Leg extensions and pull-ups: some useful technique tips to avoid injuries

We often hear conflicting guidelines and advice in the world of gyms and fitness, even on the most basic and established aspects of training. It is true that there are many valid training methods and that all exercise programming must be contextualized according to the person and his/her goal, needs, and abilities. It is also true that there are some fixed points from which no one can deviate. We talk about rules that are the basis of any training programme, whether it's strength or endurance, whether it's done with weights or bodyweight, and of course biomechanics.

Biomechanics of movement in the gym

Biomechanics is the branch of science that applies the study of mechanics to living beings, both in motion and at rest. What is the purpose of studying biomechanics in the gym? It is certainly useful, to know not just what you are doing but why, or rather, why you are doing an exercise in one way rather than another.

Many times, you will be given advice by different trainers or gym-users on the various merits for performing an exercise in one way or another, yet if you analyze the movement you find that you are not effectively training the muscle you wish to target, or that that movement risks overloading structures in a way that can lead to injury. There are also situations in which some exercises or machines are discouraged universally, without a well-founded reason. This is often the case for the leg extension machine, especially when dealing with injuries (or post-operative recovery) of the anterior cruciate ligament (ACL).

It is useful to explain the architecture of the knee very quickly in order to avoidinjuries while exercisin with leg extension machine. Joint is composed of the femur, tibia, fibula and patella, held together by the anterior cruciate ligament (which prevents the tibia from sliding forward with respect to the femur), the posterior cruciate ligament (which prevents the tibia from sliding backwards with respect to the femur) and the collateral ligaments (lateral and medial) with the function of stabilizing the knee while keeping the bone segments in line with each other. The anterior of the joint is wrapped in the quadriceps muscles which, with their common patellar tendon, have the function of extending the leg at the knee.

With an understanding of the knee joint anatomy, it is necessary to understand why leg extension is generally not recommended for individuals with an ACL injury, also explaining how the customizations that are sometimes adopted to protect the ACL are useless, if not harmful. We also offer suggestions for modifying the leg extension exercise to be safer for those with an ACL injury.

Correct execution of the leg extension exercise

A common modification to the standard leg extension set-up that is incorrectly recommended to avoid overloading the knee and its ligaments, is to raise the position of the ankle pad proximally on the tibia, moving it closer to the knee joint.  This is recommended based on the false belief that decreasing the length of the lever between the point of load and the articulation (fulcrum) will reduce the forces involved and therefore a reduce the internal load at the level of the knee.

This would be true if the exercise on leg extension machine were performed with a simple ankle weight or cable, but the leg extension machine has cams and a system of pulleys that engineer the movement to ensure that regardless of where the pad is placed, the user will experience the same internal load when a certain weight is selected. If not, a 190 cm tall person would have to work much harder than a 150 cm tall person at the same weight, as the ankle pad would be further away from the pivot point to position him/herself correctly on the machine.

Limited movement on leg extension machine with ACL injured knees

It is known that in the first part of the movement (60-75° flexion) on the leg extension machine the lowest tension is recorded on the ACL, while in the last degrees (between 45 - 0° flexion) the action of the patellar tendon causes the tibia to slide forward with respect to the femur, overloading the ACL. To avoid placing excessive strain on the ACL, therefore, it is obvious to suggest performing only the first half of the movement. This may be a good solution for safety, but how do we then go about strengthening the quadriceps muscles through the second half of the leg extension movement?

The solution to this is to change the range adjustment on the leg extension machine, moving the ankle pad about 17-20cm forward from the knee. This helps because placing the starting angle higher on the leg extension machine causes pressure at the level of the tibia that tends to push it back, reducing (or canceling out) the normal tension that would arrive at the level of the ACL, moving the pressure towards the PCL (a much stronger ligament). In this way, it is possible to complete the leg extension movement  by canceling the shearing forces at knee level.

Pull-ups: With or without elastic assistance?

Another error that is constantly made in the gym concerns the pull-ups. It is common to see people using elastics loop bands hooked to the bar and placed under their feet to assist with the pull-ups movement.

The prevalent theory on the gym floor is that if you cannot complete the pull-ups movement unassisted, the rubber band will give an extra push and counterbalance some of the body weight to allow the body to be lifted.

Among the packaged weight equipment, Vertical Traction of the Artis Line is the ideal solution to train the strength of the great dorsal and arm muscles from an ergonomic sitting position.

Stepping back to consider the strict pull-ups exercise, the target muscle is the latissimus dorsi. It is observed that this muscle expresses much strength in the first part of the adduction or extension movements and contributes little force in the final phases as it nears full contraction. This means that during pull-ups, the greatest assistance is required in the final phase of the movement when the body is closest to the bar.

Analyzing the force profile of elastic during this movement, one immediately notices how this is exactly opposite to that of the latissimus dorsi. During the pull-ups, the elastic is stretched and gives a lot of assistance but provides minimal support in the final part as it returns closer to its contracted shape.

It is undeniable that eventually, a person that regularly performs pull-ups with a band will eventually be able to complete free pull-ups. It is not the most effective way to progress, however, and once full pull-ups can be achieved, further use of the elastic will not aid additional progression effectively as the amount of assistance cannot be quantified.

If the elastic is not the most effective training aid with pull-ups, how can progression to bodyweight pull-ups be achieved more efficiently? As the problem is not being able to lift the entire body weight, the solution is to lift a lighter weight initially.

Pull-ups can start from a standing position,  pull-ups suspending the entire body from a bar, pull-ups from a sitting position with some of the bodyweight resting on the floor to reduce the effective workload. If available, use rings for your firs pull-ups, to begin with as they allow a more rectilinear trajectory and the ability to rotate the wrist makes the movement like the mechanics of pull-ups action. If rings are not available, a bar located lower than the classic ones can be used. Both options are available using Omnia.

Pull-ups from the seated position mean you can only the weight of the trunk must be lifted, rather than the combined weight of the trunk and lower limbs. At a very early stage of pull-ups, it may also help to use the legs to push slightly, further reducing the effective load.

The dynamics of the pull-ups exercise are respected, and overload can be progressed gradually simply by changing foot position to offer less assistance over time. Once pull-ups can be performed with the legs outstretched (still maintaining foot contact on the ground but not pushing to assist) the addition of a foot rise such as an aerobics step under the heels will offer a means of further progression.

As much as there are unavoidable fixed points, it is correct to say that there are not always right or wrong techniques, it is necessary to contextualize every detail of an exercise to understand why it is being performed in a way. Biomechanics helps in this, by going through the various "why" of movements, analyzing them and showing what is the safest and most functional way to move.

It is possible to achieve results by contradicting every basic law of biomechanics, but at what cost?

What matters is not simply to train today, but to obtain predictable results in a time-efficient and safe manner, improving the likelihood of achieving goals and minimizing the risks inherent in exercise.