To understand the value of leg curls, you have to understand the dual anatomical function of the hamstrings, the law of specificity, and the difference between general and specific preparation.
The hamstrings cross two joints, and as such, their proper function is dependent upon being strong in both knee flexion and hip extension. At the level of the knee, the semimembranosus and semitendinosus insert on the tibia, or medially, while the biceps femoris inserts on the fibula, or laterally. These muscles work as a team and act as a stirrup for the knee joint, holding it together. This function is called coaptation. The same thing occurs at the hip, but the joint is inherently more stable due to its ball and socket construction, so the coaptational properties of the muscles are not as important at the hip as they are at the knee.
If you want to strengthen the hamstrings to their fullest capacity, you have to perform knee flexion as well as hip extension work. Both knee flexion and hip extension exercise create more functional mass in the hamstrings and reinforce the tendons that cross the respective joints. Training both types of exercise results in a muscle belly with less weak points. Deadlifting alone does not reinforce joint integrity at both knee and hip, because the nature of the exercise does not require any flexion of the knee.
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