The number of ACL injuries in young athletes is growing; with proper training, a large number of these injuries are preventable. Many professionals agree that one explanation for the increase in ACL injuries is that many young athletes are playing a sport year-round with no time off. Taking time off from competition and following an appropriate strength-training program is paramount for injury prevention.
The primary concern when developing a strength-training program should always be “injury proofing” the athlete. A strength program should first identify the most vulnerable areas of athletes’ bodies, and implement exercises that will help to minimize the chances of injury to that area. The muscles surrounding the knee are often underdeveloped in high school athletes and even professional athletes. Weak muscles combined with improper exercise selection and poor exercise technique are putting young athletes at great risk for injury. The knee is especially vulnerable to injury, but when the appropriate exercises are selected and are performed with correct technique, some otherwise debilitating knee injuries may be prevented.
Anatomy of the Knee
There are four primary ligaments that provide stability at the knee: the anterior cruciate ligament (ACL), the posterior cruciate ligament (PCL), the medial cruciate ligament (MCL), and the lateral cruciate ligament (LCL).
The ACL prevents hyperextension and excessive rotation of the knee joint. The ACL and PCL work synergistically to enable the knee to flex and extend, without movement from side to side. The primary muscles of the knee are the quadriceps, the hamstrings and the gastrocnemius.
The quadriceps is divided into four section, or heads, the vastus medialis oblique (VMO), rectus femoris, vastus intermedius, and vastus lateralis. The four muscles of the quadriceps muscle connect the pelvis (ilium) to the femur. These muscles converge to a shared patellar tendon that passes over the front of the knee and attaches to the kneecap (patella), providing extension of the knee and flexion at the hip.
The hamstring has three heads, the semimembranosus, the semitendinosus, and the biceps femoris. The three heads of the hamstrings connect the pelvis (ischium) to the tibia and fibula (the bones of the lower leg). These muscles provide flexion at the knee, extension at the hip and rotate the leg laterally and medially.
The gastrocnemius and soleus are the large muscles of the lower leg, or calf. The gastrocnemius connects the femur (the thigh bone) to the heel by way of the Achilles tendon and the soleus connects the top of the lower leg bones (tibia and fibula) to the heel, also via the Achilles tendon.
Both the gastrocnemius and soleus muscles contribute to plantar flexion of the ankle (as in raising onto the balls of the feet), which helps in propelling the body forward when jumping, walking or running, but only the gastrocnemius contributes to knee flexion.
Understanding the function and training of each of these muscles is critical when developing a strength-training program for injury prevention. A strength training professional should always be consulted for proper technique and proper program design. Proper technique in complex exercises and understanding proper program design will yield results much faster than outdated methods.
Train the Squat Through a Full Range of Motion
Many coaches often design strength programs that do not properly work muscles through their full range of motion. For example, a football player repeatedly squatting to only parallel (where the femur is parallel to the floor) will overload the middle- and top-end range of motion of the squat, but neglect the bottom range. The result is an athlete who can squat heavy loads, but only through a limited range of motion. It has been established that athletes who squat through a full range of motion have fewer incidents of knee injuries.
Squatting to parallel is a common technique used in powerlifting. However, powerlifters are attempting to lift the heaviest weight they can, but do so in a very short range of motion. Ultimately powerlifters are training for powerlifting, not for football. Using partial ranges of motion do have their place in a strength program, but the muscle imbalances that occur when training exclusively through a partial range of motion will dramatically increase the chance of injury.
There is a misconception that squatting deep, through a full range of motion, i.e. hamstrings covering the calves, is dangerous. However, this is completely false and every legitimate study ever produced on the safety of the squat says quite the contrary: squatting deep may actually provide greater knee stability. As the saying goes, there are no “bad exercises”, only poor execution.
In the April 2012 issue of the Strength and Conditioning Journal, Brad Schoenfeld, MSc, states, “Research does not support the contention that full squats are detrimental to those with healthy knee function. Given that deep squatting confers a number of important benefits, including greater muscle activation and development, improved functional capacity, and better athletic performance, there is little reason to avoid this exercise provided no medical contraindications exist.”
In Olympic weightlifting, up to 25% of an athlete’s annual training volume can revolve around full squats. However, the rate of knee injuries that compromise knee function in Olympic weightlifting is extremely low. This important to note because these athletes “bounce” at the bottom of the “squat” portion of their contested movements. A 1999 study published in the Journal of Athletic Training analyzed the rates of injury over a 6-year period of elite Olympic weightlifters. The researchers found that injuries sustained by elite weightlifters were overuse injuries and not traumatic injuries that compromised joint integrity.
The knee joint is the most stable at full extension, as in standing, and at the bottom of a full squat. It is the least stable at the mid-way point (approximately 90 degrees), which makes it the worst possible place to stop and change the direction of movement under load. In a full squat the glutes and hamstrings absorb the forces imposed on the body, whereas with a half squat the less powerful muscles of the knee absorb a considerable amount of the forces at a point where the ligaments of the knee provide little stability.
Squatting to only parallel requires the movement to be stopped at the point where the four main ligaments of the knee are providing the least amount of support. Respected sport scientist Vladomir Zatsiorsky, PhD, states in his book Kinematics of Human Motion that, “The knee has four main protective ligaments that keeps the femur from displacing on the tibia (the ACL, PCL, MCL and LCL). These four ligaments are most effective 
at their protection during full extension and full flexion. When the knee is at 90 degrees of flexion, these four ligaments are almost completely lax and cannot exert much if any of a protective force at the knee.”
In another paper on the squat, The Biomechanics of Squat Depth, Schoenfeld states that, “Regimented resistance training confers an adaptive response in connective tissue, increasing its strength capacity. A stronger ligament serves to improve tolerance to load, thus further reducing the prospect of injury.”
In addition to strengthening the connective tissue of the knee, performing full squats also thoroughly targets the VMO. The four heads of the quadriceps all “pull” on the patella (kneecap) in slightly different directions. However, the vastus medialis oblique (VMO, or teardrop shaped muscle on the inside of the knee) is the only head of the quadriceps that crosses the knee joint, which makes developing it critical for knee stabilization. Together with the semimembranosus muscle of the hamstrings, a thoroughly developed VMO helps to protect the medial (inside) aspect of the knee.
The VMO is usually underdeveloped in football players because coaches and athletes are often overly concerned with squatting heavy loads, at the expense of proper technique. Consequently, the athlete never squats through a full range of motion, which leaves the VMO undeveloped and the knee prone to injury.
The quadriceps primarily consist of fast-twitch Type IIa fibers, also known as “fast twitch oxidative” fibers, which means they’re capable of considerable force production and use oxygen as a primary fuel source, instead of sugar. To properly develop the quadriceps a variety of loading parameters should be used. This could include anywhere from 4-10 sets of 3-20 reps, with tempos that range from 4-10 seconds taken to lower the weight, a pause of 1-2 seconds in the bottom position (a pause at the bottom does not mean relax at the bottom position!), 1-10 seconds to raise the weight and 1-2 seconds between reps. Because the squat is such a demanding exercise, allow enough rest between sets (at least 1 minute). Fewer reps per set requires longer rest periods than more reps per set because the nervous system takes longer than the muscular system to recover. In addition, the heavier loads associated with fewer reps taxes the nervous system more than higher reps.
Train the Hamstrings as Knee Flexors
Many strength programs do not spend enough time on an exercise that can prove to be quite effective for preventing knee injuries: leg curls. As a result many high school athletes have horrible hamstring development and this not only increases the risk of injury on the field, but also decreases performance.
The hamstrings are the muscles of speed and acceleration, just look at the hamstring development of any elite level short distance sprinter. The hamstrings are both knee flexors and hip extensors, which are primary movement patterns in running.
The hamstrings are key decelerators when sprinting, which means when they are properly developed an athlete can stop and change direction much quicker and much more efficiently.
While training the hamstrings as hip extensors is common practice, some strength coaches believe athletes performing leg curl variations are a waste of time because working the hamstrings as knee flexors in open chain exercises, such as leg curls, do not mimic any movement done on the field. This, however, is simply not true.
In a 2005 study, Hamstring Muscle Complex: An Imaging Review, The Journal of Continuing Medical Education in Radiology states that because the hamstrings function as both knee flexors and hip extensors their contraction cannot be localized to only one joint. Therefore, one joint must be stabilized in order to effectively act on the other. In order for the hamstrings to function properly they must be effectively developed in both functions.
The hamstrings are a dynamic stabilizer of the knee, working with the ACL to reduce shearing forces. If the hamstrings are weak as knee flexors the ACL is then forced to absorb a disproportionate amount of the shearing forces on the knee. If the hamstrings are not properly developed and the shearing forces on the knee are too high, the result is usually a torn ACL. Leg curls are a great exercise for developing the hamstrings to assist in the reduction of shearing forces on the knee.
The most versatile leg curl machine for developing the hamstrings as knee flexors is a lying leg curl. Many of the other variations, such as a standing or seated leg curl, can only be used in that specific function. The lying leg curl allows more variation because of the different permutations available: single-leg work, two legs to raise the weight and one leg to lower the weight, both legs working together, etc. If a leg curl machine is not available the glute-ham raise (see right) may be used in place of or in addition to leg curl variations.
Turning the toes a certain way can also change the recruitment pattern of the hamstrings. Performing a lying leg curl with your toes facing straight ahead places more emphasis on the semitendinosus, the middle of the hamstring muscles. Turning the toes out places more emphasis on the biceps femoris, the outermost (lateral) hamstring and turning the toes in emphasizes the innermost (medial) hamstring, the semimembranosus.
Along with the VMO, strengthening the semimembranosus is extremely important for protecting the medial aspect of the knee because it attaches to the tibia (one of the lower leg bones) in five different places. Strengthening the hamstrings, especially the semimembranosus, as knee flexors will help provide more knee stability because the hamstrings work together with the quadriceps when decelerating the knee joint. Research has shown that athletes who have adequate hamstring/quadriceps strength ratios (2:3) suffer fewer ACL injuries. Properly raining the hamstrings as knee flexors will provide stronger more stable knees.
The hamstrings (as knee flexors) consist primarily of fast-twitch muscle fibers and respond better to heavier loads for fewer reps (3-8). For leg curl variations, as an athlete gets stronger, performing more than 8 reps generally means that the loads used are too light to elicit a strength response. The hamstrings respond very well to accentuated eccentric training, or “negatives”, when trained as knee flexors. When performing leg curls use a tempo that is forceful, yet controlled, on the way up and take 4-6 seconds to lower the weight.
Train the Calves
Many football training programs utilize some variation of Olympic lifts in their strength training programs because of their ability to increase strength and power. Triple extension (extension at the hip, knee and ankle) is critical for executing these lifts with the proper technique. However, very few programs place any emphasis on developing the calves, which can also increase the risk for injury and decrease performance.
Strong, stable ankles and strong calf muscles also help stabilize the knee joint when decelerating by providing more stability from the ankle.
A muscle that spans a joint will actively move as well as stabilize that joint. The gastrocnemius spans the knee joint, originating on the femur passing over the backside of the knee joint. When properly developed the calves (gastrocnemius) provide structural integrity to the knee joint and together with the hamstrings help to keep the femur from displacing on the tibia.
When the knee is extended during a calf exercise the gastrocnemius is preferentially recruited. The muscle fibers of this muscle are primarily fast-twitch and respond best to heavy loads and 3-4 sets of 8-20 reps. Since the range of motion is rather short when training the calves, use a tempo that calls for lowering the weight for 2-3 seconds, rest for 2-6 seconds in the stretched position, raise the weight for 2-3 seconds and hold in the contracted position for 2-3 seconds. Standing calf raises done on a standing calf raise machine are a very effective exercise. Single-leg standing calf raises performed with a dumbbell are also a very effective exercise to target the gastrocnemius and help develop single-leg strength.
When the knee is bent during a calf exercise the soleus is preferentially recruited. The muscle fibers of this muscle are primarily slow-twitch and respond best to moderately heavy loads and 3-4 sets of 15-25 reps. The tempos for seated calf exercises are similar to the tempos used for the standing calf raises with a few alterations. Use tempos that call for lowering the weight for 2-3 seconds, rest for 2-3 seconds in the stretched position, raise the weight for 2-3 seconds and hold in the contracted position for 2-3 seconds. Seated calf raises done on a seated calf machine are the best exercise to target the soleus.
In closing, athletes, coaches and parents need to gain a better understanding of the importance of strength training for injury prevention. No young athlete should ever have his athletic career cut short because of an injury that could have potentially been avoided. Striving for a gain in strength should never come at the expense of injury prevention. A properly programmed and executed strength program can help keep young athletes healthy and on the playing field.
For more information on ACL injury prevention contact me at cdellasega@darisports.com.
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