Regional differences in abdominal fat loss

OBJECTIVE

This study determined if the magnetic resonance imaging (MRI) protocol used alters the estimation of change in abdominal fat with weight loss in obese type 2 diabetic women. This study also examined if there is a uniform fat loss across the abdomen.

METHODS AND PROCEDURES

Thirty-three obese postmenopausal women with type 2 diabetes (age 50-70 years, body mass index>30 kg/m(2)) had a total abdominal MRI scan pre- and post weight loss intervention. Three different MRI analysis protocols were used and compared: a single slice at L(2)-L(3) vs five slices (centered at L(4)-L(5)) vs all abdominal slices. In addition, the total abdominal scan was divided into four regions (four slices each) with region 3 (critical region) including the traditionally studied L(2)-L(3), and regions 1 and 2 superior and region 4 inferior to critical region 3. Analysis of variance (ANOVA) with repeated measures was used to compare the influence of weight loss on abdominal fat measured both regionally and using the varying number of MR slices.

RESULTS

At baseline, the ratio of visceral adipose tissue:subcutaneous adipose tissue (VAT:SAT) was significantly lower using the single-slice method compared to five slices and the total abdomen (P<0.01). Using the single-slice method, a lower %VAT was found than with the other methods (P<0.01). In regions 1, 2, 3, and 4, the absolute change in total fat was 122+/-50, 182+/-48, 182+/-55, and 155+/-40 cm(3), respectively. The regional difference in abdominal fat patterning revealed that the critical region (region 3) had a smaller VAT:SAT ratio than regions 1 and 2 (P<0.05), and the ratio at region 4 was smaller than region 3 (P<0.05). Weight loss resulted in a decrease in the VAT:SAT ratio (P<0.05) for regions 3 and 4 but not for regions 1 and 2.

CONCLUSIONS

The number of MR slices analyzed yields differential result in relative VAT distribution. Regional differences in abdominal fat loss occur with a greater relative VAT loss in the critical region, thus if only the critical region is analyzed the overall VAT loss induced by weight loss intervention may be overestimated.

Exercise is required for visceral fat loss in postmenopausal women with type 2 diabetes

This study examined the effects of aerobic exercise without weight loss, a hypocaloric high monounsaturated fat diet, and diet plus exercise (D+E) on total abdominal and visceral fat loss in obese postmenopausal women with type 2 diabetes. Thirty-three postmenopausal women (body mass index, 34.6 +/- 1.9 kg/m(2)) were assigned to one of three interventions: a hypocaloric high monounsaturated fat diet alone, exercise alone (EX), and D+E for 14 wk. Aerobic capacity, body composition, abdominal fat distribution (magnetic resonance imaging), glucose tolerance, and insulin sensitivity were measured pre- and postintervention. Body weight ( approximately 4.5 kg) and percent body fat ( approximately 5%) were decreased (P < 0.05) with the D and D+E intervention, whereas only percent body fat ( approximately 2.3%) decreased with EX. Total abdominal fat and sc adipose tissue (SAT) were reduced with the D and D+E interventions (P < 0.05), whereas visceral adipose tissue (VAT) decreased with the D+E and EX intervention, but not with the D intervention. EX resulted in a reduction in total abdominal fat, VAT, and SAT (P < 0.05) despite the lack of weight loss. The reductions in total abdominal fat and SAT explained 32.7% and 9.7%, respectively, of the variability in the changes in fasting glucose levels, whereas the reductions in VAT explained 15.9% of the changes in fasting insulin levels (P < 0.05). In conclusion, modest weight loss, through either D or D+E, resulted in similar improvements in total abdominal fat, SAT, and glycemic status in postmenopausal women with type 2 diabetes; however, the addition of exercise to diet is necessary for VAT loss. These data demonstrate the importance of exercise in the treatment of women with type 2 diabetes.

Influence of resistance exercise training on glucose control in women with type 2 diabetes

The objective of the study was to evaluate the effects of acute and chronic resistance training on glucose and insulin responses to a glucose load in women with type 2 diabetes. Subjects consisted of type 2 diabetic women (n = 7) and age-matched controls (n = 8) with normal glucose tolerance. All subjects participated in 3 oral glucose tolerance tests: pretraining, 12 to 24 hours after the first exercise session (acute) and 60 to 72 hours after the final training session (chronic). Exercise training consisted of a whole body resistance exercise program using weight-lifting machines 3 days per week for 6 weeks. Resistance training was effective in increasing strength of all muscle groups in all subjects. Integrated glucose concentration expressed as area under the curve (AUC) was 3,355.0 +/- 324.6 mmol/L. min pretraining, improved significantly (P <.01) after the acute bout of exercise (2,868 +/- 324.0 mmol/L. min), but was not improved with chronic training (3,206.0 +/- 337.0 mmol/L. min) in diabetic subjects. A similar pattern of significance was observed with peak glucose concentration (pre: 20.2 +/-1.4 mmol/L; acute: 17.2 +/- 1.7 mmol/L; chronic: 19.9 +/- 1.7 mmol/L). There were no significant changes in insulin concentrations after any exercise bout in the diabetic subjects. There were no changes in glucose or insulin levels in control subjects. An acute bout of resistance exercise was effective in improving integrated glucose concentration, including reducing peak glucose concentrations in women with type 2 diabetes, but not age-matched controls. There were no significant changes in insulin concentrations for either group. Resistance exercise offers an alternative to aerobic exercise for improving glucose control in diabetic patients. To realize optimal glucose control benefits, individuals must follow a regular schedule that includes daily exercise.

Racial differences in subcutaneous and visceral fat distribution in postmenopausal black and white women

Most studies examining racial disparities in abdominal fat distribution have focused on premenopausal women. The purpose of this report was to determine if racial differences exist in the abdominal fat distribution in postmenopausal white and black women. Fifty-four women (33 white and 21 black) were scanned by magnetic resonance imaging (MRI) to determine abdominal fat distribution, were measured by hydrostatic weighing for percent body fat, and had their fasting blood lipids, glucose, and insulin levels measured. These women were matched for age (mean age, 53.5 +/- 0.9 years) and percent body fat (black: 39.6% +/- 2.3%, white: 37.3% +/- 1.2%). When adjusted for total body fat mass and hormone replacement therapy (HRT), total abdominal fat (white: 10,352.1 +/- 535.2, black: 11,220.4 +/- 670.1 cm(3)) was not statistically different between groups, but the visceral fat content was significantly higher in the white women (white: 2,943.5 +/- 220.4, black: 2,332.6 +/- 176.1 cm(3)). The percent visceral fat was also higher in these women (white: 30.5% +/- 1.3%, black: 22.1% +/- 1.6%, P <.01). Subcutaneous adipose tissue (SAT) was significantly higher in the black women (white: 7,408.6 +/- 450.2, black: 8,887 +/- 563.1 cm(3), P <.05). No significant differences were found in the insulin concentrations or the blood lipid profile of these women. Regardless of race, visceral fat was a significant predictor of log triglyceride, low-density lipoprotein-cholesterol (LDL-C), cholesterol/LDL-C, insulin levels, and insulin resistance. Race was only found to contribute to 8% of the variability of LDL-C. HRT use had no effect on abdominal fat distribution or the blood lipid profile in this cohort of women. In conclusion, disparities in abdominal fat distribution between black and white women continue to exist in the early postmenopausal years, and the regression results indicate that the absolute amount of visceral fat, and not the relative amounts of visceral fat, is the best predictor of the blood lipid profile and insulin sensitivity. HRT use did not result in differences in abdominal fat distribution in these women. Factors, such as genetics and lifestyle, must play a larger role in explaining the increased health risk in black women.

Orientation and familiarization to 1RM strength testing in old and young women

The purpose of this study was to compare the number of testing sessions required to achieve consistent 1 repetition maximum (1RM) strength measurements in untrained old and young women. Consistency of measurement was defined as consecutive 1 RM strength measures that increased by 1 kg or less. Untrained old (n = 6, age 66 +/- 5 years) and untrained young (n = 7, age 23 +/- 4 years) women were repeatedly strength-tested for bilateral concentric knee extension 1 RM strength until consecutive measurements were increased by no more than 1 kg. At least 48 hours of rest was allowed between 1 RM measurements. The old subjects required significantly more testing sessions (8-9 sessions) compared with the young subjects (3-4 sessions) to achieve the same absolute consistency of measurement (p < 0.05). Absolute increase in strength between the first and final testing sessions did not differ between groups (young = 11 +/- 4 kg and old = 13 +/- 2 kg) (p > 0.05). The relative increase was significantly greater in the older subjects (young = 12 +/- 5%; old = 22 +/- 4%) (p < 0.05). In conclusion, older subjects require more practice and familiarization and show greater relative increases in 1RM strength when compared with younger subjects of the same experience level. This is important to consider, especially when evaluating the magnitude of strength increase in response to resistance training.

Resistance training reduces susceptibility to eccentric exercise-induced muscle dysfunction in older women

This study evaluated the effect of age on susceptibility to muscular weakness and damage caused by eccentric (ECC) exercise and determined whether this susceptibility was altered by resistance training. Young and older women performed concentric (CON) and ECC one repetition maximum (1 RM) strength tests of the quadriceps femoris. Older women also performed knee extension training for 12 weeks. An unaccustomed bout of ECC knee extension exercise was performed before and after training, and CON and ECC 1 RM were reassessed for 11 days after the ECC bout. Magnetic resonance imaging was used to evaluate changes in muscle water content associated with muscle damage. Before training, older subjects showed a larger decline in CON (p =.008) and ECC (p =.03) strength induced by the unaccustomed ECC bout, compared with the young subjects. One day following the ECC bout, the older women showed a 24% reduction in CON and a 27% reduction in ECC 1 RM, compared with only 6% (CON) and 10% (ECC) in the younger women. A magnetic resonance imaging evaluation indicated that edema or damage was significantly greater in the older untrained women than it was in young women (p <.05), but the resistance-trained older women showed no greater muscle injury than the young women (p >.05). Resistance-trained older women showed no greater decline than sedentary young women in either CON (p >.05) or ECC (p >.05) strength. In conclusion, sedentary older women are more susceptible to ECC-induced muscle dysfunction, but resistance training reduces this susceptibility.

Abdominal fat distribution in pre- and postmenopausal women: The impact of physical activity, age, and menopausal status

Age-related increases in total body fat have been reported, but the impact of menopause on abdominal fat distribution is still unclear. The purpose of this study was to determine the impact of menopausal status on abdominal fat distribution using magnetic resonance imaging (MRI). In addition, we investigated the influence of abdominal fat distribution on blood lipid profiles and leptin concentrations. Twenty-three premenopausal (PRE), 27 postmenopausal (POST), and 28 postmenopausal women on estrogen replacement therapy (ERT) had measurements of regional abdominal fat, blood lipids, and serum leptin concentrations. The women were matched for body mass index (BMI) and total body fat mass. Age and menopausal status were not found to be significant predictors of total abdominal fat, visceral fat, or subcutaneous fat, while physical activity was a significant predictor (P <.01) for total abdominal fat (R(2) =.16), visceral fat (R(2) =.32) and percent visceral fat (R(2) =.25). There was a trend for a greater visceral fat content in the POST women compared with the PRE women (2,495.0 +/- 228.4 v 1,770.4 +/- 240.8 cm(2), respectively, P =.06). The percent visceral abdominal fat was significantly lower (P <.05) in the premenopausal women than in either postmenopausal group (PRE, 23.2% +/- 1.7%; POST, 28.9% +/- 1.8%; ERT, 28.9% +/- 1.6%). Menopausal status and age did not influence any of the blood lipid values. Abdominal fat distribution was a significant predictor of cholesterol concentrations and the cholesterol/high-density lipoprotein-cholesterol (HDL-C) ratio, but only accounted for approximately 15% of the variability in these levels. Total body fat and physical activity accounted for 47% of the variability in leptin concentrations, while abdominal fat distribution, age, and menopausal status were not significant predictors. In conclusion, in early postmenopausal women, the level of physical activity accounts for the variability in abdominal fat distribution observed, while menopausal status and age do not play a significant role. ERT was not associated with additional benefits in abdominal fat distribution compared with postmenopausal women not on ERT or in the blood lipid profile in these women.

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.

Use of muscle functional magnetic resonance imaging with older individuals

Muscle functional magnetic resonance imaging (mfMRI) has been widely used to study muscle recruitment in exercise in young healthy subjects, but has not been validated or used with older subjects. This study validates and demonstrates the use of mfMRI in older subjects. Subjects consisted of apparently healthy sedentary younger (n = 7) and older (n = 6) women. Proton transverse relaxation (T2)-weighted MRI scans were obtained of the quadriceps femoris at rest and immediately following three bouts of knee extension exercise (50%, 75%, and 100% of untrained 5 x 10 repetition maximum [RM]). Older subjects performed knee extension training for 12 weeks and repeated the MRI scan protocol using the same absolute loads. Training induced a 13% increase in 1 RM and a 25% increase in 5 x 10 RM. Older subjects had higher resting T2 values compared with younger subjects; however, the T2 response to exercise (slope) was similar among groups (young = 0.063+/-0.003, older untrained = 0.055+/-0.011, older trained = 0.053+/-0.008; p > .05). In all cases, T2 increased linearly with load. Trained older subjects showed a lower T2 response when lifting the same absolute load compared with before training, which is consistent with results previously obtained from young subjects. In the older population, mfMRI is appropriate for use and offers benefits over other technologies.

Electromyographic activity of the lumbar extensor muscles: effect of angle and hand position during Roman chair exercise

OBJECTIVE

To evaluate the effects of angle and hand position during variable-angle Roman chair (VARC) back extension exercise on lumbar paraspinal electromyographic (EMG) activity.

DESIGN

Descriptive, repeated measures.

SETTING

University-based musculoskeletal research laboratory.

PARTICIPANTS

Two female and eight male volunteers recruited from a university setting.

INTERVENTION AND OUTCOME MEASURES

Surface integrated EMG activity was recorded from the L3-L4 paraspinal region during 24 10-second repetitions of dynamic back extension exercise, each consisting of a unique VARC angle (six total) and subject hand position (four total). Lumbar paraspinal surface integrated EMG activity measured in millivolts per repetition was used for analysis.

RESULTS

Significant lumbar paraspinal EMG activity was evident during each of the 24 repetitions (p < or = .05), with a 104% increase in activity noted between the lowest and highest. EMG activity increased progressively among hand positions and as the VARC angle became more horizontal. VARC angle affected EMG activity more than hand position, but the greatest impact on EMG activity was produced by modifying both angle and hand position.

CONCLUSION

Lumbar paraspinal EMG activity can be altered during VARC back extension exercise by changing angle and hand position. Clinicians can use these data to develop progressive resistance exercise programs using the VARC apparatus.

Increased vulnerability to eccentric exercise-induced dysfunction and muscle injury after concentric training

OBJECTIVE

To test whether concentric (CON)-only resistance training increases the vulnerability to eccentric (ECC) exercise-induced dysfunction and muscle injury.

DESIGN

Before-after trial.

SETTING

General community.

PATIENTS OR OTHER PARTICIPANTS

Nine men (77 +/- 3 kg body weight) who were not trained in resistance exercise.

INTERVENTION

Nine weeks of CON-only training of the left quadriceps femoris (QF); performance of 10 sets of 10 ECC actions with each QF using a load equivalent to 85% of the posttraining CON 1-repetition maximum (1RM).

MAIN OUTCOME MEASURES

CON 1RM for the untrained and trained QF measured immediately after training, and again 1, 3, 8, and 10 days after the bout of ECC exercise; cross-sectional area and spin-spin relaxation time (T2) of each QF determined by magnetic resonance imaging (MRI) before and 3 days after the ECC exercise bout.

RESULTS

As a result of the ECC bout, the CON 1RM was reduced by approximately 30% in the trained limb and 16% in the untrained limb. Strength did not return to normal in the trained limb even after 10 days of recovery; however, in the untrained limb strength was restored by day 8. As assessed by MRI, the trained limb showed a greater area of muscle injury compared with the untrained QF. The T2 of the injured area was similar in both limbs, suggesting that the intensity of injury was similar, but more widespread in the trained limb.

CONCLUSION

CON-only training increases the vulnerability to ECC exercise-induced dysfunction and muscle injury, probably by increasing the CON 1RM, thus allowing the individual to be exposed to greater ECC loading potential.

Different effects of exercise and edema on T2 relaxation in skeletal muscle

The hypothesis that increased muscle T2 after exercise is caused by increased extracellular fluid volume was tested by comparing the effects of exercise versus external leg negative pressure on muscle T2 relaxation in normal human subjects. T2 in lower leg muscles was measured by echo-planar imaging at 63 echo times from 24 to 272 ms, and the relaxation spectrum was calculated by using a non-negative least squares algorithm. T2 relaxation in anterior leg muscle before exercise was characterized by a single component with mean T2 = 29.3 +/- 0.7 (SE, n = 5). After ankle dorsiflexion exercise, this single component broadened, and mean T2 increased to 38.3 +/- 0.7 ms. In contrast, after leg negative pressure, which increased the total leg muscle cross-sectional area by 21% (range 12-32% n = 6), there was a variable appearance of much slower-relaxing components (60-500 ms). The results suggest that increased extracellular fluid can account for only a minor portion of the increase in muscle T2 observed during exercise.

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

This study investigated changes in skeletal muscle cross-sectional area (CSA) evoked by fluid shifts that accompany short-term 6 degrees head-down tilt (HDT) or horizontal bed rest, the time course of the resolution of these changes after resumption of upright posture, and the effect of altered muscle CSA, in the absence of increased contractile activity, on proton transverse relaxation time (T2). Average muscle (CSA and T2 were determined by standard spin-echo magnetic resonance imaging. Analyses were performed on contiguous transaxial images of the neck and calf. After a day of normal activity, 24 h of HDT increased neck muscle CSA 19 +/- 4(SE)% (P < 0.05) while calf 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) in the neck muscle CSA and an 8 +/- 2% decrease (P < 0.05) in the calf. Within 2 h after resumption of upright posture, neck and calf muscle CSA returned to within 0.5% (P > 0.05) of the values assessed after a day of normal activity, with most of the change occurring within the first 30 min. No further change in muscle CSA was observed through 6 h of upright posture. Despite these large alterations in muscle CSA, T2 was not altered by more than 1.1 +/- 0.6% (P > 0.05) and did not relate to muscle size. These results suggest that postural manipulations and subsequent fluid shifts modeling micro-gravity elicit marked changes in muscle size. Because these responses were not associated with alterations in muscle T2, it does not appear that simple movement of water into muscle can explain the contrast shift observed after exercise.

Vulnerability to dysfunction and muscle injury after unloading

OBJECTIVE

To test whether unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury.

DESIGN

Before-after trial.

SETTING

General community.

PATIENTS OR OTHER PARTICIPANTS

Two women and 5 men (73 +/- 3kg [mean +/- SE]) who were active college students but were not trained in lower body resistance exercise volunteered.

INTERVENTION

Five weeks of unilateral lower limb suspension (ULLS), which has been shown to decrease strength and size of the unloaded, left, but not load-bearing, right quadriceps femoris muscle group (QF) by 20% and 14%, respectively; performance of 10 sets of ten eccentric actions with each QF immediately after the ULLS strength tests with a load equivalent to 65% of the post-ULLS eccentric 1-repetition maximum.

MAIN OUTCOME MEASURE(S)

Concentric and eccentric 1-repetition maximum for the left, unloaded and the right, load-bearing QF measured immediately after ULLS and 1,4,7,9, and 11 days later; cross-sectional area and spin-spin relaxation time (T2) of each QF as determined by magnetic resonance imaging and measured the last day of ULLS and 3 days later.

RESULTS

The mean load used for eccentric exercise was 23 +/- 2 and 30 +/- 3kg for the left, unloaded and right, load-bearing QF, respectively. The concentric and eccentric 1-repetition maximum for the unloaded and already weakened left QF was further decreased by 18% (p = .000) and 27% (p = .000), respectively, 1 day after eccentric exercise. Strength did not return to post-ULLS levels until 7 days of recovery. The right, load-bearing QF showed a 4% decrease (p = .002) in the eccentric 1-repetition maximum 1 day after eccentric exercise. The left, unloaded QF showed an increase in T2 (p = .002) in 18% of its cross-sectional area 3 days after the eccentric exercise, thus indicating muscle injury. The right, load-bearing QF showed no elevation in T2 (p = .280).

CONCLUSION

Unloading increases vulnerability to eccentric exercise-induced dysfunction and muscle injury, even at relatively light loads.

Cardiorespiratory and metabolic adaptations to hyperoxic training

This study examined the effects of hyperoxic training on specific cardiorespiratory and metabolic responses. A group of 19 male subjects trained for 5 weeks on a cycle ergometer at 70 percent of hyperoxic or normoxic maximal heart rate, the hyperoxic group (HG) breathing 70 percent O2, the normoxic group (NG) breathing 21 percent O2. The subjects were tested pre- and post-training under both hyperoxia and normoxia. Measurements included cardiac output (Q(c)), stroke volume (SV), heart rate (HR), pulmonary ventilation (V(E)), oxygen consumption (VO(2)), partial pressure of oxygen (PO(2)), partial pressure of inspired carbon dioxide (PCO(2)), blood lactate concentration [La], and fiber type composition. The V(E) was significantly lower at submaximal work rates (P <0.05) and maximal V(E) increased after training in both groups for both test conditions; hyperoxic V(E) was lower than normoxic V(E) (P <0.05). The maximal V0(2) increased significantly (P <0.05) in both groups for both tests and was 11 percent - 12 percent higher during hyperoxia. Post-training maximal heart rate (HR(max)) was significantly decreased (P <0.05) at the same absolute work rate regardless of the training group or test type. The SV was increased at each work rate and Q c was unchanged. The maximal Q(c) increased significantly (P <0.05) for both groups and types of test: for normoxia: NG 27.3-30.41*min(-1) and HG 30.3-32.31*min(-1) and for hyperoxia: NG 24.7-25.6 and HG 27.9-31.21*min(-1). Although working at the same intensity relative to HR(max), HG showed significantly lower [La] following a single training session, yet maximal values were unchanged after training. Both groups showed a significant increase in the percentage of type IIA fibers post-training but HG retained a larger percentage of IIB fibers. Mitochondrial enzymes; citrate kinase, 3-hydroxyacyl CoA dehydrogenase, and cytochrome c-oxidase were increased in the normoxic trained subjects (P <0.05). In summary, training induced adaptive responses in maximal aerobic power, HR, SV, Q(c), [La], and muscle fiber type composition, independent of inspired PO(2). Intramuscular data suggested there may be some differences between hyperoxic and normoxic training and these were substantiated by mitochondrial enzyme and lactate findings. Our data would suggest that transport mechanisms may limit the ability to increase aerobic power.

Resistance exercise-induced fluid shifts: change in active muscle size and plasma volume

The purpose of this study was to test the hypothesis that the reduction in plasma volume (PV) induced by resistance exercise reflects fluid loss to the extravascular space and subsequently selective increase in cross-sectional area (CSA) of active but not inactive skeletal muscle. We compared changes in active and inactive muscle CSA and PV after barbell squat exercise. Magnetic resonance imaging (MRI) was used to quantify muscle involvement in exercise and to determine CSA of muscle groups or individual muscles [vasti (VS), adductor (Add), hamstring (Ham), and rectus femoris (RF)]. Muscle involvement in exercise was determined using exercise-induced contrast shift in spin-spin relaxation time (T2)-weighted MR images immediately postexercise. Alterations in muscle size were based on the mean CSA of individual slices. Hematocrit, hemoglobin, and Evans blue dye were used to estimate changes in PV. Muscle CSA and PV data were obtained preexercise and immediately postexercise and 15 and 45 min thereafter. A hierarchy of muscle involvement in exercise was found such that VS > Add > Ham > RF, with the Ham and RF showing essentially no involvement. CSA of the VS and Add muscle groups were increased 10 and 5%, respectively, immediately after exercise in each thigh with no changes in Ham and RF CSA. PV was decreased 22% immediately following exercise. The absolute loss of PV was correlated (r2 = 0.75) with absolute increase in muscle CSA immediately postexercise, supporting the notion that increased muscle size after resistance exercise reflects primarily fluid movement from the vascular space into active but not inactive muscle.

Effect of unweighting on skeletal muscle use during exercise

Exercise-induced spin-spin relaxation time (T2) shifts in magnetic resonance (MR) images were used to test the hypothesis that more muscle would be used to perform a given submaximal task after 5 wk of unweighting. Before and after unilateral lower limb suspension (ULLS), 7 subjects performed 5 sets of 10 unilateral concentric actions with the quadriceps femoris muscle group (QF) at each of 4 loads: 25, 40, 55, and 70% of maximum. T2-weighted MR images of the thigh were collected at rest and after each relative load. ULLS elicited a 20% decrease in strength of the left unweighted QF and a 14% decrease in average cross-sectional area (CSA) with no changes in the right weight-bearing QF. Average CSA of the left or right QF showing exercise-induced T2 shift increased as a function of exercise intensity both before and after ULLS. On average, 12 +/- 1, 15 +/- 2, 18 +/- 2, and 22 +/- 1 cm2 of either QF showed elevated T2 for the 25, 40, 55, and 70% loads, respectively, before ULLS. Average CSA of the left but not the right QF, showing elevated T2 after ULLS, was increased to 16 +/- 2, 23 +/- 3, 31 +/- 7, and 39 +/- 5 cm2, respectively. The results indicated that unweighting increased exercise-induced T2 shift in MR images, presumably due to greater muscle mass involvement in exercise after than before unweighting, suggesting a change in motor control.