Seated Leg Extension with Curl Machine: How To Do & Muscles Worked

Jason Williams
leg extension and curl machine

The Leg Extension Machine is an isolating exercise of the quadriceps, which helps condition and shape the quadricep muscle. The technique of doing the movement on the Leg Extension Machine is quite simple, however it has its own nuances which is important to be aware of to ensure you have an effective leg workout.

What Muscles Does Seated Leg Extension Work?

The main workload falls on the quadriceps femoris muscle. Quadricep are the largest muscle in the human body and its main function is to extend the knee joint and straighten the leg. This muscle is also partially involved in the hip joint flexion.

The quadriceps consist of four smaller muscles:

  • rectus femoris muscle (covers the front thigh);
  • vastus intermedius muscle (located in front under the rectus femoris muscle);
  • vastus lateralis muscle (forms the outer side of the thigh);
  • vastus medialis muscle (forms the inner side of the thigh).

The Seated Leg Extension allows you to train all the bundles of the quadricep, helping to condition and shape of the upper legs. The added advantage is that it is also an exercise that will improve your performance in basic exercises, such as the Barbell Back Squat and Leg Press.

The Leg Extension exercise is typically used as a warm-up before the main leg workout. It can also be used for pre-fatigue (at an advanced level) or for finishing off the quadriceps after basic exercises. It can also be used by beginners as a basic exercise to strengthen the upper leg muscles before moving onto basic exercises like squats which may be too difficult for them.

The Effects of Leg Extensions using the Curl Machine on Knees

There are considerations that need to be accounted for when and it’s effects on the knee joints. The anterior cruciate ligament is subjected to significant stress when flexion and extension of the legs occur under the influence of weight. This therefore creates a risk of injury which is why that it’s important to have correct technique and to use moderate weights.

With that said, this exercise also helps to strengthen the knee ligaments (when done correctly and with light weight). It is typically used for athletes who are in rehabilitation after knee injuries.  Therefore, for this exercise to have only a positive effect, it is important to perform it with correct technique.

leg extension with curl machine

How to Do the Seated Leg Extension Exercise with a Curl Machine?

Adjust the machine for your body type/proportions. There are typically two adjustments that need to be done: the position of the seat back and the location of the roller at the lower part of the legs, and yes, of course, the weight.

  1. The backrest should be adjusted so that your upper leg fits completely on the seat and the knees are positioned perfectly at the edge of the seat, and the sacrum is pressed against the back of the seat.
  2. The position of the second/lower roller should be set so that it rests against the lower part of the lower leg, just above the ankle IE against the lower shins. When you are in this position, the angle in the knee joint should be at least 90° or less.
  3. The final adjustment will be to set the weight depending on your level of training. Select a weight which will enable you to complete 3-4 sets of 15-20 reps. Heavy weights are not necessary for this exercise.

Execution of the exercise: 

  1. Sit on the Curl Machine, press your back against its back rest and put your feet/lower legs behind the roller. Grasp the handles on the sides of the seat and point the toes up.
  2. As you exhale, slowly straighten your legs at the knees. Tighten/flex the quadriceps as much as possible at the top point. Hold this position for 1-2 seconds.
  3. As you inhale, lower your legs down under control, but do not bend/lower them all the way to the end. IE: The quadriceps should always be under tension, even at the lowest point. Look out for any negative sensations in the knee joint and never throw your legs up and down sharply.
  4. Perform the required number of repetitions.

You can complete the set with partial-amplitude repetitions if your muscles are already too tired to perform a full-amplitude extension. The main thing is not to allow sudden movements and not to throw the weight down; control the weight at all times.

Technique Options

Among the different options of this exercise, you can execution of extensions in turn IE: First with one leg, then with the second. This approach helps some athletes better feel the work of the muscles, or it can be used in the case of asymmetry.

You can also do the Seated Leg Extension by turning and pointing the feet in or out. The workload is shifted towards the inner (medial) head of the quadriceps when turning outward, when turning inward towards the outer (lateral).


Doing Leg Extensions with the Curl Machine will help to give the thigh muscle the desired overall shape and condition them for further strength and muscle mass gaining exercises.  For the harmonious development of the body, you should also supplement this exercise with other workouts for the back of the thighs.

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