RESEARCH OVERVIEW
I study the physiology and biomechanics of locomotion at the whole organism level. I focus on the interaction of biomechanics and energetics.

RESEARCH GOALS

1. To understand how biomechanical factors determine the metabolic cost of locomotion.

For my doctoral research, I developed and tested a new "cost of generating force" hypothesis. I proposed that metabolic rate is proportional to the average vertical force exerted on the ground and inversely proportional to the time over which that force is exerted (foot-ground contact time). Exerting force over a shorter period of time requires faster muscle fibers that are less economical. This approach explains why metabolic rate increases when an animal runs faster and why running is cheaper for larger animals.

My students and I have continued to test and modify and extend this hypothesis. I think that the next step is to synthesize the classical mechanical work approach and the cost of generating force approach in a quantitative, empirically based manner. We have begun a series of novel experiments that systematically increase or decrease the force and work required for both walking and running. We measure the effect of these manipulations on the metabolic energy required. These experiments utilize newly developed procedures (e.g., simulated hypo- and hyper-gravity) and biomechanical equipment (e.g., a force-treadmill). These experiments are designed to clarify what fraction of the total metabolic energy demand can be attributed to the generation of force and what fraction can be attributed to mechanical work or power. These experiments will lead to a more comprehensive understanding of the overall metabolic energy demand for locomotion. This research is funded by a five year R-29 FIRST grant from NIH.

2. To understand how the mechanics of locomotion are determined by speed, body size and fundamental forces such as gravity, inertia).

I have conducted many experiments in this area on humans. I found that during simulated reduced gravity, the nervous system acts to control the mechanical stiffness of the leg rather than peak foot ground force. However, in experiments where human runners carried weights, they avoided high peak foot-ground forces by extending the foot-ground contact time. Surprisingly, adding weights to the legs had almost no effect on the kinematics of running. We have also experimentally altered centripetal forces and found that existing theories for curve running are inadequate. My graduate students and I invented and built a new force treadmill that measures both vertical and horizontal forces that we are using in these experiments.

In other human experiments in this area we have used simulated reduced gravity as a tool to understand basic principles. We have discovered that at lower levels of gravity, people choose to switch from a walk to a run at very slow speeds in a way consistent with Alexander's theory of dynamic similarity. However, we have also discovered that other aspects of the dynamic similarity theory are not valid in either human walking or running. We are developing a more comprehensive theory for the overall mechanics of locomotion. Another recent project has de-coupled mass and weight so as to understand the relative imporatnce of gravitational and inertial forces in human running. We have applied to NASA for funding for this area of experiments.

In addition to human experiments, I use a comparative approach involving many different species of animals. Medium sized animals (approx. 1-100kg) locomote in a physical world where gravity and inertial forces are the most important. For my post-doc, I studied the locomotion of insects because the locomotion of these small animals is not dominated by gravity. A fly crawling on the ceiling is the most obvious demonstration of this. By studying animals that locomote in different physical constraints, we can gain perspective on the constraints that exist on the locomotion of medium sized animals like ourselves. For example, I discovered that rhinoceros beetles can carry enormous loads (> 30x body weight) with very little metabolic cost. I also examined locomotion mechanics of different ant species ants weighing as little as 0.3 milligrams. I documented that at fast speeds ants use aerial gaits (i.e. they trot). The bulk of my research time was spent studying the mechanics of cockroach locomotion with an eye towards robotic applications. This research was supported by the Office of Naval Research.

At the other extreme of size, many biomechanical aspects of locomotion by very large animals are puzzling. In very large animals, gravity is an overwhelmingly dominant force. For example, elephants are unable to use an aerial gait (i.e. trot or gallop) without risking a broken leg. We measured the rate of energy consumption of African elephants and found that their most economical speed is about 1.3 m/sec, about the same as adult humans. This seems incongruous with the idea that an inverted pendulum mechanism acts to conserve mechanical energy during walking because elephants have much longer legs and thus large fluctuations in gravitational potential energy. We have been conducting biomechanical experiments using zoo elephants over the last few years.

Other extreme locomotor performances of interest are those of pronghorn antelope and cheetahs, the fastest animals on the North American and African continents respectively. We have conducted experiments on pronghorn antelope galloping at up to 13 m/sec on a treadmill and hope to explore even faster speeds and to study other extremely fast species.

EXPERIMENTAL APPROACHES

Direct Approach
I use direct experiments that manipulate one or more mechanical variables and I measure the mechanical or physiological effects that result. The power of these direct experiments is due to the ability to control all but the variables in question and thus they can establish cause and effect. Many of my direct experiments use human subjects. This allows for easy experimental manipulations and the findings are more likely to be applied to human health.

Comparative Approach
I also make comparisons across a wide diversity of animal species, which often span large differences in body size (e.g. ants vs. elephants) or locomotor performance abilities (e.g. sloth vs. cheetah). These experiments derive power from the natural diversity resulting from evolution. By making comparisons across a wide diversity of animals, I can elucidate general principles and basic mechanisms that we would never discover if we only studied humans. Some of these discoveries may lead to applications relevant to human health and some of them simply satisfy my curiosity about the world around us.