Should knees be extended fully at the top position of the Leg Press? I personally become horrified by this video that I've found on YouTube. It looks as if the cause of the event was putting way too much plates on the machine, but perhaps pressing the knees with hands seems to had contributed to the outcome? Ouch!
For many, the gruesome example of the leg press hyperextension injury is presumably adequate proof to the dangers of locking out during exercises. However these sort of cases offer no more proof of the dangers of locking out than it demonstrates that weight training is inherently dangerous. We obviously know that the latter would be a gross overgeneralization due to the evidence to the contrary (see Weight Training Safety). What is apparently less evident is how it is also an overgeneralization to conclude that locking out is also inherently dangerous based on this sort of evidence. This particular video is of some ego-driven knuckle head evidently using far more weight than he is capable of handling. Besides using a weight so heavy that he cannot even lower to a proper depth, this misfortune could be attributed to any number of reasons including not following sound training principles or sporadically locking out with no consistency or systematic progressions. It is abundantly clear that his knee joint had not adequately adapted to this load in a locked out position or any other range of motion. This sort of thing does not just happen because of locking out the knees!
There are countless other examples of individual's occurring certain injuries while performing other exercises or utilizing certain techniques. People often make the mistake of attributing an injury to the specific exercise or technique in which they were performing, where there were in a fact many other factors which could be attributed to the injury other than the specific exercise or utilizing full range of motion. See following resources:
Other reasons for this particular injury could include genetic / structural factors such as predisposition to hyperextension including the shape of the knee joints contributing to susceptibility of hyperextension, symmetry of leg lengths, genetic collagen deficiency, and disposition of ligament tensile weakness, likely the Posterior Cruciate Ligament in this case. Even certain drugs can also impair mechanical strength ligament strength such as Corticosteroids and Non-steroidal Anti-Inflammatory Drugs (NSAIDS) such as Ibuprofen (Hauser & Dolan 2011). We simply do not know his case history, however it is so easy to blame this horific incident on locking out conclude everyone should avoid full range of motion even under normal training conditions.
To avoid full range of motion in effort to avoid hyperextension injury is in defiance of fundamental principles of exercise including adaptation and specificity. In fact, it is the act of moving a resistance through full extension until lockout which allows for proper adaptation of joint structures to avoid this sort of injury, both in and out of the weight room.
American College of Sports Medicine (ACSM) weight training guidelines as well as other authorities recommend full range of motion during weight training exercises. Fleck & Kraemer (1986) testify to the importance of full range of motion weight training in effort to reduce injury in and out of the weight room. Also see evidence for joint and structural adaptation (cartilage, bone, ligament and tendon).
In order for adequate adaptation to occur specific Adaptation Criteria must be followed. Injury typically occurs when one or more of those criteria are violated. That being said, the leg press may have the potentially slightly greater compression and hyperextension forces on the knee than compared to squats or deadlifts. The range of motion is typically shorter than other leg movements and feet are typically place slightly high to emphasize glutes over the quadriceps so greater weight can be used. Also, the greater forces required to move a heavy weight may result in an acceleration as torque forces dissipate throughout the final range of motion until a potentially sudden lockout. Under the weight of the legs and an extreme load, greater hyperextension forces can act on the knees during the abrupt lockout. Remember that force is equal to mass multiplied by acceleration. Increasing speed of lockout outside of this adapted force increases risk of injury particularly if other risk factors are present. Under an extreme load, the joint structures may fail, not because of the range of motion per se but because of the lack of adequate adaptation of the joint structures under the greater forces. For this reason, it is particularly important to control the speed of lock out when training with very heavy weights to keep forces on knees within adapted limits.
Locking out on the leg press is obviously controversial, arguably due to misunderstandings of how adequate adaptation is achieved. There are those who cannot make the distinction between idiotic practices as seen on the video, and locking out under controlled circumstances including Adaptation Criteria necessary for adequate adaptation. As explained, avoiding locking out may ironically increase risk of injury when joints are later subjected to real world forces, particularly from sporadic or unintented occurances.
American College of Sports Medicine (1995). Principles of Exercise Prescription, William & Wilkins, 5.
Fleck SJ, Falkel J (1986). Value of Resistance Training for the Reduction of Sports Injuries. Sports Medicine, 3, 61-68.
Hauser RA & Dolan E (2011). Ligament Injury and Healing: An Overview of Current Clinical Concepts. Journal of Prolotherapy. 2011;3(4):836-846.