The way you move when you exercise can be defined by Newton's laws of motion. Physical training methods are based on the laws of inertia, acceleration and counterforce. When you are physically active, you either move in a straight line or an angular motion which includes rotation, not forward motion. These movements, how fast you perform them and gravity's effect on your body can all be identified with Newton's laws of motion.
Increase the challenge of your workout by using Newton's first law of motion. This law states that a body remains in motion unless acted upon by an outside force. Reduce inertia's effect on your workouts, since inertia keeps you moving forward, by changing the surface upon which you exercise. Exercise on surfaces that cause your forward motion to slow down so that you have to use more energy to keep moving forward. For example, run on sand instead of pavement or perform aerobic exercise on rubber mats instead of wood flooring. Bicycle through gravel instead of on the sidewalk.
Add intensity to your workout with Newton's second law of motion. This states that the velocity of a body can be changed through the addition of force. Run into the wind to add resistance to your workout. When cycling up a large hill, remain seated to increase the intensity of your ride. Attach a resistance band to your waist, secure it to the wall on the side of a pool and swim against the force placed upon you to increase the intensity of your swimming.
Apply Newton's third law of motion to your strength training workout. This law states that "for every action, there is an equal and opposite reaction." Increase the speed and intensity with which you connect with the ground to elicit a ground reaction force, or the return of an equal amount of force from the ground to your body through your foot; this will cause stronger muscle contractions. Perform jumps from a 12-inch box and land in a squat to strengthen your legs, or perform pushups with a hop at the top of the movement; lift your hands and feet from the floor before landing and repeating the pushup.