TL;DR: Take care of your knees. Short strides, faster turnover to compensate. Train your calf muscles (before they become cows and moo at you).
The idea is to counter landing ahead of your body, which is a “natural” tendency. Your knee must be slightly bent upon impact. This will allow your mid to forefoot to position itself directly under the knee and the supporting system of your body. A straight leg will not only negate your legs power potential (quads), but it will also cause a greater strain on the hamstring and calf muscles when they are called into action to unbend the joint.
Look at Kipchoge and his pacers. They're propelling themselves forward and the knees are always slightly bent, especially when landing. That is all, except two (perhaps three) pacers who look like they're heel-striking, knee-straight on landing and that's going to hurt over time.
Landing ahead of your center of mass is also what we call overstriding and is primarily an attribute of straightening the knee, but we mostly tell runners that to fix this you must stop heel-striking, which is true but it can also happen in midfoot-striking.
The problem of landing ahead of your body, is mostly caused by straightening the knees. So notice if you are straightening your knees, and instead keep them flexed. Landing under, instead of ahead of you, is not something you are born with as a runner.
University of Rochester tips for runners: Runners Knee (avoid Kneesles) on how to avoid ACL injury, namely train and condition year round. Strengthen your hamstring and quadriceps muscles. The hamstring muscle is at the back of the thigh; the quadriceps muscle is at the front. The muscles work together to bend or straighten the leg. Strengthening both muscles can better protect the leg against knee injuries.
Before using harder shoes or running shoeless, learn safe foot placement while wearing good (low mileage) shoes that absorb landing shocks and allow the development of support musculature. As musculature develops, more of the absorbed landing shock is elastically returned as motive force with less lost through heat and sound via shoes and ground.
Recovering Healthy Knees
A number of running tips can help keep knees healthy and (as "knee"ded) return them to a less compromised state. Some of these tips also help reduce skeletal stress body-wide. The tips fall into the following 4 categories:
- Minimize Impact Shock
- Reduce Body Twisting
- Avoid Habituated Damage
- Don't Cheat (like me)
Minimize Impact Shock
Probably the most important single factor is the amount of stride-shock that is introduced during running. Bended Knee, Short Stride, Hips, and Quiet Feet, together with shoes that are in good condition, help to minimize and distribute stride-shock through the body and "Flatten the Curve"TM (that's the Energy-Curve, not the other more famous curve) by spreading the stride-shock energy more evenly during the stride. A less emphasized but important feature of Quiet Feet introduces the notion that muscles can be strengthened to store impact energy and return it to a useful point in the stride. Using the elasticity of muscles (e.g. calf) to absorb stride-shock and apply that stored energy to power a later stage of the stride, diverts damaging energy away from other less resilient tissues and structures. A specific example would be to bias the foot ground-contact towards the mid-foot and away from the heel, throughout the stride, which requires developing strong calf muscles (and can take a while to achieve). The energy stored keeping the heel off the ground is returned to the powering part of the stride, automagically as the foot passes under the body.
Reduce Body Twisting
Various torsion forces need to be countered without exposing less tolerant tissues to their ravages. The major body-torsion relievers are the arms which help apply a counter twist, distributing torsion forces along the spine and redirecting some of that torsion force forward.
Avoid Habituated Damage
Road Camber contributes to laterally-uneven running forces that can place damaging forces biased towards one side, including foot angle, sideways knee forces, spine strain, etc. This is why we alternate our weekly running direction during WFR. Running towards the center of a cambered surface (erm, A Road) is not always a safe option! If possible avoid knee twisting and try to keep feet pointing forward at all times. This does not work for everyone but pressuring the outer part of the foot, as it lands, can help straighten foot placement and reduce knee strain.
Don't Cheat (like me)
Having mastered front-foot strike and trained past calf-mooing stage I noticed that when running downhill I could roll my foot onto its heel, resulting in a (not quiet) second strike and a nice bonus kick forward. Over time this too contributes to Kneesles... Don't cheat like me!
These hints and tips should be applied carefully and (gently) over time. Everyone's body is different. Please be aware of how these changes are effecting you personally.