The leg extension is a popular exercise amongst exercise enthusiasts and professionals and is used to develop the strength and endurance of the quadriceps muscle group. Defined as an isolation and open kinetic chain movement, the safety of the leg extension has for some time been questioned by some professionals. In this article we will determine the fate of the leg extension, should it be assassinated or is it worthy of the space it occupies on the gym floor? Let’s explore….
During the leg extension, the upper leg is supported by the seat and the lower leg remains unsupported throughout. From this position, the lower leg is loaded with the ankle pad which distends from the central component of the machine via a telescopic arm. Throughout the extension movement, and especially at the end of the range, the knee joint is exposed to colossal shear forces which place considerable strain on the cruciate ligaments, particularly the anterior cruciate ligament (ACL). In this position, the tibia and fibula of the lower leg are loaded, forcing them posteriorly (backwards) and in the opposite direction to the femur of the upper leg.
Compound movements, which are typically far more functional than isolation exercises, result in a co-contraction of the opposing (antagonist) muscle group- the hamstrings. The simultaneous activation of the hamstrings provides additional stability to the knee as tension is created on its anterior (front) surface by the quadriceps and on its posterior surfaces by the hamstrings. When opposing tensions are equal, the knee remains stable- when the magnitude of this tension is unequal, shear forces are created and the knees stability is compromised further.
As outlined in the previous paragraph, compound exercises (multi-joint movements) are considerably more functional than isolation exercises (single joint) because they represent more closely how the body moves on a daily basis. Unlike robots, human beings move through multiple joints and planes at the same time in order to perform daily activities. Therefore, in order for an exercise to prepare individuals for these activities, it must replicate the way in which the body generates movements. Exercises which employ a greater number of joints and muscle tissues are likely to have a greater transferability to other daily and sporting activities. In addition, exercises which either move through multiple planes of motion, or resist movement in multiple planes are again more functional and are highly transferable to other activities.
In addition to the number of joints active throughout the leg extension movement, this exercise is also performed from a static and stable platform, which requires no intervention from the body’s natural stabilising or fixator mechanisms- especially that of the core. The long-term risk of such exercises is that the strength of the peripheral muscles like the quadriceps, will be disproportionate to that which the body is able to provide adequate stabilisation for- the net result will therefore be injury.
To summarise- the leg extension exposes the stabilising structures of the knees to considerable shear forces which increase their potential for trauma. The transferability of the exercise is somewhat limited and the strength/endurance of the quadriceps is developed disproportionately to that of the muscles that provide the body with stability. As if that wasn’t enough to help you make your own mind up, from a weight management perspective, less energy is expended at the same workload as equivalent compound movements because less musculature is employed.
We could go on but there really isn’t much point- go on, pull the trigger!