Torque—the capability of a force to produce rotation—is a normal product of the movement, that occurs when a bone rotates about a joint. Torque is produced by the interaction of external loads and muscle activity. Torque can be dangerous to exercisers because it produces “grinding” motion on the joints. To visualize torque during exercise, think about a screw being driven into a board. The screw’s rotation grinds it into the board and is comparable to the rotary effect of force on joints during exercise.
Scientifically, torque is defined as the magnitude of the force multiplied by the moment arm (T = F × moment arm, where T = torque and F = force). The moment arm is the shortest perpendicular distance between the force’s line of action and the axis of rotation (joint). The contact force is the place where the force actually touches the person’s body; for example, if your client holds a dumbbell in his hand, the contact force is at the hand. The contact force originates the force’s line of action.
Using the lateral shoulder raise as an example, the force’s line of action is a parallel line to the spinal cord from the weight in the hand toward the floor. The moment arm is the perpendicular distance between the shoulder (axis of rotation) and the weight in the hand (contact force or force’s line of action). This distance changes from the start position of the exercise (Figure 1) to the middle position (Figure 2). The moment arm is not a straight line (such as a bone) between the joint and the force’s line of action, but a distance that changes during movement.
In terms of exercise, the most important part of the torque equation is force. Force is the concept used to describe the interaction of an object with its surroundings. A force is equal to the mass of an object multiplied by its acceleration (F = M × A, where F = force, M = mass and A = acceleration). Mass is the weight of the object. Acceleration is the rate of change in velocity with respect to time. Accelerating during exercise is the same as accelerating when driving a car. In driving, acceleration occurs between pressing the gas and reaching the desired speed. The concept is the same during fitness training—acceleration occurs between starting a movement and reaching the desired speed of movement.
Both mass and acceleration are important in defining how torque acts on joints during exercise. The bottom line is that the greater the mass of an object or the greater its acceleration, the greater the force and the torque that act on the joints.
To better understand force and torque during exercise, it is critical to grasp Newton’s First Law of Motion: The Law of Inertia. The Law of Inertia states that a force is required to start, alter or stop movement. This is an important concept in exercise, because we repeatedly ask clients to start, change or stop movement. For example, when a client does a leg extension, you ask her to start the exercise, hold it at the top and squeeze the quadriceps, and then return to the start position. The starting, changing or stopping of an exercise produces torque on the joints.
To grasp how body weight influences exercise and increases the torque on joints, the concept of kinetic energy must be understood. Kinetic energy is defined as the capacity of an object to perform work because of its motion. Imagine a ball or other object rolling down a hill. Its kinetic energy will enable it to continue rolling once it reaches a flat surface, owing to the momentum gained on the way down.
All people have a natural tendency to use momentum during exercise. The use of momentum during movement is adaptive and is the body’s way of conserving energy. Overweight people have an even greater reason to use momentum: Their added weight is an advantage that lets them gain more momentum, making the exercise easier. The problem with using momentum is that the muscles do not perform the work—kinetic energy does. When a person uses momentum during an exercise, the joints go through the movement, producing torque without the benefit of training the muscles.
In the previous example of the ball, a force is required to stop its motion. The heavier the ball, the more momentum it has and the more force is needed to stop it. The same is true with people. The heavier someone is, the more momentum that person can gain during an exercise and the greater the force and torque required to stop the motion. During exercise, whether it’s because of greater mass or more acceleration, plus-size people are at increased risk for joint injury and degeneration, as a result of increased torque.
Joint health is important to everyone, not just overweight persons. Give careful consideration to torque for all your clients, including those in postrehab, the elderly, kids and adolescents, athletes and apparently healthy adults.
Torque is always present and acting on joints during exercise. When you think about torque during exercise, imagine the body interacting with its environment. There are three sets of circumstances during fitness training that show the body interacting with its environment and producing torque on joints.
1. Torque Is Produced When Two Human Bodies Interact. A common example of two bodies interacting during fitness training is when a trainer applies a contact force on a client. Contact forces from trainers are essential during assisted stretching (see Figure 3). In this assisted hamstring stretch, the trainer is shown applying a contact force 1–11/2 inches below the client’s ankle, parallel to the floor. Contact forces from trainers also occur during assisted lifting. One example is when a trainer helps a client perform a seated dumbbell shoulder press; the trainer places his hands under the client’s arms inside from the elbows. Another common contact force is seen during manually resisted exercises. For example, a trainer will place her hands on a client’s hip abductor and provide resistance. Contact forces are also produced when two persons exert equal but opposite force on one another. They may push—like two linemen on a football field—or pull—as in partner-assisted stretching.
2. Torque Is Produced When a Human Body Interacts With an External “Object.” Examples of such objects include free weights, machines, bands, benches, a wall, the floor or any other training tool that makes contact with the person. The contact force is the point at which the training tool touches the body. Examples of contact forces during exercise are a free weight in the client’s hand during a biceps curl and the leg pad against the back of the lower leg during a leg curl.
3. Torque Is Produced When a Bone Rotates About a Joint. Examples of torque from movement alone are seen when a person uses his own body weight and/or gravity as training tools. For example, during a push-up, torque is produced on the elbow.
To minimize torque during fitness training, ask three questions before you begin.
1. Which joint(s) are involved in the exercise?
2. Where is the contact force during the exercise?
3. How much force acts on the joint(s) during the exercise?
The first two questions are easily answered. The third question poses a bit more of a challenge. Rather than attempt to calculate torque on a joint, it is more practical to take steps to minimize the torque.
Walk a Mile in a Biomechanist’s Shoes. The first step to minimizing torque during exercise is to use correct exercise mechanics. Biomechanics is the use of engineering mechanics to study biological systems—in this case, the human body. A biomechanist sees and defines movement in lines; when watching a client move, she sees the lines of the bones that rotate about the joints and the forces that act on those joints as a result.
Using a biomechanical approach can be helpful when working with overweight clients. It can enable you to look past the flesh and envision the bones and joints during exercise. Because everyone’s skeleton is similar—except for variations in bone length and minor variations in joint architecture—exercise mechanics are similar for most clients. The skeletal system can move in only so many different ways, regardless of body mass from muscle and/or fat.
Emphasize Correct Setup. Starting in the correct position decreases the likelihood of using dangerous mechanics during an exercise. Correct start mechanics for any exercise include neutral spine and joint alignment, plus square shoulders, hips, knees and ankles.
Stress Excellent Mechanics. Make certain your client maintains correct mechanics during exercise relative to the changes in posture the exercise requires. The lunge is an example of an exercise that is often executed incorrectly. It is common to see clients start the lunge in the correct start position, but then rotate their knees inward or outward during the step phase, producing an unnecessary torque on the joints.
Control Speed and Watch for Momentum. Control the speed of exercise movement and prevent the use of momentum. Using a counting system is helpful. For example, use a count of two for the exertion phase of the exercise and another count of two for the return-to-start phase.
Decrease Range of Motion (ROM). Another way to control for mechanics, speed and momentum is to decrease the ROM of an exercise. Get new clients acclimated to exercise or introduce new movements by using a small ROM. An exercise with smaller ROM decreases the chance for mechanics errors and dangerous torque on the joints. Once a client has the correct exercise mechanics, gradually increase ROM.
Decrease Impact Forces From Body Weight. Change the mode of exercise from the treadmill to the bike, or have your client walk instead of jog. A common error in program design is to start an overweight person’s exercise program with walking. Each step places a direct torque greater than his body weight on the ankle joints (DeVita & Hortobagyi 2003). Start an overweight client’s exercise program with time on the bike or elliptical trainer. Walking and/or jogging can be incorporated later in the program, as long as the client uses correct walking and/or jogging mechanics and reports no joint pain or discomfort.
Added weight from free weights and machines increases the total mass of the person exercising and increases the torque on joints. Loading an exercise with weight is safe as long as your client uses correct mechanics and does not use momentum. However, limit the use of weight training exercises that produce a force acting straight down on the spine. For example, limit squats with a bar on the upper spine to just once a week and vary exercises to work the leg muscles without forces that act directly on the spine.
Limit the Use of Repetitive Movements. Repetition increases the risk that your client will use momentum. Also, repetition produces torque on the same joint in the same direction repeatedly. To reduce torque on a specific joint, use a variety of exercises to work the same muscle.
Shorten the Moment Arm. Torque is the product of mass multiplied by moment arm. To reduce the torque on joints, shorten the moment arm. This can be understood by studying the leg extension (Figure 4). Many clients report knee discomfort during this exercise. In this example, the axis of rotation is the knee and the contact force is the pad above the ankle. The moment arm is the distance between the knee and the pad above the ankle during the various positions of the exercise. To decrease the pressure on the knee, shorten the leg piece so the pad is closer to the knee.
Additional Strategies. Other general rules to reduce torque during exercise are not to push down on a client during assisted stretching or during an assisted lift, and not to apply a contact force directly to a joint. Always place hands on soft tissues. Use of these training techniques decreases the direct application of a force on your client and reduces torque on joints.
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