No because your pushing your muscles foward and basically your gonna hurt yourself cause the muscles are hard and that equals more pain

121 pounds is equals to 55kg . As for 13 year old boy . You can say its healthy , as it can be counted as muscles if you have great body shape or quiet good muscles build . As muscles are heavier then fats .

These muscles are called "skeletal muscles".

Skeletal muscles.

The heart and the lungs, these are known as involuntary muscles.