Don’t Underestimate the Strength Needed for Ordinary Activities
Descending stairs (and presumably walking down a steep hill) requires 90 degrees of knee flexion ability. 90 degrees would mean that your shin becomes perpendicular to your thigh bone.
“Satisfactory flexion is needed for various daily activities: 67º for swing phase of gait, 83º for climbing up the stairs, 90º for descending stairs, and 93º for standing up from a chair.”
From this article: https://academic.oup.com/ptj/article/52/1/34/4589163
If I sit down in a chair and pull my knee to my chest, then I’m achieving something a bit greater than 90 degrees of knee flexion, but that’s without any load. Load is the amount of resistance my muscles experience. Resistance is caused by weight or drag (like water or elastic bands).
This all changes once I stand up. Now the load on my working muscles is my bodyweight. In a standing position 50% of my body weight will be on one knee and the other 50% will be on the other knee. If I squat I would need to be able to achieve a 90 degree angle between my thigh bone and my shin bone in order to get to the degree of knee flexion needed to go down a flight of stairs (or stand from a chair).
But that’s not the end of the story, because in a squat I’m distributing my bodyweight through the muscles of both legs. When someone descends a flight of stairs all of their body weight is on one leg.
Can you do a leg press with only one leg and move a weight equal to your body weight?
It’s pretty hard work.
So, what happens if you can’t stand on one leg and flex your knee to a 90 degree angle? Does that mean you can’t go up and down stairs?
You don’t have to achieve the entire 90 degrees of flexion with your full body weight because once you can reach your toes to the next stair tread you’ll start shifting some of the weight onto the lower foot. By the time your shin actually achieves a position perpendicular to your thigh most of the load will have shifted to the front foot. But you still have to squat deep enough to reach your toes to the next stair tread.
In descending stairs more load bearing flexion must be achieved than many are capable of. And in general it’s best to have an ability that is greater than the demand of the task in order to avoid injury or disability. Remember, you would still need to be able to move your bodyweight up from 90 degrees of flexion to rise from a chair.
Lots of people can still make it downstairs and not be able to do the aforementioned leg press. Once they reach the end of the range which they have the strength to control several interesting things happen.
- They can’t bend their knee any farther. This is important. Rather than collapsing, or tearing their tissues one simply can’t move. The body has a reflex arc that you will have a hard time overriding. Any time you attempt to move a joint into a position that the body feels is likely to result in rupture it will simply stop the motion. Weakness gets confused for stiffness all the time.
- The work that would have stayed in the quads gets moved to other parts of the body. The pelvis will drop on the reaching side, the side back muscles will tighten on the standing side, as will the hip. There might be more of a plop onto the front foot causing some of the work to be absorbed in the knee joint. The work of controlling the descent has to go somewhere. If it doesn’t go where it’s needed, it will land someplace else. (As an aside, this is precisely the reason that we must start our strength training at the point that’s right for us. If we think that we can do the exercise when, in fact, we can’t, the work will not go where it’s needed and will land someplace else.)
- Movement strategy and body position changes. The handrail gets used so that the upper body can do some of the work of controlling the descent. Someone might shift themselves to descend the stairs at an angle to decrease the amount of knee flexion needed, or change their knee angle in the frontal plane so that it’s no longer moving the same direction as their toes.
The body is a genius at working around our limitations and still getting us where we want to go, but there are consequences. Namely those consequences are that the work of controlling the force or load will repeatedly land in the wrong spot. When work lands in the right spot (and we allow recovery) it becomes strengthening, when it lands in the wrong spot it causes increasing pain.
The main point I’m trying to make is that it is easy to underestimate how much strength we actually need to have in order to do activities that we consider to be quite ordinary.