The term “knee extension” refers to the biomechanic wherein the leg forms a straight line, fully extending the foot beneath the knee when seen from a standing position. This is made possible through contraction of certain muscle groups in the upper leg, and is used in almost every movement found in day to day life.
For individuals who wish to reinforce this biomechanic and its related muscles - or those who require rehabilitation of the area - there are several exercises that are capable of training the joint and muscles associated.
The primary knee extension exercises are those that target the quadriceps femoris muscle group, or otherwise place mild pressure on the knee joint as it moves through the knee extension range of motion - with the most common of which being leg extensions and step-ups.
The biomechanic of knee extension is primarily driven by the quadriceps femoris, which is a four-headed muscle group located on the anterior portion of the femur, or what is otherwise known as the front of the thigh.
The quadriceps femoris both stabilizes the knee joint as it is moving through its range of motion, as well as contracts against the pelvis and femur so as to extend the knee forward, resulting in the leg straightening out.
While certain exercises that incorporate knee extension can also train other muscle groups, those that are solely focused on this biomechanic will almost exclusively recruit the quadriceps femoris by itself.
Though reinforcement of the knee extension biomechanic is usually performed by high level athletes or injured individuals, people who do not fit into such categories can nonetheless benefit quite well from exercising this particular joint.
Doing so can reduce the risk that they will develop an acute or chronic injury of the knee or quadriceps muscle, while individuals undergoing physical rehabilitation due to a knee injury will also see significant benefit by performing knee extension exercises.
Considering the fact that the primary muscle responsible for knee extension is the quadriceps femoris, any sort of exercise meant to improve or rehabilitate the knee extension biomechanic will also directly stimulate the quadriceps femoris.
In cases where the resistance or volume is sufficient enough, this will eventually result in muscular hypertrophy and several other physiological changes that translate to improved strength output of the muscle.
Though knee extension is only one half of the total range of motion of the knee joint, reinforcing it through exercise and mobility drills is an excellent way to ensure that knee mobility is retained and maximized, whether to counter the effects of aging or an injury.
Picking the right knee extension exercise requires that you first assess the reason behind needing such a type of exercise - was it prescribed to you by a physical therapist? Does your barbell squat have a sticking point due to poor knee stability? Or do you simply wish to reinforce your knee so as to avoid any future injuries that may occur?
For individuals seeking the rehabilitative or preparatory effects of knee extension exercise, it is best to avoid those that make use of additional resistance, as doing so can worsen an injury or otherwise cause one.
However, for those seeking improvement and reinforcement of their knees and quadriceps femoris, using exercises that utilize knee extension is an excellent way to build stronger muscles and tendons.
Step ups are a dynamic resistance exercise that feature the exerciser repeatedly stepping on and off an elevated platform so as to target the muscles of the legs.
Due to the movements involved in this exercise, knee extension is also a significant factor that is incorporated into each and every set. In particular, knee extension is utilized to the greatest degree during the initial start of the step up repetition, with the exerciser raising their foot so as to step onto the object.
Step ups can be performed with a plyometric block, actual stairs or a variety of other sorts of items that are sufficiently elevated enough to create a challenge for the exerciser.
Step ups are a suitable exercise for individuals who are new to resistance training but nonetheless wish to strengthen their quadriceps femoris and the rest of the legs - or for people seeking a way to reinforce their knee extension biomechanic without necessarily being injured or having a weakened knee joint.
Step ups are not appropriate for people who are currently rehabilitating their knee, as the risk of further injuring oneself during the exercise is only further compounded by the fact that placing the entire body weight of the exerciser on their injured knee will interrupt proper rehabilitation.
To perform a set of step ups, simply stand one step behind an elevated platform and raise one foot to climb atop it. Follow this action with the other foot, much like climbing stairs.
Then, reverse the motion by returning the first foot back to its original position, following suit once more with the other foot.
Perform this motion as many times as is needed to complete the set.
In order to increase the resistance of the exercise, you may grip a pair of dumbbells in your hands or otherwise pick a more elevated object to step upon.
A number of different resistance machines make use of the knee extension biomechanics, with the most popular being the leg press, the smith machine squat, and the leg extension machine itself.
For individuals seeking rehabilitation or a lower impact exercise of the knees, the majority of these machines place too much pressure and force on the joints to be safely used. However, for exercisers that wish to train their knee extension, these machines are one of the best ways to go about doing so.
There are far too many resistance machines that utilize the leg extension mechanic to list within a single article, and as such it is far more efficient to group them together for the purposes of brevity.
Machine-based leg extension exercises are more suitable for individuals seeking the anabolic or muscle-building benefits of knee extension exercises, rather than people attempting to rehabilitate any injuries of the knee joint.
Though it is entirely possible to utilize these machines during the course of physical rehabilitation, it is best to do so beneath the guidance and supervision of a medical professional so as to avoid accidentally worsening the injury.
A zero-impact exercise more akin to stretching rather than resistance training, lying leg lifts are utilized as a method of improving both knee mobility and hip joint mobility - while also aiding injured individuals in recovering their knee extension range of motion.
Furthermore, lying leg lifts are likely one of the most convenient knee extension exercises in this article, as they require little more than a comfortable space on the ground in order to perform.
Lying leg lifts are an excellent rehabilitory exercise for restoring knee extension function and hip or knee joint range of motion. A lack of pressure or force placed on either joint make the lying leg lift particularly useful for incorporating into a physical rehabilitation routine.
However, due to the lack of significant resistance and the rather disadvantageous movement involved, lying leg lifts are unlikely to result in any strength developments or muscular hypertrophy - making them unsuitable for individuals who desire these outcomes.
To perform lying leg lifts, the exerciser rests on the side opposite the knee to be worked, placing the other leg flat on the ground.
Then, they will raise the upward-facing leg outwards, extending the knee fully so as to form a straight line from foot to pelvis. If the exerciser begins to experience pain, they are advised to not raise the leg any further than this point.
Otherwise, the leg is held in place for several seconds before the exerciser slowly returns it to the starting position, completing a repetition of lying leg lifts.
Seated knee extensions are another kind of exercise that are frequently encountered in physical rehabilitation programs, wherein they are meant to isolate the knee extension mechanic so as to gently reinforce the knee joint with little to no excessive movement or impact taking place.
Seated knee extensions are employed primarily during cases where the total range of motion of the knee is rather narrow, or if other portions of the leg must remain immobile during the course of the knee’s rehabilitation.
Seated knee extensions are most suitable for individuals in the early stage of knee rehabilitation, or those who wish to ease their way into exercises targeting the knee joint in an isolated manner.
It is unsuitable as a method of inducing muscular hypertrophy or strength developments, as the short range of motion and lack of resistance means that no growth can occur in these aspects.
Sitting in a chair with the uninjured leg propped on a second object approximately leg-width away, the injured leg should be slightly raised off the floor due to the opposite leg being positioned in such a manner.
Then, the exerciser will push their injured knee downwards, avoiding usage of their hips or calves in order to complete the exercise. Once the knee has reached as much extension as its range of motion allows, the exerciser will then relax the knee once more - completing the repetition.
A variation of the standard squat exercise with only the upper range of motion remaining, this particular knee extension exercise may be performed with only bodyweight resistance for the purposes of rehabilitation, or with weight so as to induce muscular developments.
The reason why the half squat is performed instead of a full squat is simply to isolate the knee extension mechanic as much as possible, as going near or below parallel squat depth instead utilizes knee flexion - the opposite motion of knee extension.
The half squat is appropriate for all individuals wishing to work on their knee extension functionality, even those with weakened or previously injured knee joints - so long as the latter group has been cleared for exercise by a physician.
The main difference between the two uses of the half squat is in the level of resistance required, with those seeking the muscle-building effects of the half squat being better served by its weighted variation, and those seeking rehabilitation finding bodyweight resistance to be safer.
To perform the half squat, the exerciser will place their feet slightly wider than hip width apart, with the toes facing outwards.
Bracing their core and pushing their chest out, they will then lower themselves by bending at the hips and knees simultaneously - stopping once they are approximately one inch above parallel squat depth.
Then, exhaling and breathing in once more, they will push through the heels and rise back to a standing position, thereby completing a repetition of the half squat exercise.
And with that, we’ve covered some of the most common and effective knee extension exercises utilized by weightlifters and physical therapists alike.
Remember that if you are injured or were previously injured, it is vitally important to first seek the approval of a physician prior to attempting to place any sort of strain on the injured area. Doing so without medical clearance can easily result in a recurrence or even a worsening of the injury itself.
Otherwise, reinforcing your knee extension biomechanics via quadriceps femoris exercise is an excellent way to remedy sticking points and maximize the strength of your legs.
1. Yoshioka T, Kubota S, Sugaya H, Arai N, Hyodo K, Kanamori A, Yamazaki M. Feasibility and efficacy of knee extension training using a single-joint hybrid assistive limb, versus conventional rehabilitation during the early postoperative period after total knee arthroplasty. J Rural Med. 2021 Jan;16(1):22-28. doi: 10.2185/jrm.2020-024. Epub 2021 Jan 5. PMID: 33442431; PMCID: PMC7788304.
2. Huberti HH, Hayes WC, Stone JL, Shybut GT. Force ratios in the quadriceps tendon and ligamentum patellae. J Orthop Res. 1984;2(1):49-54. doi: 10.1002/jor.1100020108. PMID: 6491798.
3. Chen C, Ost JJ, Salathé EP. Biomechanical analysis of the knee extension exercise. Arch Phys Med Rehabil. 1993 Dec;74(12):1336-42. doi: 10.1016/0003-9993(93)90089-s. PMID: 8259902.