The strength curve represents the muscular force at different points throughout the range of movement of an exercise. How muscles attach to the bone around joints, and the angles these muscles create relative to joints determines the strength curve.
A simple example of these biomechanics is the biceps curl. The biceps curl has a bell shaped strength curve. At the bottom of the lift, the arm is straight. Here the biceps muscle attaching to the forearm is close to the elbow joint meaning it doesn’t have much fulcrum action to help it. Therefore it requires a lot of force making this portion of the lift difficult. Halfway up, when the arm creates a 90-degree angle, the muscle attaching to the forearm is further out from the elbow joint. This distance creates a larger fulcrum action, and less force is required making this portion of the lift easier. As the arm comes up, the fulcrum action decreases, and more force is needed making the lift difficult again.
For those not familiar with fulcrum action, a practical example is to push open a door. The door hinge is the fulcrum and the closer you push on the door to the hinge, the harder it is. As you push on the door further away from the hinge, the easier it is.
THREE MAIN TYPES
There are three main types of strength curve; bell shaped, ascending, and descending.
Biceps curl and single joint exercises tend to be bell shaped. Squat and bench press are ascending. The upright row is descending.