Fossils belonging to the genus Homo, dating as far back as two million years ago, exhibit uniquely efficient features suggesting that early humans had evolved to become exceptional endurance runners. Although they did not have the cushion or stability-control features provided in our modern day running shoes, our early human ancestors experienced far less of the running-related injuries we experience today. The injury rate has been estimated as high as 90% annually for Americans training for a marathon and as high as 79% annually for all American endurance runners. There is an injury epidemic in conventionally shod populations that does not exist in the habitually unshod or minimally shod populations around the world. This has led many to conclude that the recent advent of highly technological shoes might be the problem. Although current literature has been inconclusive, there are two main limitations in virtually all of the studies: 1) transition phases of less than three months and 2) transition phases without rehabilitation exercises. These two aspects are key to the treatment of the structural consequences on the muscles and tendons of the foot and calf that habitually shod individuals have faced. This study includes a discussion of the cumulative consequences that lifelong shoe usage has on the development of the feet and lower legs. I propose a 78-week study that addresses the limitations of past studies by implementing a gradual, 32-week, multi-shoe transition complemented by an evidence-based rehabilitation program. I believe that this approach will restore strength and elasticity to muscles and tendons that have been inhibited by lifelong usage of overconstructed shoes and adequately prepare runners for the increased demand brought on by a­­­­­ changing running mechanic. This comprehensive, multifaceted transition plan to a fully minimalist shoe will provide novel insight into the ongoing barefoot debate. Can this approach finally demonstrate the proposed benefits of losing the shoes?

Fish encounter daily cycles of hypoxia in the wild, but the physiological strategies for coping with repeated cycles of normoxia and hypoxia (intermittent hypoxia) are poorly understood. Contrastingly, the physiological strategies for coping with continuous (constant) exposure to hypoxia have been studied extensively in fish. The main objective of this thesis was to understand how Fundulus killifish cope with a diurnal cycle of intermittent hypoxia, an ecologically relevant pattern of aquatic hypoxia in the natural environment. To do this, I characterized the effects of intermittent hypoxia on hypoxia tolerance, oxygen transport, metabolism, and the oxidative stress defense system of killifish, and compared these effects to fish exposed to normoxia, a single cycle of hypoxia-normoxia, and constant hypoxia. Specifically, I studied the following topics: (i) how acclimation to intermittent hypoxia modifies hypoxia tolerance, and the hypoxia acclimation response of Fundulus heteroclitus (Chapter 2), (ii) metabolic adjustments occurring during a hypoxia-reoxygenation cycle (Chapter 3), (iii) how acclimation to intermittent hypoxia alters O2 transport capacity and maximal aerobic metabolic rate (Chapter 4), (iv) the effects of hypoxia and reoxygenation on reactive oxygen species and oxidative stress (Chapter 5), and (v) variation in hypoxia tolerance and in the hypoxia acclimation responses across Fundulus fishes (Chapter 6). Killifish rely on a unique and effective physiological strategy to cope with intermittent hypoxia, and that this strategy is distinct from both the response to a single bout of acute hypoxia-reoxygenation (12 h hypoxia followed by 6 h reoxygenation) and to chronic exposure to constant hypoxia (24 h hypoxia per day for 28 d). Key features of the acclimation response to intermittent hypoxia include (i) maintenance of resting O2 consumption rate in hypoxia followed by a substantial increase in O2 consumption rate during recovery in normoxia, (ii) reversible increases in blood O2 carrying capacity during hypoxia bouts, (iii) minimal recruitment of anaerobic metabolism during hypoxia bouts, and (iv) protection of tissues from oxidative damage despite alterations in the homeostasis of reactive oxygen species and cellular redox status. Of these features, (i) is unique to intermittent hypoxia, (ii) also occurs in fish exposed to acute hypoxia-reoxygenation, and (iii) and (iv) are observed in both fish acclimated to intermittent hypoxia as well as those acclimated to constant hypoxia. This is the most extensive investigation to date on how fish cope with the energetic and oxidative stress challenges of intermittent hypoxia, and how these responses differ from constant hypoxia. This thesis adds substantial insight into the general mechanisms by which animals can respond to an ecologically important but poorly understood feature of the aquatic environment. / Dissertation / Doctor of Philosophy (PhD) / Oxygen levels in the aquatic environment are dynamic. Many fishes routinely encounter changes in oxygen content in their environment. However, we have very little understanding of how cycles between periods of low oxygen (hypoxia) and periods of high oxygen (normoxia) affect the physiology of fish. This thesis investigated how Fundulus killifish cope with daily cycles between hypoxia and normoxia (intermittent hypoxia) by modifying oxygen transport, metabolism, and oxidative stress defense systems. I found that killifish rely on a unique and effective physiological strategy to cope with intermittent hypoxia, and that this strategy is distinct from how they respond to a single bout of hypoxia (followed by normoxia) and to a constant pattern of only hypoxia. This is the most extensive investigation to date on how fish respond to the challenges of intermittent hypoxia, an understudied but ecologically important type of aquatic hypoxia.