Oxygen Availability and Motor Unit Activity in Humans
Authors: Toshio Moritani, W. Michael Sherman, Masashi Shibata, Tamaki Matsumoto, Minoru Shinohara
DOI / Source: https://doi.org/10.1007/bf00843767
Date: November 1992
Reading level: Intermediate
Why This Matters for Freedivers
Even when the effort feels “easy,” low oxygen delivery to working muscles can force your nervous system to recruit bigger, less efficient motor units and ramp up firing rates to keep the same output. For freedivers, this helps explain why technique can suddenly get sloppy, legs can “burn,” or movement can feel harder late in a dive or late in a set: it’s not just willpower—your muscle oxygen supply is changing how your body is allowed to produce force.
Synopsis
This study asked a simple question: if you keep the force output the same, does reduced oxygen availability change how your nervous system drives the muscle?
Six men performed a standardized handgrip protocol: intermittent isometric contractions at 20% of maximal grip strength, using a 2-second squeeze + 2-second rest rhythm for 4 minutes. They did this in two conditions: 1) Normal blood flow (control). 2) Temporary arterial occlusion (experimental), where a cuff on the upper arm was inflated to 200 mmHg between minute 1 and minute 2, then released so blood flow returned.
While they squeezed, the researchers recorded: - Intramuscular motor unit spikes (what individual motor units are doing), - Surface EMG (including RMS amplitude and mean power frequency), - Venous blood lactate (from a catheter), - And the force-time integral (to confirm the actual work output stayed consistent).
In the normal blood-flow condition, the electrical signals stayed steady: motor unit spike amplitude and firing frequency stayed roughly constant, and the surface EMG showed no clear fatigue pattern. In other words, at 20% MVC with normal circulation, the muscle could keep doing the task without needing “extra help” from the nervous system.
But when blood flow was cut off, the pattern changed quickly. During the occlusion period, the researchers observed increases in motor unit spike amplitude and firing frequency, consistent with recruiting additional (higher-threshold) motor units and increasing the discharge rate of those units to maintain force. The surface EMG also shifted in a classic fatigue-like direction: RMS amplitude increased (more overall neural drive) and mean power frequency decreased (a signal change often seen when the muscle’s metabolic state worsens). Interestingly, the force-time integral stayed constant, meaning the subject was still producing the same external output even though the nervous system had to work harder to make it happen.
Blood lactate didn’t spike during the actual occlusion (likely because washout was limited), but it rose significantly after the cuff was released, consistent with lactate built up in the muscle being carried into the blood once circulation returned.
The main takeaway is important: oxygen availability influences motor unit control. Even at low force levels, when oxygen delivery drops, your body may compensate by changing recruitment and rate coding to preserve performance—until it can’t. This provides a clean physiological reason why low-oxygen states can make movements feel “more expensive,” even if the workload looks unchanged from the outside.
Abstract
Six men were studied to determine the interrelationships among blood supply, motor unit activity, and lactate concentration during intermittent isometric handgrip contractions. Subjects performed repeated contractions at 20% of maximal voluntary contraction for 2 seconds followed by 2 seconds of rest for 4 minutes, either with unhindered circulation or with arterial occlusion applied between the first and second minute. With unhindered circulation, mean motor unit spike amplitude and firing frequency, as well as surface EMG measures (mean power frequency and root mean square amplitude), remained essentially constant, showing no electrophysiological signs of fatigue. With arterial occlusion, motor unit spike amplitude and firing frequency increased significantly, accompanied by changes in surface EMG parameters and increases in venous lactate concentration after release of the occlusion, while force-time integrals remained constant. These findings suggest that the metabolic state of active muscle plays an important role in regulating motor unit recruitment and rate coding during exercise.