The Quantification and Development of an Osteogenic Jump-landing Programme and Its Chronic Effects on Bone Health in Premenopausal Women
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A review into the effects of impact forces as a bone stimulus found that mineralisation can be achieved in areas where the stress is applied, however specific exercise protocols are currently lacking. It was also apparent that very little research had investigated the factors that influence jump-landing ground reaction forces (GRF’s) in premenopausal women, and no study had investigated the use of reactive jump-landings to optimise the osteogenic stimulus for jump-landings when performed by premenopausal women. In addition, no study had attempted to develop a quantified 12-month periodised osteogenic jump-landing programme to be specifically utilised by premenopausal women to improve bone health, nor investigated the chronic effects of such a programme on functional performance parameters, body composition and bone remodelling at clinically relevant sites. Therefore, addressing these gaps and limitations from the reviews has set the framework for this thesis. The first three studies sought to quantify the full spectrum of ground reaction forces (GRF’s) associated with a variety of jumps in this population. Study’s 1 to 3 essentially involved one data collection however, they are 3 separate studies (represented as Chapters 3, 4 and 5 in the PhD thesis) given the magnitude of the data collected and the very different kinetic characteristics of the jumps. Study’s 1 and 2 investigated the main jumping and jump-landing factors deemed important in achieving greater GRFs included the cueing of participants to; (i) use a vigorous arm swing in a “countermovement” style; and, (ii) ‘land stiffly’ and ‘immediately jump again for maximal height’ (reactive jump). With these instructions provided, GRF’s for all bilateral vertical and multiplanar jump- landings for magnitude (3.9 to 5.5 Body Weights - BW’s) and rate of strain (192 to 359 BW∙s-1), easily exceeded the previously defined vertical osteogenic thresholds (>3 BW’s and 43 BW∙s-1), shown to improve bone mass at clinically relevant sites for premenopausal women. Although vertical landing forces were similar for the bilateral multiplanar jumps, substantially different medio-lateral GRF’s (↑85% to ↑466%, for star jump) and anterio- posterior (↑103% to ↑316%, for stride jump), were observed between the jumps. The multidirectional nature of these jumps presented an opportunity to maintain and enhance mechanical bone stimulation after the bone becomes saturated to vertical loading cycles,however this was poorly understood for unilateral jump-landings. Therefore, in Study 3 unilateral multiplanar jumps, including the vertical, forward and lateral hop were investigated. Unlike the bilateral jumps, the unilateral hops did not utilise a reactive component, however a vigorous countermovement arm swing was used and participants were cued to land “stiffly”. The magnitudes (4.2 to 5.1, BW’s) and rates of strain (239 to 334 BW∙sˉ1) for the jump-landings, performed on an AMTI force plate, exceeded osteogenic thresholds previously determined using bilateral jump-landings. Furthermore, significant differences were detected for GRF’s for unilateral jumps landings across all planes of motion (19% to 93%) suggesting that each landing type provided a novel force application to potentially optimise bone stimulation. In Study 4 the kinetic data previously quantified in the aforementioned studies, in combination with a review of current and relevant evidence-based literature was utilised to develop a 12-month periodised osteogenic jump-landing programme for premenopausal women. The data was organised into a bone-specific ‘stress stimulus rating’ based on GRF magnitude (>3BW), GRF rate (>43 BW∙sˉ¹), and technical difficulty (i.e. vertical bilateral to multiplanar unilateral) in a progressive manner over a 12-month period. The 12-month periodised osteogenic jump-landing programme was defined by; a limited number of ground contacts per session (< 50 per session), rest intervals inserted between each jump (15 seconds), and adequate recovery between daily (3 - 5 times per week) sessions (at least 24 hours). The final study (Study 6), utilised a longitudinal controlled trial to determine the effects of the quantified jump-landing programme, on parameters of bone health in premenopausal women for a period of 12 months. Performance testing and dual energy x-ray absorptiometry (DEXA) was performed at baseline, 3, 6, 9 and 12 months. The excellent test-retest reliability demonstrated for BMD, BMC, bone geometry and body composition in Study 5 provided the ability to determine that the changes we detected over time were statistically and clinically significant. Significant (p ≤ 0.01) group main effects (↑0.41 - ↑3.72%) in favour of the jump group were observed for bone mineral density (BMD) and bone mineral content (BMC) at the femoral neck, total hip and lumbar spine. Significant (p ≤ 0.01) group main effects (↑2.78 - ↑3.84%) for cross- sectional area, cortical thickness and section modulus at the femoral narrow neck were also in favour of the jump group. For ground contact time, improvements in the jump group over the control group were apparent (↑21.9% vs ↓8.86%) with significant (p ≤ 0.01) group and time effects being observed. Group main effects that approached significance (↑7.54% vs ↓0.24%) were towards the jump group for vertical jump performance. Our findings have shown that a brief (2 - 3 minute) jump programme, with a specific focus on the jump-landing technique has provided the required stimulus to improve bone strength at clinically relevant sites associated with osteoporosis in premenopausal women. In summary, this thesis provides an original and vital contribution to preventative health care management that in the long term has the potential to reduce the direct and indirect costs of osteoporosis to the health sector. Evidence from this research has the potential to inform future exercise recommendations used to improve bone health during the critical premenopausal period, and to both reduce and delay the incidence of osteoporotic fracture in the years post menopause.