Biology subjects

Carr, J. C.

Publications and source records attributed to Carr, J. C..

3 recordsLinked to original sources

bioRxiv2026

Unimanual fatigue increases muscle excitation and local metabolic activity in the resting contralateral forearm

Intense unilateral muscle contractions evoke measurable activity within the contralateral neuroaxis, which can be detected with surface electromyographic activity in the resting homologous muscle. Physiological mirror activity (PMA), the unintentional increase in contralateral muscle excitation, has been implicated in cross-limb interactions and adaptations. Despite longstanding observations of PMA, it remains unknown whether this low-level muscle excitation influences local muscle metabolism. We addressed this question using a vascular occlusion test in 10 healthy adults. Surface electromyography and near-infrared spectroscopy-derived measures of tissue oxygen saturation and muscle oxygen consumption (m[V]O2) were obtained from the resting left forearm during vascular occlusion at rest and during fatiguing unimanual contractions of the right hand. PMA in the contralateral resting arm was greater during unimanual fatigue than during rest (mean difference: 8.9%AA, 95% CI: 4.1 to 13.8; p = 0.002, g = 1.20). This increase was accompanied by a steeper rate of tissue oxygen desaturation (mean difference: -0.132 %{middle dot}s-{superscript 1}, 95% CI: -0.227 to -0.037; p = 0.012, g = -0.91) and greater m[V]O2 (mean difference: 0.188 mL O2{middle dot}min-{superscript 1}{middle dot}100 g-{superscript 1}, 95% CI: 0.057 to 0.320; p = 0.010, g = 0.94). Greater PMA was associated with both a faster rate of oxygen desaturation (r = -0.85, 95% CI: -0.96 to -0.46, p = 0.002) and greater m[V]O2 (r = 0.78, 95% CI: 0.28 to 0.94, p = 0.008). These findings suggest that PMA is accompanied by increased local metabolic demand, consistent with a coupling between unintentional muscle excitation and oxygen extraction in the resting limb. New and noteworthyPhysiological mirror activity is an established feature of intense unimanual contractions, yet its metabolic relevance has remained unknown. We show that unimanual fatigue increases contralateral muscle excitation, accelerates oxygen desaturation, and increases muscle oxygen consumption in the resting forearm during vascular occlusion. The magnitude of mirror activity was strongly associated with both oxygen desaturation and oxygen consumption, providing evidence that involuntary contralateral muscle excitation is accompanied by increased local metabolic demand in the resting limb.

physiology↗

bioRxiv2025

The Effects of Oral Contraceptives on Grip Strength and Neuromuscular Activation During Short-Term Immobilization and Rehabilitation of the Wrist/Hand

Females often experience greater weakness following immobilization compared to males. Hormonal fluctuations from the menstrual cycle or oral contraceptive (OC) use may contribute to sex differences and response variation. We examined differences in peak and rapid force and neuromuscular activation among females using monophasic OC and females not using OC following immobilization and rehabilitation. To examine potential sex differences, a male control group was included. Ten males, 10 OC females, and 10 non-OC females (mean {+/-} SD age = 23 {+/-} 3 years) immobilized their left wrist/hand with a brace for one week, followed by [≥] one week of rehabilitation. Participants completed grip tests to assess peak force and the rate of force development (RFD) before and after immobilization and post-rehabilitation, with electromyographic signals recorded from the extensor carpi radialis brevis (ECBR) and flexor digitorum superficialis (FDS). Grip force declined post-immobilization: males = -17.2 {+/-} 10.3%, non-OC = -22.3 {+/-} 24.7%, OC = -20.7 {+/-} 14.8%. No significant time x group interactions were observed for any dependent variables (p > 0.05, 2p [≤] 0.084). Time effects showed recovery post-rehab. RFD, particularly at 200 ms, declined posttest and rebounded post-rehab. ECBR excitation increased post-rehab; FDS trended upward. Only 5 participants required > one week of rehabilitation (2 males, 2 non-OC, 1 OC). We conclude that males and females exhibit similar declines and recovery in grip force after one week of wrist/hand immobilization, regardless of OC use. OC use does not appear to affect outcomes in female patients undergoing musculoskeletal rehabilitation. NEW & NOTEWORTHYPrevious studies report greater strength losses in females than males after immobilization, yet the underlying mechanisms remain unclear. This is the first study to examine whether monophasic OC use influences neuromuscular responses to disuse and recovery. We found no significant differences in the loss or recovery of peak force, RFD, or sEMG excitation between OC users, non-users, and males. Short-term immobilization and rehabilitation outcomes are not meaningfully affected by OC use in healthy, young females.

physiology↗

bioRxiv2025

Resistance training-induced appendicular lean tissue mass changes are largely unrelated to pre-training bone characteristics in a larger cohort of untrained adults

We sought to determine if pre-intervention bone characteristics measured by dual-energy x-ray absorptiometry (DXA) were associated with changes in bone-free lean tissue mass following a period of resistance training in a large cohort of untrained adults (n=119, 62M/57F, 26.0{+/-}4.7 kg/m2, age range = 18-70 years old). Participants completed 10-12 weeks of supervised whole-body resistance training twice weekly, and DXA scans were obtained approximately the same time of day prior to the intervention and 48-72 hours following the final training bout. Associations between baseline skeletal measures (e.g., appendicular bone characteristics, shoulder and hip widths) and training induced changes in appendicular lean mass were examined by estimating correlations between participant-level random slopes (reflecting change over time) and baseline skeletal measures. The same approach was used to evaluate associations between other participant attributes (e.g., age, training volume-load, self-reported energy intake) and appendicular lean tissue mass changes. Modeling was also used to explore whether baseline skeletal characteristics (e.g., shoulder and hip widths) moderated the change in appendicular lean tissue mass from training. All analyses used a Bayesian framework, and interpretation focused on estimated effect sizes and their associated credible intervals rather than formal null hypothesis testing. Strong positive associations were observed between pre-intervention characteristics including dual-arm lean tissue mass and dual-arm bone mineral content (r=0.90), dual-leg lean tissue mass and dual-leg bone mineral content (r=0.86), dual-leg lean tissue mass and pelvic mineral content (r=0.73), and dual-arm lean tissue mass and shoulder width (r=0.76). In contrast, weak associations were observed between training-induced changes in appendicular lean tissue mass versus bone characteristics, training volume-load, self-reported energy intake, self-reported protein intake, BMI, and age (-0.08[≤]r[≤] 0.24). After adjusting for sex, multivariable analyses indicated minimal evidence that skeletal characteristics moderated the hypertrophic response to training. These findings do not support a meaningful role of pre-training bone characteristics in influencing the lean tissue mass adaptations to shorter-term resistance training.

physiology↗