Resistance Load Crosses a Perfusion Threshold in Knee Extension Exercise
A new physiology preprint argues that resistance exercise is not a smooth continuum of vascular stress. In this knee extension study, leg blood flow and muscle oxygenation fell once external load reached 40% 1RM and above, while lighter work at 20% and 30% 1RM preserved much higher perfusion. The mechanism is mechanical rather than mysterious: contraction-induced rises in intramuscular pressure squeezed the vasculature enough to blunt convective oxygen delivery.
The result fits a long-standing physiological idea, but the data make the threshold more concrete. At lower loads, the muscle still contracts hard enough to drive blood flow above rest. Once the load rises into the higher band, the expanding force inside the muscle appears to crowd out the vessels that should carry oxygenated blood into the working tissue.
Intramuscular Pressure Rose Stepwise With Load
Twelve participants completed knee extension bouts across a range of loads while investigators tracked vastus lateralis intramuscular pressure, femoral blood flow by Doppler, blood pressure, muscle oxygenation by near-infrared spectroscopy, and electromyography. Mean intramuscular pressure increased in parallel with the relative load: 19.1 mmHg at 20% 1RM, 22.4 mmHg at 30% 1RM, 27.0 mmHg at 40% 1RM, 31.5 mmHg at 50% 1RM, and 41.14 mmHg at 60% 1RM. The progression is important because it ties the external weight directly to a local compressive force inside the muscle belly.
One sentence captures the physiology bluntly: higher load meant higher pressure, and higher pressure meant less room for blood to move.
That sequence explains why the investigators focused on the concept of the critical occluding tension, or COT. For knee extension, the COT has been estimated around 30% to 40% 1RM, the range where contraction begins to compromise intramuscular vessels. This preprint places the observed threshold squarely in that zone and shows the vascular consequence in the same bout rather than inferring it indirectly from force production alone.
Blood Flow Stayed High Below 30% 1RM, Then Flattened
Leg blood flow told the clearest story. Relative to rest, flow rose to 2.6-fold at 20% 1RM and 1.8-fold at 30% 1RM. Once load reached 40% 1RM, flow was only 1.4-fold of rest, and the values at 50% and 60% 1RM hovered around 1.1-fold of rest. The authors grouped the data into a lower-load band of 20% to 30% 1RM and a higher-load band of 40% to 60% 1RM, and the divergence between those groupings was statistically significant.
The implication reaches beyond a single exercise model. Skeletal muscle needs both force and oxygen, and the vascular system must keep pace with the metabolic demand imposed by contraction. When the muscle is asked to work against heavier resistance, the internal squeeze can cancel out the very rise in perfusion that exercise normally provokes. In practical terms, the muscle may be working harder while receiving proportionally less oxygenated blood.
Muscle Oxygenation Dropped Once Compression Outran Delivery
Muscle oxygenation tracked with the blood flow findings. Oxygenation was lower at 40%, 50%, and 60% 1RM than at 20% and 30% 1RM. Near-infrared spectroscopy does not measure the same thing as direct tissue oxygen tension, but it offers a useful window into the balance between supply and extraction during exercise. Here, the signal was consistent with a supply bottleneck rather than a metabolic artifact.
That pattern matters because oxygenation is the final common pathway linking vascular delivery to muscle performance. Even if motor units remain active and blood pressure climbs, perfusion can still fail locally when intramuscular pressure compresses the vessels. The muscle then relies more heavily on anaerobic pathways and intramuscular reserves, a shift that may help explain why heavier resistance produces a different physiological environment from lighter repetitions performed to similar subjective effort.
Why the Threshold Matters for Resistance Exercise Physiology
This preprint adds a mechanical explanation to a familiar training observation: not all resistance loads challenge the muscle in the same way. Below 30% 1RM, exercise increased blood flow well above rest, suggesting that vascular delivery remained open enough to support the working tissue. At 40% 1RM and above, the pressure generated by contraction was sufficient to impair perfusion and limit the rise in blood flow.
For exercise physiologists, that is a direct test of the COT concept. For clinicians and rehabilitation specialists, it offers a reminder that external load is only part of the story. The same movement pattern can create very different intramuscular environments depending on the weight on the bar or machine. A heavier set may compress blood vessels enough to reduce oxygen delivery even when the muscle is still fully engaged.
The study does not answer every practical question. Twelve participants is a small sample, and the findings come from a preprint that has not yet undergone peer review. The work also focuses on one exercise, one muscle group, and a controlled laboratory setting, so the same threshold may not map cleanly onto other movements, patient groups, or training conditions. Even so, the core signal is hard to ignore: once knee extension load reached 40% 1RM, rising intramuscular pressure began to outrun the circulation that exercise depends on.