In response to the enquiry: “Fluid on the knee flying [sic]” and “knee pain flying”.
During a long flight seated in economy class with little leg room, bodily fluids can accumulate in the feet, ankles, lower leg, and knees. They become swollen and they feel stiff. I often feel my feet swell up and my shoes feel much tighter after about three hours in flight.
The lack of physical movement brings on the accumulation of bodily fluids. Walking to the rear lavatory alleviates the stiffness and loosens the joints.
The lack of physical movement in flight also reduces efficient blood flow to the feet and legs, and this also reduces oxygen flow. Muscles stiffen when oxygen flow is hindered and reduced.