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Leg Curls and Extensions

Tuesday, June 6, 2023 - 2 Likes

Brent Brookbush

Brent Brookbush


Leg Curls and Extensions

By Brent Brookbush MS, PES, CES, CSCS, ACSM H/FS

Seated and Prone Leg Curls:

Although the risk of these exercises are fairly low; so is the reward. These exercises do not require more than optimal flexibility, and they do not force an individual to assume a position that increases the strain on passive structures (provided the equipment has been set up properly).

These exercises do run the risk of reinforcing common compensation patterns by increasing the strength of synergists that are overactive in LPHC and lower-leg dysfunction. Specifically, hyperactivity of the biceps femoris dominates hip extension when the gluteus maximus is inhibited resulting in SI joint dyskinesis and tibia external rotation (feet turn out) during gait. Strengthening these muscles reinforces compensation patterns whether seated or prone. You may have noted that during hamstring curls the feet turn-out (biceps femoris dominance as an external rotator of the knee). My suggestion to the average “gym-goer” is to limit isolation exercises to the activation of commonly underactive musculature (gluteus maximus, gluteus medius, tibialis anterior and posterior, etc.), and concentrate on compound movements during resistance training.

Seated Leg Extension Machine:

This exercise does not require more than optimal flexibility so the risks associated are generally low. There is concern when this exercise is used during strength training. There is potential risk to passive structures during full extension, and this exercise may reinforce compensation patterns.

The increased risk to passive structure during strength training is due to the downward force created by the leg pad during full extension. Although the increased tension in the quadriceps increases tension in the patellar tendon and compresses the tibia into the femur it does not fully balance the posterior shear force created during heavy lifting. The inability of the quadriceps to create an anterior shear force on the tibia leaves much of this force to be balanced by the ligaments of the knee specifically the PCL.

Further, without a movement prep/corrective exercise warm-up this single joint activity can reinforce imbalances between quadriceps muscles. Specifically, the vastus lateralis often becomes overactive and bound in lower-leg and LPHC dysfunction due to its role in TFL/ITB/VL complex (Tensor-fascia latae/Iliotibial band/Vastus lateralis). This complex becomes synergistically dominant during hip flexion and abduction and should be released in cases of dysfunction before participation in strength training. The rectus femoris is often short and tight as well. In LPHC dysfunction the rectus femoris may be overactive due to an anterior pelvic tilt, and during knee dysfunction the rectus femoris may develop trigger points in response to knee pain.

This exercise may have benefits when used in a corrective program for knee dysfunction, but this exercise would not be my first choice. Research show an increase in vastus medialis obliquus (VMO) activity in the last 10 degrees of knee extension. Because the VMO is an important medial stabilizer of the knee and generally underactive, we could use this exercise with light weight to strengthen the VMO by focusing on the last 10 degrees of movement. However, when you compare a machine extension to the increase in muscle activity found less stable environments, like leg extensions off the side of a table with an ankle weight, it is hard to recommend the machine. Further exercises like a TKE (Total Knee Extension) provide additional benefit - improved tibiofemoral mechanics (allows for femoral external rotation) and incorporation of the commonly underactive glute musculature.

© 2011 Brent Brookbush

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