Time Course and Muscular Correlates of Sit-to-Stand Adaptations Following Intermittent or Continuous Light-Intensity Activity in Older Adults: A Two-Arm Quasi-Randomised Trial

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.