Many factors are linked to developing osteoporosis;​ however, the inadequate accumulation of peak bone mass prior to skeletal maturity is considered an important factor. As peak bone mass is often achieved by the end of the third decade, young adulthood provides an important and possibly final opportunity for its augmentation. Physical activity has been proposed as one of the most effective strategies to improve bone mass, however, the bone response to physical activity is highly dependent on the nature of the loads imposed. The most osteogenic loads are those that induce high magnitude bone strains at high loading rates. As most mechanical forces acting on the skeleton are generated through either gravitational-derived loads or muscle-derived loads, debate exists as to the predominant source of the adaptive stimulus. Despite this, few direct comparisons of the effect of gravitationally-derived (impact) loading and muscle-derived (resistance training) loading on bone have been undertaken, with none examined under true high-intensity training conditions. Consequently, the primary goal of the current work was to compare the bone response to two known osteogenic loading methods, impact loading exercise versus resistance training exercise in young women with below average bone mass.

The thesis comprises six manuscripts, presented as three published papers, two under review and one submitted. The first paper (Chapter 3) details the protocol for the Osteoporosis Prevention Through Impact and Muscle-loading Approaches to Exercise (OPTIMA-Ex) trial. Therein, the methods of the three-arm randomised controlled trial, comparing the bone responses of 10-month, twice-weekly, either supervised high-intensity impact training, high-intensity resistance training, or unsupervised home-based low intensity exercise (active control) in young adult women with lower than average bone mass have been described. The two subsequent papers describe the outcomes of methodological studies related to the measurement of forces associated with the exercise interventions, specifically the impact training intervention. The first study (Chapter 4), determined the validity and reliability of a Gym-aware linear positional transducer to track the progression of the novel punching exercise of the impact training arm of the OPTIMA-Ex trial. The second (Chapter 5) quantified the intensity of mechanical loading associated with the lower limb exercises of the impact training exercise program. We found that vertical ground reaction forces generated were greater than four times body weight for all seven training stages and increased over time, suggesting that the impact intervention arm of the OPTIMA-Ex trial achieved its goal of being both high-intensity and progressive in nature.

The fourth manuscript (Chapter 6) comprises the overall findings of the between-group comparisons of the OPTIMA-Ex trial. We found that high-intensity resistance training may provide a broader osteogenic loading stimulus than high- intensity impact training for young adult women with lower than average bone mass, although findings were site-specific. More specifically, high-intensity resistance training had a greater osteogenic effect for cortical bone than impact training, while impact training provided a greater stimulus for trabecular bone. Furthermore, resistance training evoked greater improvements in both body composition and physical performance measures. Manuscript five (Chapter 7), presents a comparison of upper and lower limb responses to high-intensity impact training and high-intensity resistance training on indices of bone strength. Only those who completed the intervention arms of the OPTIMA-Ex trial were included in this analysis. Similar site-specific responses were observed from these analyses, irrespective of upper or lower extremity. Resistance training elicited greater effects at the proximal femur and along the shaft of long bones, while the effects of impact training were greater in the distal segments of long bones nearest to the points of impact.

The final manuscript (Chapter 8) describes a mixed-methods study to explore participant experiences during the OPTIMA-Ex trial, determine enjoyment and acceptability of each exercise mode, and identify barriers and facilitators to bone- targeted exercise in young adult women. Overall, the program was well received, with all groups demonstrating an improvement in the ‘mental health’ domain of the quality of life measure, however, the two supervised exercise groups reported greater levels of physical activity enjoyment. Qualitative analysis revealed the impact and resistance training groups had ‘richer’ exercise experiences than controls, through perceived improvements in emotional and mental well-being along with the an overall sense of achievement through progression and skill acquisition. Furthermore, clear group differences were observed in terms of willingness to recommend their training mode to a friend and intention to continue the program after the trial period, with resistance training participants appearing most positive for both outcomes.

Overall, the current work demonstrates site-specific effects of high-intensity impact training and high-intensity resistance training in young adult women with lower than average bone mass. Within the limits of the modest sample size, we conclude that both impact training and resistance training have important beneficial effects on bone in the participant demographic and are safe, enjoyable and well received. High-intensity resistance training, however, may have a slightly broader effect on bone outcomes than impact training, as well as on body composition, strength and acceptability in young adult women.