Step width and peak knee forces.

Forget older adults, this is for everyone. If you have a step width that affords knee over foot, hip over knee, and you load those stacked joints, there will be less peak adduction and abduction loads at the knees . . . . and less risk for frontal plane drift of the hip-pelvis, improved control of limb rotation during loading, and reduced risks for over pronation at the foot-ankle complex. These are anti-cross over gait principles.
And, this is obviously not just a stair descent or ascent issue, these are normal fundamental gait (walk and run) principles that just make good common sense !

Knee. 2014 Aug;21(4):821-6. doi: 10.1016/j.knee.2014.03.006. Epub 2014 Apr 3.

Effects of increased step width on frontal plane knee biomechanics in healthy older adults during stair descent.

Paquette MR1, Zhang S2, Milner CE3, Fairbrother JT2, Reinbolt JA4.

Landing strategies focusing on the control of tibial rotation in the initial contact period of one-leg forward hops - Chen - 2016 - Scandinavian Journal of Medicine & Science in Sports - Wiley Online Library→

“If the knee is whining and doing things it should not be doing, the wise clinician first looks at the foot-ankle and the hip-pelvis complexes, where the blood has dried. Don’t look for the fresh blood at the knee” - Dr. Allen

If you cannot control pelvis position on the femoral head, or hip rotation or initial foot arch mechanics, the knee is going to give in to the directional loading response and that typically means medial valgus movement. This is internal tibial rotation or spin.  

Here is an analogy i use with all my patients. The knee is like the middle child. In the simplest terms, you have 3 lower limb joint complexes. The foot/ankle, the knee and the hip. The knee is the middle of these 3 joint complexes.  

Similarly if you put 3 children in the back of the car, the one sitting in the middle is the one directly impacted by the child on the right and the left.  When you hear the middle child screaming and whining, the smart parent first looks at the two apparently “innocent” children looking out the windows (with blood dripping off their elbows). 

Similarly, the knee takes this same seat. IF the knee is whining and doing things it should not be doing, the wise clinician first looks at the foot-ankle and the hip-pelvis complexes, where the blood has dried. Don’t look for the fresh blood at the knee

Changing landing strategies with the focus of control of tibial rotation, requires the astute clinician to look at all the children.

Dr. Shawn Allen, one of the gait guys.