Knee biomechanics: part 2

Biomechanics helps explain why the knee behaves the way it does during a squat, extension and flexion. If you know the principles, it is easier to choose exercises, maintain good technique and spot the points where unnecessary stress builds up on the knee.

Why does knee biomechanics matter?

At first glance, the knee joint seems simple: the leg extends and flexes. In reality, several things happen at once with every repetition. The femur, tibia, patella, muscles and tendons all have to work in the same rhythm. If one part moves incorrectly or is overloaded, the knee lets you know fairly quickly.

That is why it is not enough to ask whether an exercise is “good” or “bad”. The same movement can suit one person and be too demanding for another. What matters is the position of the leg, whether the foot is fixed, how long the range of motion is and which muscles have to do the work.

Although we are talking about the knee here, the same principles apply to other joints. The shoulder, hip and ankle also follow the laws of force, movement and muscle tension. The knee is simply a good example, because the squat, leg extension and lunges are familiar to most people who train.

How does knee biomechanics work?

The first important concept is the kinetic chain. In an open kinetic chain, the far end of the limb moves freely. For the knee, this means, for example, a leg extension on a machine: the thigh stays still and the lower leg moves. In exercises like this it is easy to feel a specific muscle working, but the training load is distributed differently in the knee than in movements performed standing up.

In a closed kinetic chain, the far end is fixed. The squat is the classic example: the feet rest on the floor and the body moves relative to them. Here it is not only the knee that works. The hip, ankle, trunk position and balance also influence how force travels through the knee.

The simple truth: the effect of a knee exercise is not determined only by which muscle “burns”. It also depends on which chain the movement takes place in. An open-chain exercise can be useful for feeling a muscle work or for rehabilitation. A closed-chain exercise teaches the body to use several joints at once, as is often needed in everyday life and sport.

The patella as an anatomical pulley

An anatomical pulley changes the direction of muscle force. In the knee, the patella is a good example. The patella does not make the muscle stronger the way training does, but it improves the mechanics of the quadriceps muscle, because it changes the angle at which force is applied.

When the muscle’s line of action moves farther from the joint axis, the muscle’s ability to produce movement increases. This is a very practical part of biomechanics. A small anatomical detail can change how effectively the muscle extends the knee and how much pressure is placed on the patella.

Length-tension relationship and elastic energy

A muscle does not produce the same amount of force at every length. The length-tension relationship describes how a muscle can usually work best when it is neither too short nor stretched too long. The greatest total tension often occurs when the muscle is in a stretched position, because in addition to the contraction, the elastic properties of the tissues also contribute.

According to the original source, the greatest total tension can be produced at between 120-130% of resting length. This does not mean every trainee should deliberately seek out extreme pre-stretch in knee exercises. On the contrary, caution is needed. Pre-stretching the thigh muscles can affect force production during knee extension, but it can also increase patellar compression and raise the risk of injury.

Practical biomechanical cues in the gym

In practice, it is worth checking whether the knee moves in line with the direction of the foot. If the knee repeatedly caves in or moves uncontrollably, the problem may not lie only in the knee. Hip control, ankle mobility and too much weight often play a part.

Another sign is where the pain is located. Sharp pain around the patella, uncomfortable pressure at the front of the knee, or a feeling that the knee cannot tolerate the bottom part of the movement is a signal to change your technique. This does not mean immediate panic, but it does mean the body is giving you information.

Two-joint muscles add another layer to the picture. The rectus femoris crosses both the hip and the knee. This means it can struggle to shorten enough to produce strong knee extension and hip flexion at the same time. This is called active insufficiency.

Passive insufficiency is the opposite situation. The muscle does not lengthen enough to allow a large movement in two joints at the same time. For example, the back of the thigh may not allow full knee extension and hip flexion at once. That is why mobility and stretching exercises have their place, especially if your training includes a lot of squatting, flexion and extension.

Honestly, not every trainee needs to memorize these terms. But understanding the principle helps you make better decisions: when to reduce the weight, when to change the range of motion, and when to choose a squat instead of a machine exercise, or vice versa.

FAQ

Is the machine knee extension a bad exercise?

Not automatically. It is an open-chain exercise and can be useful for feeling the quadriceps working. What matters is the training load, the range of motion, and whether your knee tolerates the exercise without pain.

How does a squat differ from a leg extension?

In a squat, your feet are fixed and the movement takes place in a closed chain. In a leg extension, the lower leg moves freely. Both train knee extension, but force and load are distributed differently.

Can stretching help improve how the knee joint works?

Stretching can help when movement is limited by muscle stiffness. It won’t solve every knee problem, but better flexibility may reduce passive insufficiency that sets in too early.

Source: www.ifbb.com
Professor Mauricio de Arruda Campos
IFBB Educational Commission

Translated by Jupp, Author Professor Mauricio de Arruda Campos

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