The lying leg press, like other types of leg presses, is a common strength-training machine that works your lower body. Unlike the squat, which is performed with free weights, the leg press provides your back with support. The leg press is often viewed as a safe and effective exercise, which is true, provided that the machine is set properly and you perform the exercise with proper form. However, if you bend your legs too deeply, the leg press can cause injuries or be harmful to your health.
Knee Hyperflexion
Bending your legs too deeply can place a potentially injurious stress on your knees. The anterior cruciate ligaments and patella tendon will be placed under tremendous pressure as your knees are bent to an acute angle. If this hyperflexion, or over bending of the legs, is performed forcefully or with speed, it could cause severe injury to the connective tissue within your knees. This type of ligament and tendon damage takes a long time to heal and may even need surgical intervention. An excessive range of knee joint movement can also result in increased knee wear and tear. This can lead to chronic degenerative joint conditions, such as arthritis.
Rounding of the Lower Back
Your spine has inward and outward curves, which help to dissipate any stress evenly throughout your vertebrae. The correct alignment of your vertebrae is called a neutral spine. If you bend your knees too deeply when using the leg press, the inward curve of your lower back is likely to become an outward curve as your pelvis curls under and your hips roll forward. This loss of neutral spine places an inordinate amount of stress on the passive structures in your lower back -- specifically your inter vertebral discs and spinal ligaments. Permanent injury can result if you place excessive stress on these tissues.
Elevated Blood Pressure
The more deeply you bend your knees, the greater the pressure your legs will exert on your internal organs. This, in turn, will drive up your blood pressure. In addition to elevating your blood pressure through abdominal compression, your blood pressure is further elevated by the action of holding your breath as you strain to lift the weight. This is called the Valsalva manoeuvre. While it is normal for your blood pressure to increase when exercising, elevating it excessively, especially if you have a history of hypertension, can cause cardiovascular complications, including ruptured blood vessels and even strokes.
Leg Press Safety
To minimize your risk of suffering any ill effects when using the leg press, avoid bending your knees too deeply. Stop the descent of the weight when your knees are bent to around or slightly less than a 90-degree angle. Keep your feet flat, your shins perpendicular to the foot plate and avoid holding your breath. Inhale as you bend your legs and exhale as you extend them. If you find your lower back becomes rounded when you perform the leg press, it may be because your hamstrings are overly tight. If this is the case, spend some time developing your hamstring flexibility.
References
- "Sports Injuries: Diagnosis and Management"; Christopher M. Norris; 2004
- "Designing Resistance Training Programs"; Steven Fleck, et al.; 2003
- "Essentials of Strength Training and Conditioning"; National Strength and Conditioning Association; 2008
- "Ultimate Back Fitness and Performance"; Stuart McGill; 2004
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