Motion has a crucial role in joints. Long-term joint immobilization causes contracture and cartilage degeneration, whereas joint motion exercise is effective to alleviate osteoarthritis symptoms. However, the molecular mechanisms of these effects have been unclear. Here, we investigated how joint motion regulates joint homeostasis. We first established a minimized mechanical stress (MMS) model by knee joint immobilization of mice that were suspended by their tails. Histological examination showed synovitis reached a peak at 2 weeks, while cartilage degeneration had occurred by 6 weeks. When we cancelled immobilization at 2 weeks, synovitis was reversed and cartilage degeneration was prevented. Bulk RNA-seq of the synovium and cartilage of MMS model mice showed remarkable increases in expression of catabolic factors in synovium and remarkable decreases of matrix proteins in cartilage. On the basis of the cartilage gene expression alterations, Ingenuity Pathway Analysis estimated dozens of upstream cytokines and growth factors, including Spp1 and Il-1β, whose expressions were increased in the MMS model synovium. We also conducted scRNA-seq of synovium from control and MMS model mice. A few specific subsets were identified among activated fibroblasts and activated macrophages, which remarkably increased in the MMS model synovium. These subsets expressed cytokines and growth factors responsible for cartilage degeneration, which was consistent with the bulk RNA-seq and histological findings. Moreover, ligand-receptor analysis indicated dynamic interactions between the two MMS-specific subsets through these secreted molecules. In summary, loss of mechanical loading on joints induces specific subsets of activated fibroblasts and macrophages in synovium, which probably cause cartilage degeneration through secretion of catabolic and inflammatory factors. The synovial changes are reversible by resuming joint motion, which indicates that the two MMS-specific subsets or the pathological interaction between them are mechanosensitive. Taken together, we conclude that motion is essential to maintain the synovium and consequently joint homeostasis.