Articulated legs enable the selection of robot gaits, including walking in different directions such as forward or sideways. For longer distances, the best gaits might maximize velocity or minimize the cost of transport (COT). Interestingly, while animals often adapt their morphology for walking either forward (like insects) or sideways (like crabs), robots that can walk forward or sideways often pick a direction by convention. In this paper, we compare walking in forward and sideways directions. To do this, a simple gait design method is introduced for determining forward and sideways gaits with equivalent body heights and step heights. Specifically, the frequency and stride lengths are tuned within reasonable constraints to find gaits that represent a robot’s performance potential in terms of speed and energy cost. Experiments are performed in both dynamic simulation in Webots and a laboratory environment with our 18 degree-of-freedom (DOF) hexapod robot, Sebastian. With the common three joint leg design, the results show that sideways walking is overall better (75% larger walking speed and 40% lower COT). The performance of sideways walking was better on both hard floors and granular media (dry play sand). This supports the development of future crab-like walking robots for future applications. In future work, this approach may be used to develop nominal gaits without extensive optimization, and to explore whether the advantages of sideways walking persist for other hexapod designs.

Natural terrain is uneven and walking over these substrates may benefit from grasping into the depressions or "valleys" between obstacles. To examine how leg geometry influences walking across obstacles with valleys, we (1) modeled the performance of a two-linkage leg with parallel axis "hip" and "knee" joints to determine how relative segment lengths influence stepping across rocks of varying diameter and (2) measured the walking limbs in two species of intertidal crabs, \textit{Hemigrapsus nudus} and \textit{Pachygrapsus crassipes}, which live on rocky shores and granular terrains. We idealized uneven terrains as adjacent rigid hemispherical "rocks" with valleys between them and calculated kinematic factors such as workspace, limb angles with respect to the ground, and body configurations needed to step rocks. We first find that the simulated foot tip radius relative to the rock radius is limited by friction and material failure. To enable force closure for grasping and assuming that friction coefficients above 0.5 are unrealistic, the foot tip radius must be at least 10 times smaller than that of the rocks. However, ratios above 15 are at risk of fracture. Second, we find the theoretical optimal leg geometry for robots is with the distal segment 0.63 of the total length, which enables traversal of rocks with a diameter 37\% of the total leg length. Surprisingly, the intertidal crabs' walking limbs cluster around the same limb ratio of 0.63, showing deviations for limbs less specialized for walking. Our results can be applied broadly when designing segment lengths and foot shapes for legged robots on uneven terrain, as demonstrated here using a hexapod crab-inspired robot. Furthermore, these findings can inform our understanding of the evolutionary patterns in leg anatomy associated with adapting to rocky terrain.