Though both the leg extension and leg curl may initially seem to share many similarities due to their identical nature and training stimulus, they are in fact quite distinct. So much so that they are actually incomparable within certain contexts, as they target different muscle groups and utilize differing biomechanics.
In terms of simply acting as leg muscle group exercises however, there are certain points of conflict between the two; the majority of which surround the effectiveness and exclusivity of the leg extension and the leg curl.
For the most part, the comparable characteristics between the leg extension and the leg curl surround their shared usage of machine-based resistance, their relatively similar intensity and complexity, as well as their similarity in knee biomechanics.
The leg extension is a machine-based resistance exercise primarily performed at a low to moderate intensity for the purposes of training the quadriceps femoris muscle group by way of loaded knee extension movement.
It is a particular favorite of novice exercisers and athletes requiring training stimulus within a certain range of motion in relation to the knee, as it is entirely an isolation type exercise and thus provides a specificity that other leg exercises cannot.
The leg extension’s primary benefits are reinforcement of the patellar joint and its surrounding tissues (up to a point) by way of improved mobility and hypertrophic effect.
In addition to this, the relative risk of injury of the leg extension is quite low in comparison to other exercises that recruit knee flexion, making one of its benefits a high margin of safety alongside its relatively low exercise complexity.
Being an isolation exercise, the leg extension solely trains the quadriceps femoris muscle group; four large muscles that run along the anterior portion of the femur and possess attachment points at or around the patellar joint and the hip socket joint.
These four muscle groups, ordered from the largest to smallest, are the vastus lateralis, vastus medialis, rectus femoris and vastus intermedius - with the rectus femoris in particular responding extremely well to the training stimulus of moderate intensity leg extension sets.
The leg curl is also a machine-based resistance exercise of the isolation type muscle group activation set, wherein it is performed by exercisers for the purposes of inducing training stimulus in the various muscles of the hamstring muscle group, with the gluteus muscle group and the calves acting as secondary or stabilizer muscle groups.
The leg curl is often performed either seated or lying, depending on the preference of the exerciser and any available equipment - though both the seated and lying leg curl are performed with a low to moderate level of intensity, much like the leg extension.
This particular exercise is more commonly utilized as an adjunct or accessory exercise meant to reinforce posterior chain mobility and development during a workout session without excessive fatigue or risk of injury being found.
The name leg curl refers to both the prone and seated leg curl exercise, both of which recruit the same muscle groups and utilize similar pieces of equipment - with the sole difference being the relative angle of resistance and range of motion between the two exercises.
The prone or lying leg curl recruits the glutes muscle group to a somewhat greater extent as the angle of resistance and range of motion are placed further away from the exerciser’s hips, forcing the entire posterior chain to be recruited to a greater extent.
Whereas in the seated leg curl, the exerciser curls their heels beneath them under the seat, drawing the source of resistance beneath the hips and thus reducing gluteus muscle group recruitment in favor of greater hamstring muscle group recruitment instead.
Regardless of what particular leg curl variation is being performed however, the majority of their characteristics are practically identical - so much so that they may practically be considered the same exercise outside of highly advanced training programming.
The leg curl is highly effective at providing targeted hamstring muscle group activation, as well as enhancing effective knee range of motion mobility - aiding in the relative physical strength capacity and size of the muscles found therein.
The leg curl, unlike the leg extension, is not known to cause any direct acute injuries to the exerciser’s muscle groups or connective tissues - though overuse injuries are still a factor, just as they are in any sort of exercise.
As such, one other major benefit to the leg curl is its significantly low risk of injury when performed in an appropriate manner.
Being an isolation exercise, the leg curl solely trains the hamstrings muscle group that run along the posterior side of the femur bone, with attachment points at the hips and the rear of the patellar joint.
Much like the quadriceps femoris, the hamstrings muscle group is responsible for the movement of the knee in relation to the hips and the rest of the body - though, in the case of the hamstrings, it is to put the knees in a state of flexion instead of extension, drawing the hips parallel to the knees if utilized from an upright position.
The hamstrings muscle group consists of three muscles; namely, the biceps femoris, the semitendinosus, and the semimembranosus - all of which surround the rear of the leg and ensure stable and effective movement of the legs, especially in a squatting or stair-climbing motion and are equally activated by the leg curl in a significant capacity.
The leg extension and the leg curl position the exerciser’s body in distinctly different states, requiring different biomechanics and biokinetic movements be made in order to complete the exercise.
The leg extension is performed in an upright and seated position, with the exerciser hooking their feet beneath a padded handle and extending their knees so as to activate the quadriceps femoris. This is a relatively simple movement, and so long as the exerciser refrains from completely extending their knees, should rarely ever result in injury.
In the case of the leg curl, the exerciser is either in a face-down prone position or seated upright in a manner similar to the leg extension. In both instances, the source of resistance is placed behind their ankles, with the exerciser entering a state of knee flexion while stabilizing their hips and torso so as to prevent any carry-over or improper muscular activation from occurring.
Though both the leg curl and leg extension make use of adjacent muscle groups and the knee joint, they are in fact quite distinct and as such are incomparable in this particular respect.
Even in instances of exercisers with a history of knee injury, both the leg extension and the leg curl are equally capable of aggravating such injuries depending on their severity and the manner in which these exercises are performed.
The difference between the two exercises is so much so that they are in fact more often performed together within the same workout session, even to the extent that they are part of the same super-set, with one exercise being immediately performed after the other in quick succession.
Both the leg extension and leg curl present the same shared disadvantage; they both make use of self-stabilizing machine-based resistance as a form of training stimulus, resulting in greatly reduced synergist or stabilizer muscle group recruitment and thus less real-world or athletic strength carryover and the stabilizer muscles are left untrained by the exercise.
In addition to this, both exercises possess similar safety mechanisms built into the machine - that is to say, very little, as both the leg curl and leg extension are equally safe so long as a low to moderate amount of resistance is utilized.
However, it is the leg extension that serves a somewhat greater risk of injury than the leg curl, as the knee is more likely to result in connective and osseous tissue injury when placed under significant strain during a state of extension instead of flexion.
In order to avoid this, the exerciser will simply not complete a full knee extension range of motion while performing the leg extension exercise.
Apart from the leg extension’s relatively small risk of resulting in knee injury, both exercises may otherwise be considered quite safe isolation movements that unfortunately cannot be entirely relied on to provide leg training stimulus due to their lack of stabilizer muscle group recruitment and small number of muscle groups worked.
Due to the significant differences of the leg extension and the leg curl, they are in fact quite effective when performed together - fully training the upper leg and resulting in an excellent amount of secondary training stimulus when combined with a leg-focused compound exercise prior in the workout session.
This is due to the fact that the leg extension trains the quadriceps femoris while the leg curl trains the hamstring muscle group, two opposing muscular structures on either end of the femur bone that act in an opposing and synergistic capacity throughout the majority of leg-driven movements.
As such, supersetting or even simply programming the leg extension and the leg curl together in a workout is far more advisable than simply choosing one exercise over the other; thereby providing a more well-rounded and effective leg workout than what only a single one of the two exercises can achieve.
Though a nearly complete leg training stimulus is achieved by combining the leg curl and the leg extension, the squat is still far superior to either (or both) exercises for a variety of reasons, such as a wider-encompassing muscle group activation set, greater activation intensity, stabilizer muscle group recruitment and a number of other factors difficult to achieve with the leg extension or leg curl.
The leg curl is doubtless a highly effective and safe hamstring training exercise - but this is not to say that it is capable of replacing posterior chain compound movements like the deadlift or barbell hack squat, both of which are superior to the leg curl for much the same reasons that the squat is superior to the leg extension.
As such, it is more advisable for the exerciser to simply perform the leg curl after they have performed these hamstring-activating compound movements in order to better develop their posterior chain instead of simply relying on the comparatively less effective leg curl training stimulus.
The leg extension and leg curl are incomparable within the context of most circumstances, and are far better off combined instead of having one picked over the other - with the overuse of one exercise instead of its counterpart easily resulting in muscular imbalances and weakness.
When in doubt, the exerciser is always better served simply resorting to a compound movement that trains both the quadriceps and the hamstrings muscle groups equally, as neither the leg extension or leg curl are essential in the pursuit of proper muscular development.
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