At this year's Radiological Society of North America's annual meeting (did any of you make it?) a study on exercise and knee health was presented.
The question: Does activity level (low, medium, or high) impact the health of the knee joint?
The activity level: Low was deemed as sedentary to exercising less that once per week. Medium was exercising regularly, but only low to non-impact activities. High activity levels featured both high-impact activities as well as large quantities of movement (multiple hours per day of high impact or repetitive motion type exercises).
The results: Looking at MRIs of 236 adults, ages 45-55, showed that 93% of the high-activity level group had knee cartilage damage as opposed to 60% in the low activity group. The high-activity group's knee damage was three times more severe than that of the couch potatoes (note: not the group's official name).
The conclusion: Excessive mileage and impact forces are contributing to the increasing levels of osteoarthritis, knee surgeries, and knee replacements in the United States.
But before you go logically extrapolating the fact that your LA-Z-Boy is a healthy choice, allow me to comment.
Firstly, we need to keep in mind that while "movement" is a physical requirement to physiological function, "exercise" is a modern creation designed to deal with the waning health of our immobile population. Cramming a whole day's worth of movement into a one or two hour-long intense bout has the same physiological impact of starving yourself all day then following it up with a 2500 calorie meal. Your body doesn't work that way!
Secondly, please understand that alignment matters. The position of your toes, feet, ankles, knees, hips, chest, and head all dictate the loading in the knee (and all the joints, if we're going to be correct). If you are not balanced, the incorrect and non-symmetrical wearing patterns will result in an injury. If you are more active (time-wise) and choose activities with high loads (running vs. walking or riding hills vs. flats) the damage will accumulate faster.
The moral of today's story? It is much better for the health of your joints (and surprise surprise, your metabolism) if you move throughout the day as opposed to all at once. The weight-loss affect of exercise is the same (in fact, it's actually easier to lose weight by getting your movement in smaller doses but more often), and your joints have time to adapt to the loading. Imagine your knees, completely unloaded while you sit in your desk chair/car/recliner and then all of a sudden you get up and start jamming the bones into the cartilage at 3-G forces. Not very considerate. Also, get yourself aligned. If you have one foot that points slightly out to the side, a knee that doesn't bend the same as the other, or a torso that tends to lean, don't make the problem worse by jumping up and down on it...FIX IT! We align the tires on our cars because we want our car to last. Extend the same courtesy to the human body.
And...she's off the soap box.