Background Disuse of a muscle group, which occurs during bedrest, spaceflight, and limb immobilization, results in atrophy. It is unclear, however, if the magnitude of decline in skeletal muscle quality is similar to that for muscle size. The purpose of this study was to examine the effects of two weeks of knee joint immobilization on vastus lateralis and rectus femoris echo intensity and cross-sectional area. Methods Thirteen females (mean ± SD age = 21 ± 2 years) underwent two weeks of left knee joint immobilization via ambulating on crutches and use of a brace. B-mode ultrasonography was utilized to obtain transverse plane images of the immobilized and control vastus lateralis and rectus femoris at pretest and following immobilization. Effect size statistics and two-way repeated measures analyses of variance were used to interpret the data. Results No meaningful changes were demonstrated for the control limb and the rectus femoris of the immobilized limb. Analyses showed a large increase in vastus lateralis echo intensity (i.e., decreased muscle quality) for the immobilized limb (p = .006, Cohen’s d = .918). For vastus lateralis cross-sectional area, no time × limb interaction was observed (p = .103), but the effect size was moderate (d = .570). There was a significant association between the increase in vastus lateralis echo intensity and the decrease in cross-sectional area (r =  − .649, p = .016). Conclusion In female participants, two weeks of knee joint immobilization resulted in greater deterioration of muscle quality than muscle size. Echo intensity appears to be an attractive clinical tool for monitoring muscle quality during disuse.

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Background: Aging results in adaptations which may affect the control of motor units. Objective: We sought to determine if younger and older men recruit motor units at similar force levels. Design: Cross-sectional, between-subjects design. Setting: Controlled laboratory setting. Participants: Twelve younger (age = 25 ± 3 years) and twelve older (age = 75 ± 8 years) men. Measurements: Participants performed isometric contractions of the dominant knee extensors at a force level corresponding to 50% maximal voluntary contraction (MVC). Bipolar surface electromyographic (EMG) signals were detected from the vastus lateralis. A surface EMG signal decomposition algorithm was used to quantify the recruitment threshold of each motor unit, which was defined as the force level corresponding to the first firing. Recruitment thresholds were expressed in both relative (% MVC) and absolute (N) terms. To further understand age-related differences in motor unit control, we examined the mean firing rate versus recruitment threshold relationship at steady force. Results: MVC force was greater in younger men (p = 0.010, d = 1.15). Older men had lower median recruitment thresholds in both absolute (p = 0.005, d = 1.29) and relative (p = 0.001, d = 1.53) terms. The absolute recruitment threshold range was larger for younger men (p = 0.020; d = 1.02), though a smaller difference was noted in relative terms (p = 0.235, d = 0.50). These findings were complimented by a generally flatter slope (p = 0.070; d = 0.78) and lower y-intercept (p = 0.009; d = 1.17) of the mean firing rate versus recruitment threshold relationship in older men. Conclusion: Older men tend to recruit more motor units at lower force levels. We speculate that recruitment threshold compression may be a neural adaptation serving to compensate for lower motor unit firing rates and/or denervation and subsequent re-innervation in aged muscle.

Physiological systems exhibit a substantial degree of plasticity in response to imposed physical demands. Whereas increased external loading results in several benefits to the musculoskeletal system, disuse causes dramatic declines in muscle strength, atrophy, cardiorespiratory impairments, and decreases in bone mineral density (Campbell et al., 2019).

Clinical trial participant populations fail to adequately represent target populations that drugs are intended to serve. Improving racial and ethnic diversity of clinical trial participants is essential for generalizable, quality clinical research results and ensuring social and medical equity. Site-level clinical research professionals (CRPs) have unique insights on diversity improvement strategies for clinical trial enrollment. A survey was distributed to current CRPs working at clinical research sites in the United States to describe current practices and perceptions of the impact these practices have on participant diversity. Subsequently, descriptive quantitative analysis and inductive content analysis were performed. For the practices surveyed, there are discrepancies between frequency of use and perceived impact on diversity enrollment. Common current practices include phone-based or telemedicine study visits, electronic/digital data collection, and participant compensation. However, we report travel reimbursement and services, translated documents and translator services, and adequate participant compensation as most impactful on diverse enrollment. A multistakeholder approach is necessary to enhance diversity and inclusion (D&I) of study participants. Besides large-scale solutions such as countering community distrust, actionable steps are needed by sponsors and study sites to improve D&I of trial participants. Study leadership at the sponsor, contract research organization (CRO), and site-level should create diversity plans prior to study start, and CRO and sponsor budgets should consider D&I strategies during study planning. Planning should incorporate strategies to improve D&I including adequate participant compensation, translated documents and translator services, and travel reimbursements.