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Bicycling Biomechanics One complete circular movement of the pedals around the bottom bracket is one two-phase pedal cycle. In the power phase, the cyclist pushes down on the pedal and transfers the greatest amount of energy to move the bicycle forward. The power phase begins with the pedal in the 12-o'clock position and ends with the pedal at the 6-o'clock position. The power phase is followed by the recovery phase, which progresses from 6-o'clock back to the 12-o'clock position. During the pedal cycle, the knee goes through approximately 75° of motion. The knee begins the power phase flexed about 110° and extends to about 35° of flexion. The quadriceps muscle provides most of the force, with input from the hamstring and gluteal muscles. While the knee extends, it also adducts because of the normal valgus angulation of the distal femoral condyles relative to the femoral shaft and foot motion during the power phase. This motion leads to medial translation of the knee during the pedal stroke while the knee extends. Additionally, the foot pronates during the power phase, causing an internal rotation of the tibia that increases stress on the medial knee. Also, an increased Q angle, seen in females, may further stress the medial joint. During the recovery phase of the pedal stroke, the knee flexes and moves laterally while the tibia externally rotates to ready the leg for the power phase of the next pedal cycle. Biomechanic abnormalities may arise from anatomic as well as functional factors. Muscular inflexibility is a leading contributor to injury. Most cyclists' quadriceps and hamstrings will tighten with prolonged riding. Inflexibility of the quadriceps, hamstrings, or iliotibial band (ITB) may restrict range of motion around the knee and are likely to increase the forces on the knee.6 Weakness in the leg muscles may lead to fatigue-induced alterations in pedaling technique, which will also alter the forces on the knee. Inappropriate saddle height or improperly aligned shoe cleats transmit increased repetitive forces through the knee, with a greater likelihood of injury.12
