Pain does not track knee load, review finds

Knee loading is associated with how severe knee osteoarthritis is and how fast it progresses, while knee pain is what drives reduced function and quality of life. It is tempting to assume the two travel together — that the activities which load the joint hardest are the ones that hurt most — and advice to avoid particular movements often rests on that assumption. But loading and pain are measured by different means and may not line up. This review set out to quantify both across ordinary daily activities and to work out why published studies disagree with one another.

The authors searched PubMed, Web of Science and Scopus, plus manual searching, up to July 2024, for studies measuring both knee loading and knee pain in people with knee osteoarthritis during walking, sit-to-stand and stair climbing. A total of 183 studies were included. The team combined a descriptive synthesis of the data with a meta-analysis using a mixed-effect model, and examined methodological reasons for the differences between studies. Alongside the pooled estimates, they looked at how activities ranked against each other for loading and for pain, and at the relationship between the two within walking.

Walking produced the greatest knee loading, followed by stair climbing and then sit-to-stand, but only sit-to-stand was significantly lower than the others (p < 0.001) — the overall trend across activities was not statistically significant. The pain ranking was different: stair ascent was most painful, followed by walking, then stair descent, then sit-to-stand. Within walking, there was a strong negative correlation between knee loading and pain (r = -0.507), meaning higher measured loading went with lower reported pain across studies. The authors note that a substantial number of studies examined only a single activity, most often walking, which could bias the comparisons.

The loading trend across activities, although non-significant here, matched the authors' earlier experimental work, which lends it some consistency. Where this analysis differs from other studies is likely explained by high heterogeneity in the populations tested and by differences in method, which the review flags as the central problem in this literature. The negative loading-pain correlation complicates any simple model in which more load means more pain, but as a cross-study association it cannot establish direction or cause. The authors recommend that future work report participant characteristics and experimental design in detail and include control groups to account for loading offsets.

The intuitive equation of heavier joint loading with more pain does not hold up neatly in this pooled picture, and the activity that loaded the knee most was not the one that hurt most. That is a reason to be cautious about ruling activities in or out purely on the assumption that they must be grinding the joint. It is not a reason to conclude that loading is irrelevant to osteoarthritis, since loading remains associated with severity and progression. The wide variation between studies means these rankings are best read as a map of an unsettled research area rather than guidance for an individual knee.