Adverse bone health negatively impacts quality of life and healthcare systems worldwide. Millions of people, particularly the elderly and postmenopausal women, suffer from osteoporotic fractures each year, costing healthcare systems $17.9 billion in the USA and £4 billion in the UK.¹

However, poor bone health is not exclusive to older populations. Bone mineral density (BMD) contributes to bone strength and low BMD has also been observed in younger individuals, especially endurance athletes, with studies reporting its prevalence in adolescent female runners² and cyclists.³ Sub-optimal accrual of bone mineral during early life increases the risk of bone injuries and osteoporosis during adulthood, highlighting the need for preventive measures.

WEIGHT-BEARING EXERCISE AND ENERGY DEFICIT

Positive effects of habitual weight-bearing exercise on bone health are well established. Longitudinal intervention research has demonstrated the benefits of weight-bearing exercise on BMD and bone structure.⁴ The anabolic effects of exercise on bone are evidenced in racquet sports, where the dominant arm has been shown to have 7–11% greater cortical bone content compared with the non-dominant ‘control’ arm.⁵ This may seem paradoxical, given the prevalence of low BMD in athletes from certain sports. However, low BMD is most common in athletes who participate in non-weight-bearing sports,⁶ where the loading stresses applied to bone tissue are insufficient to stimulate bone adaptation.

Additionally, athletes may be in a prolonged state of energy deficit, a condition known to negatively affect bone health,⁷ and this should be avoided for those looking to optimise bone health. It is important to note that prolonged states of energy deficiency (and associated key nutrient deficiencies) are more likely in sports which demand high energy expenditures and often promote restrictive energy intakes, such as distance running and cycling, possibly creating the ‘perfect storm’ for suboptimal bone health.

WHAT IS THE BEST EXERCISE FOR BONE?

The optimal mode of exercise for enhancing bone health remains unclear. Methodological differences in populations (bone responds differently in the young versus the old), exercise types and duration, and the magnitude of bone stress make it difficult to compare studies and establish a hierarchy of exercises that benefit bone health.

Nevertheless, it has been shown that sports involving high load magnitudes, a high rate of load application (i.e. load rate) and irregular movement patterns consistently produce greater osteogenic effects than those involving lower magnitudes and impacts and more repetitive movement patterns (e.g. team sports such as football versus running or cycling).⁶

Adult males who regularly participate in football increased cortical BMD, cross-sectional area, circumference and tibial thickness compared with age-matched participants engaged in regular resistance training.⁶ The anabolic bone response to football is probably due to high load magnitudes, rates, frequencies and multidirectional movements required during football training and match play. Additionally, nuanced differences in how an activity is performed can affect the extent of bone adaptation. For example, footballers who performed a greater number of decelerations during training and match play showed a greater increase in tibial strength compared with those who performed fewer decelerations.⁸

Interestingly, the bone anabolic response seems to plateau after a relatively low number of loading repetitions,⁹ although the exact number will depend on a complex interplay between several factors, including load magnitude, rate, frequency and direction. Once plateaued, the mechanosensitivity of bone becomes fully restored after several hours of rest,¹⁰ such that the time between exercise bouts also seems to be a determinant of the bone response to exercise. A recent in vitro study has shown that greater bone formation occurs in osteoblast cells subjected to intermittent loading bouts compared with continuous cyclic loading.¹¹ Furthermore, a larger bone adaptive response has been found when 14 seconds of rest are taken between each application of load, compared with shorter rest periods.⁹

Taken together, these data suggest that short bouts of high magnitude and high impact loads, with appropriate rest between each load and each exercise bout, may be most beneficial for optimising positive bone adaptation.

AN AREA FOR EXPERIMENTAL IMPROVEMENT

A limitation of current research assessing the optimal exercise for bone adaptation is the lack of quantification of exercise load. While studies have shown that various forms of exercise (jumping, hopping, football, etc.) can increase bone size and structure, few studies have accurately quantified the loading stress that has been applied to cause the response. Instead, researchers often rely on imprecise metrics, such as the number of jumps, time spent partaking in an activity, or retrospective recall, to quantify load. Although using these metrics can offer information about the characteristics of exercise (e.g. the amount of impact exercise, the multi-directional nature of the activity, and sometimes even the ground reaction forces), they do not provide objective data on factors such as magnitude, rate, frequency, direction and distribution of loading stress experienced at different sites on the bones of interest, all of which are influential in bone adaptation. If bone loading data could be more accurately quantified through technologies like force sensors, the prescription of osteogenic exercise could be improved.

IN SUMMARY

Weight-bearing exercise is known to improve bone health, but the optimal type of exercise for this purpose remains unclear due to methodological variations. Sports with high impact and irregular movements, like football, produce better osteogenic effects compared with low impact, repetitive activities. Deficiencies in energy or key nutrients may interfere with the benefits of exercise on bone health. Additionally, factors such as rest periods and specific movement patterns, like decelerations, can influence the degree of bone adaptation. However, more precise measurement of exercise load is needed to fully optimise osteogenic exercise prescriptions.

IAN VARLEY
Associate Professor in Exercise Physiology, Nottingham Trent University

MARK HUTSON
Lecturer in Sport and Exercise Nutrition Practice, Ulster University, Belfast

REFERENCES

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