Hello, A severe deep leg pain mainly present on sitting is indicating more towards positional in nature which occur mostly in form of Sciatica ( occurs due to prolong compression & pressure on sciatic nerve). Pain can also develop due to compression of nerve root at its exit from vertebral canal at certain posture, or due to formation of clot at the site which can lead to a severe condition known as pulmonary thromboembolism. From my point of view your pain which is more on sitting position is least likely to be associated with arthritis in which lesion mostly located in joints, so it can be an accidental finding. But diagnosis of arthritis can not be ignored. So consult a orthopaedician for more evaluation and investigations like Doppler USG, MRI, blood test for antibody detection. Avoid improper position& sitting for prolong period, use local hot packs, do some light exercises under guidance of physiotherapist, NSAID Ibuprofen/ naproxen can be used for pain relief. Thank for consulting HCM.