I know muscles need time to rest from weight training, but why doesn't cardo effect this healing process? For example say you do legs one day and the next day you jump on a treadmill. Does this not effect the leg muscle in healing?
Cardiovascular activity (activities) can effect muscle recovery (dependent on diet circumference,amount of calories burned, etc, etc) ; however, its one physical event among a soup-bowl of "things" that can effect recovery, here are some examples: (this assumes equal ground, and post newbie gains)
1. Diet, being deficient in calories; macro-nutrients/micro-nutrients
2. Inefficient hydration
3. The Volume/Intensity of training (For example, too high of volume/intensity while in a caloric deficit which erodes the ability to recover properly)
4. Age, and physiological/biological genetic potential/capability of the individual
5. Inappropriate rest in accordance with diet and training Volume/Intensity/frequency (even diet in surplus)
While its absolutely true that muscle and/or strength rebuilds/improves away from the gym during time off/rest period (s), it is not the only factor (s) involved.
For example, lets say (hypothetically) that an individual has been training for a while (post newbie gains), and is dieting to lose fat tissue. The ability to recover/rebuild is "less than optimal (sub-par)", even if adequate rest is given away from the gym. The cause (primarily, keeping things equal), is the deficiency in calories/energy, and muscle rebuilding can be caloric/energy expensive process. And, its flat deficient in this hypothetical environment.
Best wishes,
Chillen