What Grateful player describes is normal muscle physiology and trying to train muscle development. This needs to obey the rules of muscle development, through muscle physiology and biochemistry principles. I taught muscle physiology and biochemistry at a medical school for 27 years. I have been able to use this knowledge to apply it to a method of developing a safe and optimal trumpet embouchure, as is outlined below:
When control is lost and repetitively increases in intensity, it is time to put down the horn. Loss of control means fatigue has set into embouchure muscle. Initial fatigue is not damaging but is giving advanced warning that blood flow to muscle groups is compromised. At this point, lactic acid builds. If this continues without rest, the acid environment leads to muscle fiber strain. If this continues then the condition progresses to sprain. This then begins a process of remodeling (disrupting the initial efficient architecture of the motor units connection to on another). That process then progresses to negatively impact the embouchure from that point on disrupting the initial efficient natural architecture, irreversibly.
Carmine himself may be good for playing 40 minutes a day and that may be what he finds is needed to progress, but he is making this recommendation for an individual that has an embouchure that has matured and had the chance to develop and adapt to handle more stressful challenges.
My recommendation would be to start the series but as soon as control is lost to end the session. Note the time it had taken to get to that point. Keep practicing to that point for the next 2 weeks. Then in the 2 weeks that follow, try to increase the session by another 5 minutes. Then stay there again for another 2 weeks. Then add another 5 minutes and repeat the cycle. After a month or so, you can then meet the initial threshold of the 20-40 minute practice range.
