Isaac Newton’s law of reaction is a very important one in the fitness world.
The laws of reaction state that every applied action is accompanied by an equal and opposite reaction. A bird flying uses it’s wings to propel itself by pushing the air downwards with each flap, yet when the wings are moving back up, the air is then pushing under the wings, keeping the bird stable.
In fitness, you have to consider the impact a force will have during various movements made. So, let’s say you want to go for a run. That is a ground-reaction force, and you have to select the right flooring in order to prevent injuries. You would choose a treadmill, or running outside on cement. Running on carpet might pose a threat to getting injured. Excessive ground-reaction forces may place someone at a higher risk for getting hurt, or developing overuse injuries.
Now, with this law of reaction comes motive and resistive forces (movements). A MOTIVE force is one that will cause an increase in speed or direction. RESISTIVE forces will hold off the movement of another external force. It makes sense, when you look at the two main words. Motive is movement, Resistive is resistant to change.
If you are working out, and doing an overhead dumbell shoulder press, during the upward movement you are creating a motive force. Muscles are contracting (moving) and what is resisting you would be gravity, hence the resistive force. While bringing the arms back down to complete the movement, the motive force would be gravity (since it’s pulling your arms back down) and the resistive force would be the contracting muscles.
Going along these lines, let’s talk about CLOSED-CHAIN exercises and OPEN-CHAINED exercises. If you are performing a squat, you are doing a closed-chain exercise. CLOSED-CHAIN is where you have an extremity that is locked in place and cannot move, like your feet would be during a squat. These extremities (your feet) are in constant contact with an unmovable surface. Closed-chain is considered a safer choice over OPEN-CHAINED, where the extremity is free to move. So, during a biceps curl, your arm swivels down and then is pulled back up. Your arm is not locked into place, and your hand is free to move down and then back up again.