Utilizing Lee's Training Philosophy Motivation provides the energy needed until success is achieved. Success does not have to mean finishing first, but satisfaction that you've done your best and stayed focused toward your personal goals. It is permissible to miss a workout if you are physically under the weather. However, if you have had a long day, just cut back on the intensity and number of sets. For example, instead of doing four sets per exercise, only do two.
Muscle responds to resistance and stimulus.
It doesn't know the difference between 100 lb. and 400 lb. So train the muscle by using proper form and technique. And keep the ego in check. In order to stimulate muscle mass, basic, explosive training of the muscle groups must be performed. Be mindful that it doesn't take a ton of weight to stimulate a muscle to grow, just the correct stimulus.