Crank speed (power output when combined with gearing) is a result of a coordinated effort of hip flexion/extension and knee flexion/extension. On the downstroke, peak joint speed (hip and knee extension) attained when the crank arm reaches the position of 45 to 90 degrees vertical. This position correlates to the position at which the crank arm reaches it's peak rotational speed.
On the upstroke, peak joint speed (hip and knee Flexion) is attained in a sequential way whereby the knee reaches peak speed first as the pedal begins to rise out of the bottom of the pedal stroke, and peak hip speed is attained when the pedal reaches the top quarter of the pedal stroke.
Joint speed production and subsequent crank speed and power output stem from the pelvis and depend on good core stability as a foundation for pushing (extension) and pulling (flexion). Symmetry between the joints on the left and right sides and the proper sequence of downstroke and upstroke mechanics result in a cyclist with good pedal stroke efficiency with respect to crank speed and power output. The most effective crank speed and power are created when extension on one side works in conjunction with flexion on the opposite side. When one side does not contribute effectively, the other side does more work to create the same crank speed and power. Then compensatory mechanisms begin to occur to accommodate for the asymmetry, leading to more energy expenditure and a resultant loss of crank speed and power output. Compensations can also result in peak joint and crank speeds occurring in less than optimal positions in the crank cycle This can affect cycling performance and the ability to maintain a certain power output over a period of time, leading to an increase in the rate of fatigue.
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