Each exercise has a strength curve. A strength curve is a model that shows how much force you can produce at the joint angles throughout a range of motion. It assumes that you start the exercise at the beginning of the positive or lifting phase.
Ascending, Descending, and Parabolic Strength Curves
An ascending strength curve feels easier typically as you near full extension. The muscle tension required decreases throughout the range of motion. Exercises such as the bench press and the squat have ascending strength curves.
A descending strength curve feels harder typically as you near full flexion. The muscle tension required increases throughout the range of motion. Exercises such as the row and the leg curl have descending strength curves.
A parabolic, concave, or bell-shaped strength curve feels easier at the endpoints. The muscle tension required increases and then decreases. An exercise such as the arm curl has a parabolic strength curve.
The length-tension relationship establishes that a muscle creates the most tension, the main stimulus for more size and strength, at the midpoints for the ranges of motion at all of the attached joints. The parts that bind together to form a muscle contraction match up best here. At the endpoints, these components either spread too far apart or overlap excessively to allow as much tension.
The length-tension relationship can never change through training. Some studies show selective strengthening along certain portions of the range of motion based on how you perform the exercises. Perhaps it comes through some change in the nervous system but this seems questionable. The exception may come from non-uniform motions where the contribution of the muscles could differ throughout the range of motion, such as on a dead-lift.
Strength curves depend not just on the length-tension relationship but on leverage. A mechanical advantage will make that portion of the range of motion feel easier. This confuses some trainees because they feel stronger in positions that their muscles actually operate weakly.
Focusing on strength curves without understanding the underlying science has led to some poor practices.
Range of Motion
Regardless of how range of motion applies to strength curves, too much of it brings problems. You should never include the extreme endpoints for any exercise. This could harm you and also reduces the stimulus for more strength and size.
Consider the bench press. The involved muscles should work hardest at the point when you possess the worst leverage, or when your elbows are furthest away from your shoulders when viewed horizontally. Tension is greatest here since the resistance’s line of action is perpendicular, or at a 90° angle, to your body’s lever arm. Although you feel weaker at the bottom as you push the barbell at or near your chest, realize that you do not need to drop too deeply in the first place. Descend just low enough to benefit from the pre-stretch. The lockout feels much easier due to leverage but this area fails to stimulate much tension anyway, so you can include this portion but need not focus on it.
Trainees assume they need a full range of motion. This places the joints, especially the shoulders on the bench press, in a vulnerable position. Authorities also design machines incorrectly to make the lockout harder. The more sensible designers try to implement a parabolic strength curve, but this still encourages too much movement, often fails to match our natural strength curve, and carries the other disadvantages associated with machines. Others may add chains and bands onto free weights that make the exercise harder precisely where the least benefit exists.
Our bodies evolved to deal with the demands imposed by gravity. Any alteration intending to make a basic free weight exercise better has no value. They also evolved to use lots of muscle at once with compound movements for heavy or fast actions. The tools we use should reflect this reality.
A sticking point occurs at the low point of the strength curve. The exercise will feel toughest at this point. If you use the right range of motion and speed though, the sticking point occurs precisely where it should, in the middle where the muscles produce the most tension. This also addresses the criticism that free weights can maximally tax only the weakest joint angles since the resistance remains constant. This only occurs if you ignore the suggestions here.
Move neither too fast nor too slow. Too slow will have you spend too much time at weaker lengths and also ignores the pre-stretch. Too fast risks your safety with spikes in force and also reduces the storage and release of elastic energy to overcome a sticking point. Avoiding this tip can shift the sticking point to a poor region throughout the range of motion.
The parabolic or bell-shaped curve, according to the length-tension relationship, would appear as the ideal strength curve. Once again though, this only matters if you use too great a range of motion anyway. Many exercises that possess this curve are bad isolation options.
Partial range of motion movements that do not include the ideal middle length will reduce gains in strength and size. Remember that we do need some range of motion, just not too much.
Ignore Strength Curves
I suggest focusing entirely on the barbell back squat, the barbell bench press, and the single-armed dumbbell row.
Choose a consistent range of motion without the extreme endpoints. Perform the exercise neither too fast nor too slow. As long as you use these free weight, compound exercises and pursue them as described, you will build size and strength effectively and safely.
Avoid manipulating strength curves.