Question

Does the resistance torque change as leg position is changed? Explain, based on the average scores for perceived exertion, which leg position is easiest to perform and which is hardest. Why are there differences in perceived exertion for the different leg positions?

Answer 1

The first step of understanding and calculating torque is identifying the moment arm.

The moment arm (lever arm) of a force system is the perpendicular distance from an axis to the line of action of a force. In other words, moment arm determines the quality of the torque. An important concept to remember is that the moment arm changes with the angle of application of the force (angle of insertion).

Simply stated, torque is the ability of a force to cause rotation on a lever (moment of force).

The more detailed definition of torque is that it is a force applied over a distance (lever arm) that causes rotation about a fulcrum (axis of rotation).

To calculate force you must first draw a detailed free-body diagram of the force system, including the all force components. Then torque can be calculated using on of the following formulas:

• Torque =Lever Arm x Fy (or Force sin())
• Torque = Force (Fm) x Moment Arm
• Torque is what creates biomechanical movement. It is what creates the movement of the lever system (bones). This is important to understand. Being able to maximize the amount of torque a muscle can generate will allow for optimal strengthening of that muscle.
• The greater the torque a muscle can produce, the greater the movement it will produce on the body’s levers. If your goal of treatment is to increase movement, you can manipulate the torque variables to maximize the efficiency of the muscles to move the body part. The barbell biceps curl exercise provides a great example of this. It’s much harder to move the bar when your elbows are fully extended compared to when they’re at 90°.
• This is because of the angle-torque relationship. In this relationship, the greatest amount of torque is always when the force is applied at a 90 degree angle to it’s lever.
• This concept can also be used with the opposite goal in mind. By adjusting the angle of application and moment arm, you can change the force vector components and increase the amount of compressive force. Increasing the compressive force is often the goal when attempting to maximize stability. The rotator cuff demonstrates this type of force vector when the muscles synergistically contract to create a compressive force couple to stabilize the humeral head in the glenoid fossa.