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Background: Modern physical activity (PA) and exercise guidelines have evolved significantly, from early 20th-century physical education aimed at military preparedness to guidelines promoting PA for disease prevention and health enhancement. Despite this development, adherence among older adults remains low, exacerbating age-related health declines and increasing the prevalence of non-communicable diseases. The demographic shift towards a growing number of older adults highlights the need for tailored PA recommendations to promote healthy aging. Therefore, there is an urgent need to evolve and expand our knowledge base on PA and exercise recommendations for older adults. Improving and broadening our understanding of effective guidelines and promotion among older adults can contribute to a more active and healthier aging population. Aim: The overarching aim of this dissertation was to integrate insights from resistance and aerobic exercise interventions to inform current PA and exercise prescription guidelines for older adults. This aim was approached sequentially, beginning with an in-depth exploration of power training as a form of resistance training. The first objective was to investigate the associations between power training-induced changes in body composition and physical function in older men (Study I). Following this, the focus transitioned to aerobic exercise, where the second objective was to compare the effect of moderate- versus high-intensity aerobic exercise on cardiorespiratory fitness in older adults, taking into account the volume of exercise completed (Study II). Additionally, the third objective aimed to assess the impact of exercise frequency, time, type, and volume on aerobic exercise-induced changes in cardiorespiratory fitness in older adults (Study II). The final phase of the dissertation involved the practical application of a home-based high-intensity interval training (HIIT) intervention. The fourth objective was to investigate the effectiveness of a six-month home-based HIIT intervention to improve peak oxygen uptake (V̇O2PEAK) and lactate threshold in older adults (Study III). Furthermore, the fifth objective aimed to evaluate the adherence levels within the six-month home-based HIIT intervention, applying the FITT principle (Frequency, Intensity, Time (duration), and Type) to explore individual variations and outcomes (Study III). Methods: This thesis is based on data from three different studies involving older adult populations. The Force-Velocity study (Study I) included 56 home-dwelling older men from Kristiansand, Norway. The systematic review and meta-analysis (Study II) included data from 1322 older adults. The Fitness, Aging, and Bilingualism study (Study III) involved 233 home-dwelling older adults from South Norway and the West Midlands region of the United Kingdom. Study I was conducted as a randomized controlled trial, with participants performing two pre- and two post-intervention test sessions before and after a 10-week biweekly power training period with two sessions per week. Body composition was measured using dual-energy X-ray absorptiometry, and physical function was assessed as a composite Z-score combining measures from Sit-to-stand power, Timed Up and Go time, and loaded and unloaded Stair-climbing time. Study II was performed and reported according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). First, a systematic search based on predefined inclusion and exclusion criteria was performed, followed by data extraction and risk-of-bias assessment of included studies. Further, studies eligible for the systematic review were included in the meta-analysis if adequate data could be extracted. Study III was performed as a randomized controlled trial with participants randomly assigned to either a home-based HIIT group or a passive control group stratified by age and sex. Thereafter, the participants performed pre- and post-test sessions before and after a 26-week period. The HIIT group exercised three times a week, while the passive control group was asked to keep their normal PA level. Exercise sessions were monitored using a Polar watch and a logbook, and outcomes were assessed using a modified Balke protocol combining V̇O2PEAK and lactate threshold measures. Results: Objective 1: In Study I, we discovered a significant non-linear relationship between changes in body fat percentage and physical function improvements from pre- to post-intervention (R2=0.15, F(2, 45)=4.94). Individuals maintaining or slightly modifying their body fat percentage demonstrated the most substantial gains in physical function from pre- to post-intervention. Objectives 2 & 3: In Study II, we revealed a moderate, albeit not statistically significant, relationship between the intensity of exercise and enhancements in V̇O2PEAK, taking into account the volume of exercise completed (β=0.31, 95% CI=[−0.04;0.67], z=1.74). Furthermore, in studies that directly compared moderate- and high-intensity exercise, a small, yet not statistically significant, advantage was noted for high-intensity exercise (Hedges' g=0.20, 95% CI=[−0.02;0.41]). Finally, no significant differences in V̇O2PEAK improvements were found across exercise groups employing various methods, modalities, and intensity monitoring strategies. Along with no significant association between the duration and volume of weekly exercise and the increase in V̇O2PEAK (β= −0.00, 95% CI=[−0.01;0.00], z= −1.66; and β= −0.00, 95% CI=[−0.00; 0.00], z= −1.39, respectively). Objectives 4 & 5: In Study III, we discovered a significant between-group difference in the pre-to-post change in V̇O2PEAK (1.8 [1.2;2.3] mL/kg/min). Compared to controls, the exercise group had lower blood lactate concentration (−0.7 [−0.9;−0.4] mmol/L), % of peak heart rate (−4.4 [−5.7;−3.0]), and % of V̇O2PEAK (−4.5 [-6.1;−2.9]) at the intensity corresponding to pre-intervention lactate threshold, and achieved a higher treadmill stage (% incline) at lactate threshold (0.6 [0.3;0.8]), following the intervention. Furthermore, the findings revealed an overall mean adherence level of 122%. Specifically, when stratified based on the FITT principle, adherence to frequency was 86%, adherence to intensity was 98%, and adherence to time (duration) was 135%. Conclusions: By prioritizing improving muscular strength and power while maintaining a healthy to overweight BMI, offering adaptable aerobic exercise intensity options, prioritizing regular aerobic exercise for its primary benefits to cardiorespiratory fitness, advocating for accessible home-based HIIT programs, and underscoring the importance of individualized prescriptions, the insights from this dissertation can help guide the development of more effective, inclusive, and individualized PA and exercise guidelines. Such strategies are crucial for promoting healthier and more active aging, addressing one of this century's significant public health challenges.
