Last month, I risked heresy by suggesting that the King of all exercises (the barbell squat) takes a backseat to a single-joint movement (leg extensions) if you're trying to isolate and strengthen the muscles of the front thigh (quadriceps). Squats also target the muscles of the rear thigh.
Three muscles comprise the muscle group that resides on the rear portion of the thigh. Collectively called the hamstrings, the semimembranosus, semitendinosus and biceps femoris cross both the hip and knee, and are responsible for creating motion at both of these joints. The hamstrings are active during squats because the femur (thigh bone) is drawn back to a neutral position (straight at the hip) from flexion (bent to the front). The gluteus maximus also contributes to this motion (hip extension), so even if you eschew compound movements like squats and opt for single-joint hip extension, your hamstrings will not be the sole recipient of the training effect.
Hamstrings can also be worked by isolating their function at the knee. In this case, they develop tension and shorten to draw the lower leg toward the femur against opposition. Therefore, if we start with resistance that pushes the knee into a straightened position and pull the lower leg up to overcome that opposition, a concentric (shortening) muscle action by the hamstrings will have taken place. Exercises that present this challenge are called leg curls.
The classic example of a leg curl machine is one where you lay face-down with your knees straight and legs positioned so that the pads attached to the machine's movement arm are resting on the backs of your lower legs. Many of these machines are angled so that your thighs are flexed at the hips when you assume this position. This position places your hamstrings on stretch to allow for greater muscle activation as you curl the weight up and also helps to defeat the temptation for you to lift your hips to achieve this facilitation. But regardless of whether your thighs are neutral or flexed, your job when performing leg curls is the same: You should curl the weight up with motion occurring at your knees exclusively.
Leg curls also engage another muscle group. In addition to the ankle, one of the muscles of the calf (the gastrocnemius) also crosses and acts at the knee, assisting the hamstrings with flexion. If your goal is to minimize this involvement when performing leg curls, you should avoid dorsiflexing your feet at the ankles (drawing the tops of your feet toward your shin) as you curl the weight up. This action places the gastrocnemius on stretch and facilitates its involvement. However, calves are typically stronger than hamstrings, so even if you allow them to maximally contribute by moving your feet in this manner, chances are your hamstrings will still derive the training effect because they will be the weaker muscular link that will break first.
Lying leg curls are the most common version of this exercise, but there are a number of other alternatives that are also available. One involves the exerciser maintaining a standing posture while performing the movement. Obviously, these can only be done unilaterally (one leg at a time). In recent years, seated leg curl machines have also become quite popular. One advantage these provide is an ability to train past momentary muscular failure (perform forced reps) without partner assistance by leaning forward, placing your hands on your ankles and pushing down to assist your hamstrings in flexing your knees past the sticking point. They also provide a more comfortable option for individuals with lower back problems and those who are not suited to lying face down.
Mention hamstrings and the first thing many people think of is injury. It is safe to say anyone who has pulled one won't soon forget the excruciating pain that followed. And just about every sports fan has experienced the agony of waiting for a key player's slow-to-heal tear to mend. These injuries are not random occurrences, however, so an ounce of prevention can definitely be worth a pound of cure in this case.
Hamstring strains often occur because the balance between the tension-generating capacity of the quadriceps and hamstrings is skewed toward quads. So, to avoid this injury, strengthening the hamstrings is critical, but this enhancement must be done without running the risk of making the quads more capable in the process. Compound movements like squats work both muscle groups, so employing these maneuvers to prevent this injury runs the risk of exacerbating the imbalance that causes it. On the other hand, single-joint leg curls can be used to ensure a hamstrings-exclusive training effect.
Tight hamstrings are also problematic because they are more susceptible to tears. In addition, they can wreak havoc on pelvic alignment, thereby placing undue stress on the lower back. Consequently, hamstring stretching should be incorporated along with strengthening exercises like leg curls in order to maintain adequate range of motion in this muscle group.
Fred Dimenna, a Certified Strength and Conditioning Specialist and lifestyle and Weight Management Consultant is a two-time Natural Mr. United States and a WNBF drug-free professional bodybuilder. Visit him at www.freddimenna.com or email him: firstname.lastname@example.org