Adequate sleep can easily become compromised as student-athletes try to balance the multiple demands on their time. People with sleep deficiency are at increased risk for acute illnesses, traumatic sports injuries, and development of chronic diseases. Training sessions or competitions during extremely early or late hours can interfere with circadian and homeostatic rhythms. Adjusting the training schedule to improve sleep duration has a significantly positive impact on several aspects of athletic performance. Pediatricians should increase the time dedicated in well-child visits for sleep hygiene and evaluate for sleep disorders at all ages. Parents, coaching staff, teachers, and pediatricians should advocate for improved education on the importance of sleep during adolescence. Future sleep research specific to adolescent athletes can further delineate requirements specific to sport, gender, training times, and surrounding competitions. [Pediatr Ann. 2017;46(3):e106-e111.].

The relative oxygen saturation of hemoglobin (%HbO2) and the rate of perfusion (θ̇) are important physiological quantities, particularly in organs such as skeletal muscle in which oxygen delivery and use are tightly coupled. The purpose of this study was to demonstrate the image-based calculation of %HbO2 and quantification of perfusion in skeletal muscle during isometric contractions. This was accomplished by establishing an empirical relationship between the rate of RF-reversible dephasing (R2′) and near infrared spectroscopy (NIRS)-observed oxyhemoglobin saturation (%HbO2) under conditions of arterial occlusion and constant blood volume. A calibration curve was generated and used to calculate %HbO2 from R2′ changes measured during contraction. Twelve young healthy subjects underwent 300 seconds of arterial occlusion and performed isometric contractions of the dorsiflexors at 30% of maximal contraction for 120s. Muscle perfusion was quantified during contraction by arterial spin labeling and measures of muscle T1. Comparisons between the %HbO2 values predicted from R2′ and that measured by NIRS revealed no differences between methods (p = 0.760). Muscle perfusion reached a value of 34.7 mL 100g−1 min−1 during contraction. These measurements hold future promise in measuring muscle oxygen consumption in healthy and diseased skeletal muscle.

Blood oxygenation level dependent (BOLD) contrast in skeletal may reflect the contributions of both intravascular and extravascular relaxation effects. The purpose of this study was to determine the significance of the extravascular BOLD effect in skeletal muscle at 3 T. In experiments, R 2 * was measured before and during arterial occlusion under the following conditions: (1) the leg extended and rotated (to vary the capillary orientation with respect to the amplitude of static field) and (2) with the blood's signal nulled using a multiecho vascular space occupancy experiment. In the leg rotation protocol, 3 min of arterial occlusion decreased oxyhemoglobin saturation from 67% to 45% and increased R 2 * from 34.2 to 36.6 sec 21 , but there was no difference in the R 2 * response to occlusion between the extended and rotated positions. Numerical simulations of intra-and extravascular BOLD effects corresponding to these conditions predicted that the intravascular BOLD contribution to the R 2 * change was always > 50 times larger than the extravascular BOLD contribution. Blood signal nulling eliminated the change in R 2 * caused by arterial occlusion. These data indicate that under these experimental conditions, the contribution of the extravascular BOLD effect to skeletal muscle R 2 * was too small to be practically important. Magn Reson Med 64:527-535, 2010. V C 2010 WileyLiss, Inc.Key words: mfMRI; extravascular BOLD effect; oxygenation; vessel orientation; VASO Variations in the rates of permanent and effective transverse relaxation (R 2 and R 2 *, respectively) or in T 2 -weighted signal may be used to evaluate the hemodynamic, metabolic, and structural changes associated with muscle contractions (1), an approach known as muscle functional MRI (2). During and following muscle contractions, changes in muscle R 2 and R 2 * result from increases in intracellular water content, secondary to metabolite accumulation (3,4); variations in intracellular pH (4-6); and changes in the concentrations of paramagnetic molecules such as deoxyhemoglobin, as reflected in the blood oxygenation level dependent (BOLD) effect (7,8). The magnitude of each of these changes and thus the variations in R 2 and R 2 * depend on the exercise and intensity (9), the muscle fiber type composition (3), and whole-body aerobic capacity (10). Since these factors affect R 2 and R 2 * to different degrees, quantifying the independent contributions of and interactions among these factors is important to interpret physiologic and pathologic variations in muscle R 2 and R 2 *.Regarding the source of oxygen-level dependent contrast in skeletal muscle, it is noteworthy that skeletal muscle cells themselves contain several potential magnetic field perturbers that may also contribute to susceptibility-related signal decay in skeletal muscle. However, both theoretical calculations and experimental observations suggest that a true BOLD effect, and not a muscle oxygenation level dependent effect, occurs in skeletal muscle (8). The presence of a muscle BOLD ...

The purpose of this study was to determine whether there are differences in postisometric contraction blood volume and oxygenation responses among groups of type 2 diabetes mellitus (T2DM), obese, and lean individuals detectable using MRI. Eight T2DM patients were individually matched by age, sex, and race to non-T2DM individuals with similar body mass index (obese) and lean subjects. Functional MRI was performed using a dual-gradient-recalled echo, echo-planar imaging sequence with a repetition time of 1 s and at two echo times (TE = 6 and 46 ms). Data were acquired before, during, and after 10-s isometric dorsiflexion contractions performed at 50 and 100% of maximal voluntary contraction (MVC) force. MRI signal intensity (SI) changes from the tibialis anterior and extensor digitorum longus muscles were plotted as functions of time for each TE. From each time course, the difference between the minimum and the maximum postcontraction SI (ΔSI) were determined for TE = 6 ms (ΔSI(6)) and TE = 46 ms (ΔSI(46)), reflecting variations in blood volume and oxyhemoglobin saturation, respectively. Following 50% MVC contractions, the mean postcontraction ΔSI(6) values were similar in the three groups. Following MVC only, and in the EDL muscle only, T2DM and obese participants had ∼56% lower ΔSI(6) than the lean individuals. Also following MVC only, the ΔSI(46) response in the EDL was lower in T2DM subjects than in lean individuals. These data suggest that skeletal muscle small vessel impairment occurs in T2DM and body mass index-matched subjects, in muscle-specific and contraction intensity-dependent manners.

The purpose of this study was to assess the repeatability of a dual gradient-recalled echo (GRE) muscle functional MRI technique. On two days, subjects (n=8) performed 10 s isometric dorsiflexion contractions under conditions of: 1) maximal voluntary contraction (MVC), 2) 50% MVC (50%MVC) or 3) 50% MVC with concurrent proximal arterial cuff occlusion (50%MVCcuff). Functional MRI data were acquired using single-slice dual GRE (TR/TE=1000/6, 46 ms) EPI for 20 seconds before, during, and for 180 seconds after each contraction. The mean signal intensity (SI) time courses at each TE (SI6 and SI46, reflecting variations in blood volume and %HbO2, respectively) from the tibialis anterior (TA) and extensor digitorum (EDL) muscles were characterized with the post-contraction change in SI and the time to peak SI (ΔSI and TTP, respectively). ΔSI6 and ΔSI46 were 36% and 31% higher following an MVC than after a 50%MVC (p = 0.05 and p = 0.07 respectively). For ΔSI6 the highest intraclass correlation coefficients (ICC) were observed for the TA muscle at the 50%MVC and MVC condition, with values of 0.83 (p = 0.01) and 0.88 (p = 0.005), respectively. Bland-Altman plots revealed repeatability coefficients (RC) for the 50%MVC and MVC conditions in the TA muscle of 1.9 and 1.4, respectively. The most repeatable measures for ΔSI46 were obtained for the 50%MVC and MVC conditions in the EDL muscle (p = 0.01 and p = 0.04, respectively). Bland-Altman plots revealed RC’s for 50%MVC and MVC conditions in the EDL muscle of 3.9 and 5.7, respectively. ΔSI6 and ΔSI46 increased as a function of contraction intensity. The repeatability of the method depends on the muscle and contraction condition being evaluated, and in general, is higher following an MVC.