Two types of exercises

I know that exercise is important.
I knew there are two types of exercise, one is to strengthen the muscle, the other is to soften the muscle.
However I didn't know  doing the same exercise, whether using tools or not the effect is different.

For example, while we are sitting on the chair, we can do some exercises.
Sit on a chair, your back not touching the backrest, raise your hands above your head, and lean back, this exercise strengthens your back muscle.
On the other hand when you  lean on the back of the chair applying your upper back or the bottom of your shoulder blade to the backrest, and do the same pose, this exercise relaxes your back bone.

After this exercise, you should do a reverse pose.
Sit  on the chair, legs open wide,  put your head down between your legs, close to the floor and look at your background from under the chair. Your back muscle and thigh muscle are strengthened. On the other hand when you do this pose holding the legs of the chair, your back muscle is relaxed.

Many elderly people have troubles with their knees. So many people try to do squat exercise.
Without holding anything, this exercise strengthens the knee muscle, but it may put an extra load on your back muscle.
On the other hand holding the back of the chair while squatting, this exercise relaxes your back muscle.

I learnt these things from a physical therapist at a manipulative clinic.
I hope you understand my English.