HP Compaq Presario,Presario 1515 Supplementary Manual page 15

Make sure your feet can rest solidly and comfortably on the floor while sitting. Use
an adjustable work surface and chair that allow your feet to rest firmly on the floor, or
use a footrest. If you use a footrest, be sure it is wide enough to accommodate
different leg positions within your comfort zone.
Be sure you have sufficient space under your work surface for your knees and legs.
Avoid concentrated pressure points along the underside of your thigh near the knee
and the back side of your lower leg. Stretch your legs and vary your leg posture
throughout the day.
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Rest your feet firmly on the floor or a footrest.
Don't dangle your feet and compress your thighs.