For too long, bones were viewed merely as inert structural components of the body, and there was little appreciation for their broader biological functions and responsiveness to stimuli. Today, however, we know that bones are dynamic, biologically active tissues with the innate ability to respond and adapt to various individual and environmental stimuli.
Bone tissue undergoes continuous turnover via the processes of resorption (breakdown) and formation. Functional adaptations occur via four adaptive pathways, differentiated by whether the bone is exposed to heightened loading or disuse, and whether resorption and formation occur independently (modelling) or are coupled (remodeling). Formation modelling (targeted deposition of new bone) and targeted remodeling (the coupled breakdown and removal of old bone, followed by replacement with newly formed bone at the same site) both occur with increased loading, whereas resorption modelling and disuse-mediated remodeling occur with disuse.
A wide range of modifiable and non-modifiable factors influence these adaptive pathways, the most important of which is the extent and nature of the applied mechanical load. As such, an individual’s exercise habits are critical contributors to their whole-bone mechanical properties. The adage “Use it or lose it” applies to bone, and reduced loading, whether due to inactivity, injury or special environments such as spaceflight, can lead to bone loss. In contrast, the bone benefits of engaging in regular physical activity are plentiful, with more active people tending to have greater bone cross-sectional area, greater bone mass and enhanced microarchitecture compared to their less active counterparts. Moreover, bone responds best to high-impact, multidirectional muscular or gravitational loads, and athletes who compete in sports that convey greater and unusual loading patterns, including ball sports like basketball and soccer, tend to have denser, stronger bones than athletes that compete in low-impact, repetitive loading-type sports such as cycling and swimming.
Yet many questions remain about exercise and bone health, particularly in relation to the occurrence of bone stress injuries (BSIs) in athletes. Despite the osteogenic benefits of exercise, BSIs — including their more advanced manifestation, stress fractures — are common, estimated to account for up to 20% of injuries seen in sports medicine clinics. These injuries and resultant sequelae can wreak havoc on an athlete’s career due to long recovery times and high recurrence rates.
Research into the pathophysiology of BSI is ongoing, but in general, BSIs occur when the rate of microdamage accumulation exceeds the bone’s capacity to remove and replace the damaged sections with new bone via targeted remodeling. Thus, strategies aimed at reducing BSI risk should focus either on reducing the applied repetitive load or on increasing bone’s resistive strength and repair capacity. Important factors include optimizing training, nutrition and sleep. Biomechanical assessment and optimization may be required as well to ensure that loads are not disproportionately and repeatedly applied to bone areas vulnerable to stress injuries. And appropriate strength and conditioning programs, too, are important, ensuring that the surrounding musculature can adequately absorb and distribute applied loads. Further, it is essential to recognize that bone has a slower turnover rate than the surrounding tissues and, as such, may require a longer recovery period and slower return to play than other types of musculoskeletal injury.
Considering nutritional factors, low energy availability (LEA) is perhaps the greatest threat to bone health for highly active individuals and is an important BSI risk factor. LEA can impact bone through three main pathways: (a) suppressed bone metabolism as an energy-conserving measure, which can lead to a weakened structure over time; (b) suppression of reproductive and metabolic hormones, which are essential for bone health; and (c) deficiencies in key nutrients, including carbohydrates, protein, calcium, vitamin D and iron. Finally, adequate sleep is vital to support bone metabolism and adequate recovery, especially in athletes who may have elevated bone turnover due to intense training.
Eimear Dolan, PhD, is a researcher within the Applied Physiology and Nutrition Research Group at the Faculty of Medicine of the University of São Paulo in Brazil. She is currently supported by a research fellowship from the São Paulo Research Foundation (FAPESP) and leads a team of postgraduate students on a range of studies investigating the influence of exercise and nutrition on bone health in varying populations, from individuals with clinical conditions to elite athletes. She was nominated the Bone and Osteoporosis Interest Group’s Emerging Inspiring Leader at this year’s ACSM annual meeting. Her main research areas include bone, energy availability, sex-specific physiology, and the application of evolutionary theory to investigate topical exercise and nutrition questions.
Nicole M. Sekel, PhD, is an Oak Ridge Institute for Science and Education (ORISE) postdoctoral fellow supporting the U.S. Army Research Institute of Environmental Medicine (USARIEM) in Natick, Massachusetts. Dr. Sekel is an active ACSM member at the national and regional levels and received the Young Investigator’s Award from the Bone and Osteoporosis Interest Group at last year’s annual meeting. Dr. Sekel’s primary areas of research expertise includes advanced imaging (HR-pQCT, pQCT, iDXA), exercise physiology, peripheral bone adaptation to military training, aging, and micronutrient deficiency, specifically iron and vitamin D deficiency.
