Tight ankles indicate edema in the ankles. Edema in the ankles is the most common cause of tight ankles. This happens with women during menstruation, and sitting for long periods of time. Tight ankles also occur very frequently when one is on a long flight sitting in the economy class seat on United-Continental planes. Long distance driving in a luxury car with automatic transmission also tightens the ankles. Tight ankles also indicate that the lower leg and feet muscles are tense and tight.
The ankles become tight when the butt is lower than the knee while sitting.
Throw away the chair. Stand up and walk about every 20 minutes or so. Do not continue to sit for more than 20 minutes. Sit in a chair without a seatback. Elevate the chair so that the butt is at least at the same level as the knees.