For those wishing to understand the effects of exercise on joints...
The thick layers of cartilage at joints do provide a measure of shock absorbtion.
Although most of cartilage is composed of material that is not living, the cells interspersed throughout the cartilage are still capable of manufacturing more of the material making up the cartilage, as well as the quality of that cartilage.
Mechanical loads that are not excessive stimulate the cells in cartilage to secrete more cartilage. [By excessive I mean plyometric jumping.]
"3.1. Moderate Mechanical Loading Plays a Role in Normal Tissue Remodelling
Several investigators have used a range of approaches to examine the effect of moderate exercise in maintaining cartilage homeostasis (Table 2). Indeed, there is sound evidence that individuals engaging in regular activity are less prone to incidence of OA, since frequent dynamic loading in the physiological range will increase cartilage thickness and maintain normal cartilage integrity [36, 37].
There is also evidence that exercise therapy in the form of aerobic and strengthening activities reduced pain and disability, enhances GAG content, and protects against cartilage degeneration in subjects with knee OA [4, 38–41]. However, the protective effect of recreational exercise has been reported to be dependent on a number of risk factors including age, body mass index, history of knee injury, smoking, and education [42–44]. Clinical observations suggest that healthy subjects as well as OA patients, in general, can pursue a high level of physical activity, provided that the activity is not painful and does not predispose to trauma .
In most animal studies, load bearing exercise minimises the development of OA. For example, daily exercise increased proteoglycan content and cartilage thickness in hamster and rodent models [26, 50]. In dogs, moderate exercise augments GAG content particularly in younger animals [51, 52]. In hamsters, early joint loading advances the maturation of matrix proteins, improved the integrity of the collagen network and the tissue resistance against OA in older animals [53–56]. In general, exercise and loading of joints within a physiological range appears to have beneficial effects over normal day to day activities characterised by modest movement.
The anabolic changes induced by exercise appear to enhance the load bearing properties of cartilage and may help explain how lifelong physical activity protects the joint from OA during later periods in life.