This model locks together the arm and leg crankarms. The 690 and 660 share the first-time features of longer, upper (arm) crankarms and short, lower (leg) crankarms first to give your arms more cycling power. Next, this crankarm system alternates your arm-leg motions. Your arms and legs raise and lower in an alternating rhythm, and not a jerky "right arm-leg" and "left arm-leg" monotonous series. Third, your legs flex, however their shorter crankarms mean they move slower than the arms. You get a smoother, coordinated, and balanced arm-leg workout only from Saratoga's Spirit design! Foot plates w/ankle-toe straps included.