Are leg curls a valid exercise to strengthen the hamstrings?

I am curious about the choice of leg curls to strengthen the hamstrings. Why bother? Isn’t deadlifting enough? Eric Cressey says...

"Strengthening the posterior chain with closed chain movements like deadlifts also reduces the risk of injury. Weak hamstrings are a serious risk factor for anterior cruciate ligament (ACL) injuries, patellofemoral pain, and a host of other problems in the knee, hip, ankle, and lower back. Conversely, leg curls simply won’t get the job done because they don’t require co-contraction of the glutes and hamstrings. They’re open chain and occur in a fixed line of motion. Our body is far smarter than some piece of inflexible equipment."

Deadlifts vs Leg Curls - Which is Better?

Whenever you read anything, you have to evaluate it within the context that it was said. That quote is from an article espousing the deadlift as a superior posterior chain exercise. And it is. Typically, the population that will read that type of content is not interested in reading about general preparation. General preparation is inherently boring, especially when compared to the relative glamour associated with hefting 800 lbs off the floor while blood shoots from your nose. While there are some truths to what he says, the article addresses one part of training, without looking at the whole picture.

Let's start with what is completely accurate. It is true that training the hamstrings with hip extension work and closed chain movements like the deadlift prevents injury... in some cases. It is also accurate that strong hamstrings help to prevent knee injuries. It is furthermore very true that deadlifting is a superior exercise for training the posterior chain. 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.

Anatomy of the Hamstrings

The hamstrings cross two joints, not one, 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 tendinosus insert on the tibia, or medially, while the biceps femoris inserts on the fibula, or laterally. The 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.

Function of the Hamstrings

If you want to strengthen the hamstrings to improve the integrity of the knee, 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. Doing so results in a muscle belly with less weak points. Deadlifting alone does not do this, because the nature of the exercise does not require any flexion of the knee.

The Law of Joint Integrity

Strong Agonists Lead to Strong Antagonists – The Law of Joint Integrity
Ultimately, a stronger hamstring at the level of the knee allows for stronger quadriceps, because of the law of joint integrity, which states that the body will shut down continued strength development on one side of the joint if there is not enough strength on the other side to maintain relative balance. From a standpoint of deadlifting then, leg curls actually make a great deal of sense, because of the secondary indirect contribution to greater quadriceps strength-potential.

The Law of Specificity

The above is an example of the concept of specificity – that form follows function. So, though it is true that a deadlift competitor would never approach the limits of his ability by performing only leg curls, to say that they are ineffective hamstring exercise makes no sense. They are half of the hamstring equation, and address an entirely separate function of the hamstrings from pulls.

General vs Specific Preparation

Finally, there is a continuum in performance training between general preparation, or structural balance - what the French and French Canadians call musculation, or "general strength training that is specific to the purpose of the competition" - and specific preparation, or peaking. General preparation is concerned with exercise that improves the structural integrity of the system, or balance; specific preparation is concerned with taking that new, balanced platform to a higher level of performance. Long term planning demands that attention be paid to both of these aspects of training, or injury or overtraining invariably occurs.

In the example of the deadlifter, then, leg curls could be a useful general preparation exercise. During specific preparation, however, they would not make such sense. The more advanced the lifter becomes, and the closer he gets to the upper level of his weight class, and the closer he is to competition, the less important leg curls become.

If the athlete were a sprinter, however, the same rule would not apply. In running, the hamstrings serve as both active knee flexors and hip extensors. As such, leg curls become much more important.

To understand the concept, think of the sprinter, running down the track. Now take away the posterior muscles that cross the knee (we're talking about the hamstrings) and see what happens when there is no eccentric force to counteract the powerful concentric forces of knee extension from the vastus medialis. Indeed, sprinters at the world class level report less incidence of hamstring pulls when leg curls are added to their strength training routines, so leg curls are actually, depending upon the sport you are involved in, a very appropriate general and specific preparation exercise.