Help clients optimize strength training and avoid injury by discouraging the use of momentum.
One of the most common mistakes exercisers make during strength training is to use momentum. For everyday movements, the use of momentum is normal and adaptive. It is the body’s way of conserving energy, particularly during running, throwing or pushing activities. But during strength training, momentum is counterproductive because it decreases the work a muscle does, thereby decreasing the effectiveness of the exercise. What’s worse, it is dangerous to the joints and spinal cord, since it overloads these areas, causing unnecessary “wear and tear.”
To understand what the use of momentum looks like during strength training, picture a string with a ball suspended from it. The muscle joint is the point of rotation, or the top of the string; and the leg, arm or torso (with or without weight) is the ball at the end of the string. Now picture the ball swinging back and forth at the end of the string. When momentum is used during strength training, the exercise is characterized by this same swinging motion; the exerciser may even appear to be bouncing or swaying.
One exercise in which momentum is frequently employed—and easy to see— is the back extension. When momentum is used, this exercise looks like a “drop” of the torso (due to gravity) followed by a “swing up” that is stopped only by the mechanical limits of the spine. This technique fails to work the low-back muscles—and places a dangerous load on the spine.
To understand the way momentum works, it’s important to grasp the concept of Newton’s Second Law of Motion, the Law of Acceleration, which states that a net force on an object will accelerate it, or change its velocity, in proportion to the magnitude of the force and in the same direction as the force. Simply stated, the rate of change in the momentum of a body is proportional to the applied force and takes place in the direction in which the force acts (Enoka 1994). During strength training, momentum is additive—it gains strength with each repetition. The exercise becomes faster, the muscle works less, and the joints and spine work more because the exerciser is using momentum rather than forcing the muscle to do the work.
Momentum is defined by two things: (1) mass, or size (of the exerciser and the weight); and (2) velocity, or speed. The larger the exercisers are, the more weight they lift, the faster they move and/or the fewer constraints that are placed on their movements during exercise—the greater are the risks caused by the use of momentum.
To help your clients make the most of strength training exercise and avoid unnecessary risks, you should know how to stop the use of momentum before it gets started.
Applying the following four principles during strength training will counteract the effects of momentum.
1. The Position Principle. Make certain that the client is in the correct spine and joint position before starting an exercise. The risk of poor mechanics is lower when the client is in the correct start position.
2. The Discrete Movement Principle. A discrete movement is one that has definable start, middle and end positions. (The end position is always the same as the start position.) Identify the start, middle and end positions for each exercise and treat each repetition as a discrete movement. This decreases the likelihood of swinging through one repetition and bouncing into the next.
3. The Peak Contraction Principle. A peak contraction is an isometric muscle contraction performed at the midpoint of a repetition. Employing the peak contraction principle forces the exerciser to stop during a repetition—thereby reducing the use of momentum—and recruit additional motor units to increase force generation.
4. The Velocity of Contraction Principle. Velocity of contraction is the speed at which a muscle contracts and performs a movement. Controlling the speed of a repetition will stop the use of momentum and force the muscles to do the work, since slow movements cannot build momentum like fast movements. Another advantage is that doing slow, controlled movements encourages clients to think about what they are doing and which muscles they are working.
Employ verbal cues throughout each repetition to remind clients to avoid the use of momentum. Here are some suggestions:
Start to Mid Position. Count (“1-one-thousand, 2-one-thousand”) to control the rate of contraction.
Mid Position. Direct your client to stop and “squeeze” (or “hold”), to incorporate the use of the discrete movement and peak contraction principles. During the squeeze or hold, continue to count.
Mid Position to End Position. Count to control the rate of contraction back to the start. Instruct the client to stop at the end of each rep to take advantage of the discrete movement principle.
See “Suggested Cues for Specific Exercises” on page 39.
By using the principles discussed above and employing cuing to put the principles into action, you can help your clients avoid the use of momentum, increase their muscle work, improve their results and prevent injury.