This thesis presents advances in the state-of-the-art in legged locomotion through the development of bounding and galloping gaits as well as new modes of hybrid wheeled-leg modes of locomotion. Two four-legged running robots, Scout II and PAW, are examined, the latter of which is distinguished by actuated wheels at the ends of its legs. / First, hybrid modes of locomotion are demonstrated which use legs to dynamically reposition wheels at specific locations with respect to the body. These modes improve the stability and tire-wear of turning and braking manoeuvres and allow pitch-controlled slope ascent and descent in a wheeled-leg vehicle such as the PAW robot. / Second, through hip actuation, passive leg compliance and controlled wheel action it is possible to make the same vehicle run using a dynamically stable legged gait called the bound. Experimental evidence of this is presented and compared to similar experiments on the same robot with mechanically blocked wheels, a 3D simulation of the same, as well as bounding on a completely different quadrupedal robot, Scout II. While a casual observer finds no difference in blocked-wheel and active wheel control modes, detailed examination of the gaits reveals lower speeds and efficiency as well as decreased repeatability when the wheels are actively controlled. / A new method of forward speed control is presented for the bounding gait using liftoff, as opposed to touchdown, leg angles. The liftoff angle method of speed control is shown to be particularly suited to fine-tuning of certain gait performance indices. / Third, the underactuated bounding gait is extended to demonstrate, for the first time, that robotic galloping is possible and that it can be achieved in two underactuated quad-rupedal robots and with varying levels of decoupled control. In the Scout II robot the front leg pair and rear leg pairs function independently; while in the PAW robot galloping is achieved with no controlled coupling between any of the four legs. The rotary gallop gait demonstrated by both robots is characterized by a significant yaw component and is compared to another bound-derived turning gait which uses liftoff angles to produce yaw. In particular, the correspondence of lead leg to yaw direction in both cases is found to match results from biology. In contrast, while it is thought that animals pivot about their lead leg to turn, the rotary gallop demonstrated by these robots shows that yaw occurs primarily in the leg behind the lead leg.

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The ability to control balance and stability are essential to prevent falls during locomotion. Maintenance of stable locomotion is challenging especially when complicated by amputation and prosthesis use. Humans employ several motor strategies to maintain stability during walking on complex terrain: decreasing walking speed, adjusting stride length and stance width, lowering the center of mass, and prolonging the double support time. The mechanisms of selecting these motor strategies by the primary motor cortex are unknown and cannot be studied directly in humans. There is also little information about dynamic stability of prosthetic gait with bone-anchored prostheses, which are thought to provide sensory feedback to the amputee through osseoperception. Therefore, the Specific Aims of my research were to (1) evaluate dynamic stability and the activity of the primary motor cortex during walking with different constraints on the base of support and (2) develop an animal model to evaluate mechanics and stability of prosthetic gait with a bone-anchored prosthesis. To address these aims, I developed a feline model that allows for investigating (1) the role of the primary motor cortex in regulation of dynamic stability of intact locomotion, (2) skin and bone integration with a percutaneous porous titanium implant facilitating prosthetic attachment, and (3) dynamic stability of walking on a bone-anchored prosthesis. The results of Specific Aim 1 demonstrated that the area and shape of the base of support influence the margins of dynamic stability during quadrupedal walking. For example, I found that the animal is dynamically unstable in the sagittal plane and frontal plane (although to a lesser degree) during a double-support by a forelimb and the contralateral hindlimb. Elevated neuronal activity from the right forelimb representation in the primary motor cortex during these phases suggests that the motor cortex may contribute to selection of paw placement location and thus to regulation of stability. The results of Specific Aim 2 on the development of skin-integrated bone-anchored prostheses demonstrated the following. Skin ingrowth into 3 types of porous titanium pylons (pore sizes 40-100 μm and 100-160 μm and nano-tubular surface treatment) implanted under skin of rats was seen 3 and 6 weeks after implantation, and skin filled at least 30% of available implant space. The duration of implantation, but not implant pore size (in the studied range) or surface treatment statistically influenced skin ingrowth; pore size and time of implantation affected the implant extrusion length (p<0.05). The implant type with the slowest extrusion rate (pore size 40-100 μm) was used in a feline model of prosthetic gait with skin-integrated bone-anchored prosthesis. The developed implantation methods, rehabilitation procedures and feline prostheses allowed 2 animals to utilize skin- and bone-integrated prostheses for dynamically stable locomotion. Prosthetic gait analysis demonstrated that the animals loaded the prosthetic limb, but increased reliance on intact limbs for weight support and propulsion. The obtained results and developed animal model improve the understanding of locomotor stability control and integration of skin with percutaneous implants.