Abstract
The epidemiological evidence that increased activity does not increase the risk of joint disease suggests that the favorable effects of exercise counteract the detrimental effects of mechanical wear. One of the most likely contributors is articulation-induced cartilage rehydration: whereas exudation due to compressive joint loading causes cartilage dehydration, articulation actively restores hydration and its dependent biomechanical outcomes such as thickness, interstitial pressure, load support, mechanical stiffness, and lubrication. To date, however, there have been no studies investigating how patterns of intermittent activity might affect the biomechanical functions of articular cartilage. In this study, we leverage a new phenomenon, tribological rehydration, to elucidate how intermittency affects the biomechanical functions of cartilage under well-controlled sliding conditions. We quantified the biomechanical response of explants in the convergent stationary contact area (cSCA) over ‘equivalent days’ of varying activity patterns. Breaking the active portion of a day (30 minutes of sliding) into longer and less frequent bouts markedly increased the loss of interstitial hydration, pressure, and lubrication. Thus, the results demonstrate that the regularity of the activity regimen, specifically the duration of each sedentary bout, has a dominant effect on the biomechanical functions of cartilage. In more practical terms, the results suggest that brief but regular movement patterns (e.g. every hour) are biomechanically preferred to long and infrequent movement patterns (e.g. a long walk after a sedentary work day) when controlling for daily activity volume (e.g. 30 minutes).