Displacing Sedentary Behaviour (SB) with light intensity physical activity (LIPA) is increasingly viewed as a viable means of health enhancement. It is, however, unclear whether any behavioural compensations accompany such an intervention. Therefore, the aim of this study was to identify any dietary changes that accompany SB displacement. We hypothesised that SB displacement would improve dietary quality. Thirty-five elderly females (73 ± 5 years) were randomly allocated to one of three groups: (1) sedentary behaviour fragmentation (SBF) (n = 14), (2) continuous LIPA (n = 14), or (3) control (n = 7). Habitual diet (four-day food diary) and physical behaviour (accelerometery) were assessed at weeks 0 and 8. Out of 45 nutrients examined, only glucose exhibited a group × time interaction (p = 0.03), mediated by an exclusive reduction following SBF (−31%). SBF was also the sole experimental group to increase nutrients promoting bone health (SBF: 17%, LIPA: −34%. control: 21%), whereas both experimental groups consumed more nutrients promoting anabolism (SBF: 13%, LIPA: 4%, control: −34%) (z-scores). New ambulators (n = 8) also consumed more nutrients promoting bone health (16%)/anabolism (2%) (z-scores), including significantly increased Zinc intake (p = 0.05, 29%). Displacing SB with LIPA improves dietary quality in older females. Furthermore, SB fragmentation appears advantageous for various dietary outcomes.

Prolonged sedentary behaviour (SB) i.e. longer bouts, is suggested to have a range of negative health effects, independent of habitual light and medium-to-vigorous physical activity (LIPA or MVPA). Any effect on musculoskeletal size, architecture or morphology has seldom been reported in older adults. Moreover, no study has yet determined if any association would persist following adjustment for covariates. Therefore, the aim of the present study was to investigate the associations between SB, and properties of the Gastrocnemius Medialis (GM) muscle, in a cross-sectional sample of older adults using compositional data analysis. 105 healthy older adults (73±6y) wore a thigh mounted tri-axial accelerometer for seven consecutive days, and underwent ultrasound [e.g. muscle length (L_m), anatomical cross-sectional area (ACSA), muscle volume (V_M), fascicle length (L_F), & physiological cross-sectional area (PCSA)], body composition (e.g. DEXA) and health (e.g. medical history) assessments. In-unadjusted models, SB time was negatively associated with ACSA at 75% of L_m (R²_adj = 0.085), V_M (R²_adj = 0.020), and PCSA (R²_adj = 0.039). Standing was positively associated with pennation angle (R²_adj = 0.110), which persisted following co-variate adjustment (R²_adj = 0.296). In fully adjusted models, both SB & LIPA time were associated with ACSA at 75% of L_m (Both R²_adj = 0.393). Standing and light activity time were also associated with L_F, V_M, & PCSA (R²_adj 0.116–0.573). In fully adjusted models, SB pattern parameters (i.e. the manner in which sedentary behaviour is accumulated daily throughout waking hours such as the timing, duration and frequency of sedentary bouts), were associated with GM muscle properties (R²_adj 0.156–0.564) including L_M, L_F, and V_M. The pattern, rather than accumulated daily SB time, was associated with the size and architecture of the GM. Our results suggest that regardless of co-existing habitual physical activities, SB bouts should be kept short and frequently interrupted to offset some of the deleterious ageing-related muscle architecture characteristics changes.

The optimal pattern of sedentarism displacement and mechanisms underlying its health effects are poorly understood. Therefore, the aim of this study was to quantify muscle-tendon adaptation in response to two different sedentarism displacement interventions and relate any adaptations to functional outcomes. Thirty-four older women (73±5yrs) underwent skeletal muscle-tendon size and functional assessments. Participants were randomly allocated to: Sedentary behaviour fragmentation (SBF), Light intensity physical activity (LIPA), or Control groups. Measures were taken at weeks 0 and 8. Gait speed significantly increased (p=0.003), in both experimental groups (SBF: 0.06 ± 0.08m/s, 6±10%, LIPA: 0.06 ± 0.07m/s, 6±6%), but not control (-0.02 ± 0.12m/s, -2±9%). Accordingly, the relative change in Vastus Lateralis muscle volume, accounted for 30% (p=0.027), and 45% (p=0.0006) of the explained variance in the relative change in gait speed, for SBF and LIPA respectively. Gastrocnemius Medialis fascicle length changes were positively associated with gait speed changes, following LIPA exclusively (R2= 0.50, p=0.009). This is the first study to show SBF and LIPA are adequate loading in older women, with related muscle adaptation and clinically relevant gait speed improvements. Such adaptations appear similar irrespective of whether sedentarism displacement is prescribed in a single bout (LIPA) or in frequent micro-bouts (SBF).

Free-living physical behaviour (PB), from sedentarism through to vigorous physical activity (PA), is increasingly studied due to its links to health outcomes. However, it remains unclear whether pre-existing day-to-day regularity in certain PB patterns influences intervention responsiveness. Therefore, this study hypothesized that (1) inter-day variability in certain PBs would decrease following a sedentary behaviour (SB) reduction intervention, and (2) those with high inter-day variability (low regularity) at baseline would be less likely to alter their behaviour compared to those with low inter-day variability (high regularity). Thirty-six older women (73 ± 5 years) were allocated to one of three groups: (1) daily SB fragmentation (SBF) (n = 14), (2) single daily bout of continuous light-intensity PA/LIPA (n = 14), or (3) control (n = 8), where no instructions vis-à-vis altering daily physical activity or sedentary behaviour were given. PB was objectively assessed (weeks 0 and 8) using three-dimensional accelerometry. Participants (48% of the study sample) with high regularity at baseline (<25th sample percentile for SB and PA bout length), showed greater SB reduction, and increased average PA bout length (p < 0.05) at week 8. These findings suggest that baseline regularity in physical behaviour may enhance intervention responsiveness. This aligns with theories of habit formation and self-regulation, indicating that personalised interventions would benefit a wider range of populations.

Background: Replacing sedentary behavior (SB) with light-intensity physical activity (LIPA) may improve sit-to-stand (STS) ability in older adults, but whether the LIPA pattern influences the time course (or physiological basis) of adaptation remains unclear. Objectives: To compare the time course of changes in STS performance, including repetition duration, following intermittent versus continuous LIPA in older women, and to examine associations between functional changes and changes in lower-limb muscle morphology and tissue quality. Methods: Twenty-eight older women = (74 ± 5 years, BMI: 26.1 ± 3.6 kg·m⁻², waist: height ratio: 0.58 ± 0.06) completed eight weeks of either intermittent (2 min bouts of LIPA following each 30 min of uninterrupted SB between 09:00 and 21:00, approximately 44 min·day⁻¹ under a maximally sedentary pattern of LIPA); or continuous (one 45–50 min daily bout of LIPA) LIPA. Compliance with LIPA interventions was objectively assessed using accelerometry at weeks 0, 4 and 8. Thirty-second STS (30STS) performance and repetition duration were assessed at baseline, week 4 and week 8. Muscle morphology was assessed by ultrasonography at baseline and week 8. This two-arm, parallel-group quasi-randomized trial analyzed changes using two-way mixed ANOVA or ANCOVA, with non-parametric within- and between-group tests used where appropriate (two-sided α = 0.05). Exploratory correlations were Bonferroni-corrected. Results: 30STS repetitions increased from baseline to week 8 (p < 0.001), without a difference between LIPA patterns (p = 0.25). In participants assessed at all time points, 30STS performance improved between baseline and week 8 (p = 0.002) and weeks 4 and 8 (p = 0.017), while the baseline–week 4 comparison showed a trend (p = 0.076). Mean repetition duration decreased from baseline to week 8 (p = 0.02), with no between-pattern difference (p = 0.67). Time-course analyses suggested that temporal adaptations may begin earlier, although corrected baseline–week 4 comparisons were not significant. The ultrasound sarcopenia index changed differently between groups (interaction p = 0.041), decreasing with intermittent LIPA and increasing with continuous LIPA. In exploratory subgroup analyses, one hypothesis-generating association survived Bonferroni correction: change in distal GL echo intensity (25% of muscle length) was positively associated with mean interval change in the repetition 7 vs. 8 duration difference within the intermittent-LIPA group (n = 12, rs = 0.82, p = 0.001). Conclusions: Both intermittent and continuous LIPA were associated with improved STS performance in older women, with no evidence of a difference between patterns. Time-course trends suggested that reductions in repetition duration may emerge before increases in total repetitions; however, this temporal ordering was not confirmed statistically. Improvements in total repetitions were evident after eight weeks. USI responses differed between LIPA patterns, whereas the isolated exploratory ultrasound–STS association requires confirmation.