Single-joint exercises provide a positive transfer into improved performance and injury risk reduction. For example, a comprehensive hamstring program should also contain at least one exercise where movement is focused at the hip joint (like the deadlift) and one exercise where movement is focused at the knee joint (like the leg curl machine).
This is because different regions of the hamstring complex can be regionally targeted through exercise selection. So, each type of exercise offers unique but complementary training benefits.
So it makes sense that the same applies to the benefit of regularly using knee extension-based exercises like machine leg extensions or backward sled pulls, along with more multi-joint knee-oriented exercises like squats, lunges, and step-ups.
Research shows the leg extension creates much higher levels of activation in the rectus femoris compared to the squat (1), which is likely why other research shows the rectus femoris seems to grow more from single-joint, machine-based knee extension training relative to the other three quadriceps. (2)
Even if you're not motivated by that research, we can all agree that muscles respond (make strength adaptations) to how they're loaded, which is the principle of specificity.
Well, as you descend into the bottom of the squat or lunge (in hip flexion and knee flexion), the rectus femoris is trying to lengthen at the knee but shorten at the hip, and ends up staying roughly the same length. Then as you ascend (performing hip extension and knee extension), the muscle is trying to shorten at the knee but lengthen at the hip, and again ends up staying about the same length. (3)
In other words, to improve your strength in movements like backpedaling, decelerating forward momentum to change direction, or to walk down stairs or downhill, you need to train such actions. Reverse sled pulls and leg extensions are my top two options for the task.
Additionally, many of the arguments against utilizing the knee extension in healthy populations (out of concern for patella femoral joint forces and ACL health) are unfounded and logically inconsistent. (4)
Not to mention, when it comes to strengthening the quads, there's a multitude of studies showing better quadriceps strength gains (even in post ACL reconstruction patients) when combining open-kinetic chain exercises like leg extensions along with closed-kinetic chain exercises like squats and lunges over using only closed-kinetic chain exercises. (5)
- Ebben WP et al. Muscle activation during lower body resistance training. Int J Sports Med. 2009 Jan;30(1):1-8.
- Ema R et al. Inhomogeneous architectural changes of the quadriceps femoris induced by resistance training. European Journal of Applied Physiology. 2013 Aug;113(11):2691-2703.
- Beardsley C. Can you "just squat" for maximal leg development? Retrieved 2018 from www.strengthandconditioningresearch.com.
- Tumminello N et al. Are the Seated Leg Extension, Leg Curl, and Adduction Machine Exercises Non-Functional or Risky? NSCA Personal Training Quarterly. 2017 Jun;4(4):50-53, 2017.
- Treubig D. Why You Should Be Using Knee Extensions After ACL Reconstruction. Modern Manu Therapy. 2018.