The leg curl is a machine based compound exercise that, true to its name, places significant emphasis on the various muscle groups of the legs so as to induce a safe yet effective level of training stimulus.
It is most commonly seen in bodybuilding workout regimens as an auxiliary exercise adjunct to free weight compound exercises such as the squat or romanian deadlift - generally for the purposes of squeezing out any available training stimulus that may be left over.
Despite its role as an auxiliary exercise, the leg curl is nonetheless an excellent exercise all on its own, and is a suitable choice for practically any resistance training program that requires lower body muscle group recruitment.
In more technical terms, the leg curl is an isolation exercise with an open kinetic chain and is generally performed with the use of a machine simply dubbed the leg curl machine (though certain versions of the leg curl may be performed with bodyweight alone).
In terms of relative rate of perceived exertion, the leg curl is usually meant to be performed at a rating anywhere between 11 to 15, depending on the exerciser’s own physical strength and experience in resistance exercises.
The leg curl is also occasionally referred to as a hamstring curl, which is only partially accurate as it is capable of activating more muscle groups than just the hamstring muscles alone - though its related machine may also be referred to as a hamstring curl machine, confusingly enough.
The leg curl is generally rated at a novice level of complexity and injury risk, meaning that it may be performed by practically any individual save for those with a history of injuries in the posterior chain or other parts of the lower body.
The leg curl primarily activates the hamstrings muscle group, of which are located along the back of the femur or thigh bone, and are responsible for the extension and contraction of the knees, as well as the extension of the hips by moving the upper leg backwards.
In addition to its usage of the three hamstring muscles as primary mover muscles, the leg curl also recruits muscle fibers in the calf muscle groups, glute muscle groups and quadriceps femoris muscle groups to a certain extent - with the quadriceps and calves being utilized primarily as stabilizer muscle groups for the most part.
Conversely, the leg extension machine directly targets the quadriceps femoris muscle group by way of loaded knee extension movement.
The leg curl is performed in bodybuilding routines so as to increase relative training stimulus and therefore induced muscular hypertrophy in the workout session - especially in the capacity of an exercise that does not otherwise activate other parts of the posterior chain that may already be fatigued by previous exercises.
In the case of powerlifting routines or similar training programs meant to improve the athleticism and strength of the exerciser, the leg curl may be used either as a preparatory exercise prior to an athletic event so as to precondition the tendons and skeletal muscles of the posterior chain, or as a form cue exercise for other hamstring related movements, like the deadlift or squat.
As the leg curl is generally performed either with the use of a lying leg curl machine or a seated leg curl machine, it is best to separate the two in terms of form cues and mechanics so as to avoid confusion and allow the exerciser to maximize any benefits they may receive from either variation.
In order to begin performing repetition of the prone or lying leg curl, the exerciser must first position themselves face down over the machine with their ankles hooked into the distal foam padding at the opposite end of the machine from their head.
Some adjustment may be required for the positioning of said foam padding, as well as the handles from which the exerciser will grip so as to stabilize their trunk during the exercise.
Once comfortably positioned, the exerciser will then contract their hamstrings and squeeze their buttocks (thereby recruiting the glutes) which should have the intended effect of curling their legs backwards, drawing the ankles and subsequently the foam padding towards the exerciser’s lower back.
Acting as the concentric portion of the exercise, the exerciser will then squeeze their hamstrings and glutes once more, before proceeding to the eccentric phase of the repetition.
To do so, the exerciser will slowly allow their legs to extend once more, straightening the knee joint and relaxing the glutes muscles as their ankles return to the starting position.
With this, a repetition of the lying leg curl is completed, with subsequent repetitions requiring that the exerciser simply repeat the motion once more until the set is finished.
A variation of the leg curl that places the exerciser in a seated position with their shins and knees locked between two adjustable pads, the seated leg curl is as simplistic and low impact as its lying counterpart.
To begin performing a set of seated leg curls, the exerciser must first adjust the two pads (as well as the seat) so that they remain stable and in contact with the exerciser’s legs throughout the repetition without limiting their range of motion by being too narrow.
Once properly adjusted, the exerciser will then squeeze the two handles at either side of their hips with their hands, brace their core, inhale, and draw their legs inwards, pulling their ankles backwards until they are nearly parallel with the knees vertically.
Squeezing their hamstrings and glutes at this point in the repetition, the exerciser will then proceed to the eccentric portion of the exercise, having already completed the concentric portion of the movement.
The exerciser will slowly allow the weight to raise their legs back to full extension - all the while maintaining a level of tension in their hamstrings and calves throughout this portion of the repetition.
Once the pads and the exerciser’s legs have returned to their original starting position, they have successfully completed a repetition of the seated leg curl, with subsequent repetitions in the set requiring that they simply repeat the motion once more.
The leg curl, like many other exercises, can present a variety of benefits if performed in an appropriate manner and on a regular basis.
However, some of these benefits in particular are only truly applicable in the case of the leg curl or exercises similar to it, and as such it is these that are best focused upon when choosing the leg curl for its variety of beneficial effects.
Though the hamstrings are recruited to some degree in a large number of exercises, few exercises are as capable of isolating them as the leg curl itself - making it one of the only possible choices for bodybuilders and athletes seeking to induce a training stimulus in such a manner.
The capacity of an exercise to isolate the hamstrings is not unique to the leg curl, but it is considered one of the most easily accessible and simplistic exercises that can achieve such a feat, allowing it to be performed in practically any gym or by any individual of healthy bodily function.
A common side effect of modern sedentary living is the shortening of the tendons connecting to the hamstring and the subsequent reduction in its available range of motion, something that is otherwise known as reduced flexibility.
The leg curl may help remedy this problem by inducing muscular hypertrophy and improving venous blood flow to the hamstring muscles, thereby allowing it to remain stable at a wider range of motion and reducing the risk of said hamstring muscles tearing or otherwise becoming damaged if overextended past their flexible range.
As the hamstring muscles are responsible for the extension and flexion of the knee, as well as the adduction of the hips, reinforcement of said hamstring muscles and any associated connective tissues therein will have a marked reduction in the severity (or even incidence) of injuries sustained in those areas.
This is only further added to by the improvement in the relative static strength capacity of the glutes, hamstrings and calves - all of which make up part of a kinetic chain of muscle groups known as the posterior chain.
Leg curls directly improve and strengthen the stability of the posterior chain, allowing for significantly improved lower body stability and balance; thereby resulting in further reduced incidence of injury, both in and out of athletic activities.
As previously mentioned in the last section of this article, the hamstring muscle and its connecting tendons are responsible for certain capacities of motion involving the knees and hips - with the most obvious result being a subsequent strengthening of said bodily structures, greatly improving their ability to function and withstand the rigors of daily usage.
As such, the leg curl is not only an excellent exercise for bodybuilders and athletes seeking to improve the appearance and function of their hamstrings, but also as a rehabilitory and preparatory exercise for physical therapy patients and recovering exercisers alike.
Though the leg curl is relatively simplistic in the majority of its variations, certain mistakes are commonly encountered in concerns to the manner at which it is performed or the programming of the exercise itself.
Fortunately, these mistakes are quite easy to remedy, and may only require a small adjustment in the total amount of repetitions performed per set by the exerciser, or a minor adjustment in the positioning of the leg curl machine’s padding.
Regardless of the complexity behind these particular mistakes, it is important for the exerciser to remedy them as soon as possible - not only to avoid any injuries that may occur, but to also maximize the training stimulus they may develop with the leg curl.
Not a mistake solely reserved for the leg curl, the usage of rushed repetitions during the exercise is nonetheless one of the largest mistakes novice exercisers can make - sabotaging any training stimulus they may be attempting to induce and placing themselves at risk of connective and skeletal tissue injury.
A repetition of the leg curl should be performed in a slow and focused manner, with the exerciser directing their attention to the proper recruitment of the calves, hamstrings and glutes throughout the motion while avoiding the usage of any momentum whatsoever.
Leg curls may provide insufficient training stimulus if performed in volumes of repetitions that are too low, while conversely providing the incorrect form of training stimulus when performed at volumes that are in excess of what is required in order to induce muscular hypertrophy.
This is due to the fact that excessive volume is usually an indicator of insufficient resistance utilization, shifting the resistance loading of the exercise and altering the muscular activation pattern involved therein.
When performing leg curls, the exerciser should choose to keep their volume of repetitions anywhere between 8 to 20 repetitions per set, with higher repetitions per set being best reserved for warm up or rehabilitory leg curl variations instead.
Though not as significant as the previous two mistakes mentioned in this article, improper adjustment of the various portions of the leg curl machine may result in unsecure usage of the machine that may result in injury - or otherwise uncomfortable performance of the exercise.
It is important for the padded portions of the machine to remain in constant contact with the appropriate portions of the exerciser’s legs, as this will aid in maintaining their position within the machine, and prevent any slipping or excessive momentum from occurring.
If one is unsure of how to adjust their leg curl machine, they are best left consulting an athletic coach or similarly experienced individual.
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