Increase In Muscle Cross-sectional Area Research Articles

Rate Of Velocity Development Positively Correlates With Quadriceps Cross Sectional Area

Although isokinetic dynamometers purport to move a limb at a constant speed, the tested limb achieves a variety of velocities through the range of motion. To perform an isokinetic movement, the tested limb must overcome its inertia through increased torque until the limb angular velocity equals the velocity prescribed by the isokinetic device. The rate of change of velocity over time necessary to achieve the prescribed angular velocity is known as RVD. The purpose of this study was to determine the relationship between muscle cross-sectional area (CSA) and RVD of the quadriceps in recreationally trained males during isokinetic concentric knee extension. On two separate visits, 57 male (age 23.67 ± 2.17 years; height 178.39 ± 7.58 cm; mass 80.49 ± 15.33 kg) subjects performed three maximal concentric knee extension repetitions of their dominant leg on an isokinetic dynamometer at 15 randomized angular velocities (30-500°·s−1). Investigators calculated muscle cross sectional area (CSA) via the Housh equation using skinfold and circumference of the anterior thigh. A Pearson correlation calculated the relationship between muscle CSA of the quadriceps and RVD. Muscle CSA of the quadriceps significantly (p < 0.05) correlated with RVD measures from 90°·s−1 through 500°·s−1 with r values increasing from 0.33 to 0.56, respectively. However, no significant correlation existed between muscle CSA and RVD at speeds of 30°·s−1 and 60°·s−1. Muscle CSA significantly correlated with RVD at the tested angular velocities between 90°·s−1 through 500°·s−1 with an increasing trend. Consequently, increased muscle quantity may lead to increased RVD during an isokinetic knee extension exercise. Therefore, training at high velocities and with sufficient loads to induce muscle hypertrophy might improve performance during high velocity dynamic movements. Training designed to increase muscle CSA might increase RVD during isokinetic knee extension movements. Increasing RVD (and therefore reducing time to achieve a desired velocity) might represent a desirable adaptation for any dynamic movement.

Longitudinal study of muscle strength, quality, and adipose tissue infiltration

BackgroundSarcopenia is thought to be accompanied by increased muscle fat infiltration. However, no longitudinal studies have examined concomitant changes in muscle mass, strength, or fat infiltration in older adults. ObjectiveWe present longitudinal data on age-related changes in leg composition, strength, and muscle quality (MQ) in ambulatory, well-functioning men and women. We hypothesized that muscle cross-sectional area (CSA) and strength would decrease and muscular fat infiltration would increase over 5 y. DesignMidthigh muscle, subcutaneous fat (SF), and intermuscular fat (IMF) CSAs and isokinetic leg muscle torque (MT) and MQ (MT/quadriceps CSA) were examined over 5 y in the Health, Aging, and Body Composition study cohort (n = 1678). ResultsMen experienced a 16.1% loss of MT, whereas women experienced a 13.4% loss. Adjusted annualized decreases in MT were 2–5 times greater than the loss of muscle CSA in those who lost weight and in those who remained weight-stable. Weight gain did not prevent the loss of MT, despite a small increase in muscle CSA. Only those who gained weight had an increase in SF (P < 0.001), whereas those who lost weight also lost SF (P < 0.001). There was an age-related increase in IMF in men and women (P < 0.001), and IMF increased in those who lost weight, gained weight, or remained weight-stable (all P < 0.001). ConclusionsLoss of leg MT in older adults is greater than muscle CSA loss, which suggests a decrease in MQ. Additionally, aging is associated with an increase in IMF regardless of changes in weight or SF.

Greater growth hormone and insulin response in women than in men during repeated bouts of sprint exercise

In a previous study, sprint training has been shown to increase muscle cross-sectional area in women but not in men [Eur J Appl Physiol Occup Physiol 74 (1996) 375]. We hypothesized that sprint exercise induces a different hormonal response in women than in men. Such a difference may contribute to explaining the observed gender difference in training response. Metabolic and hormonal response to three 30-s sprints with 20-min rest between the sprints was studied in 18 physically active men and women. Accumulation of blood lactate [interaction term gender (g) x time (t): P = 0.022], and plasma ammonia (g x t: P < 0.001) after sprint exercise was greater in men. Serum insulin increased after sprint exercise more so in women than in men (g x t: P = 0.020), while plasma glucose increased in men, but not in women (g x t: P < 0.001). Serum growth hormone (GH) increased in both women and men reaching similar peak levels, but with different time courses. In women the peak serum GH level was observed after sprint 1, whereas in men the peak was observed after sprint 3 (g x t; P < 0.001). Serum testosterone tended to decrease in men and increase in women (g x t: P = 0.065). Serum cortisol increased approx. 10-15% after sprint exercise, independent of gender (time: P = 0.005). Women elicited a greater response of serum GH and insulin to sprint exercise. This may contribute to explaining the earlier observed muscle hypertrophy in women in response to sprint training.

Steroid use and human performance: Lessons for integrative biologists

While recent studies have begun to address how hormones mediate whole-animal performance traits, the field conspicuously lags behind research conducted on humans. Recent studies of human steroid use have revealed that steroid use increases muscle cross-sectional area and mass, largely due to increases in protein synthesis, and muscle fiber hypertrophy attributable to an increased number of satellite cells and myonuclei per unit area. These biochemical and cellular effects on skeletal muscle morphology translate into increased power and work during weight-lifting and enhanced performance in burst, sprinting activities. However, there are no unequivocal data that human steroid use enhances endurance performance or muscle fatigability or recovery. The effects of steroids on human morphology and performance are in general consistent with results found for nonhuman animals, though there are notable discrepancies. However, some of the discrepancies may be due to a paucity of comparative data on how testosterone affects muscle physiology and subsequent performance across different regions of the body and across vertebrate taxa. Therefore, we advocate more research on the basic relationships among hormones, morphology, and performance. Based on results from human studies, we recommend that integrative biologists interested in studying hormone regulation of performance should take into account training, timing of administration, and dosage administered when designing experiments or field studies. We also argue that more information is needed on the long-term effects of hormone manipulation on performance and fitness.

The Effect Of 4-week Low-intensity Ischemic Training On Quadriceps Size, Performance And Oxygen Availability

Low-intensity/high-repetition exercise combined with restricted blood flow has been reported effective for increasing muscle mass and muscle strength. In contrast, ischemic exercise has been also shown to have an opposite effect: enhanced muscle aerobic metabolism and endurance capacity. PURPOSE: To evaluate the effect of ischemic training on quadriceps performance, cross-sectional area (CSA) and O2 availability. METHODS: Ten healthy males trained on a leg-extension machine at loads equal to 15% of maximal voluntary contraction (MVC) force for 16 training sessions. One leg (I-leg) was trained with vascular occlusion induced by cuffs (≥230mmHg), whereas the other leg (C-leg) completed the same training volume without blood flow restrictions. Peak muscle strength was assessed with isometric MVC force, while endurance capacity was evaluated with one control and one ischemic test of repetitive load lifting to volitional failure. Oxygen availability in v. lateralis and activation of rectus f. and v. medialis during the endurance trials were measured with near-infrared spectroscope and surface EMG. Muscle CSA was measured with a series of slices acquired by MRI. All tests and measurements were performed prior to and after training. Differences in means were tested using factorial ANOVA. RESULTS: After training, a 9±2%, 5±1% and 7±1% increase (P<0.05) in CSA of rectus f., v. lateralis and v. medialis was observed in the I-leg, respectively. There was no change in MVC force in either I-leg or C-leg. Number of repetitions during control endurance test with I-leg and C-leg increased (P<0.01) by 63±9% and 36±8%, respectively. Root mean square EMG amplitudes of all muscles remained similar in both leg. The attained plateau of relative decrease in oxy-haemoglobin was reduced (P<0.01) by 15.7±0.4 μmol in I-leg and 6.7±0.4 μmol in C-leg. In contrast, the mean relative increase in total haemoglobin was higher (P<0.01) by 9.3±0.2 μmol in I-leg and 5.4±0.2 μmol in C-leg. The slope of change in both oxy-and deoxy-haemoglobin at the onset of exercise increased (P<0.05) in both legs, but more in I-leg. CONCLUSION: Ischemic training substantially augments endurance capacity of m. quadriceps, which can be attributed mainly to enhanced muscle blood supply and O2 availability. Increased muscle CSA may be an index of ischemia-induced capilarisation.

Postmenopausal hormone replacement therapy modifies skeletal muscle composition and function: a study with monozygotic twin pairs

We investigated whether long-term hormone replacement therapy (HRT) is associated with mobility and lower limb muscle performance and composition in postmenopausal women. Fifteen 54- to 62-yr-old monozygotic female twin pairs discordant for HRT were recruited from the Finnish Twin Cohort. Habitual (HWS) and maximal (MWS) walking speeds over 10 m, thigh muscle composition, lower body muscle power assessed as vertical jumping height, and maximal isometric hand grip and knee extension strengths were measured. Intrapair differences (IPD%) with 95% confidence intervals (CI) were calculated. The mean duration of HRT use was 6.9 +/- 4.1 yr. MWS was on average 7% (0.9 to 13.1%, P = 0.019) and muscle power 16% (-0.8 to 32.8%, P = 0.023) greater in HRT users than in their cotwins. Thigh muscle cross-sectional area tended to be larger (IPD% = 6%, 95% CI: -0.07 to 12.1%, P = 0.065), relative muscle area greater (IPD% = 8%, CI: 0.8 to 15.0%, P = 0.047), and relative fat area smaller (IPD% = -5%, CI: -11.3 to 1.2%, P = 0.047) in HRT users than in their sisters. There were no significant differences in maximal isometric strengths or HWS between users and nonusers. Subgroup analyses revealed that estrogen-containing therapies (11 pairs) significantly decreased total body and thigh fat content, whereas tibolone (4 pairs) tended to increase muscle cross-sectional area. This study showed that long-term HRT was associated with better mobility, greater muscle power, and favorable body and muscle composition among 54- to 62-yr-old women. The results indicate that HRT is a potential agent in preventing muscle weakness and mobility limitation in older women.

Passive properties of the muscle‐tendon unit: The influence of muscle cross‐sectional area

The purpose of the present study was to examine the relationships among the passive properties of the muscle-tendon unit and muscle cross-sectional area (CSA). Our findings indicated significant relationships between muscle CSA and passive stiffness (P < 0.05; r = 0.830) and muscle CSA and passive energy absorption (P < 0.05; r = 0.765). Since passive stiffness and the ability to absorb passive energy decreases with many diseases, these findings may support the need to maintain or increase muscle CSA.

CrossRef Listing of Deleted DOIs

Resistance training is recommended by national health organizations for incorporation into a comprehensive fitness program that includes aerobic and flexibility exercise. Its potential benefits on health and performance are numerous; it has been shown to reduce body fat, increase basal metabolic rate, decrease blood pressure and the cardiovascular demands to exercise, improve blood lipid profiles, glucose tolerance, and insulin sensitivity, increase muscle and connective tissue cross-sectional area, improve functional capacity, and relieve low back pain. Many improvements in physical function and athletic performance are associated with the increases in muscle strength, power, endurance, and hypertrophy observed during resistance training. The key element to effective resistance training is supervision by a qualified professional and the proper prescription of the program variables. Proper program design, ie, that which uses progressive overload, variation, and specificity, is essential to maximize the benefits associated with resistance training.

Randomized Controlled Trial of Intradialytic Resistance Training to Target Muscle Wasting in ESRD: The Progressive Exercise for Anabolism in Kidney Disease (PEAK) Study

To determine whether prolonged (24 weeks) intradialytic progressive resistance training (PRT) could counteract muscle wasting more effectively than short-duration training (12 weeks) in patients with end-stage renal disease. Randomized controlled trial. 49 patients (age, 62.6 +/- 14.2 years; 0.3 to 16.7 years on hemodialysis therapy) were randomly assigned to PRT plus usual care for 24 weeks (24WK group) or a crossover control group that received usual care for the first 12 weeks, then PRT plus usual care for the latter 12 weeks (12WK group). Two sets of 10 free-weight PRT exercises were performed at a high intensity during routine thrice-weekly hemodialysis treatment under direct supervision. Primary outcomes include thigh muscle cross-sectional area by means of computed tomography and intramuscular lipid content estimated through attenuation. Secondary outcomes include muscular strength, exercise capacity, and C-reactive protein level. The 24WK group increased muscle cross-sectional area (+1.82 +/- 3.25 cm(2)) compared with losses in the 12WK group (-1.37 +/- 6.87 cm(2); relative effect size, 0.59; 95% confidence interval [CI], -0.27 to 6.65; P = 0.04). However, this outcome did not achieve the level of statistical significance required (P = 0.025) after Bonferroni correction for multiple primary outcomes. There was no significant change in intramuscular lipid content between groups (+0.19 +/- 1.32 versus +0.16 +/- 1.69 Hounsfield units in the 24WK and 12WK groups, respectively; P = 0.31). Log C-reactive protein level tended to decrease in the 24WK group compared with the 12WK group (relative effect size, -0.63; 95% CI, -0.27 [-0.54 to 0.00]; P = 0.05). The 24WK group improved muscular strength measures and exercise capacity throughout the trial. Single geographic site used; no control group without exercise exposure; unblinded assessment of some secondary outcome measures. Prolonged intradialytic PRT did not significantly improve muscle cross-sectional area or intramuscular lipid content compared with a shorter duration of exercise. Future trials are required to more thoroughly investigate the clinical importance and magnitude of myogenic adaptations to PRT in this cohort.

The Influence of Frequency, Intensity, Volume and Mode of Strength Training on Whole Muscle Cross-Sectional Area in Humans

Strength training is an important component in sports training and rehabilitation. Quantification of the dose-response relationships between training variables and the outcome is fundamental for the proper prescription of resistance training. The purpose of this comprehensive review was to identify dose-response relationships for the development of muscle hypertrophy by calculating the magnitudes and rates of increases in muscle cross-sectional area induced by varying levels of frequency, intensity and volume, as well as by different modes of strength training. Computer searches in the databases MEDLINE, SportDiscus and CINAHL were performed as well as hand searches of relevant journals, books and reference lists. The analysis was limited to the quadriceps femoris and the elbow flexors, since these were the only muscle groups that allowed for evaluations of dose-response trends. The modes of strength training were classified as dynamic external resistance (including free weights and weight machines), accommodating resistance (e.g. isokinetic and semi-isokinetic devices) and isometric resistance. The subcategories related to the types of muscle actions used. The results demonstrate that given sufficient frequency, intensity and volume of work, all three types of muscle actions can induce significant hypertrophy at an impressive rate and that, at present, there is insufficient evidence for the superiority of any mode and/or type of muscle action over other modes and types of training. Tentative dose-response relationships for each variable are outlined, based on the available evidence, and interactions between variables are discussed. In addition, recommendations for training and suggestions for further research are given.

Body weight supported treadmill training in acute spinal cord injury: impact on muscle and bone

Longitudinal prospective case series. To evaluate the impact of early introduction post-spinal cord injury (SCI) of twice-weekly body-weight supported treadmill training (BWSTT) on muscle and bone. Centre for Health Promotion and Rehabilitation, McMaster University, Canada. Five individuals who had sustained traumatic SCI within 2-6 months participated in the study. Bone mineral densities (BMD) of proximal femur, distal femur, proximal tibia and lumbar spine were measured before and after training, as well as muscle cross-sectional area (CSA), BMD and bone geometry at mid-femur and proximal tibia. Serum osteocalcin and urinary deoxypyridinoline were measured at baseline, and after 24 and 48 sessions of training. All participants experienced increased muscle CSAs, ranging from 3.8 to 56.9%. Reductions in BMD were evident in all participants at almost all lower limb sites after training, ranging in magnitude from -1.2 to -26.7%. Lumbar spine BMD changes ranged from 0.2 to -7.4%. No consistent changes were observed in bone geometry. BWSTT did not alter the expected pattern of change in bone biochemical markers over time. The individual with the greatest improvement in ambulatory ability demonstrated the smallest reduction in lower limb BMD. Conversely, the individual who completed the fewest BWSTT sessions demonstrated the greatest reductions in BMD. Twice-weekly BWSTT appeared to partially reverse muscle atrophy after SCI, but did not prevent bone loss. Larger, controlled trials should evaluate whether relative preservation of bone loss occurs with regular BWSTT following acute SCI. Ontario Neurotrauma Foundation.

Muscular adaptations to computer-guided strength training with eccentric overload.

In order to investigate the muscular adaptations to a novel form of strength training, 18 male untrained subjects performed 4 weeks of low resistance-high repetition knee extension exercise. Nine of them trained on a conventional weight resistance device (Leg curler, CON/ECC group), with loads equivalent to 30% of the concentric one-repetition maximum (1RM) for both the concentric and eccentric phase of movement. The other nine trained on a newly developed computer-driven device (CON/ECC-OVERLOAD group) with the concentric load equivalent to 30% of the concentric 1RM and the eccentric load equivalent to 30% of the eccentric 1RM. Training resulted in significantly (P < or = 0.05) increased peak torque and a tendency (P=0.092) to increased muscle cross-sectional area for the CON/ECC-OVERLOAD but not the CON/ECC group, while strength endurance capacity was significantly (P < or = 0.05) increased in the CON/ECC group only. RT-PCR revealed significantly increased myosin heavy chain (MHC) IIa and lactate dehydrogenase (LDH) A mRNAs, a tendency for increased MHC IIx mRNA (P = 0.056) and high correlations between the changes in MHC IIx and LDH A mRNAs (r=0.97, P=0.001) in the CON/ECC-OVERLOAD group. These results indicate a shift towards a more type II dominated gene expression pattern in the vasti laterales muscles of the CON/ECC-OVERLOAD group in response to training. We suggest that the increased eccentric load in the CON/ECC-OVERLOAD training leads to distinct adaptations towards a stronger, faster muscle.

Age-related muscle loss and progressive dysfunction in mechanosensitive growth factor signaling.

Loss of muscle mass and function (sarcopenia) is one of the most marked problems associated with aging because it has major healthcare as well as socioeconomic implications. The growth hormone/IGF-I axis is regarded as an important regulator of muscle mass. However, it is now appreciated that other tissues in addition to the liver express IGF-I. Also, there are local as well as systemic forms of IGF-I that have different functions. We cloned two different IGF-Is that are expressed by skeletal muscle, and both are derived from the IGF-I gene by alternative splicing. One of these is expressed in response to physical activity, which has now been called "mechanogrowth factor" (MGF). The other is similar to the systemic or liver type (IGF-IEa) and is important as the provider of mature IGF-I required for upregulating protein synthesis. MGF differs from systemic IGF-IEa in that it has a different peptide sequence that is responsible for activating muscle satellite (stem) cells. Therefore, it appears these two forms of IGF-I have different actions and that they are important regulators of muscle growth. Growth hormone treatment apparently upregulates the level of IGF-I gene expression, and when it is combined with resistance exercise more is spliced toward MGF. This results in an increase in muscle cross-sectional area in the elderly subjects who otherwise would produce less MGF. The possibility of ameliorating sarcopenia using MGF delivered as a peptide or by gene therapy will be discussed.

Within-train neuromuscular propagation varies with torque in paralyzed human muscle.

Electromyographic (EMG) recordings may serve an important role in predicting torque during repetitive activation of paralyzed muscle. We compared the initial M-wave to the subsequent M-waves of the same train under fatigued and recovered conditions in the paralyzed human soleus muscle. Sixteen individuals with chronic (n = 13) or acute paralysis (n = 3) had the tibial nerve activated before and after a repetitive supramaximal stimulation protocol. The mean within-train M-wave amplitude and median frequency increased approximately 20%, whereas the duration decreased approximately 15% compared with the initial M-wave of each train. During fatigue, there was a linear decrease in the difference between the initial M-wave amplitude and subsequent train ( approximately 20% to 8%). Following fatigue, this difference recovered to approximately 12%. The difference between the M-wave train average and the initial M-wave for amplitude, duration, and median frequency closely followed torque (Pearson correlations = 0.99, 0.94, and 0.98, respectively) during fatigue. We conclude that the difference between the later-occurring M-waves (average of the train) and initial M-wave is large when muscle torque is high and less when torque is low and, therefore, predicts torque during activation of paralyzed muscle. This difference in the within-train M-wave amplitude, duration, and median frequency may reflect a mechanical change, such as muscle shortening and increased muscle cross-sectional area during isometric contractions. Electromyographic feedback may assist in the optimization of neuromuscular electrical stimulation of paralyzed muscle.

Aging does not attenuate plantaris muscle hypertrophy in male Fischer 344 rats.

The present study examined the effect of extreme old age on the plasticity of the rat plantaris muscle in response to an increase in mechanical load. Male Fischer 344 rats, aged 7 months (adult) and 25 months (old) underwent bilateral surgical ablation of the gastrocnemius muscle to functionally overload (OV) the fast-twitch plantaris muscle for 8 wk At 27 months of age, plantaris wet weight and cross-sectional area (CSA) were unaffected by age, but aging decreased peak isometric tension (Po) 27% (P < 0.05). Plantaris muscle myosin heavy chain composition indicated a loss of faster myosin heavy chains (MHC) isoforms with concomitant increases in slower MHC in old rats (P < 0.05). In adult rats, OV increased muscle CSA and Po 72% and 83%, respectively (P < 0.05). Similarly, OV increased CSA and Po 69% and 73%, respectively, in old rats (P < 0.05). Average fiber CSA increased 57% and 68% in adult-OV and old-OV rats, respectively (P < 0.05). Collectively, our data indicate that plantaris muscle mass and plasticity in response to increased mechanical load are well conserved in very aged male Fischer 344 rats.

Resistance training for health and performance.

Resistance training is recommended by national health organizations for incorporation into a comprehensive fitness program that includes aerobic and flexibility exercise. Its potential benefits on health and performance are numerous; it has been shown to reduce body fat, increase basal metabolic rate, decrease blood pressure and the cardiovascular demands to exercise, improve blood lipid profiles, glucose tolerance, and insulin sensitivity, increase muscle and connective tissue cross-sectional area, improve functional capacity, and relieve low back pain. Many improvements in physical function and athletic performance are associated with the increases in muscle strength, power, endurance, and hypertrophy observed during resistance training. The key element to effective resistance training is supervision by a qualified professional and the proper prescription of the program variables. Proper program design, ie, that which uses progressive overload, variation, and specificity, is essential to maximize the benefits associated with resistance training.

Different modes of hypertrophy in skeletal muscle fibers.

Skeletal muscles display a remarkable diversity in their arrangement of fibers into fascicles and in their patterns of innervation, depending on functional requirements and species differences. Most human muscle fascicles, despite their great length, consist of fibers that extend continuously from one tendon to the other with a single nerve endplate band. Other mammalian muscles have multiple endplate bands and fibers that do not insert into both tendons but terminate intrafascicularly. We investigated whether these alternate structural features may dictate different modes of cell hypertrophy in two mouse gracilis muscles, in response to expression of a muscle-specific insulin-like growth factor (IGF)-1 transgene (mIGF-1) or to chronic exercise. Both hypertrophic stimuli independently activated GATA-2 expression and increased muscle cross-sectional area in both muscle types, with additive effects in exercising myosin light chain/mIGF transgenic mice, but without increasing fiber number. In singly innervated gracilis posterior muscle, hypertrophy was characterized by a greater average diameter of individual fibers, and centralized nuclei. In contrast, hypertrophic gracilis anterior muscle, which is multiply innervated, contained longer muscle fibers, with no increase in average diameter, or in centralized nuclei. Different modes of muscle hypertrophy in domestic and laboratory animals have important implications for building appropriate models of human neuromuscular disease.

Effect of glycogen loading on skeletal muscle cross-sectional area and T2 relaxation time.

This study was performed to investigate if glycogen loading of skeletal muscles, by binding water, would effect the cross-sectional area (CSA) and if an altered water content would alter the transverse relaxation time (T2) measured by magnetic resonance imaging (MRI). Five healthy volunteers participated in a programme with 4 days of extremely carbohydrate-restricted meals followed by 4 days of extremely high carbohydrate intake. The CSA and T2 of thigh and calf muscles were related to the intramuscular glycogen content evaluated at days 4 and 8. An increase in glycogen content from 281 to 634 mmol kg(-1) dry wt increased the CSA of the vastus muscles by 3.5% from 78 +/- 11 to 80 +/- 12 cm2 and the thigh circumference by 2.5% from 146 +/- 20 to 150 23 cm2. Calf circumference increased non-significantly by 4% from 78 +/- 15 to 82 +/- 19 cm2. Mono-exponential T2 decreased in m. tibialis anterior from 27.8 +/- 1.2 to 26.9 +/- 1.7 ms, did not change in m. vastus lateralis 26.5 +/- 1.9 ms/26.6 +/- 1.3 ms or in m. gastrocnemius 29.5 +/- 1.0 ms/29.8 +/- 1.9 ms. Glycogen loading increased the signal intensity mainly at different echo times (TE) 15 and 30 ms. The study shows that increased glycogen filling in the muscles increases muscle CSA and that this can be detected by MRI. The signal intensity increased the most at shorter TEs suggesting a more tight intracellular binding of water in glycogen loaded muscles.

Long-term metabolic and skeletal muscle adaptations to short-sprint training: implications for sprint training and tapering.

The adaptations of muscle to sprint training can be separated into metabolic and morphological changes. Enzyme adaptations represent a major metabolic adaptation to sprint training, with the enzymes of all three energy systems showing signs of adaptation to training and some evidence of a return to baseline levels with detraining. Myokinase and creatine phosphokinase have shown small increases as a result of short-sprint training in some studies and elite sprinters appear better able to rapidly breakdown phosphocreatine (PCr) than the sub-elite. No changes in these enzyme levels have been reported as a result of detraining. Similarly, glycolytic enzyme activity (notably lactate dehydrogenase, phosphofructokinase and glycogen phosphorylase) has been shown to increase after training consisting of either long (>10-second) or short (<10-second) sprints. Evidence suggests that these enzymes return to pre-training levels after somewhere between 7 weeks and 6 months of detraining. Mitochondrial enzyme activity also increases after sprint training, particularly when long sprints or short recovery between short sprints are used as the training stimulus. Morphological adaptations to sprint training include changes in muscle fibre type, sarcoplasmic reticulum, and fibre cross-sectional area. An appropriate sprint training programme could be expected to induce a shift toward type IIa muscle, increase muscle cross-sectional area and increase the sarcoplasmic reticulum volume to aid release of Ca(2+). Training volume and/or frequency of sprint training in excess of what is optimal for an individual, however, will induce a shift toward slower muscle contractile characteristics. In contrast, detraining appears to shift the contractile characteristics towards type IIb, although muscle atrophy is also likely to occur. Muscle conduction velocity appears to be a potential non-invasive method of monitoring contractile changes in response to sprint training and detraining. In summary, adaptation to sprint training is clearly dependent on the duration of sprinting, recovery between repetitions, total volume and frequency of training bouts. These variables have profound effects on the metabolic, structural and performance adaptations from a sprint-training programme and these changes take a considerable period of time to return to baseline after a period of detraining. However, the complexity of the interaction between the aforementioned variables and training adaptation combined with individual differences is clearly disruptive to the transfer of knowledge and advice from laboratory to coach to athlete.

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.