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Publication Development and Evaluation of Novel Accelerometer-based Methods to Estimate the Relative Intensity of Physical Behaviors under Free-living Conditions(2024-05) Marcotte, Robert TAccelerometers are widely used to measure physical activity (PA) levels, offering insights into PA intensity and volume necessary for health benefits. Often expressed in absolute terms (e.g. metabolic equivalents), activity intensity expressed relative to cardiorespiratory may be more suitable for individuals with varying cardiorespiratory fitness and functional status. Limited device-based methods exist to predict relative PA intensity, often requiring laboratory-based exercise testing for individual-specific method calibration. Additionally, the free-living validity of relative PA estimates using individualized cutpoints has yet to be determined. This study aimed to 1) evaluate the validity of relative PA estimates using individualized cutpoints derived from exercise testing and novel random forest methods developed from free-living activities and 2) compare the dose-response association between PA and cardiovascular disease risk factors (CVDRF) when using absolute or relative intensity. Participants (n = 44, age = 40.8 ± 15.9, 52.3% female) completed laboratory-based exercise testing while undergoing metabolic measurement and completed free-living activities for up to three days while wearing a heart rate monitor on the chest, activPAL on the right thigh, and accelerometers at the right hip and both dominant and non-dominant wrists. Compared to activPAL-derived sedentary behavior and heart rate monitor-determined %HRR (criterion relative PA intensity), intensity-specific classification accuracy for MPA and VPA were low regardless of wear location, ranging from 19% to 33%. Although estimates were biased for MPA and VPA, sensitivity analyses demonstrated that biases were lower during locomotion periods compared to non-locomotion periods using individualized cutpoints. Random forest method estimates of SB and LPA were unbiased. MPA and VPA were significantly underestimated using random forest models by ≥59.9% and did not improve after inclusion of individual-specific covariates. Findings from NHANES data indicated that while both absolute and relative intensity scales showed lower odds of CVDRF with higher MVPA participation, the dose-response association differed between the two scales. Minimal MVPA participation (2-6 minutes/day) was associated with protective benefits on relative intensity scales, while ≥50 minutes/day on absolute intensity scales was required to observe significant risk reduction. Future research should focus on refining methods for accurately assessing relative PA intensity and elucidating its implications for health outcomes.Publication Human Muscle Fiber Function when Altering Phosphate, Hydrogen Ion, and Regulatory Light Chain Phosphorylation Status at Physiological Temperatures(2024-05) Momb, Brent A.Introduction: The impact of skeletal muscle fatigue, which is the contraction-induced decline in muscle force or power, is associated with elements that are temperature-sensitive, such as myosin regulatory light chain (RLC) phosphorylation, increased phosphate (Pi) and hydrogen (H+) ion accumulation. However, to maintain protein stability, molecular and cellular experiments that illuminate the underlying mechanisms of muscle fatigue are typically examined at temperatures from 15-30°C, which is lower than in vivo (37°C). This study sought to characterize the molecular and cellular effects of fatiguing conditions, including the rarely-studied effect of RLC phosphorylation, at physiological temperatures. Methods: Biopsies of the vastus lateralis muscle of females (n = 8) aged 71.8 + 1.3 years and males (n = 5) aged 69.4 + 1.7 years in ongoing studies (Cultivating Healthy Aging in Older Adults and Understanding Fatigue in Older Adults) were completed and single fibers used for mechanical testing. Maximal calcium-activated cellular force production and molecular-level interactions (myosin-actin cross-bridge kinetics and mechanical myofilament properties) in slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibers were tested at physiological temperatures (37°) to examine the effects of elevated Pi, H+, and RLC phosphorylation. To explore the effects of Pi and H+, repeated measures were performed on single fibers under control (pH = 7, Pi = 5 mM), high phosphate (pH = 7, Pi = 30 mM), high hydrogen ion (pH = 6.2, Pi = 5 mM) and fatigue (pH = 6.2, Pi = 30 mM) conditions. To explore the effects of RLC phosphorylation, repeated measures were performed on single fibers under control and fatigue, then control with RLC phosphorylation and fatigue with RLC phosphorylation. Results: At 37°C, specific tension (force normalized to cross-sectional area) was greater than 25°C, apparently due to greater numbers of strongly-bound cross-bridges, which in turn were established by quicker cross-bridge kinetics. With fatigue, specific tension was lower at 37° and 25°C, presumably due to fewer strongly-bound cross-bridges and slowed cross-bridge kinetics observed compared to control conditions in MHC I fibers. In MHC IIA fibers at 25°C, fatigue was accompanied by a reduction in strongly-bound cross-bridges and slower cross-bridge kinetics, but at 37°C due to increases in the work-absorbing properties of single fibers, and faster cross-bridge kinetics compared to control. When examining Pi and pH independently at 37°C, no change in specific tension was found due to increased myofilament stiffness and decreased strongly-bound cross-bridges in both MHC I and IIA fibers. In both cases for MHC I and IIA fibers, oscillatory work and power were depressed with alterations to Pi and pH, but recovered completely for MHC I fibers with fatigue due to alterations in the cross-bridge kinetic ratio, and slightly in MHC IIA due to maintenance of strongly-bound cross-bridges and faster cross-bridge kinetics. With RLC phosphorylation, specific tension was reduced compared to control conditions due to a loss of strongly-bound cross-bridges in both MHC I and IIA fibers, with an additional drop in myofilament stiffness in MHC IIA fibers. However, the relative drop in specific tension from control to fatigue was lower with RLC phosphorylation in both MHC I and IIA fibers. Shifts in the cross-bridge kinetic ratios lead to differing results for oscillatory work and power, such that MHC I fibers had dramatically increased work and power under fatigue with RLC, and in MHC IIA fibers work and power were dramatically increased under control with RLC phosphorylation. Summary: Fiber type specific changes occurred with alterations in temperature in fatiguing conditions indicate the need to conduct experiments at physiological temperatures when attempting to extrapolate to in vivo conditions. While the alterations in specific tension may not occur with changes in phosphate or hydrogen ion concentration, the change in oscillatory work and power production, i.e. force transmission or generation, may be substantial. Additionally, RLC phosphorylation brought about differing effects dependent upon the fiber type examined and fatigue status, thus the relevance of phosphorylation combined with other fatiguing metabolites, should be further questioned and quantified.Publication The Effects of Acute Sleep Restriction on Subsequent Physical Activity, Sedentary Behaviors, and Sleep(2024-05) Chase, John DavidThe purpose of the present study was to determine the impact of a single night of early awakening sleep restriction on subsequent physical behaviors, compared with a full night of sleep in healthy young adults. Twenty-three (62%F, 20.7±2.8 years) healthy young adults participated in a free-living, randomized, counter-balanced study where they were assigned a night of early awakening sleep restriction (SR, 3 hours) and a night of full sleep (FULL, 8 hours) during a 14-day period. Participants wore an ActiGraph GT9X accelerometer on the non-dominant wrist. The Hildebrand 2014 method was used to identify sedentary behavior (SED) and total physical activity (TPA), which was subdivided into light physical activity (LPA) and moderate-to-vigorous physical activity (MVPA). Average bout durations were calculated for SED bouts (≥30 min), as well as LPA, MVPA, and TPA bouts (≥10 min). Linear mixed regression models were used to examine the effect of sleep condition on 1) average daily time spent in SED, LPA, MVPA, TPA, and activity counts, 2) average timing (6-hour windows) and bout duration of SED, LPA, MVPA, and TPA bout duration, and 3) cognitive performance on the Go/No-Go and Visuospatial Tasks. Awake time was included as a covariate in regression models to account for additional wake time on Day 1 post-SR. Time spent in SED was higher on Days 1 and 3 post-SR, with proportional reductions in TPA and activity counts (Day 1), compared with Days 1 and 3 post-FULL. There were no significant differences between conditions for LPA or MVPA, despite trends towards reduced LPA (‘Condition’, p=0.1) and MVPA [Day 1 (p=0.07); Day 3 (p=0.09)]. There were no differences in timing or bouts of physical behaviors. Go/No-Go Task accuracy was lower on Day 1 post-SR. There were no other significant main or mediating effects associated with cognitive performance. The primary result was an increase in SED and proportional decrease in TPA on Days 1 and 3 post-SR, with decreased activity counts on Day 1. The present study provides meaningful insight for researchers and clinicians on the importance of obtaining adequate sleep, as well as the relationships between sleep and subsequent physical behaviors.