Abstract

Kram, R., Wong, B. and Full, R.J. 1997. Three dimensional kinematics and limb kinetic energies of running cockroaches. J. exp Bio. 200, 1919-1929.

We tested the hypthesis that fast-running hexapeds must generate high levels of kinetic energy to cycle their limbs rapidly compared with bipeds and quadrupeds. We used high-speed video analysis to determine the three-dimensional movements of the limbs and bodies of cockroaches running on a motorized treadmill at 21cm s-1 using an alternating tripod gait. We combined these kinematic data wth morphological data to calculate the mechanical energy produced to move the limbs relative to the overall center of mass and the mechanical energy generated to rotate the body (head + thorax + abdomen) about the overall center of mass. The kinetic energy involved in moving the limbs was 8 Jstride-1 (a power output of 21mWkg-1), which was only approximately 13% of the external mechanical energy generated to lift and accelerate the overall center of mass at this speed. Pitch, yaw and roll rotational movements of the body were modest (less then +/- 7 degrees), and the mechanical energy required for these rotations was surprisingly small (1.7 Jstride-1 for pitch, .5 Jstride-1 for yaw, and .4 Jstride-1 for roll) as was the power (4.2, 1.2, and 1.1mWkg-1, respectively). Compared at the same absolute forward speed, the mass-specific kinetic energy generated by the trotting hexaped to swing its limbs was approximately half of the predicted from data on much larger two- and four-legged animals. Compared at an equivalent speed (mid-trotting speed), limb kinetic energy was a smaller fraction of total mechanical energy for cockroaches than for large bipedal runners and hoppers and for quadrupedal trotters. Cockroaches operate at relatively high stride frequencies, but distribute ground reaction forces over a greater number of relatively small legs. The relatively small leg mass and inertia of hexapeds may allow relatively high leg cycling frequencies without exceptionally high internal mechanical energy generation.