Lower extremity motions during cycling are often assumed to occur in the sagittal plane. While seemingly logical, this assumption has not been rigorously tested. Frontal plane rotation of the ankle joint (inversion/eversion) has been studied extensively during gait but infrequently during cycling despite the suggestion that excessive eversion or pronation may be related to overuse knee injuries. Two-dimensional sagittal plane hip, knee, and ankle joint kinematics were generally found to be similar to simultaneously measured 3-D values. Despite the similarity in motion patterns, maximum hip angle was 34° more flexed in 2-D than 3-D. Maximum and minimum frontal plane ankle joint angles were similar in 2-D and 3-D. However, during the middle of the pedal cycle, 2-D frontal plane ankle joint motion deviated from 3-D, such that maximum ankle eversion was reached 36% of the pedal cycle later in 2-D versus 3-D. The discrepancy at the hip was due primarily to differences in hip angle definition for 2-D and 3-D approaches, and an alternate convention for hip angle in 2-D is suggested. Discrepancies in frontal plane ankle joint motion are due to weaknesses in the planar approach and would be difficult to overcome without resorting to 3-D measurement.
The authors are with the Exercise & Sport Research Institute at Arizona State University, Tempe, AZ 85287.