ZANI, P.A.*; GOTTSCHALL, J.S.; LAFAVE, A.A.; KRAM, R.; Univ. of Colorado; Univ. of Colorado; Univ. of Colorado; Univ. of Colorado: The Biomechanics of Giant-Tortoise Locomotion: Walking Without an Inverted Pendulum

Tortoises are derived terrestrial forms of turtles, some of which have evolved into gigantic forms that walk extremely slowly. To date, all walking vertebrates studied utilize an inverted-pendulum mechanism that conserves total mechanical energy by converting between kinetic energy (KE) and gravitational-potential energy (GPE) during each step. However, we hypothesized that due to their large size and slow walking speed, giant Galápagos tortoises would not use an inverted-pendulum mechanism of energy conservation. We studied 5 adult Geochelone elephantopus (body-mass range = 103 � 196 kg; avg. = 143 kg) as they walked across a large force platform. To calculate the KE and GPE of the center of mass, we integrated ground-reaction forces over integral numbers of steps. Tortoises walked slowly (velocity range = 8.2 – 21.9 cm/s; avg. = 15.8 cm/s; Froude-number range = 0.002 � 0.013; avg. = 0.008), nearly pausing with each step, yielding an average ΔKE_forward = 7.8 J. Their centers of mass had average vertical amplitudes of 2.0 cm corresponding to an average ΔGPE = 28.4 J. Therefore, the amount of KE at the beginning of a step was woefully insufficient to be converted into enough GPE to lift the tortoise up and over each limb. Rather than operate like a passive inverted pendulum, tortoises need to lift their center of mass actively during each step. Yet, tortoise muscle is known to be extraordinarily efficient and turtles are known to walk with remarkable metabolic economy. Leading to the question: is an effective inverted pendulum necessary for economical walking?

Authors thank Oklahoma City Zoo; supported by NIH F32 AR08615 to PAZ.