Biology subjects

Kent, J. A.

Publications and source records attributed to Kent, J. A..

5 recordsLinked to original sources

bioRxiv2026

Age-Related Differences in Carbohydrate and Fat Oxidation Over 24 Hours and During Daily Activities in Humans

The effects of older age on the oxidation of energy substrates (carbohydrates, fats) over 24 hr and during activities of daily living (ADL) may have substantial implications for metabolic health. To examine this issue, we evaluated age-related differences in substrate oxidation for 24 hr and periods of sleep, ADLs, and a 30-min treadmill walk (30MTW, 1.3m{middle dot}s^-1) in 17 young (29 {+/-} 5 yr, mean {+/-} SD, 6F) and 16 older (72 {+/-} 5, 8F) healthy active adults. Oxygen consumption (&VdotO2) and carbon dioxide production were measured using a 32,500 L room calorimeter to quantify substrate oxidation, which was normalized to body mass (kJ{middle dot}kg^-1) and total energy expenditure (%). Separately, &VdotO2peak was measured during an incremental treadmill test to exhaustion. &VdotO2peak (ml{middle dot}kg^-1{middle dot}min^-1) was lower in older adults compared with young adults. Older adults oxidized fewer carbohydrates than young over 24 hr (kJ{middle dot}kg^-1, %), sleep (kJ{middle dot}kg^-1), and ADLs (kJ{middle dot}kg^-1, %); and fat oxidation was greater in older than young over 24 hr (kJ{middle dot}kg^-1, %) and ADLs (kJ{middle dot}kg^-1, %). Carbohydrate oxidation did not differ by group during the 30MTW (kJ{middle dot}kg^-1, %), but fat oxidation was greater in older (kJ{middle dot}kg^-1) despite their walking at a greater relative intensity. These results indicate that older adults derived oxidative energy differently than young over 24 hr and during daily tasks. The lower carbohydrate and greater fat contribution to daily energy requirements may be meaningful in the development of metabolic disease in older adults.

physiology↗

bioRxiv2026

Plantar flexor muscle size, composition and strength and the redistribution of joint work in gait of healthy older adults

BackgroundOlder adults exhibit hallmark changes in gait mechanics that may contribute to mobility loss. These changes are suggested to be precipitated by decreased mechanical output from the ankle plantar flexor muscles. However, the extent of age-related changes within individual triceps surae muscles and the consequences for plantar flexor specific torque (torque per unit muscle) and walking mechanics remains unclear. The study aims were to quantify age-related differences in triceps surae muscle cross-sectional area (CSA), fat fraction and peak absolute and specific plantar flexion torque, and to evaluate their relationships with positive hip and ankle work distribution during walking. MethodsEleven younger (36.2 +/-3.4 years, 5 female) and 12 older (73.3+/-2.8 years, 6 female) adults completed dynamometry, magnetic resonance fat-water imaging of the shank, and overground gait analysis at a prescribed (1.2 m/s) and fast walking speed. ResultsOlder adults had a smaller fat-free muscle CSA for the soleus and medial gastrocnemius, resulting in {bsim}25% smaller triceps surae CSA. There was no difference by age in fat fraction for any muscle. Peak plantar flexor torque was lower in older and proportional to the differences in cross-sectional area, thus specific torque did not differ by age. Older adults generated greater positive hip work at both speeds and less positive ankle work at the fast speed leading to a greater redistribution ratio compared to younger. However, no significant correlations were found between the redistribution ratio and triceps surae muscle morphology or plantar flexion absolute or specific torque in older adults. ConclusionsThese results suggest that triceps surae muscle morphology and function may not be a primary source of age-related changes in gait mechanics in healthy older adults.

physiology↗

bioRxiv2025

Phosphate and acidosis cause fiber-type specific changes to cellular and molecular contractile mechanics at 37°C in skeletal muscle from older adults

Intracellular accumulation of hydrogen ions (H+) and inorganic phosphate (Pi) have temperature-dependent effects on single fiber contractile function between 10-30{degrees}C. In vivo, human skeletal muscle temperatures range between 35-38{degrees}C, and although contractile function is highly dependent on temperature, the effects of fatigue-inducing [H+] and [Pi] on contractile mechanics at 37{degrees}C is unknown. Using sinusoidal analysis, the independent and combined effects of these metabolites on cellular and molecular contractile function were determined at 37{degrees}C in slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibers from vastus lateralis muscle of 13 older adults (8 females), under four conditions: maximal calcium activation ("control"; 5 mM Pi, pH 7.0), high Pi (30 mM), low pH (6.2), and fatigue (30 mM Pi and pH 6.2). Specific tension (force/cross-sectional area, mN/mm2) in both fiber types was reduced only under fatigue conditions (20-26%). MHC I fibers had slower cross-bridge kinetics with fewer or less stiff strongly-bound myosin-actin cross-bridges in high Pi, low pH, and fatigue. In contrast, fatigued MHC IIA fibers had faster cross-bridge kinetics with increased myofilament and/or cross-bridge viscosity. Single fiber oscillatory work was reduced in both fiber types when Pi or pH alone was altered. However, fatigue conditions returned oscillatory work values toward control through alterations to cross-bridge kinetics in MHC I fibers and changes to work absorption and production processes in MHC IIA fibers. These findings quantify fiber-type specific mechanical and kinetic mechanisms of fatigue in human skeletal muscle at 37{degrees}C, thus advancing our understanding of metabolite-based muscle fatigue in vivo. KEY POINTS SUMMARYO_LIWorking skeletal muscle increases intracellular concentrations of hydrogen ion and inorganic phosphate, leading to fatigue, or loss of force-generating capacity C_LIO_LITemperature plays a well-established role in the muscle response to hydrogen ion and/or inorganic phosphate accumulation, but has not previously been studied at human body temperature (37{degrees}C) C_LIO_LIAt 37{degrees}C, reduced force generation only occurs when high phosphate and hydrogen ions are combined, not when changed individually C_LIO_LIIn slow-contracting fibers, fatigue slowed myosin-actin cross-bridge kinetics and reduced the number or stiffness of strongly-bound cross-bridges. In fast-contracting fibers, fatigue increased myosin-actin cross-bridge kinetics and increased myofilament viscosity. C_LIO_LIThe distinct responses by fiber type to fatigue provides new insight into its mechanisms and advances our understanding of the whole muscle and body responses to fatigue C_LI Abstract Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=125 SRC="FIGDIR/small/672942v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@e4ece3org.highwire.dtl.DTLVardef@17c62eforg.highwire.dtl.DTLVardef@1434e30org.highwire.dtl.DTLVardef@1c2446c_HPS_FORMAT_FIGEXP M_FIG C_FIG

physiology↗

bioRxiv2025

Slow- and fast-contracting skeletal muscle fibers have more similar cellular and molecular contractile function at 37°C than at 25°C in older adults

As human skeletal muscle cellular and molecular contractile properties are temperature-sensitive, the ability to perform experiments at body temperature ([~]37{degrees}C) may lead to a better understanding of their in vivo responses and potentially their effects upon whole-muscle and whole-body performance. We quantified molecular (myosin-actin cross-bridge mechanics and kinetics) and cellular (specific tension; force divided by cross-sectional area) function in slow-contracting myosin heavy chain (MHC) I and fast-contracting MHC IIA fibers from older adults (n=13, 8 female) at 37{degrees}C and compared these to results at 25{degrees}C. MHC I fibers were more temperature-sensitive than MHC IIA fibers, showing greater increases in cross-bridge kinetics (MHC I: 4.9-8.7x; IIA: 4x) and number or stiffness of strongly-bound cross-bridges (MHC I: 86%; IIA: 34%), leading to increased specific tension in MHC I (19%), with no change in MHC IIA fibers. The expected relationship between fiber force and size (cross-sectional area, CSA) was stronger at 37{degrees}C in both fiber types, explaining 80-82% of the variance compared to 51-52% at 25{degrees}C. Specific tension was unchanged with size at 37{degrees}C in both fiber types, showing that force increases proportionally with CSA, which may be due to the increased number or stiffness of strongly-bound cross-bridges at this temperature. At 25{degrees}C, specific tension decreased with size in agreement with previous experiments. Overall, MHC I and IIA fibers at body temperature (37{degrees}C) became more analogous, including similar specific tension and closer cross-bridge kinetics, and force production was more strongly correlated with fiber size compared to a non-physiological temperature. Abstract Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=103 SRC="FIGDIR/small/670366v1_ufig1.gif" ALT="Figure 1"> View larger version (20K): org.highwire.dtl.DTLVardef@70fe6aorg.highwire.dtl.DTLVardef@4ef5e4org.highwire.dtl.DTLVardef@1f1551forg.highwire.dtl.DTLVardef@10ba6d1_HPS_FORMAT_FIGEXP M_FIG C_FIG KEY POINTS SUMMARYO_LIAlthough skeletal muscle function is highly sensitive to temperature, human single fiber studies have only been conducted at [≤]30{degrees}C. C_LIO_LISmall-amplitude sinusoidal perturbations were utilized to elucidate mechanisms of single fiber force production at human physiological temperature (37{degrees}C). C_LIO_LIWe found that functional differences in slow-contracting myosin heavy chain (MHC I) and fast-contracting MHC IIA fibers observed at 25{degrees}C were less apparent at 37{degrees}C, as force, crossbridge kinetics, and strongly-bound crossbridges increased more in MHC I fibers than MHC IIA fibers at 37 vs. 25{degrees}C. C_LIO_LIThese results indicate that, given the different sensitivity of each fiber type to changes in temperature, functional assessments of muscle should be conducted at 37{degrees}C to better translate to vivo conditions. C_LI

physiology↗

bioRxiv2020

Effects of old age on knee extensor fatigue and recovery from high-velocity muscle contractions

PurposeAlthough high-velocity contractions elicit greater muscle fatigue in older than young adults, the cause of this difference is unclear. We examined the potential roles of resting muscle architecture and baseline contractile properties, as well as changes in voluntary activation and low-frequency fatigue in response to high-velocity knee extensor work. MethodsVastus lateralis muscle architecture was determined in quiescent muscle by ultrasonography in 8 young (23.4{+/-}1.8 yrs) and 8 older women (69.6{+/-}1.1). Maximal voluntary dynamic (MVDC) and isometric (MVIC), and stimulated (80Hz and 10Hz, 500ms) isometric contractions were performed before and immediately after 120 MVDCs (240{degrees}{middle dot}s-1, one every 2s). ResultsArchitecture variables did not differ between groups (p[≥]0.209), but the half-time of torque relaxation (T1/2) was longer in older than young women at baseline (151.9{+/-}6.0 vs. 118.8{+/-}4.4 ms, respectively, p=0.001). Older women fatigued more than young (to 33.6{+/-}4.7% vs. 55.2{+/-}4.2% initial torque, respectively; p=0.004), with no evidence of voluntary activation failure ({Delta}MVIC:80Hz torque) in either group (p[≥]0.317). Low-frequency fatigue ({Delta}10:80Hz torque) occurred in both groups (p<0.001), as did slowing of T1/2 (p=0.001), with no differences between groups. Baseline T1/2 was inversely associated with fatigue in older (r2=0.584, p=0.045), but not young women (r2=0.147, p=0.348). ConclusionsThese results indicate that differences in muscle architecture, voluntary activation, and low-frequency fatigue do not explain the greater fatigue of older compared with young women during high-velocity contractions. The inverse association between baseline T1/2 and fatigue in older women suggests that factors related to slower muscle contractile properties may be protective against fatigue during fast, repetitive contractions in aging.

physiology↗