With age, muscles can become weaker. this is due to
a. connections between nerves, which cause more pain and therefore less use
b. damage from everyday use and injuries that accumulate over time
c. disconnections between motor nerves and muscle fibers
d. difficulties that occur during muscle growth and repair

The answer is letter C.

As we age the connection of our motor nerves to our muscle fibers tend to deteriorate. When this happens we tend to lose our balance and coordination as well as our overall muscular strength and endurance through time. This explains why we cannot perform some activities that we do when we were younger when we turn into adults. The brain is already having a hard time to recruit the necessary stimulus to be sent to the muscle fibers to perform necessary actions. This is why the human body is always compared to a ticking clock where any moment it could just give up because of its deteriorating nature. 

Subtract your 2-minute  heart rate  from the  heart rate you took immediately after exercising. the faster your heart rate  recovers the fitter and healthier your  heart.