Structure-to-human interaction during rhythmic jumping

Rhythmic jumping is the largest human-induced load that occurs on assembly structures. It involves a repeated alternation between contact and flight phases at a given frequency. Human loading on the structure takes place during the contact phase while the load is zero during the flight phase. When a structure vibrates, the displacement profile of the structure interacts with human movement during the contact phase. This structure-to-human interaction could result in an alteration in the jump height and the contact duration, and thus the peak ground reaction force (GRF) attained. This paper presents an experimental study that investigates the influence of jump timing relative to the platform position in its vibration cycle on rhythmic jumping. The vertical harmonic vibrations of the platform had a magnitude of 2.0 m/s2 and a frequency of 2.8 Hz. Data were collected from a test subject who was instructed to jump at a frequency of 2.8 Hz in such a way as to land at four different instances of the vibration cycle. These were landing on the platform while at its (i) reference position and on the way down (mid-down), (ii) lowest position (trough), (iii) reference position and on the way up (mid-up), and (iv) highest position (peak) in the vibrating cycle. The timings were presented to the test subject through a metronome beat. The peak toe clearance, impact ratio, contact ratio and frequency of jumping were compared on a cycle-by-cycle basis between the four cases of target landing timing as well as on a non-vibrating platform. The impact ratio and the peak toe clearance were positively correlated with each other and negatively correlated with the contact ratio. Furthermore, the jump height and consequently the peak GRF were higher, and the contact ratio was lower for landing at the trough and the mid-up positions, compared to those on the non-vibrating platform. This is the result of the platform helping the test subject to perform jumps of greater impact in case of landing near the trough and the mid-up positions of the platform.