A TWO-DIMENSIONAL KINEMATIC ANALYSIS OF A "BUNNY HOP" PERFORMED BY A SKILLED MOUNTAIN BIKER

Abstract

The motions taken by a skilled mountain biker during the temporal stages of a bunny hop over an obstacle were determined in this study. A "bunny hop" is defined as a hop (usually over an obstacle) performed by a cyclist in which there is no ramp or other means of launching the cyclist and bicycle. This study is of value because it provides qualitative and quantitative information about a skill in which there is little or no research available. One male subject (age 21, height 185 cm, weight 75 Kg) performing a bunny hop over an obstacle (height 33 cm) was analyzed. A Panasonic WV-D5 100HS camera was placed in line with the subject's plane of motion and was used strictly for qualitative analysis to assure that the subject went over (not around) the obstacle and did not twist. The quantitative analysis was done using a tape from a Panasonic WV-D5 100HS camera placed perpendicular to the subject's plane of motion (15 meters from the obstacle) and Peak 5 Performance Motion Analysis Software. Both cameras were genlocked and recorded the trial with a shutter speed of 111000 sec., a filming speed of 60 fields (30 frames) per sec. with a SMPTE time code generated on both tapes. The 31 points on the subject and bicycle were digitized for 82 fields of videotape (1.394 sec.) then conditioned using a Butterworth filter with a cutoff frequency set at the optimum Hertz for each point on the subject and bicycle. The time between each field was 0.017 sec. The bunny hop was divided into a prejump, airborne and post-jump phase. The phases lasted 0.547,0.476 and 0.340 sec., respectively. The kinematic variables analyzed were linear velocity and acceleration and joint angular velocity and acceleration. The subject and bicycle traveled at an average horizontal velocity of 4.6032 + 0.1052 meters per sec. All points on the subject above the knee achieved maximum upward vertical velocity immediately prior to the airborne phase with the exception of the wrist (0.017 sec. into the airborne phase). All points on the subject and bicycle achieved maximum downward vertical velocity during the last 0.068 sec. of the airborne phase. Maximum upward vertical acceleration was achieved prior to the final 0.102 sec. of the pre-jump phase for the points at or above the shoulder. Analysis of joint angular velocities showed that just prior to the airborne phase, the elbow and knee achieved maximum extension velocity and the ankle achieved maximum plantar flexion velocity. Around the point of landing, the shoulder achieved maximum extension velocity while the ankle achieved maximum dorsi flexion velocity. The results of this study can be compared to that of King (1991). King compared vertical standing jump height with and without arm swing and concluded that arm swing can contribute up to a 25% gain in jump height. The gain in jump height was caused by the transfer of momentum from the arms to the total body. Similarly, a cyclist performing a bunny hop must have an upward acceleration of a portion of his or her body mass prior to a deceleration, or reversal, of that portion of body mass just before the hop. This study concludes with a description of how to perform a bunny hop based on the qualitative and quantitative findings.