Background: Recent findings suggest that higher levels of intermuscular adipose tissue (IMAT) are associated with glucose dysregulation, lower levels of muscle strength, and a heightened risk of disability. Although several studies have described adaptations in muscle after reduced physical activity, the change in IMAT in healthy young adults is unknown. Objective: The objective was to determine whether reduced lower limb activity alters IMAT in healthy young adults and to assess whether this change affects muscle strength loss. Design: The subjects (6 men and 12 women aged 19 -28 y) underwent a 4-wk control period, which was followed by 4 wk of unilateral lower limb suspension. Volumes of whole muscle, subcutaneous adipose tissue, and IMAT were assessed by using magnetic resonance imaging in the thigh and calf. Muscle strength was assessed during maximal voluntary isometric contractions. Results: No changes were observed in the control period. Reduced physical activity decreased thigh and calf muscle volumes by 7.4% and 7.9% (P 0.001), respectively; no significant change in subcutaneous adipose tissue was observed. Additionally, IMAT increased in both regions; the increase was larger in the calf (20%) than in the thigh (14.5%) (P ͨ 0.005) and was partially explained by the loss in muscle (R 2 ҃ 26%). The loss in strength was greater in the thigh (20.4%) than in the calf (15%). Strength loss was associated with increases in IMAT (P ҃ 0.039) after adjustment for the loss in muscle, initial strength, initial IMAT, and initial muscle volume. Conclusions: IMAT accumulates markedly after reduced activity in healthy young adults. Increases in IMAT may contribute to losses in muscle strength associated with reduced physical activity, but the mechanism responsible is yet to be determined. 2007;85:377-84. Am J Clin Nutr

Exercise has shown little success in mitigating bone loss from long-duration spaceflight. The first crews of the International Space Station (ISS) used the ''interim resistive exercise device'' (iRED), which allowed loads of up to 297 lb f (or 1337 N) but provided little protection of bone or no greater protection than aerobic exercise. In 2008, the Advanced Resistive Exercise Device (ARED), which allowed absolute loads of up to 600 lb f (1675 N), was launched to the ISS. We report dietary intake, bone densitometry, and biochemical markers in 13 crewmembers on ISS missions from 2006 to 2009. Of these 13, 8 had access to the iRED and 5 had access to the ARED. In both groups, bone-specific alkaline phosphatase tended to increase during flight toward the end of the mission (p ¼ 0.06) and increased 30 days after landing (p < 0.001). Most markers of bone resorption were also increased in both groups during flight and 30 days after landing (p < 0.05). Bone densitometry revealed significant interactions (time and exercise device) for pelvis bone mineral density (BMD) and bone mineral content (p < 0.01), hip femoral neck BMD (p < 0.05), trochanter BMD (p < 0.05), and total hip BMD (p < 0.05). These variables were unchanged from preflight only for ARED crewmembers, who also returned from flight with higher percent lean mass and lower percent fat mass. Body mass was unchanged after flight in both groups. All crewmembers had nominal vitamin D status (75 AE 17 nmol/L) before and during flight. These data document that resistance exercise, coupled with adequate energy intake (shown by maintenance of body mass determined by dual-energy X-ray absorptiometry [DXA]) and vitamin D, can maintain bone in most regions during 4-to 6-month missions in microgravity. This is the first evidence that improving nutrition and resistance exercise during spaceflight can attenuate the expected BMD deficits previously observed after prolonged missions. ß

The purpose of this study was to identify quadriceps femoris strength thresholds below which performance on ambulatory tasks is compromised. A second purpose was to evaluate whether self-reported functioning matches evaluated performance of the activities. Subjects (N = 100; age 73 +/- 0.9 years) participated in isometric knee extension strength tests, performed three functional ambulatory tasks (chair rise, gait speed, and stair ascent and descent), and answered standard survey questions assessing physical function. Significant relationships were observed between functional performance and the ratio of isometric leg extension peak torque to body weight (STR/WT) for each activity (p <.0001). For each activity, thresholds of STR/WT between 3.0 and 3.5 N m/kg were observed, below which the likelihood for success was reduced. Thresholds were determined by calculating the value of STR/WT that minimized the classification error. Individuals with a STR/WT < 3.0 N m/kg are at a substantial risk for impaired function in chair rise, gait speed, and stair ascent and descent. Sensitivity and specificity of STR/WT as a predictor of functional success ranged from 76% to 81% and from 78% to 94%, respectively, depending on activity. This is of clinical significance, as the STR/WT thresholds can identify individuals with preclinical disability (beginning to have difficulty with ambulatory tasks) as opposed to those in whom an outright disability is observed. This may be useful for targeting individuals for strengthening interventions and developing specific intervention goals.

The purposes of this study were 1) to evaluate gender differences in back extensor endurance capacity during isometric and isotonic muscular contractions, 2) to determine the relation between absolute load and endurance time, and 3) to compare men [n = 10, age 22.4 +/- 0.69 (SE) yr] and women (n = 10, age 21.7 +/- 1.07 yr) in terms of neuromuscular activation patterns and median frequency (MF) shifts in the electromyogram (EMG) power spectrum of the lumbar and hip extensor muscles during fatiguing submaximal isometric trunk extension exercise. Subjects performed isotonic and isometric trunk extension exercise to muscular failure at 50% of maximum voluntary contraction force. Women exhibited a longer endurance time than men during the isometric task (146.0 +/- 10.9 vs. 105.4 +/- 7.9 s), but there was no difference in endurance performance during the isotonic exercise (24.3 +/- 3.4 vs. 24.0 +/- 2.8 repetitions). Absolute load was significantly related to isometric endurance time in the pooled sample (R(2) = 0.34) but not when men and women were analyzed separately (R(2) = 0.05 and 0.04, respectively). EMG data showed no differences in neuromuscular activation patterns; however, gender differences in MF shifts were observed. Women demonstrated a similar fatigability in the biceps femoris and lumbar extensors, whereas in men, the fatigability was more pronounced in the lumbar musculature than in the biceps femoris. Additionally, the MF of the lumbar extensors demonstrated a greater association with endurance time in men than in women (R(2) = 0.45 vs. 0.19). These findings suggest that gender differences in muscle fatigue are influenced by muscle contraction type and frequency shifts in the EMG signal but not by alterations in the synergistic activation patterns.