Fig. 6. The components of the energy in a single unloaded tarsometatarsal segment as a function of time during one stride in a representative bird running at 1.5 m s^-1. Horizontal (E_K,x; solid black line), vertical (E_K,y; broken red line) and rotational (E_rot; dotted blue line) kinetic energies are indicated. The green broken and dotted line indicates the gravitational potential energy (E_g) referenced to zero at the lowest point in the stride. The vertical line indicates the stance swing transition. (Please note that the dip in E_K,x shown during in swing phase is representative in that a decrease in this mechanical energy term occurred during mid-swing in 4 of the 5 birds measured. However, the magnitude of the mid-swing decrease in E_K,x in the example shown was the largest found.)