The Oxygen Conforming Response & Skeletal Muscle Hypertrophy
This picture shows an athlete’s response to 10 reps x4 sets of Back Squats at a challenging load [~2 reps in reserve] captured with a Moxy Monitor.
In each work set [titled 'Load'] you can observe a rapid drop in muscle oxygen saturation down to ~10% as the athlete approaches failure, followed by a re-oxygenation of tissue as they perform an extended rest period [titled 'Recovery'] to ensure sufficient recovery between sets. When force output is maintained during exercise, muscle activation and perception of effort rapidly increase as muscle oxygen saturation decreases and blood flow is impaired. In simple terms, if we're repping out squats with a load that is heavy enough to restrict blood flow (over 30-40% 1RM for most people) we will get more and more muscle activation and a higher perceived exertion as oxygen in the working muscles decrease. This suggests that an ‘oxygen-conforming’ responses exists and is able to modify an individual's perception of effort and muscle fiber force production in the working muscle. So, does this mean that metabolic stress is a trigger for muscle growth? Perhaps, but I think it would be premature to make any conclusions. In the applied research we see that… (1) Shorter rest intervals provide less of a growth stimulus than longer rest intervals when volume is equated between groups performing each intervention. (2) Blood flow restriction [BFR] at the end of a set of curls does not lead to more muscle growth than standard curls, even though there is more metabolic stress. (3) BFR in lieu of exercise does not increase muscle protein synthesis despite reduced muscle oxygenation. For now, it seems that metabolic stress is only anabolic in the presence of muscular contractions/ mechanical tension, which is why it is often called a ‘back door’ path to hypertrophy, but we can’t discredit the fact that metabolic stress is ridiculously hard to quantify and as a result, it’s difficult to make strong conclusions either way. What does appear to be clear though is that a decrease in oxygen saturation (down to ~ <15% on a 0-100% scale) during moderate to heavy resistance training can be a good indication that someone is training within an effective intra-set volume, intensity, and effort range for eliciting hypertrophy. However, it's unclear if the inherent 'metabolic stress' associated with doing so is a trigger for growth, or if it is due to the fact that a high degree of O2 desaturation can lead to increased mechanical tension in the muscle [Local muscle desaturation --> Peripheral fatigue --> Increased motor until recruitment --> Increased mechanical tension]. “The implication of the oxygen-conformer response for exercise where force production must be maintained is that integrated electromyography (iEMG) must increase when muscle oxygenation is reduced.” -Patrick Drouin “The importance of mechanical tension in promoting muscle growth is indisputable" -Brad Schoenfeld