Muscle building: Squats, leg press or knee extensions - which
exercise is best for the quads?
Training the quadriceps muscles is an integral part of most sports strength programmes. The quadriceps are
important for cycling, swimming, running, jumping, sprinting, throwing - in fact, virtually every full-body athletic
movement. Three of the most common quadriceps exercises are the squat, the leg press and the knee extension. But
although all three exercises target the quads, they all vary in terms of knee joint forces, muscle activity and
functionality. There are even variations within an exercise through changes in technique or equipment.
The squat and the leg press are considered to be a different type of exercise from the knee extension. The squat and
the leg press are known as closed kinetic chain exercises (CKC), whereas the knee extension is considered an open
chain kinetic exercise (OKC). CKC exercises are distinguished by the foot being fixed and the knee joint moving in
conjunction with the hip and ankle in a predictable manner. With the squat, for example, the foot is on the floor. and
ankle, knee and hip all flex and then extend in sync. OKC exercises, on the other hand, are distinguished by the foot
being free to move and the knee joint working independently of any other joints. With the knee extension, the hip
joint is fixed and the knee flexes and extends with the foot freely rotating. (Recently, researchers have argued that
this classification system of exercises is too simplistic, but for the purposes of this article, the simple distinction is
What the researchers say
Researchers and physiotherapists seem to be agreed that CKC exercises are superior to OKC ones. CKC knee
exercises are considered safer and more effective since they place less strain on the anterior cruciate ligament (ACL)
and elicit a hamstrings co-contraction together with the quadriceps. Researchers from the Mayo Clinic (New York)
showed that leg press placed no strain on the ACL and elicited significant hamstring co-contraction, whereas the knee
extension placed strain on the ACL at 30° of flexion. The decreased ACL strain makes CKC knee exercises important
for ACL rehabilitation programmes.
The Mayo Clinic team also argue that CKC exercises are superior because they are more functional than OKC
exercises. Walking, jumping and running movements all involve the kinetic chain of ankle, knee and hip. Thus it is
advantageous to strengthen the quadriceps in a similar manner to real movements - specificity of training is an
accepted principle in sports science. During the squat and leg press, the knee and hip extend together. While the knee
extends, the rectus femoris shortens and the hamstrings lengthen, but while the hip extends, the rectus femoris
lengthens and the hamstrings shorten. The result is a simultaneous concentric and eccentric contraction at the
opposite ends of each muscle. This is known as the 'concurrent shift', and is a specific neuromuscular pattern which
occurs during all multi-joint leg movements. This concurrent shift does not take place in OKC exercises. Theoretically,
training the quadriceps in isolation, without normal muscular recruitment patterns, could lead to inefficient
neuromuscular coordination in athletic movements. Training movements that involve the concurrent shift are very
important, so CKC knee exercises are recommended.
Other studies have compared the muscle electromyographic (EMG) activity during the squat, leg press and knee
extension exercises. EMG activity is an objective measure of the amount of muscle activity during the exercise. This
allows exercises to be compared. Joseph Signorile and a team from the University of Miami investigated the EMG
activity of the quadriceps during the squat and knee extension. They used experienced lifters and determined the 10-
repetition maximum weight for each exercise. This guaranteed that both exercises required the same relative effort.
The team found that the squat elicited significantly more quadriceps EMG activity compared to the knee extension.
Signorile et al concluded that because of this the squat should be seen as the superior quadriceps exercise, especially
as it is a more functional movement.
More support the squat
Kevin Wilk and a team from the American Sports Medicine Institute investigated the EMG activity of the quadriceps
and hamstrings during the squat, leg press and knee extension. They also used experienced lifters and determined
the 12-repetition maximum weight for each exercise. Like Signorile, they found that the squat produced the most
quadriceps activity, peaking at 60 per cent of maximum activity levels. The leg press produced slightly less, peaking
at 52 per cent, with the knee extension less still, peaking at 46 per cent.
Wilk's team also investigated the knee joint forces during the exercises. They confirmed the Mayo Clinic findings
regarding ACL strain forces. The CKC exercises, the leg press and squat, placed no strain on the ACL, whereas below
40° of flexion the knee extension did place a strain on the ACL. However, the leg press and squat did place a strain on
the posterior cruciate ligament (PCL), and should therefore be avoided by PCL injury patients. The leg press and squat
also produced significantly greater knee compression forces than the knee extension, with the squat producing the
highest. Compression force refers to the vertical force between the surfaces of the femur and tibia, and excessive
compressive forces can cause knee injury.
Wilk et al also found that the squat was the only exercise of the three to elicit a significant hamstring co-contraction.
During the squat, the hamstring activity peaked at 36 per cent of maximum compared with the leg press and knee
extension in which hamstring activity peaked at 12 and 13 per cent respectively. This finding contradicts the Mayo
Clinic research which showed a hamstring co-contraction during the leg press. This suggests that just because the leg
press is a CKC exercise, it does not guarantee that there will be significant hamstring co-contraction. Other factors,
such as body position and angle of force application, affect whether CKC exercises elicit co-contraction of the
hamstrings, and are therefore functional to other movements.
The athlete's position is important
In a recent review paper, Wilk and his team summarised findings from research into co-contraction of hamstrings
during leg press and squat exercises. The most important factor seems to be the technique used or body position of
the athlete when performing the exercise. For example, with the squat performed normally with a bar across the back
of the shoulders, as the knee and hip flex, the trunk leans forward. At the bottom of the lowering phase, the bar is
positioned in front of the hips. This means that, as well as the quadriceps working to extend the knee, the hamstrings
must work to extend the trunk back upright.
By contrast, with the seated leg press, the athlete sits with the body fixed upright and the footplate is level with the
hips. Thus when the legs extend, the quadriceps work to extend the knee but the hamstrings need not work, because
the trunk is fixed and the weight is in line with the hips. This biomechanical difference explains why Wilk found co-
contraction with the squat and not with the leg press.
With a lying leg-press machine, the body position changes once again. The feet are placed above the hips and so the
weight is in front of the hips. Thus, when the leg extends, the hamstrings must work to extend the hip along with the
quadriceps which are working to extend the knee. So with the lying leg press there is hamstring co-contraction as with
the squat. This is the type of leg press the Mayo team used in their study, which showed leg-press hamstrings
Changes in squat technique can reduce the hamstrings co-contraction - for instance, by placing one's back against a
support. This change will isolate the quadriceps since the trunk is supported and the hamstrings do not have to work
to keep it upright. Other studies have shown that wide-stance squats produce more hamstrings and gluteal activity,
and narrow-stance squats more quadriceps activity. Again, changes in technique result in different patterns of muscle
From the research discussed above, we can draw some conclusions about the efficacy of the three quadriceps
1. The squat
This is probably the best exercise for the quadriceps. Studies have shown that the squat elicits the highest quadriceps
EMG activity compared to the leg press and the knee extension. This means that the squat works the quadriceps the
hardest. In addition, the squat is a CKC multi-joint exercise which elicits co-contraction of the hamstrings.
Researchers have argued that this makes the exercise functional to athletic movements and therefore a sports-
specific strength exercise. The co-contraction of the hamstrings means that the squat trains the 'concurrent shift'
pattern, which is very important biomechanically. The squat is also safe for ACL patients, although it is not safe for
Variations such as narrowing the stance will concentrate activity on the quadriceps, while widening the stance will
allow more gluteal and hamstrings activity. Leaning against a back support will isolate the quadriceps.
The major disadvantage of the squat is that it results in the highest knee-joint compression forces of all the exercises.
This may cause problems for those with weak knees because of the extra pressure on the surfaces of the femur and
tibia. For this reason, a correct squatting technique is vital to safety. Athletes also need to be strong in the low back
and abdominals, because the squat works the low-back muscles hard and a high intra-abdominal pressure is required
to support the spine. For heavy squatting, the athlete will need a training partner or a squat frame to train safely.
I conclude from the evidence that the squat is a very effective and sports-specific quadriceps strengthening exercise.
However, it is probably best for well-conditioned athletes only.
2. Leg press
This is a good quadriceps exercise. Wilk's research showed that the quadriceps activity was lower than with the squat,
but the knee compressive forces are not quite as high. The leg press is also safe from a technique viewpoint as the
machine is easy to use. Thus the leg press can be seen as a safer, easy alternative to the squat.
The major disadvantage of the leg press is that it is not necessarily functional simply because it is a CKC multi-joint
exercise. Wilk showed that with the seated leg press there was no hamstrings co-contraction. This means the
concurrent shift pattern is not trained as it is with the squat. However, hamstring co-contraction is possible with a
lying leg-press position with the feet placed higher than the hips. The lying leg press would be a good sport-specific
exercise, just like the squat, only a little safer and easier.
The complication with the lying leg press is that the feet should not be placed too high. Ideally, they should be placed
so that they are above the hip but level with the knee when the knee is fully extended. In this position, both the
quadriceps and the hamstrings will work. If the feet are too high, the knee can go below them, which means the
quadriceps stop working.
I conclude that the lying leg press, with the feet placed correctly, is a good alternative to the squat., potentially not
quite as effective but safe and easy to use, making it more suitable for weight-training beginners. The seated leg
press with feet low is not as good because it lacks the functional relevance.
3. Knee extension
This is shown by research to be the least effective of the three exercises, as it elicits the lowest quadriceps activity. In
addition, because it is an OKC single-joint movement, it has no functional relevance for most athletic movements.
The advantage of the knee extension is that compression forces are lower than with the CKC exercises. And, although
the knee extension places a strain on the ACL, the level of strain is safe for a healthy knee. The knee extension is
therefore a safe quadriceps exercise for athletes without ACL problems, but the fact that it works the quadriceps in
isolation makes it much less effective than other exercises. If variation in strength exercises is required, then
dumbbell lunges, barbell step ups, and single-leg squats are much better choices since they are CKC multi-joint
movements. The only athletic movement the knee extension is functional for is kicking, which requires a powerful
isolated quadriceps contraction.
I conclude that the knee extension exercise is the least effective and least functional of the three. However, it is safe
(for non-ACL patients) and would be useful for football and rugby players to improve kicking power.
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