump and the jumper's weight. It was found that the height of the jump and Epos increased with increasing amounts of Eneg, up to a certain level (jumping down from 0.40 m). The gains in Epos over that in condition 1, were expressed as a percentage of Eneg and found to be 22.9 % in condition 2, and 13.2, 10.5, and 3.3 % in the three situations of condition 3. It is suggested that the elastic energy is stored in the active muscles, and it is demonstrated that the muscles of the legs are activated in the downward jumps before contact with the platform is established.

These experiments were designed to investigate the effects of O2 breathing on limb blood flow and metabolism during exercise. Six subjects took part in the study. Four subjects breathed air or 100% O2 while pedaling a Krogh bicycle at 150 W (55-70% of maximal aerobic capacity). Two subjects breathed either 60% or 100% O2 while working at a power output at or slightly in excess of their maximal aerobic capacities. The major findings of the study were 1) leg blood flow is reduced during exercise when comparing hyperoxia with normoxia; 2) VO2 of the exercising limb is not different during hyperoxia; 3) O2 delivery to the leg (the product of blood flow and arteriovenous O2 difference) is not significantly different in the two conditions; and 4) blood pressure is not markedly affected in the experiments at 150 W. Since BP was not different during hyperoxia, at a time when flow was reduced by 11%, this suggests an increased resistance to flow in the exercising limb. In general, these findings are consistent with those reported for the in situ dog muscle but are at variance with results of experiments with humans, especially the reports indicating substantial increases in O2 uptake during hypertoxic conditions.

ASMUSSEN, E. and F. BONDE-PETERSEN. Apparent efficiency and storage of elastic energy in human muscles during exercise. Acta physiol. scand. 1974. 92. 537-545.3 subjects ran on the treadmill (10 km/h) against varying horizontal impeding forces. One subject was further studied during the same kind of walking and bicycling on the treadmill, and during work consisting in lowering and lifting the body by flexing and extending the legs from a standing or sitting position a t varying frequencies, with or without rebound in the deepest position. Workpower (W kcal/min) and the corresponding steady state metabolic rate (E kcal/min, Douglas bag method) were measured. Apparent efficiency ( N ) was calculated as AW/AEx 100 %. During load running N was 53.8, 37.6 and 41.2 %, respectively, in the 3 subjects. In the subject more extensively studied N was: running 53.8, walking 32.3, bicycling 25.1, knee-flexions (deep or half) with rebound 39.4 or 41.0, without rebound 26.1 or 21.9 70. These variations in N 740 were explained in accordance with the possibilities forre-using the energy, absorbed and stored in the muscles as elastic energy during a phase of negative exercise, in a subsequent phase of positive exercise. The condition of this is that the positive phase follows immediately after the negative. A calculation showed that during running 35-53 7% of the energy absorbed during the negative phase was re-used. Corresponding figures for walking and rebounding knee-extensions were 23 ?6 and 34 "lo, respectively, while in bicycling and knee-extensions without rebound all of the negative work degenerated into heat.

The endurance during sustained contraction of elbow, flexors, elbow extensors, and back extensors was tested in 3 human subjects. The force level used was varied between ca. 15 and ca. 75% of maximal isometric strength (IS). The clearance of 133Xe from contracting muscles was registered during and after the endurance test. In this way it was possible to determine whether muscle blood flow (MBF) was increased or had stopped during the contraction. Experiments with artificial ischaemia of the upper arm together with MBF measurements showed that MBF was of no importance for continuing sustained contractions above a certain force level, which was 50,25, and 40% of IS for elbow flexors, elbow extensors and back extensors, respectively. However, the level, where longer lasting ( greater than 15 min) sustained contraction is possible is directly related to MBF. These levels were 22, 15, and 20% IS for elbow flexors, elbow extensors, and back extensors, respectively.