Gastric gas and fluid emptying assessed by magnetic resonance imaging

Magnetic resonance imaging (MRI) was used to characterize the volumes and rates of gastric emptying of both liquid and gas following the ingestion of beverages of varying carbonation and carbohydrate levels. Eight subjects drank 800 ml each of four test beverages in a counterbalanced order: water, a non-carbonated carbohydrate-electrolyte solution (NC), a lightly carbonated carbohydrate-electrolyte solution (PC), and a carbonated cola (CC). T2-weighted, echoplanar images (25-30 contiguous slices, 1 cm thick, 256 x 128 matrix, TE = 80, 40 cm FOV) of the abdomen were collected at minutes 3,110, 20, 30, 45, and 60 following beverage ingestion. Images were analyzed for gas and liquid volumes. Water and NC emptied the most rapidly, with half times of 21(3) and 31(3) min, respectively [mean (SE)]. PC emptied significantly slower [47 (6) min] and CC slower yet [107 (8) min]. The carbonation content of the beverage accounted for 84% of the variation in emptying time, whereas carbohydrate content did not account for any significant variation. The gastric gas volume of the CC was higher at 2 min post-ingestion compared with all other drinks; however, the rate of emptying of the gas was the same among all beverages. Significantly greater total gastric volumes (gas+ liquid) were associated with the ingestion of CC, and accordingly produced a greater severity of gastric distress, as evaluated with a gastric distress inventory. The high gastric gas volumes (approximately 600 ml) after ingestion of CC suggested a potential source of error in body composition using standard hydrostatic weighing methods. This prediction was tested in nine additional subjects. Ingestion of 800 ml of CC prior to hydrostatic weighing resulted in a 0.7% underestimate of body density and thus an 11% overestimate of percentage body fat compared to measurements made before beverage consumption.     

Reliability of knee and ankle strength measures in an older adult population

Strength training for older adults is increasingly common, yet surprisingly little research has evaluated the reliability of strength testing protocols in this population. Thirty-three volunteers (17 women, 16 men; 72 +/- 6 years) were tested for strength of the knee and ankle using a Biodex 3 dynamometer on 3 separate occasions. The peak torque and work for each test was analyzed for reliability over the last 2 visits using limits of agreement (LOA). The magnitude of the systematic bias was 8 Nm or less for the peak torque and 5 J or less for the work measures. The random error ranged from 9 to 20 Nm and 6 to 24 J for peak torque and work, respectively. Heteroscedasticity was present in 8 of the 20 measures. The ratio LOA ranged from 21% to 43% for these peak torque and work measures. The total error of each strength measure, which was mostly comprised of random error, can be applied to interpretation and development of training protocols for the older adult.     

Short-term training effects on diastolic function in obese persons with the metabolic syndrome

The aim of this study was to determine the effects of a short-term high-intensity exercise program on diastolic function and glucose tolerance in obese individuals with and without metabolic syndrome (MetSyn). Obese men and women (BMI > 30 kg/m(2); 39-60 years) with and without the MetSyn (MetSyn 13; non-MetSyn 18) underwent exercise training consisting of 10 consecutive days of treadmill walking for 1 h/day at 70-75% of peak aerobic capacity. Subjects performed pre- and post-training testing for aerobic capacity, glucose tolerance (2-h meal test), and standard echocardiography. Aerobic capacity improved for both groups (non-MetSyn 24.0 +/- 1.6 ml/kg/min vs. 25.1 +/- 1.5 ml/kg/min; MetSyn 25.2 +/- 1.8 ml/kg/min vs. 26.2 +/- 1.7 ml/kg/min, P < 0.05). Glucose area under the curve (AUC) improved in the MetSyn group (1,017 +/- 58 pmol/l/min vs. 883 +/- 75 pmol/l/min, P < 0.05) with no change for the non-MetSyn group (685 +/- 54 pmol/l/min vs. 695 +/- 70 pmol/l/min). Isovolumic relaxation time (IVRT) improved in the MetSyn group (97 +/- 6 ms vs. 80 +/- 5 ms, P < 0.05), and remained normal in the non-MetSyn group (82 +/- 6 ms vs. 86 +/- 5 ms). No changes in other diastolic parameters were observed. The overall reduction in IVRT was correlated with a decrease in diastolic blood pressure (DBP) (r = 0.45, P < 0.05), but not with changes in glucose tolerance. Body weight did not change with training in either group. A 10-day high-intensity exercise program improved diastolic function and glucose tolerance in the group with MetSyn. The reduction in IVRT in MetSyn was associated with a fall in blood pressure. These data suggest that it may be possible to reverse early parameters of diastolic dysfunction in MetSyn with a high-intensity exercise program.     

Effect of acute head-down tilt on skeletal muscle cross-sectional area and proton transverse relaxation time

Conley, Michael S., Jeanne M. Foley, Lori L. Ploutz-Snyder,Ronald A. Meyer, and Gary A. Dudley. Effect of acute head-down tilt on skeletal muscle cross-sectional area and proton transverse relaxation time. J. Appl. Physiol.81(4): 1572-1577, 1996.This study investigated changes inskeletal muscle cross-sectional area (CSA) evoked by fluid shifts thataccompany short-term 6° head-down tilt (HDT) or horizontal bedrest, the time course of the resolution of these changes afterresumption of upright posture, and the effect of altered muscle CSA, inthe absence of increased contractile activity, on proton transverserelaxation time (T₂). Averagemuscle CSA and T₂ were determinedby standard spin-echo magnetic resonance imaging. Analyses wereperformed on contiguous transaxial images of the neck and calf. After aday of normal activity, 24 h of HDT increased neck muscle CSA 19 ± 4 (SE)% (P < 0.05) whilecalf muscle CSA decreased 14 ± 3%(P < 0.05). The horizontal posture(12 h) induced about one-half of these responses: an 11 ± 2%(P < 0.05) increase in neck muscleCSA and an 8 ± 2% decrease (P < 0.05) in the calf. Within 2 h after resumption of upright posture, neckand calf muscle CSA returned to within 0.5% (P > 0.05) of the values assessedafter a day of normal activity, with most of the change occurringwithin the first 30 min. No further change in muscle CSA was observedthrough 6 h of upright posture. Despite these large alterations inmuscle CSA, T₂ was not altered bymore than 1.1 ± 0.6% (P > 0.05)and did not relate to muscle size. These results suggest that posturalmanipulations and subsequent fluid shifts modeling microgravity elicitmarked changes in muscle size. Because these responses were notassociated with alterations in muscleT₂, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.

     

Growth hormone and muscle function responses to skeletal muscle ischemia.

We examined the effects of ischemia (ISC) alone and with low-intensity exercise (ISC+EX) on growth hormone (GH) and muscle function responses. Nine men (22 +/- 0.7 yr) completed 3 study days: an ISC day (thigh cuff inflated five times, 5 min on, 3 min off), an ISC+EX day [knee extension at 20% maximal voluntary contraction (MVC) with ISC], and a control day. MVCs and submaximal contraction tasks (15 and 30% MVC) were performed before and following the perturbations. Surface electromyogram signals were collected from thigh muscles and analyzed for median frequency and root mean square alterations. Blood samples were collected every 10 min (190 min total) and analyzed for GH concentrations. Peak GH concentrations and GH area under the curve were highest (P < 0.01) on the ISC+EX day (7.5 microg/l and 432 microg.l(-1).min(-1), respectively) compared with the ISC (0.9 microg/l and 76.4 microg.l(-1).min(-1)), and CON (1.1 microg/l and 83.8 microg.l(-1).min(-1)) days. A greater GH pulse amplitude, mass/pulse, and production rate were also observed on the ISC+EX day (P < 0.05). Following the intervention, force production decreased on the ISC and ISC+EX days by 16.1 and 55.8%, respectively, and did not return to baseline values within 5 min of recovery. During the submaximal contractions, median frequency shifted to lower frequencies for most of the muscles examined, and root mean square electromyogram was consistently elevated for ISC+EX day. In conclusion, ISC coupled with resistance exercise acutely increases GH levels and reduces MVC, whereas ISC alone decreases force capacity, without alterations in GH levels.     

Gender differences in skeletal muscle fatigability are related to contraction type and EMG spectral compression.

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.     

Fiber type and metabolic dependence of T2 increases in stimulated rat muscles.

This study examined the relationships between muscle fiber type, metabolism, and blood flow vs. the increase in skeletal muscle (1)H-NMR transverse relaxation time (T2) after stimulation. Triceps surae muscles of anesthetized rats were stimulated in situ at 1-10 Hz for 6 min, and T2 was calculated from (1)H-NMR images acquired at 4.7 T immediately after stimulation. At low-to-intermediate frequencies (1-5 Hz), the stimulation-induced T2 increase was greater in the superficial, fast-twitch white portion of the gastrocnemius muscle compared with the deeper, more aerobic muscles of the triceps surae group. Although whole triceps muscle area changed in parallel with T2 after stimulation when blood flow was intact, clamping of the femoral artery during stimulation prevented an increase in muscle area but not an increase in T2. Partial inhibition of lactic acid production with iodoacetate diminished intracellular acidification (measured by (31)P-NMR spectroscopy) during brief (1.5 min) stimulation but had no significant effect either on estimated osmolite accumulation or on muscle T2 after stimulation. Depletion of muscle phosphocreatine content by feeding rats beta-guanidinopropionate decreased both estimated osmolite accumulation and T2 after 1.5-min stimulation. The results are consistent with the hypothesis that the T2 increase in stimulated muscle is related to osmotically driven shifts of fluid into an intracellular compartment.     

Adaptations in human neuromuscular function following prolonged unweighting: I. Skeletal muscle contractile properties and applied ischemia efficacy.

Strength loss following disuse may result from alterations in muscle and/or neurological properties. In this paper, we report our findings on human plantar flexor muscle properties following 4 wk of limb suspension (unilateral lower limb suspension), along with the effect of applied ischemia (Isc) on these properties. In the companion paper (Part II), we report our findings on the changes in neurological properties. Measurements of voluntary and evoked forces, the compound muscle fiber action potential (CMAP), and muscle cross-sectional area (CSA) were collected before and after 4 wk of unilateral lower limb suspension in adults (n = 18; 19-28 yr). A subset of subjects (n = 6) received applications of Isc 3 days/wk (3 sets; 5-min duration). In the subjects not receiving Isc, the loss in CSA and strength was as expected ( approximately 9 and 14%). We observed a 30% slowing in the duration of the CMAP, a 10% decrease in evoked doublet force, a 12% increase in the twitch-to-doublet force ratio, and an altered postactivation potentiation response (11% increase in the postactivation potentiation-to-doublet ratio). We also detected a 10% slowing in the ability of the plantar flexor to develop force during the initial phase of an evoked contraction, along with a 6% reduction in in vivo specific doublet force. In the Isc subjects, no preservation was observed in strength or the evoked muscle properties. However, the Isc group did maintain CSA of the lateral gastrocnemius, as the control subjects' lateral gastrocnemius atrophied 10.2%, whereas the subjects receiving Isc atrophied 4.7%. Additionally, Isc abolished the unweighting-induced slowing in the CMAP. These findings suggest that unweighting alters the contractile properties involved in the excitation-contraction coupling processes and that Isc impacts the sarcolemma.     

Test battery designed to quickly and safely assess diverse indices of neuromuscular function after unweighting

Adequately describing the functional consequences of unweighting (e.g., bed rest, immobilization, spaceflight) requires assessing diverse indices of neuromuscular function (i.e., strength, power, endurance, central activation, force steadiness). Additionally, because unweighting increases the susceptibility of muscle to damage, testing should consider supplementary safety features. The purpose of this study was to develop a test battery for quickly assessing diverse indices of neuromuscular function. Commercially available exercise equipment was modified to include data acquisition hardware (e.g., force plates, position transducers) and auxiliary safety hardware (e.g., magnetic brakes). Ten healthy, ambulatory subjects (31 ± 5 years, 173 ± 11 cm, 73 ± 14 kg) completed a battery of lower- and upper-body neuromuscular function tests on 3 occasions separated by at least 48 hours. The battery consisted of the following tests, in order: (1) knee extension central activation, (2) knee extension force steadiness, (3) leg press maximal strength, (4) leg press maximal power, (5) leg press power endurance, (6) bench press maximal strength, (7) bench press force steadiness, (8) bench press maximal power, and (9) bench press power endurance. Central activation, strength, rate of force development, maximal power, and power endurance (total work) demonstrated good-to-excellent measurement reliability (SEM = 3-14%; intraclass correlation coefficient [ICC] = 0.87-0.99). The SEM of the force steadiness variables was 20-35% (ICC = 0.20-0.60). After familiarization, the test battery required 49 ± 6 minutes to complete. In conclusion, we successfully developed a test battery that could be used to quickly and reliably assess diverse indices of neuromuscular function. Because the test battery involves minimal eccentric muscle actions and impact forces, the potential for muscle injury has likely been reduced.     

Reliability of techniques to assess human neuromuscular function in vivo.

The purpose of this study was to comprehensively evaluate the reliability of a large number of commonly utilized experimental tests of in vivo human neuromuscular function separated by 4-weeks. Numerous electrophysiological parameters (i.e., voluntary and evoked electromyogram [EMG] signals), contractile properties (i.e., evoked forces and rates of force development and relaxation), muscle morphology (i.e., MRI-derived cross-sectional area [CSA]) and performance tasks (i.e., steadiness and time to task failure) were assessed from the plantarflexor muscle group in 17 subjects before and following 4-weeks where they maintained their normal lifestyle. The reliability of the measured variables had wide-ranging levels of consistency, with coefficient of variations (CV) ranging from approximately 2% to 20%, and intraclass correlation coefficients (ICC) between 0.53 and 0.99. Overall, we observed moderate to high-levels of reliability in the vast majority of the variables we assessed (24 out of the 29 had ICC>0.70 and CV<15%). The variables demonstrating the highest reliability were: CSA (ICC=0.93-0.98), strength (ICC=0.97), an index of nerve conduction velocity (ICC=0.95), and H-reflex amplitude (ICC=0.93). Conversely, the variables demonstrating the lowest reliability were: the amplitude of voluntary EMG signal (ICC=0.53-0.88), and the time to task failure of a sustained submaximal contraction (ICC=0.64). Additionally, relatively little systematic bias (calculated through the limits of agreement) was observed in these measures over the repeat sessions. In conclusion, while the reliability differed between the various measures, in general it was rather high even when the testing sessions are separated by a relatively long duration of time.