If you or your athlete cannot get the knee to full extension (straight) it's a problem - seriously. It can cause numerous performance deficits - both acute and chronic - a big problem if you want to produce maximal force in the lower extremities and avoid injury. Check out the video and discussion below:
It's clear in the video that sprint coach Jared Krout of KroutPerformance.com, is lacking full (terminal) knee extension. Normal knees are able to extend past neutral into hyperextension approximately 5 degrees. Restoring terminal knee extension is one of the first tasks your PT will take on post-surgically and it's one of the first things I look for in my athletes. You may have seen or even done the above exercise for quad-strengthening albeit with less resistance. However, the advantage of the way I've done it is to provide both an anterior glide to the tibia parallel to the joint surface in the Maitland/Kaltenborn method of joint mobilization and cue the quads to contract in a closed-chain function allowing what is essentially a two-for-one: mobilizing the joint into the position of restriction and activating the quads (which have an anterior joint glide moment in terminal extension) at this end range against some considerable resistance. Do NOT do this if you are early post-op ACL, patellar/quad tendon repair, or have some funky unstable knee (listen to your MD and PT - don't be a bozo). You can find these bands here.
So why is terminal knee extension so important?
1. If your joint does not have full range of motion in all directions, it is not normal/healthy. Now, an athlete can manage this restriction in terminal knee extension for some time, until they can't and they come to me with pain - in the knee or elsewhere. Without this critical range of motion they are predisposing the joint to high loads at a mechanical disadvantage.
2. If your knee is unable to get straight and then we stack some loading on top of it, think deadlift or jumping/landing, the joint is unable to fully disperse this load across the joint surface. A fully extended knee is considered the close-packed position of the knee because it is the position in which the articular surfaces are most congruent and the ligaments are the most taut. If you cannot obtain full extension, the load is poorly dispersed and cartilage does not do well with repetitive loading to a localized area. Here is some great evidence on the correlation of knee motion and early arthritic changes. Don't wear a hole in your knee!
3. In addition to the cartilage loading, lack of terminal knee extension means that you are unable to lock out the knee and the angle of pull on the patella and patellar tendon is altered, causing increased compression of the patella against the knee. This may lead to Patellofemoral Pain Syndrome and possibly patellar tendinitis due to the increased off-axis pull on the patella.
4. Perhaps most importantly, you need full knee extension to generate optimal quad force production. We will typically see poor quad function toward knee extension in those who are restricted in this motion. I don't need to tell you how important force development is for your quads. Any loss in quad function can destabilize the knee and increase joint loading.
Here's the kicker, loss of terminal knee extension will likely cause deficits up and down the kinetic chain. If you're unable to get your knee straight during triple extension (ankle plantarflexion, knee and hip extension) which is needed in basically all athletics, the hip cannot get into neutral extension causing you to overextend at the lumbar spine in order to keep you upright. Blow through some reps like that with hip flexors that are now super tight and it's hello spondy, plus it just looks like weak sauce. You may also have difficulty with appropriate foot strike during running, but that's a whole other topic.
Bottomline: Athletes must have full knee extension for optimal performance of the entire kinetic chain. If you are walking around and can't get your knee straight, get it together! Try the video above as a starting point (I realize that the screw-home rotational component may still need addressed for full extension, though not discussed here).