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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 sufficient.)
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 activity.
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 activity.
From the research discussed above, we can draw some conclusions about the efficacy of the three quadriceps exercises.
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 PCL patients.
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.
Raphael Brandon
Useful Links
common knee injuries, knee injuries symptoms, runners knee, iliotibial band syndrome (ITBS), knee injuries treatments
knee brace, sport braces for knee injuries
acl knee injury, anterior cruciate ligament, knee injury symptoms, knee injury prevention, knee injury exercises
exercises for muscle building
knee strengthening exercises
quadriceps exercises
leg press
leg exercises
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 activity.
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 activity.
From the research discussed above, we can draw some conclusions about the efficacy of the three quadriceps exercises.
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 PCL patients.
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.
Raphael Brandon
Useful Links
common knee injuries, knee injuries symptoms, runners knee, iliotibial band syndrome (ITBS), knee injuries treatments
knee brace, sport braces for knee injuries
acl knee injury, anterior cruciate ligament, knee injury symptoms, knee injury prevention, knee injury exercises
exercises for muscle building
knee strengthening exercises
quadriceps exercises
leg press
leg exercises
This article was taken from the Peak Performance newsletter, the number one source of sports science, training and research. Click here to access these articles as soon as they are released to maximise your performance
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