Leg Press vs Leg Extensions: 3 Major Differences Explained

published by: Debbie Luna
Last Updated:
August 4, 2022

Among the various machine-based leg exercises available, few are as frequently used as the leg press and the leg extension - two widely differing movements that are nonetheless often compared to one another in error, as the differences are too vast to merit an actual comparison.

For the most part, these differences revolve around the fact that the leg extension possesses a far smaller number of muscle groups activated than the leg press, placing both exercises in entirely different categories.

However, there are certain instances where the leg press and the leg extension may be of equal value to the exerciser, and as such it is vital that they understand the main differences between the two leg movements so as to pick what best fits their training program.

What is the Leg Press?

The leg press is a compound machine-based resistance exercise most notable for being considered a possible alternative to the standard barbell back squat, making it both an excellent secondary compound exercise as well as the primary source of lower body training stimulus in certain training programs.

leg press machine

It is most often used for the purposes of inducing muscular hypertrophy and strength development in every muscle group of the legs, providing a complete activation of all muscles therein without the usual risks associated with intense compound leg exercises that make use of knee and hip flexion.

Much like any other machine-based resistance exercise, the leg press provides a longer time under tension and reduced usage of stabilizer muscle groups than its free weight counterparts, as well as several safety mechanisms that may make it safer to use for novice exercisers or recovering athletes.

What are Leg Extensions?

The leg extension exercise is a machine-based isolation exercise primarily targeting the quadriceps femoris located along the anterior side of the femur bone, or what is otherwise known as the outer thigh.

leg extension machine

Unlike the leg press, the leg extension is more often utilized for its specificity of muscle group activation, its relatively low risk of injury and its capacity for developing strength within a more exact range of motion, eliminating the recruitment of synergist or secondary muscle groups.

However, despite this higher level of specificity in muscle group activation, there is little evidence that the leg extension recruits muscle fibers within the quadriceps femoris at a greater level than in the leg press, making the leg extension more of a situational alternative rather than a concrete one.

1. Muscle Group Activation Differences

The largest and most obvious difference between the leg press and the leg extension is in the muscle groups trained by either exercise, with the leg press training the glutes, hamstrings, quadriceps and a variety of other smaller muscle groups while the leg extension solely activates the quadriceps femoris.

leg extension muscles

This is the primary factor behind the wide disparity in characteristics of the leg press and the leg extension, and as such should also be the deciding factor on which exercise one chooses to incorporate into their training routine.

2. Positioning Differences

Another point of distinction between the leg press and the leg extension is the difference in the equipment of the two exercises, with one being more conducive to natural human biomechanics and the other placing stress on parts of the body that may lead to injury if used improperly.

The leg press places the exerciser in a seated position with their legs raised upward and ahead of their torso, allowing the back to be braced against the cushioning of the seat and eliminating most of the risk of spinal injury while still allowing for a full range of motion to be achieved in the hips and knees.

This is not so much the case with the leg extension, which - while also seating the exerciser - does so in an upright manner, placing the feet immediately below the hips in the resting position of the exercise, wherein the exerciser will extend the knees and “kick up” their feet in a slow manner.

With mechanics such as this, the exerciser will force the patellar joints to absorb a significant amount of tension as they are placed between the source of resistance at the feet and the buttocks braced against the seat.

While at lower levels of resistance this presents little danger, performing a leg extension with a similar level of weight to the leg press will quickly result in knee injuries - or with excessively high amounts of volume.

3. Biokinetic and Biomechanical Differences

Despite the fact that the leg press and leg extension are both lower body exercises, their relative biomechanical cues are in fact quite different - with the only similarity between the two exercises being the usage of knee extension as a means of activating the quadriceps femoris.

The leg press recreates much of the same movements as a standard squat, wherein the knees and feet point outwards and thus require the hips to be placed in a state of adduction as the weighted portion of the machine bears downwards onto the heels, creating a more neutral angle of resistance and evenly distributing the mechanical stress across much of the lower body.

The leg extension, on the other hand, involves none of these biomechanics and instead relies solely on the knee extension mechanic of the quadriceps femoris, with the mechanical tension of the exercise placing at a shearing angle along the femur and knee joint due to the fact that the source of resistance rests atop the ankles and feet.

This mechanical tension may be somewhat mitigated in terms of focus by preventing the exerciser from fully extending the legs, though this will also consequently fatigue the quadriceps femoris to a greater degree and produce a less complete training stimulus in the muscle group.

Which Exercise is Safer?

In terms of built-in safety mechanisms of the two exercise machines, it is the leg press that possesses far more and at a greater complexity - though this is a necessity as the risk of injury with the leg press is also somewhat higher if excessive weight or poor form adherence is encountered by the exerciser.

Conversely, the leg extension machine possesses few - if any - safety mechanisms built in, as the risk of injury is of a more mechanical nature than having anything to do with poor form being utilized by the lifter.

In both the leg press and the leg extension, fully extending the knees or otherwise locking out said knee joint can result in excessive physical stress being placed throughout the leg, as well as reduced circulation to the brain - both of which can easily result in injury.

However, despite this particular risk, it should be noted that the leg press and the leg extension are both reasonably safe exercises that are suitable for novices and actively recovering physical rehabilitation patients, as their machine-based nature and relatively low impact in comparison to free weight exercises make them excellent candidates in this particular regard.

Which Exercise is Better for Building Strength?

In terms of total lower body strength output, it is the leg press that is the clearly superior choice - as it activates not only the quadriceps femoris but a variety of other muscle groups that contribute to the strength and power of the exerciser’s legs.

While the leg extension is capable of strengthening the quadriceps femoris in an equally effective capacity as the leg press, the quadriceps femoris is only responsible for a certain portion of the leg’s effective range of motion, and as such will only strengthen the exerciser in the performance of specific tasks that recruit this muscle group alone.

Ideally, an exerciser aiming to develop leg muscle strength will combine both the leg press and the leg extension - alongside several other exercises - in order to develop both compound and isolation-based training stimulus, achieving a greater hypertrophic effect.

Free Weight Exercise Carry-over

One caveat that both the leg press and leg extension share in this regard is the fact that they are both of the machine-based nature, equating to a greatly reduced recruitment of stabilizer or synergist muscle groups, and therefore reducing their carry-over to free weight or real life movements.

In order to remedy this, the exerciser will find that including a free weight compound exercise alongside the leg press or leg extension will recruit their stabilizer muscle groups enough to offset the deficit of stimulus caused by solely utilizing machine-based resistance exercises.

Can the Leg Press and Leg Extension be Combined?

Due to the differences of muscle group activation and specificity between the two exercises, the leg press and the leg extension may in fact be combined within the same workout session - yielding greater quadriceps femoris development and reinforcement of knee extension biomechanics.

While the leg press is a more quadriceps dominant exercise than its free weight counterparts (as a result of the exerciser’s position within the machine), it is nonetheless still capable of activating the posterior chain muscle groups to a certain extent, training the entirety of the lower body in a manner that is only furthered by the addition of the leg extension later in the training session.

Doing this requires some alteration in the structure of the training program, as it is far more advisable that the exerciser perform the leg press with greater resistance and lesser volume, while the leg extension is instead left with high amounts of repetition volume per set at a relatively low level of resistance so as to avoid injury and overtraining.

Frequently Asked Questions

In Totality, Which Exercise is Better?

Save for a specific few circumstances or training goals, it is the leg press that is clearly superior in terms of training effectiveness, efficiency, muscle group activation and safety. 

While this is true in most cases, one should not immediately discount the usefulness of the leg extension, as it serves the purpose of an accessory quadriceps exercise quite well, aiding in the specificity of training stimulus that is so often needed in many compound-focused training programs.

Can the Leg Press Replace Barbell Squats?

For the most part, the leg press can in fact replace barbell squats - so long as the exerciser ensures they are taking sufficient steps to address the deficits in training that are produced by swapping out such an essential compound exercise.

barbell full squat

The addition of more posterior chain exercises, as well as certain free weight compound movements that summarily activate the stabilizer muscle groups of the lower body are all a necessity when substituting the barbell squat with the leg press.

What Happens if You Only Do Leg Extensions?

As leg extensions are an isolation exercise that solely train the quadriceps muscle group, solely utilizing the leg extension as a source of lower body training stimulus will quickly result in muscular imbalances and injury to the knee as supportive and synergist muscle groups are overpowered by the resistance.

If one chooses to add the leg extension to their training routine, they should ensure that their lower body is receiving an equal distribution of training stimulus, with posterior chain exercises or compound movements that also activate the rest of the leg’s muscle groups making up for the difference in muscle group activation.

In Conclusion

It is quite clear that the leg press is superior to the leg extension as a general exercise - though the leg extension’s relatively smaller range of motion and single muscle group activation make it more suitable for specificity of training or rehabilitation of leg-related injuries.

As such, it is our advice that the exerciser first incorporate the leg press into their training routine, only adding the leg extension in if they find that further quadriceps femoris training stimulus is required to reach their goals.

Regardless of whether the exerciser has chosen to utilize the leg press, leg extension or even both - solely relying on machine-based resistance exercises as a source of muscular development for the lower body can easily lead to many issues, and as such the exerciser should also utilize certain free weight compound exercises such as the back squat or split squat in order to complement these exercises.

References

1. Martín-Fuentes I, Oliva-Lozano JM, Muyor JM. Evaluation of the Lower Limb Muscles' Electromyographic Activity during the Leg Press Exercise and Its Variants: A Systematic Review. Int J Environ Res Public Health. 2020 Jun 27;17(13):4626. doi: 10.3390/ijerph17134626. PMID: 32605065; PMCID: PMC7369968.

2. Alkner BA, Tesch PA, Berg HE. Quadriceps EMG/force relationship in knee extension and leg press. Med Sci Sports Exerc. 2000 Feb;32(2):459-63. doi: 10.1097/00005768-200002000-00030. PMID: 10694132.

3. Escamilla RF, Fleisig GS, Zheng N, Lander JE, Barrentine SW, Andrews JR, Bergemann BW, Moorman CT 3rd. Effects of technique variations on knee biomechanics during the squat and leg press. Med Sci Sports Exerc. 2001 Sep;33(9):1552-66. doi: 10.1097/00005768-200109000-00020. PMID: 11528346.

4. Pincivero DM, Gandhi V, Timmons MK, Coelho AJ. Quadriceps femoris electromyogram during concentric, isometric and eccentric phases of fatiguing dynamic knee extensions. J Biomech. 2006;39(2):246-54. doi: 10.1016/j.jbiomech.2004.11.023. PMID: 16321626.

Debbie (Deb) started powerlifting and Olympic lifting in High School as part of her track team's programming; She continues to train in order to remain athletic. Inspire US allows Deb to share information related to training, lifting, biomechanics, and more.
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