What influences the gain of muscle mass? : The type and level of training - Recovery and rest :

The age
The type and level of training :
To develop muscle, you have to adapt your training plan and make sure to follow a progressive program targeting strength resistance.

Recovery and rest :
Making new fabrics is work! To allow the body to synthesize new muscle fibers, it must be allowed periods of recovery between training. During sleep, the body secretes growth hormone more abundantly. However, this hormone is necessary for an optimal development of the muscle mass. So make sure you have nights long enough to recover.

* Avoid training more than two consecutive days. Ex: 2 days of muscular training and a day of rest or even better, a day of intense training followed by a day of rest.
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