Are the Seated Leg Extension, Leg Curl, and Adduction Machine Exercises Non-Functional or Risky?

Many single-joint, machine exercises, like the seated hip adduction, lying hamstring curl, and leg extension, are commonly believed to be “non-functional” and even “risky.” Many personal trainers base this conclusion on the criteria that such exercises are 1) performed in the seated or lying position and 2) are isolation exercises that are designed to create a resistance challenge mostly on a single-joint action.  

This article highlights the scientific evidence on exercises like the seated leg extension, leg curl, and adduction machines to highlight their potential benefits on enhanced performance and potentially reducing injury risk. This article will also demonstrate that simply because a given exercise is performed in the seated position and targets a single-joint action (i.e., an isolation exercise) does not necessarily mean that a given exercise provides little-to-no positive training transfer or, alternatively, is universally risky. Therefore, such conclusions or claims made from using such highly questionable criteria are less likely to be sound. 

Functional Benefits of the Lying Leg Curl and Seated Hip Adduction Exercise

For clarity of communication, it is important to first define exactly what “functional training” is—and what it is not—when discussing the functional benefits of given exercises, such as the lying leg curl and seated hip adduction machine. According to the Oxford English Dictionary, the word functional is defined as, “of or having a special activity, purpose, or task,” or, alternatively, “designed to be practical and useful, rather than attractive,” (12). With this definition in mind, functional training for improved performance and injury risk reduction has nothing to do with what the exercise looks like, nor does it have to do with the type of equipment one is using; instead, functional training is all about positive transfer to one’s training goals, which is the purpose of training. In other words, the goal of exercise programming for enhanced performance and the reduction of injury risk is to maximize training transfer. 

With the above in mind, since exercises such as the seated hip adduction and leg curl machine do not generally mimic specific movement patterns of many common actions in athletics, their benefits for improved performance and injury risk reduction are less obvious (24). This fact can lead fitness professionals into mistakenly labeling them as “non-functional” and, therefore, not valuable. The research provided in this section suggests that seated exercises that target a single-joint action (i.e., an isolation exercises), such as seated hip adductions and leg curls, may indeed offer a positive training transfer. Thus, it is misguided and inaccurate to believe such exercises to be non-beneficial when it comes to improving performance and reducing injury risk. 

Askling et al. studied the effects of prescribing elite soccer players additional specific eccentric hamstring training using the lying leg curl machine (1). In their study, 30 players from two of the best premier-league division teams in Sweden were divided into two groups: one group received additional specific hamstring training using the lying leg curl machine, whereas the other group did the same strength and conditioning programs without the additional specific hamstring training using the lying leg curl machine. The results showed that the occurrence of hamstring strain injuries were clearly lower in the group (3 out of 15) that did additional specific hamstring training using the lying leg curl machine than in the control group (10 out of 15) that did not get the additional hamstring training (1). Additionally, compared to the group that was not prescribed lying leg curls, the group that received the addition of lying curl increased sprint speed (1). Furthermore, one should also consider the principle of specificity, which dictates that the adaptations to training will be specific to the demands the training puts on the body. According to Dr. Everett Harman in the NSCA’s Essentials of Strength Training and Conditioning (3rd Edition), “The simplest and most straightforward way to implement the principle of specificity is to select exercise similar to the target activity with regard to the joints about which movement occur and the direction of the movements. In addition, joint ranges of motion in the training should be at least as great as those in the target activity,” (2). In sporting activities, such as running, sprinting, and changing direction, the hamstrings are controlling forces concentrically and eccentrically from various angles of knee flexion. So, it stands to reason why the addition of the lying leg curl machine exercise created superior results in both improved performance and hamstring injury risk reduction. 

A comprehensive approach to hamstring training would incorporate at least one exercise focused on the hip joint movement (such as the deadlift or other similar compound exercises) and one exercise focused on the knee joint (such as the leg curl machine or other similar isolation exercises), as each offer unique but complimentary training benefits. Another example of using isolation exercises for performance are exercises for hip adductors. A scientific review found that hip adductor strength was one of the most common risk factors for groin injury in sport (26). One particular study on professional ice hockey players found that athletes were 17 times more likely to sustain groin injury if their adductor strength was less than 80% of their abductor strength (25). This study’s results do not necessarily mean that stronger adductors always decrease injury. However, strong evidence exists that strength training can help reduce the risk of sports injury and overuse injury, so exercises designed to help strengthen the adductor musculature are reasonable to include in a resistance training program (17). 

Many fitness professionals think that specific exercises to target the adductors are unnecessary, believing compound exercises like squats and lunges will effectively and sufficiently target them. However, research in this arena, along with the universal principle of specificity, call this common belief into question. First, one must consider that different adductor muscles have different actions and that during hip extension-based movements, they may not be adequately stressed to create training adaptation (8). With respect to both eliciting greater adductor electromyography (EMG) amplitude and the principle of specificity, exercises developed to train the hip adductors directly—specifically, all of the fibers that partake in hip adduction rather than just those that assist in hip extension—such as standing hip adductions with a band or cable, the Copenhagen hip adduction exercise, or the seated hip adductor machine may be superior to exercises like wide-stance squats, single-leg squats, and lunges (3,5,9). It is important to note that, while EMG amplitude itself may not be a marker of adaptation, when considered in conjunction with the principle of specificity and basic biomechanics work, it does strengthen the overall rationale for isolated adductor work. 

It is important to see how scientific and anecdotal evidence falls in line with universal training principles in order to make decisions on practical programming strategies. For example, research demonstrates that strength gains are highly specific to the part of the movement one trains in, with limited transfer to the rest of the untrained ranges of the movement that may not be addressed in a given exercise (13,18). These results are in line with the principle of specificity. With this in mind, exercises developed to train the hip adductors directly, such as standing hip adductions with a band or cable, the Copenhagen hip adduction exercise, and the seated hip adductor machine, involve moving through larger ranges of motion than exercises such as squats, single-leg squats, and lunges. Therefore, as with training the hamstrings, when training the adductor musculature, it makes sense to also incorporate such exercises designed to target the adductors into a comprehensive resistance training program in order to train in ranges of motions that may not be sufficiently addressed by more compound exercises. 

Is The Knee Extension Dangerous?

Another controversial single-joint exercise, the knee extension, has been on the receiving end of heavy criticism by personal trainers and strength and conditioning coaches. The concerned is about the forces that are placed upon structures within the knee during its execution including the anterior cruciate ligament (ACL) and patellofemoral joint (PFJ) (4). Some argue that the knee extension places tensile forces on the ACL, which can be considered dangerous, and instead, recommend exercises like squats (4). This perspective is not only shallow, but is also logically inconsistent. Indeed, tensile forces are placed on the ACL during the knee extension exercise (11). However, when examining these forces, it is important to bear in mind that they exist on a continuum, and injury is not a concern until a certain threshold is reached. For the ACL, this threshold has been reported to be upwards of 2,000 N, depending on an individual’s age (27). Thus, one can see that the tensile forces experienced by the ACL during the knee extension (158 – 396 N, using loads ranging from a dynamic 12 repetition-maximum to a maximum voluntary isometric effort) are less than one-fifth of its ultimate strength (11). Moreover, ACL forces during the knee extension are less than or equal to many other “functional” tasks, such as walking or landing (11,22). Therefore, knee extensions do not appear to be any more unsafe for the ACL than tasks like walking. Moreover, the recommendation of the squat being a safer exercise for knee ligament health is also logically inconsistent. For example, one study suggests that posterior cruciate ligament (PCL) forces in the squat are about an order of magnitude, or ten-times, greater than those placed on the ACL during the knee extension (10). To put this into perspective, the PCL:ACL ultimate strength ratio does not approach such a difference (10:1) and is closer to 1.5 – 2:1 (14,15). Thus, many of the arguments against utilizing the knee extension in healthy populations out of concern for ACL health are unfounded and logically inconsistent. 

PFJ forces have been considered to be a biomechanical contributor to the development or maintenance of patellofemoral pain (16). While this perspective is likely overly simplistic, due to the complex, biopsychosocial nature of pain, one should address the biomechanical inconsistencies with the argument (6,7,18). First, when it comes to the PFJ, speaking about forces is not enough; instead, one must consider the stresses, as the force is distributed over a given area of tissue. Indeed, such comparisons have been made, and they suggest that bodyweight squats to 90° knee flexion elicit greater peak PFJ stress than EMG amplitude-matched knee extensions, especially with greater knee flexion (21). Nevertheless, if working with symptomatic individuals, exercise selection and range of motion should be tailored to the individual as per the recommendation of a physical therapist or medical professional; otherwise, the knee extension, like the squat, should not be considered universally contraindicated. 

Conclusion

The research discussed in this article, along with the principle of specificity, highlights the importance of incorporating exercises focused on a single-joint action (i.e., isolation exercises), such as movements targeting the hamstring and adductor musculature, in addition to the other types of exercises when training for improved performance in order to make one’s programming more comprehensive and effective. Resistance exercise, from isolation exercises to integrated, multi-joint exercises, should not be looked at as mutually exclusive. Rather, they should be viewed as complementary training components, because each type of resistance exercise offers unique benefits the other types may lack (23). Furthermore, fear-mongering surrounding exercise variations is also unfounded and little more than conjecture; rather, nearly all exercises are safe, but should be prescribed in a context-dependent manner.

This article originally appeared in Personal Training Quarterly (PTQ)—a quarterly publication for NSCA Members designed specifically for the personal trainer. Discover easy-to-read, research-based articles that take your training knowledge further with Nutrition, Programming, and Personal Business Development columns in each quarterly, electronic issue. Read more articles from PTQ »