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Bicycle Crank Length — Go Short

Robert Clark and Christiane O’Hara stand next to a bicycle set up to a monitor

Cycling is a low impact activity used for fitness, commuting, competition or rehabilitation. A bicycle’s crank typically ranges in length between 170-175 mm. The length of the crank could impact several biomechanical and physiological measures which may affect cycling performance and potentially reduce injury.

Under the direction of kinesiology faculty Robert Clark and Christiane O’Hara, undergraduate Frost Research Fellows Cameron Swick, Natalie Grohmann, Austin Bohn and Ashley Shen conducted research with 18 elite level cyclists during summer 2018. The purpose of the study was to examine the effects of six different crank lengths on gross efficiency, heart rate, maximum power, optimal pedaling rate (cadence at maximum power) and joint kinematics of the hip and knee.

During steady-state cycling, shorter cranks had a higher gross efficiency (150 mm: 22.1%) compared to longer cranks (180 mm: 21.6%), (p < 0.001). There was an increase in optimal pedaling rate between 150 mm cranks (130 rpm) and all other lengths (180 mm: 120 rpm) (p < 0.001). 150 mm cranks have a significantly smaller hip (45 degrees) and knee (67 degrees) range of motion than all other cranks measured (180 mm: hip 51 degrees, knee 75 degrees) (p < 0.001). These results show that shorter cranks can be advantageous.

Preliminary results were presented at the Southwest American College of Sports Medicine (ACSM) Conference in fall 2018, and the students will present further findings at the National ACSM Conference in Orlando during spring 2019.

Students interested in similar biomechanics research can take a new, interdisciplinary class called Interdisciplinary Studies in Biomechanics. This activity-based class involves hypothesis-driven research in interdisciplinary teams, including written proposal development and written and oral communication of results to a scientific audience.

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