Quantitative measurement of muscle fiber composition in a normal population

Acute ingestion of acetaminophen improves cognitive and repeated high intensity short-term maximal performance in well-trained female athletes: a randomized placebo-controlled trial

This study examined the effect of acute acetaminophen (ACTP) ingestion on physical performance during the 5 m shuttle run test (5mSRT), attention, mood states, and the perception of perceived exertion (RPE), pain (PP), recovery (PRS), and delayed onset of muscle soreness (DOMS) in well-trained female athletes. In a randomized, placebo-controlled, double-blind, crossover trial, fifteen well-trained female athletes (age 21 ± 2 years, height 165 ± 6 cm, body mass 62 ± 5 kg) swallowed either 1.5 g of ACTP or 1.5 g of placebo. The profile of mood states (POMS) and digit cancellation (DCT) were assessed 45 min postingestion, and 5mSRT was performed 60 min postingestion. The RPE and PP were determined immediately after each 30-s repetition of the 5mSRT, and the PRS and DOMS were recorded at 5 min and 24 h post-5mSRT. For the 5mSRT, ACTP ingestion improved the greatest distance (+ 10.88%, p < 0.001), total distance (+ 11.33%, p = 0.0007) and fatigue index (+ 21.43%, p = 0.0003) compared to PLA. Likewise, the DCT score was better on the ACTP (p = 0.0007) than on the PLA. RPE, PP, PRS, and DOMS scores were improved after ACTP ingestion (p < 0.01 for all comparisons) compared to PLA. POMS scores were enhanced with ACTP ingestion compared to PLA (p < 0.01). In conclusion, this study indicates that acute acetaminophen ingestion can improve repeated high intensity short-term maximal performance, attention, mood states, and perceptions of exertion, pain, recovery, and muscle soreness in well-trained female athletes, suggesting potential benefits for their overall athletic performance and mood state.

Sex differences in human performance

Sex as a biological variable is an underappreciated aspect of biomedical research, with its importance emerging in more recent years. This review assesses the current understanding of sex differences in human physical performance. Males outperform females in many physical capacities because they are faster, stronger and more powerful, particularly after male puberty. This review highlights key sex differences in physiological and anatomical systems (generally conferred via sex steroids and puberty) that contribute to these sex differences in human physical performance. Specifically, we address the effects of the primary sex steroids that affect human physical development, discuss insight gained from an observational study of 'real-world data' and elite athletes, and highlight the key physiological mechanisms that contribute to sex differences in several aspects of physical performance. Physiological mechanisms discussed include those for the varying magnitude of the sex differences in performance involving: (1) absolute muscular strength and power; (2) fatigability of limb muscles as a measure of relative performance; and (3) maximal aerobic power and endurance. The profound sex-based differences in human performance involving strength, power, speed and endurance, and that are largely attributable to the direct and indirect effects of sex-steroid hormones, sex chromosomes and epigenetics, provide a scientific rationale and framework for policy decisions on sex-based categories in sports during puberty and adulthood. Finally, we highlight the sex bias and problem in human performance research of insufficient studies and information on females across many areas of biology and physiology, creating knowledge gaps and opportunities for high-impact studies.

Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status

BACKGROUND

Many sports require maximal strength and endurance performance. Concurrent strength and endurance training can lead to suboptimal training adaptations. However, how adaptations differ between males and females is currently unknown. Additionally, current training status may affect training adaptations.

OBJECTIVE

We aimed to assess sex-specific differences in adaptations in strength, power, muscle hypertrophy, and maximal oxygen consumption ( V ˙ O2max) to concurrent strength and endurance training in healthy adults. Second, we investigated how training adaptations are influenced by strength and endurance training status.

METHODS

A systematic review and meta-analysis was conducted according to PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, and a Cochrane risk of bias was evaluated. ISI Web of science, PubMed/MEDLINE, and SPORTDiscus databases were searched using the following inclusion criteria: healthy adults aged 18-50 years, intervention period of ≥ 4 weeks, and outcome measures were defined as upper- and lower-body strength, power, hypertrophy, and/or V ˙ O2max. A meta-analysis was performed using a random-effects model and reported in standardized mean differences.

RESULTS

In total, 59 studies with 1346 participants were included. Concurrent training showed blunted lower-body strength adaptations in males, but not in females (male: - 0.43, 95% confidence interval [- 0.64 to - 0.22], female: 0.08 [- 0.34 to 0.49], group difference: P = 0.03). No sex differences were observed for changes in upper-body strength (P = 0.67), power (P = 0.37), or V ˙ O2max (P = 0.13). Data on muscle hypertrophy were insufficient to draw any conclusions. For training status, untrained but not trained or highly trained endurance athletes displayed lower V ˙ O2max gains with concurrent training (P = 0.04). For other outcomes, no differences were found between untrained and trained individuals, both for strength and endurance training status.

CONCLUSIONS

Concurrent training results in small interference for lower-body strength adaptations in males, but not in females. Untrained, but not trained or highly trained endurance athletes demonstrated impaired improvements in V ˙ O2max following concurrent training. More studies on females and highly strength-trained and endurance-trained athletes are warranted.

CLINICAL TRIAL REGISTRATION

PROSPERO: CRD42022370894.

The Biological Basis of Sex Differences in Athletic Performance: Consensus Statement for the American College of Sports Medicine

ABSTRACT

Biological sex is a primary determinant of athletic performance because of fundamental sex differences in anatomy and physiology dictated by sex chromosomes and sex hormones. Adult men are typically stronger, more powerful, and faster than women of similar age and training status. Thus, for athletic events and sports relying on endurance, muscle strength, speed, and power, males typically outperform females by 10%-30% depending on the requirements of the event. These sex differences in performance emerge with the onset of puberty and coincide with the increase in endogenous sex steroid hormones, in particular testosterone in males, which increases 30-fold by adulthood, but remains low in females. The primary goal of this consensus statement is to provide the latest scientific knowledge and mechanisms for the sex differences in athletic performance. This review highlights the differences in anatomy and physiology between males and females that are primary determinants of the sex differences in athletic performance and in response to exercise training, and the role of sex steroid hormones (particularly testosterone and estradiol). We also identify historical and nonphysiological factors that influence the sex differences in performance. Finally, we identify gaps in the knowledge of sex differences in athletic performance and the underlying mechanisms, providing substantial opportunities for high-impact studies. A major step toward closing the knowledge gap is to include more and equitable numbers of women to that of men in mechanistic studies that determine any of the sex differences in response to an acute bout of exercise, exercise training, and athletic performance.

Are Whiplash-Associated Disorders and Temporomandibular Disorders in a Trauma Related Cause and Effect Relationship? A Review

Background: Whiplash is associated with a wide variety of clinical manifestations, including headache, neck pain, cervical rigidity, shoulder and back pain, paresthesia, vertigo, and temporomandibular disorders (TMDs). Previous studies reported that TMDs are more common in individuals with chronic whiplash-associated disorders (WAD) than in the general population; however, the pathophysiology and mechanism of this relationship are still not well understood. Methods: A PubMed and Ovid EMBASE review was performed to identify all studies addressing the trauma related cause and effect relationship between WAD and TMDs from January 2003 to March 2023. Results: After screening for eligibility and inclusion criteria, a total of 16 articles met the selection criteria. The various included studies discussed different aspects of the association between WDA and TMDs, including changes in the coordination and amplitude of jaw opening, the severity of the associated symptoms/signs in cases of WAD, the degree of fatigue and psychological stress, difficulty in feeding, cervical and myofascial pain, changes in the MRI signal at various muscle points, muscle tenderness, and quality of life. Conclusions: In this review, we summarized the clinical evidence of any trauma related cause and effect relationship between whiplash and TMDs. An accurate screening of the previous literature showed that, in conclusion, the relationship between whiplash and TMDs is still unclear.

Narrative Review of Sex Differences in Muscle Strength, Endurance, Activation, Size, Fiber Type, and Strength Training Participation Rates, Preferences, Motivations, Injuries, and Neuromuscular Adaptations

ABSTRACT

Nuzzo, JL. Narrative review of sex differences in muscle strength, endurance, activation, size, fiber type, and strength training participation rates, preferences, motivations, injuries, and neuromuscular adaptations. J Strength Cond Res 37(2): 494-536, 2023-Biological sex and its relation with exercise participation and sports performance continue to be discussed. Here, the purpose was to inform such discussions by summarizing the literature on sex differences in numerous strength training-related variables and outcomes-muscle strength and endurance, muscle mass and size, muscle fiber type, muscle twitch forces, and voluntary activation; strength training participation rates, motivations, preferences, and practices; and injuries and changes in muscle size and strength with strength training. Male subjects become notably stronger than female subjects around age 15 years. In adults, sex differences in strength are more pronounced in upper-body than lower-body muscles and in concentric than eccentric contractions. Greater male than female strength is not because of higher voluntary activation but to greater muscle mass and type II fiber areas. Men participate in strength training more frequently than women. Men are motivated more by challenge, competition, social recognition, and a desire to increase muscle size and strength. Men also have greater preference for competitive, high-intensity, and upper-body exercise. Women are motivated more by improved attractiveness, muscle "toning," and body mass management. Women have greater preference for supervised and lower-body exercise. Intrasexual competition, mate selection, and the drive for muscularity are likely fundamental causes of exercise behaviors in men and women. Men and women increase muscle size and strength after weeks of strength training, but women experience greater relative strength improvements depending on age and muscle group. Men exhibit higher strength training injury rates. No sex difference exists in strength loss and muscle soreness after muscle-damaging exercise.

Association of muscle fiber type with measures of obesity: A systematic review

Obesity derives from an extended period of positive energy imbalance due to a complex interplay of environmental and biological factors. Muscle fiber type and physiology have been hypothesized to affect metabolism and energy expenditure and thus to affect an individual's propensity to gain weight. However, there have been conflicting reports regarding a relationship between muscle fiber type and obesity. Here, we systematically reviewed literature investigating this topic from PubMed, Web of Science, and EMBASE. Of these, 32 articles were included in our final review and analysis. Most studies (22/32) reported a significant relationship between muscle fiber-type proportion and a measure of obesity. Overall, there was a significant negative relationship between the proportion of type I fibers and body mass index (BMI) and a significant positive relationship between the proportion of type IIX fibers and BMI. Moreover, between-group comparisons indicated a greater prevalence of type IIX fibers and a lower prevalence of type I fibers in patients living with obesity relative to lean individuals. These significant relationships were confirmed in a meta-analysis of these data. The causal nature of these associations remains to be evaluated.

Rating of perceived exertion (RPE) in studies of fatigue-induced postural control alterations in healthy adults: Scoping review of quantitative evidence

BACKGROUND

Amongst the literature researching the effects of exercise-induced fatigue on postural control in healthy adults, many studies have used the Borg scales to document the rating of perceived exertion (RPE) and have shown a broad range of RPE values. Our main aim was to map fatigue-induced RPE values in included publications. Secondary aims were to summarize the preference and purpose for the use of Borg scales within the included publications and to explore the potential associations between fatigue-induced RPE values and postural control changes.

METHODS

Five databases (Ovid Medline, PubMed, CINAHL, Scopus, and SPORTDiscus) were systematically searched for synthesizing data among the publications that reported RPE values on the Borg RPE- and Category-Ratio (CR) 10 scales and also found fatigue effects on postural control in healthy adults. Spearman's rank correlations were conducted to assess potential associations between fatigue-induced RPE values and maximal postural control changes across the included publications (group data).

RESULTS

45 of 51 studies included in this review reported maximal RPE values following exercise and ranged from 10.4-20 (6-20 Borg RPE) or 0.9-10 (CR10) indicating "very light" or "very weak" to "maximal" exertions. The 6-20 Borg and CR10 scales were mainly used to assess cardiovascular and muscular exertion, respectively. The scales were used mostly to estimate fatigue levels (n = 45), and to a lesser extent to produce a specific exercise intensity (n = 5) and as the criterion for exercise termination (n = 1). In general, there was no significant association between RPE and postural control changes across studies.

CONCLUSION

The broad range of RPE values and weak correlations may suggest that various fatigue levels can lead to postural control changes. However, one should be careful in comparing the extent of fatigue from RPE values and its potential effect on postural control in the light of many confounding factors.

Developmental Considerations for the Use of Naltrexone in Children and Adolescents

Naltrexone (NTX) is a well-tolerated drug with a wide safety margin and mechanism of action that affords use across a wide variety of indications in adults and children. By antagonizing the opioid reward system, NTX can modulate behaviors that involve compulsivity or impulsivity, such as substance use, obesity, and eating disorders. Evidence regarding the disposition and efficacy of NTX is mainly derived from adult studies of substance use disorders and considerable variability exists. Developmental changes, plausible disease-specific alterations and genetic polymorphisms in NTX disposition, and pharmacodynamic pathways should be taken into consideration when optimizing the use of NTX in the pediatric population. This review highlights the current state of the evidence and gaps in knowledge regarding NTX to facilitate evidence-based pharmacotherapy of mental health conditions, for which few pharmacologic options exist.

Performance fatigability and the bilateral deficit during maximal, isokinetic leg extensions in men and women

BACKGROUND: Few studies have examined sex differences in performance fatigability and the bilateral deficit in a dynamic modality. OBJECTIVES: The purpose of this study was to examine: 1) Leg-, mode-, and sex-specific differences in performance fatigability during maximal, dynamic leg extension muscle actions and; 2) the time course of fatigue-induced changes in the bilateral deficit for both men and women. METHODS: Eleven men and 11 women participated in 3 test visits consisting of 50 maximal, concentric, isokinetic leg extensions at 60∘/s. Each visit was randomized to perform either unilateral right leg only (RL), unilateral left leg only (LL), or bilateral (BL) leg extensions. RESULTS: The BL performance fatigability was significantly (p< 0.001) less than RL and LL. Both men and women demonstrated significant (p< 0.001) declines in moment and an attenuation of the bilateral deficit throughout the fatiguing task. There were no differences between sex for performance fatigability (p= 0.128) or the bilateral deficit (p= 0.102). CONCLUSIONS: Unilateral muscle actions were more susceptible to fatigue than BL muscle actions. Men exhibited an earlier decline in moment than women, however, men and women exhibited similar magnitudes and patterns of decline in the bilateral deficit.

The Influence of Movement Tempo on Acute Neuromuscular, Hormonal, and Mechanical Responses to Resistance Exercise-A Mini Review

Wilk, M, Tufano, JJ, and Zajac, A. The influence of movement tempo on acute neuromuscular, hormonal, and mechanical responses to resistance exercise-a mini review. J Strength Cond Res 34(8): 2369-2383, 2020-Resistance training studies mainly analyze variables such as the type and order of exercise, intensity, number of sets, number of repetitions, and duration and frequency of rest periods. However, one variable that is often overlooked in resistance training research, as well as in practice, is premeditated movement tempo, which can influence a myriad of mechanical and physiological factors associated with training and adaptation. Specifically, this article provides an overview of the available scientific literature and describes how slower tempos negatively affect the 1-repetition maximum, the possible load to be used, and the number of repetitions performed with a given load, while also increasing the total time under tension, which can mediate acute cardiovascular and hormonal responses. As a result, coaches should consider testing maximal strength and the maximal number of repetitions that can be performed with each movement tempo that is to be used during training. Otherwise, programming resistance training using various movement tempos is more of a trial-and-error approach, rather than being evidence or practice based. Furthermore, practical applications are provided to show how movement tempo can be adjusted for a variety of case study-type scenarios.

Human skeletal muscle fiber type percentage and area after reduced muscle use: A systematic review and meta-analysis

The main objective of this systematic review was to examine the effect of reduced muscle activity on the relative number of type 1 muscle fibers (%) in the human vastus lateralis muscle. Other objectives were changes in type 2A and 2X percentages and muscle fiber cross-sectional area. We conducted systematic literature searches in eight databases and included studies assessing type 1 fiber percentage visualized by ATPase or immunohistochemical staining before and after a period (≥14 days) of reduced muscle activity. The reduced muscle activity models were detraining, leg unloading, and bed rest. Forty-two studies comprising 451 participants were included. Effect sizes were calculated as the mean difference between baseline and follow-up and Generic Inverse Variance tests with random-effects models were used for the weighted summary effect size. Overall, the mean type 1 muscle fiber percentage was significantly reduced after interventions (-1.94%-points, 95% CI [-3.37, -0.51], P = .008), with no significant differences between intervention models (P = .86). Meta-regression showed no effect of study duration on type 1 fiber percentage (P = .98). Conversely, the overall type 2X fiber percentage increased after reduced muscle activity (P < .001). The CSA of the muscle fiber types decreased after the study period (all P-values < 0.001) with greater reductions in type 2 than type 1 fibers (P < .001). The result of this meta-analysis display that the type 1 muscle fiber percentage decrease as a result of reduced muscle activity, although the effect size is relatively small.

Functional and Structural Adaptations of Skeletal Muscle in Long-Term Juvenile Dermatomyositis: A Controlled Cross-Sectional Study

OBJECTIVE

To compare muscle strength and endurance of the knee extensors between patients with long-term juvenile dermatomyositis (DM) and controls and between patients with active disease and those with inactive disease, and to explore associations between strength/endurance and 1) clinical parameters, 2) physical activity, and 3) humoral/structural adaptation in the skeletal muscle of patients.

METHODS

In a cross-sectional study (44 patients and 44 age- and sex-matched controls), we tested isometric muscle strength (peak torque, in Nm) and dynamic muscle endurance (total work, in Joules) of the knee extensors, physical activity (measured by accelerometer), and serum myokine levels (by enzyme-linked immunosorbent assay). Patients were examined with validated tools (clinical muscle tests and measures of disease activity/damage and inactive disease) and using magnetic resonance imaging of the thigh muscles, which included evaluation of the quadriceps cross-sectional area (CSA). Needle biopsy samples of the vastus lateralis muscle (obtained from 12 patients ages ≥18 years) were assessed by histochemistry.

RESULTS

After a mean ± SD disease duration of 21.8 ± 11.8 years, peak torque was lower in patients with juvenile DM compared to controls (mean difference 29 Nm, 95% confidence interval 13-46; P = 0.001). Similarly, total work of the knee extensors was lower in patients compared to controls (median 738J [interquartile range 565-1,155] versus 1,249J [interquartile range 815-1,665]; P < 0.001). Both peak torque and total work were lower in patients with active juvenile DM compared to those with inactive disease (both P < 0.019); in analyses controlled for quadriceps CSA, only total work remained lower in patients with active disease. Moreover, peak torque and total work correlated with findings from clinical muscle tests in patients with active disease (r = 0.57-0.84). Muscle biopsy results indicated that the fiber type composition was different, but capillary density was similar, between patients with active disease and those with inactive disease.

CONCLUSION

In patients with long-term juvenile DM, both muscle strength and endurance of the knee extensors were lower when compared to matched controls, and also lower in patients with active disease compared to those with inactive disease. Our results indicate a need for more sensitive muscle tests in this clinical setting. We hypothesize that impaired muscle endurance in patients with active juvenile DM may be influenced by structural/functional adaptations of muscle tissue independent of muscle size.

Sex-related differences in symptoms of temporomandibular disorders and structural changes in the lateral pterygoid muscle after whiplash injury

Whiplash injury is an initiating or aggravating factor of temporomandibular disorder (TMD). Although there are sex-related differences in the mechanism of pain perception and pain control, there is a lack of research on differences in TMD after whiplash injury. We aimed to evaluate sex-related differences in the clinical symptoms and magnetic resonance imaging (MRI) findings of patients with TMD attributed to whiplash injury. This retrospective, cross-sectional study included 100 patients (50 women; 50 men; mean age, 37.60 years) who visited our oro-facial pain clinic with symptoms of TMD after whiplash injury. All patients underwent detailed evaluations for history of trauma, and their clinical and MRI findings were comprehensively assessed. Women with TMD after whiplash injury perceived more pain and presented more tenderness upon palpation than did men with TMD. In addition, women showed higher volume (58% vs 26%) and signal changes (54% vs 20%) in the lateral pterygoid muscle (LPM) and more anterior disc displacement without reduction (ADDWoR) (40% vs 20%) than did men. The presence of ADDWoR (odds ratio, 10.58; P = 0.007) and condylar degeneration (odds ratio, 9.30; P = 0.015) predicted LPM volume; stressful conditions (beta = 1.34; P = 0.011) correlated with increased visual analogue scale scores, and sleep problem was associated with an increased palpation index (PI) (beta = 0.42; P < 0.001) and neck PI (beta = 0.49; P < 0.001) scores only in women. Our results showed sex-specific differences in pain intensity, distribution of clinical and abnormal MRI findings, and their relationships, and these differences should be considered when treating patients with TMD.

The Relevance of Sex Differences in Performance Fatigability

Performance fatigability differs between men and women for a range of fatiguing tasks. Women are usually less fatigable than men, and this is most widely described for isometric fatiguing contractions and some dynamic tasks. The sex difference in fatigability is specific to the task demands so that one mechanism is not universal, including any sex differences in skeletal muscle physiology, muscle perfusion, and voluntary activation. However, there are substantial knowledge gaps about the task dependency of the sex differences in fatigability, the involved mechanisms, and the relevance to clinical populations and with advanced age. The knowledge gaps are in part due to the significant deficits in the number of women included in performance fatigability studies despite a gradual increase in the inclusion of women for the last 20 yr. Therefore, this review 1) provides a rationale for the limited knowledge about sex differences in performance fatigability, 2) summarizes the current knowledge on sex differences in fatigability and the potential mechanisms across a range of tasks, 3) highlights emerging areas of opportunity in clinical populations, and 4) suggests strategies to close the knowledge gap and understanding the relevance of sex differences in performance fatigability. The limited understanding about sex differences in fatigability in healthy and clinical populations presents as a field ripe with opportunity for high-impact studies. Such studies will inform on the limitations of men and women during athletic endeavors, ergonomic tasks, and daily activities. Because fatigability is required for effective neuromuscular adaptation, sex differences in fatigability studies will also inform on optimal strategies for training and rehabilitation in both men and women.

Reduced Appendicular Lean Body Mass, Muscle Strength, and Size of Type II Muscle Fibers in Patients with Spondyloarthritis versus Healthy Controls: A Cross-Sectional Study

Introduction. The purpose of this study was to investigate body composition, muscle function, and muscle morphology in patients with spondyloarthritis (SpA). Methods. Ten male SpA patients (mean ± SD age 39 ± 4.1 years) were compared with ten healthy controls matched for sex, age, body mass index, and self-reported level of physical exercise. Body composition was measured by dual energy X-ray absorptiometry. Musculus quadriceps femoris (QF) strength was assessed by maximal isometric contractions prior to test of muscular endurance. Magnetic resonance imaging of QF was used to measure muscle size and calculate specific muscle strength. Percutaneous needle biopsy samples were taken from m. vastus lateralis. Results. SpA patients presented with significantly lower appendicular lean body mass (LBM) (p = 0.02), but there was no difference in bone mineral density, fat mass, or total LBM. Absolute QF strength was significantly lower in SpA patients (p = 0.03) with a parallel trend for specific strength (p = 0.08). Biopsy samples from the SpA patients revealed significantly smaller cross-sectional area (CSA) of type II muscle fibers (p = 0.04), but no difference in CSA type I fibers. Conclusions. Results indicate that the presence of SpA disease is associated with reduced appendicular LBM, muscle strength, and type II fiber CSA.

Changes in Non-Enzymatic Antioxidants in the Blood Following Anaerobic Exercise in Men and Women

Purpose

The aim of this study was to compare changes in total oxidative status (TOS), total antioxidative capacity (TAC) and the concentration of VitA, VitE, VitC, uric acid (UA), reduced (GSH) and oxidized glutathione (GSSG) in blood within 24 hours following anaerobic exercise (AnEx) among men and women.

Methods

10 women and 10 men performed a 20-second bicycle sprint (AnEx). Concentrations of oxidative stress indicators were measured before AnEx and 3, 15 and 30 minutes and 1 hour afterwards. UA, GSH and GSSH were also measured 24 hours after AnEx. Lactate and H⁺ concentrations were measured before and 3 minutes after AnEx.

Results

The increase in lactate and H⁺ concentrations following AnEx was similar in both sexes. Changes in the concentrations of all oxidative stress indicators were significant and did not differ between men and women. In both sexes, TOS, TAC, TOS/TAC and VitA and VitE concentrations were the highest 3 minutes, VitC concentration was the highest 30 minutes, and UA concentration was the highest 1 hour after AnEx. GSH concentration was significantly lower than the initial concentration from 15 minutes to 24 hour after AnEx. GSSG concentration was significantly higher, while the GSH/GSSG ratio was significantly lower than the initial values 1 hour and 24 hour after AnEx.

Conclusions

With similar changes in lactate and H⁺ concentrations, AnEx induces the same changes in TAC, TOS, TOS/TAC and non-enzymatic antioxidants of low molecular weight in men and women. Oxidative stress lasted at least 24 hours after AnEx.

A model for determining baseline morphometrics of skeletal myofibres

The minimum diameter method of morphometry (MDM) is used to measure and detect changes in myofibre diameters (FD). The MDM is used to identify pathology in skeletal muscle. In such studies, an assumption is made that the mean FD in a particular muscle in both limbs is essentially the same. This study explored this premise to determine the accuracy of MDM as a means of morphometric analysis. Muscle biopsies were obtained from the left (G1) and right (G2) tibialis anterior of four vervet monkeys and from the massaged left (G3) and untreated right (G4) tibialis anterior of four animals. Wax sections were prepared for MDM and FD was measured. Three specimens were re-measured on four occasions. The mean FD of each biopsy from G1 and G2 limbs were compared and the number of measurements necessary to produce a meaningful result determined. Repeated measurement showed a difference of &lt; 3.0% in FD means between the first and three subsequent measurements. There was no significant difference of FD means between G1 and G2, whilst the difference between G3 and G4 was 11.2%. When &gt; 175 FD were measured, the difference from the final mean was less than 2.0%. These data show that, (1) FD data derived from a muscle in an untreated limb can be used as a control for experiment mediated changes of FD in the other, (2) MDM is a reliable means of measuring FD and (3) 150-175 FD are needed to provide a dependable result.

Striated muscle fiber size, composition, and capillary density in diabetes in relation to neuropathy and muscle strength

OBJECTIVE

Diabetic polyneuropathy (DPN) leads to progressive loss of muscle strength in the lower extremities due to muscular atrophy. Changes in vascularization occur in diabetic striated muscle; however, the relationship between these changes and DPN is as yet unexplored. The aim of the present study was to evaluate histologic properties and capillarization of diabetic skeletal muscle in relation to DPN and muscle strength.

METHODS

Twenty type 1 and 20 type 2 diabetic (T1D and T2D, respectively) patients underwent biopsy of the gastrocnemic muscle, isokinetic dynamometry at the ankle, electrophysiological studies, clinical examination, and quantitative sensory examinations. Muscle biopsies were stained immunohistochemically and muscle fiber diameter, fiber type distribution, and capillary density determined. Twenty control subjects were also included in the study.

RESULTS

No relationship was found between muscle fiber diameter, muscle fiber type distribution, or capillary density and degree of neuropathy or muscle strength for either patient group. Muscle fiber diameter and the proportion of Type II fibers were greater for T1D patients than both T2D patients and controls. The T2D patients had fewer capillaries per muscle fiber than T1D patients and controls.

CONCLUSIONS

Striated muscle fiber size, muscle fiber distribution, and vascularization are unrelated to DPN and muscle strength in diabetes.

Sex differences in human fatigability: mechanisms and insight to physiological responses

Sex-related differences in physiology and anatomy are responsible for profound differences in neuromuscular performance and fatigability between men and women. Women are usually less fatigable than men for similar intensity isometric fatiguing contractions. This sex difference in fatigability, however, is task specific because different neuromuscular sites will be stressed when the requirements of the task are altered, and the stress on these sites can differ for men and women. Task variables that can alter the sex difference in fatigability include the type, intensity and speed of contraction, the muscle group assessed and the environmental conditions. Physiological mechanisms that are responsible for sex-based differences in fatigability may include activation of the motor neurone pool from cortical and subcortical regions, synaptic inputs to the motor neurone pool via activation of metabolically sensitive small afferent fibres in the muscle, muscle perfusion and skeletal muscle metabolism and fibre type properties. Non-physiological factors such as the sex bias of studying more males than females in human and animal experiments can also mask a true understanding of the magnitude and mechanisms of sex-based differences in physiology and fatigability. Despite recent developments, there is a tremendous lack of understanding of sex differences in neuromuscular function and fatigability, the prevailing mechanisms and the functional consequences. This review emphasizes the need to understand sex-based differences in fatigability to shed light on the benefits and limitations that fatigability can exert for men and women during daily tasks, exercise performance, training and rehabilitation in both health and disease.

Reference values for vastus lateralis fiber size and type in healthy subjects over 40 years old: a systematic review and metaanalysis

Skeletal muscle atrophy is a major systemic impairment in chronic diseases. Yet its determinants have been hard to identify because a clear research definition has not been agreed upon. The reduction in muscle fiber cross-sectional area (CSA) is a widely acknowledged marker of muscle atrophy, but no reference values for the muscle fiber CSA at the age of the onset of chronic disease have ever been published. Thus, we aimed to systematically review the studies providing data on fiber CSA and fiber type proportion in the vastus lateralis of the quadriceps of healthy subjects (age >40 yr) and then to pool and analyze the data from the selected studies to determine reference values for fiber CSA. We followed the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) and identified 19 studies, including 423 subjects that matched the inclusion criteria. On the basis of fiber type and gender, the mean fiber CSA and the lower limits of normal (LLNs) were (%type I*60) + 1,743 μm(2) and (%type I*60) - 718 μm(2), respectively, for men; and (%type I*70) + 139 μm(2) and (%type I*70) - 1,485 μm(2), respectively, for women. There was no significant heterogeneity among subgroups of fiber type and gender. The pooled type I fiber proportion was 50.3% (LLN = 32.9%). In multivariate analysis, fiber CSA was significantly correlated with Vo2 peak (r = 190.92; P = 0.03), and type I fiber proportion was correlated with age (r = -0.024; P = 0.005), body mass index (r = 0.096; P = 0.005), and Vo2 peak (r = -0.053; P = 0.005). Our metaanalysis of a homogeneous set of studies is the first to provide valuable LLNs for fiber CSA according to fiber type and gender. This analysis will be improved by prospective assessment in well-characterized healthy subjects.

Cardiac resynchronization therapy improves minute ventilation/carbon dioxide production slope and skeletal muscle capillary density without reversal of skeletal muscle pathology or inflammation

AIMS

We evaluated the effects of cardiac resynchronization therapy (CRT) on skeletal muscle pathology and inflammation in patients with heart failure.

METHODS AND RESULTS

Stable patients (n = 21, 14 males, mean age 70 ± 7 years) with symptomatic heart failure (mean left ventricular ejection fraction 24 ± 6%) and an indication for CRT were included. Ergospirometry, skeletal muscle open biopsy, and blood sampling were performed prior to implantation and after 6 months of CRT. After CRT there was a reduction in both left ventricular end-diastolic diameter (LVEDD; 6.8 ± 0.8 vs. 6.3 ± 0.7 cm, P < 0.001) and native QRS duration (D) minus biventricular paced QRSD (172.9 ± 23 vs. 136.3 ± 23 ms, P ≤ 0.001). These changes were associated with an increase in peak slope oxygen uptake (consumption) (VO₂) (13.3 ± 2.2 vs. 14.5 ± 2.6 mL/kg/min, P = 0.07) and an improvement in the minute ventilation/carbon dioxide production slope (VE/VCO₂) slope (41.6 ± 7.4 vs. 39.1 ± 5.6, P = 0.012). There were no statistically significant changes in levels of pro-inflammatory cytokines, in mediators of mitochondrial biosynthesis or skeletal muscle pathology, except for an increase in skeletal muscle capillary density (4.5 ± 2.4 vs. 7.7 ± 3.3%, P = 0.002). Both the reduction of QRS duration and the increase in peak VO₂ correlated significantly with the change in mitochondrial density (r = 0.57, P = 0.008 and r = 0.54, P = 0.027, respectively).

CONCLUSION

Cardiac resynchronization therapy, with improved functional status and reduced LVEDD resulted in increased peak VO₂, improvement in VE/VCO₂ slope and capillary density in skeletal muscle, with no reduction in systemic pro-inflammatory cytokines, increase in intramuscular levels of mediators of mitochondrial biosynthesis or improvement in skeletal muscle ultrastructure per se. ClinicalTrials.gov Identifier: NCT01019915.

Abdominal and lumbar multifidus muscle size and symmetry at rest and during contracted States. Normative reference ranges

OBJECTIVES

The purpose of this study was to establish reference values for abdominal and lumbar multifidus muscles at rest and while contracted in a sample of active healthy adults.

METHODS

Three hundred forty participants (mean age ± SD, 21.8 ± 3.9 years; 96 females and 244 males) completed the study. Ultrasound imaging was used to assess the thickness of the transversus abdominis, internal and external oblique, rectus abdominis, and lumbar multifidus muscles. Additionally, the cross-sectional area of the rectus abdominis was assessed.

RESULTS

Although males had significantly thicker muscles than females (P < .05), the relative change in thickness during specified tasks was equivalent. Overall, relative muscle thickness and symmetry were similar to previous studies using smaller sample sizes.

CONCLUSIONS

These findings provide a robust data set of muscle thickness values measured by ultrasound imaging and can be used for comparison to those with pain, abnormal function, and pathologic conditions.

Effects of prolonged vibration on H-reflexes, muscle activation, and dynamic strength

Neural activation is generally lower during maximal voluntary lengthening compared with shortening and isometric muscle actions, but the mechanisms underlying these differences are unclear. In maximal voluntary isometric actions, reduced Ia-afferent input induced by prolonged tendon vibration has been shown to impair neural activation and strength.

PURPOSE

This study aimed to investigate whether reducing Ia-afferent input influences neural activation in maximal voluntary dynamic muscle actions and, if so, whether it affects shortening and lengthening muscle actions differently.

METHODS

Eight women participated in three familiarization sessions and two randomly ordered experiments. In one experiment, 30-min vibration at 100 Hz was applied to the Achilles tendon to decrease Ia-afferent input as measured by the H-reflex. In the control experiment, rest substituted the vibration. Root mean square EMG from plantar and dorsiflexor muscles and plantar flexor strength were measured during maximal voluntary plantar flexor shortening and lengthening actions (20°·s(-1)) before and after vibration and rest, respectively. Soleus H-reflexes and M-waves were elicited before each set of strength tests.

RESULTS

The vibration caused a decrease in H-reflex amplitude by, on the average, 33%, but root mean square EMG and plantar flexor strength remained largely unaffected in both action types.

CONCLUSIONS

The findings suggest that Ia-afferent input may not substantially contribute to maximal voluntary dynamic muscle strength of the plantar flexor muscles, as tested here, and thus, the results do not support the notion that Ia-afferent excitation would contribute differently to neural activation in maximal voluntary lengthening and shortening muscle actions.

Higher proportion of fast-twitch (type II) muscle fibres in idiopathic inflammatory myopathies - evident in chronic but not in untreated newly diagnosed patients

OBJECTIVE

Polymyositis and dermatomyositis are idiopathic, inflammatory myopathies characterized by proximal muscle fatigue. Conventional immunosuppressive treatment gives a variable response. Biopsies from chronic patients display a low proportion type I and a high proportion of type II muscle fibres. This raised a suspicion that the low proportion of type I fibres might play a role in the muscle fatigue.

AIM

To investigate whether the muscle fibre attributes evident in chronic myositis are characteristic for the polymyositis and dermatomyosistis diseases themselves.

METHODS

Muscle biopsies were obtained from thigh muscle from untreated patients (n = 18), treated responders (n = 14) and non-responders (n = 6) and from healthy controls (n = 11), respectively. For clinical evaluations, creatine kinase, functional index of myositis and cumulative dose of cortisone were established.

RESULTS

  Chronic patients had a lower proportion of type I fibres and a higher proportion of type II fibres compared to untreated myositis patients and healthy controls. Fibre cross-sectional area (CSA) did not differ between patients and healthy individuals but all women had a 20% smaller type II fibre CSA compared to men.

CONCLUSIONS

Untreated polymyositis and dermatomyositis patients and healthy controls have a different fibre type composition than chronic polymyositis and dermatomyositis patients. Fibre CSA did not differ between healthy controls or any of the patient groups. A low proportion of oxidative muscle fibres can therefore be excluded as a contributing factor causing muscle fatigue at disease onset and the gender difference should be taken into consideration when evaluating fibre CSA in myositis.

Expression of neurotrophic factors in diabetic muscle--relation to neuropathy and muscle strength

Diabetic polyneuropathy can lead to atrophy and weakness of distally located striated muscles due to denervation. Lack of neurotrophic support is believed to contribute to the development of diabetic neuropathy. In this study, we measured the expression of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3), neurotrophin 4 (NT-4) and ciliary neurotrophic factor (CNTF) in muscle biopsies taken from the gastrocnemic and deltoid muscles in 42 diabetic patients and 20 healthy control subjects. To express the distal neuropathic gradient and to reduce interindividual variation, a distal/proximal ratio between expression levels in the gastrocnemic and deltoid muscles was calculated for all neurotrophic factors. Neuropathic status was determined by clinical examination, electrophysiological studies and quantitative sensory examination in diabetic patients, and muscle strength at both the shoulder and ankle was assessed by isokinetic dynamometry. Distal/proximal ratios for NT-3 were lower in diabetic patients [median (range) 110.7 (39.8-546.8)] than in controls [157.6 (63.3-385.4); (P < 0.05)], and in neuropathic diabetic patients [107.1 (39.8-326.0)] versus patients without neuropathy [134.5 (46.6-546.8); (P < 0.005)]. Further, ratios for NT-3 were related to muscle strength (r(s) = 0.41, P < 0.01) and showed a tendency towards a negative relationship to the combined score of all measures of neuropathy [Neuropathy rank-sum score (NRSS)] (r(s) = -0.27, P = 0.09). Similar trends were observed for ratios for NT-4. Ratios for NGF (r(s) = -0.32, P < 0.05) and BDNF (r(s) = -0.32, P < 0.05) were related to NRSS, but not to muscle strength. Ratios for CNTF were higher in diabetic patients [64.6 (23.7-258.7)] compared with controls [50.2 (27.2-186.4); (P < 0.05)], but showed no relationship to neither NRSS nor muscle strength. Our results show that the expression of NT-3 is reduced in striated muscles in diabetic patients and is related to muscle weakness and neuropathy. We suggest that lack of NT-3 contributes to insufficient re-innervation leading to the loss of muscle strength in diabetic neuropathy.

Estudo comparativo da força muscular da mão entre cadetes homens e mulheres da Força Aérea Brasileira

O objetivo do estudo foi quantificar a força muscular da mão, nos movimentos de preensão palmar e três tipos de pinça, comparando os cadetes homens e mulheres. Foram avaliados 31 cadetes da Academia da Força Aérea Basileira, sendo 17 homens e 14 mulheres, com os dinamômetros Jamar e Preston Pinch Gauge. A média de preensão das mulheres foi de 31,4 kgf no lado dominante e 29,3 kgf no lado não-dominante; nos homens, foi 43,6 e 41,2 kgf, respectivamente (p<0,01). Na pinça lateral, a média feminina foi de 7,4 kgf e masculina, 10,2 kgf, no lado dominante (p<0,01); na pinça trípode, a média das mulheres foi de 7,2 kgf no lado dominante e 7,1 kgf do lado não-dominante; nos homens, foi respectivamente de 9,8 kgf e 9,5 kgf (p<0,01); na pinça polpa-a-polpa, as médias femininas foram de 4,9 e 4,6 kgf nos lados dominante e não-dominante; dos homens, foram de 6,5 e 5,7 kgf nos lados dominante e não-dominante, respectivamente (p<0,01). A força de preensão palmar, pinça lateral, pinça trípode e pinça polpa-a-polpa foi significantemente maior nos cadetes masculinos do que nas mulheres. Ambos os sexos mostraram mais força do lado dominante do que no não-dominante, exceto na pinça trípode. Quando comparados à população em geral, há pouca diferença dos valores obtidos nos cadetes, tanto em relação ao sexo quanto à dominância. Esses achados podem ser relevantes na avaliação fisioterapêutica e para o desempenho desses pilotos.

Relationship between the hip and low back pain in athletes who participate in rotation-related sports

CONTEXT

Hip function has been proposed to be related to low back pain (LBP) because of the anatomical proximity of the hip and lumbopelvic region. To date, findings have been inconclusive, possibly because the samples studied were heterogeneous. Sub-grouping samples based on characteristics such as activity demand, LBP classification, and sex might clarify research findings.

OBJECTIVE

To describe and summarize studies that examine 3 factors proposed to be important to the study of the hip-LBP relationship.

DESIGN

Review of cross-sectional studies.

SETTING

Academic health-care center and research laboratory.

SUBJECTS

3 groups: athletes with a history of LBP who regularly participate in rotation-related sports, athletes without a history of LBP who are active but do not regularly participate in rotation-related sports, and athletes without a history of LBP who participate in rotation-related sports.

MAIN OUTCOME

Hip range of motion and hip-lumbopelvic region coordination.

MEASURES

Hip range of motion was measured with an inclinometer. Coordination was examined based on kinematics obtained with a 3-dimensional motion-capture system.

RESULT

Differences among groups were found based on activity demand, LBP classification, and sex.

CONCLUSION

When assessing athletes with and without LBP, characteristics such as activity demand, LBP classification, and sex should be considered.

Sex differences in time to task failure and blood flow for an intermittent isometric fatiguing contraction

The purpose of this study was to compare the time to task failure, postcontraction hyperemia, and vascular conductance of young men and women for a submaximal intermittent fatiguing contraction performed with the handgrip muscles. Twenty men and 20 women (mean +/- SD: 22 +/- 4 years) performed an isometric contraction at 50% of maximal voluntary contraction (MVC) (6-s contraction, 4-s rest) until task failure. Forearm venous occlusion plethysmography was used to estimate the peak blood flow (after 10-min occlusion) and blood flow at rest after 6-s submaximal contractions of varying intensities, and during an intermittent fatiguing contraction at 1-min intervals and task failure. The time to task failure was longer for the women compared with the men (408 +/- 205 s vs. 297 +/- 57 s, P < 0.05). Postcontraction hyperemia and vascular conductance were greater for men than for women after nonfatiguing 6-s submaximal contractions performed at 20%, 40%, 50%, 60%, and 80% of MVC force (P < 0.05). In contrast, hyperemia and vascular conductance were similar for both genders when measured at 50 s into the fatiguing contraction, at each minute thereafter, and at task failure. Regression analysis indicated that the rate of electromyographic activity and perceived exertion were the significant predictors of the time to task failure. The longer time to task failure for women compared with men for an intermittent fatiguing contraction with handgrip muscles was not explained by postcontraction hyperemia or vascular conductance with fatigue.

Effect of physical training on the proportion of slow-twitch type I muscle fibers, a novel nonimmune-mediated mechanism for muscle impairment in polymyositis or dermatomyositis

OBJECTIVE

To compare muscle fiber type composition and muscle fiber area in patients with chronic polymyositis or dermatomyositis and healthy controls, and to determine whether physical training for 12 weeks could alter these muscle characteristics.

METHODS

Muscle fiber type composition and muscle fiber area were investigated by biochemical and immunohistochemistry techniques in repeated muscle biopsy samples obtained from 9 patients with chronic myositis before and after a 12-week exercise program and in healthy controls. Muscle performance was evaluated by the Functional Index (FI) in myositis and by the Short Form 36 (SF-36) quality of life instrument.

RESULTS

Before exercise, the proportion of type I fibers was lower (mean +/- SD 32% +/- 10%) and the proportion of type IIC fibers was higher (3% +/- 3%) in patients compared with healthy controls. After exercise, percentage of type I fiber increased to 42% +/- 13% (P < 0.05), and type IIC decreased to 1% +/- 1%. An exercise-induced 20% increase of the mean fiber area was also observed. The functional capacity measured by the FI in myositis and the physical functioning subscale of the SF-36 increased significantly. Improved physical functioning was positively correlated with the proportion of type I fibers (r = 0.88, P < 0.01) and type II muscle fiber area (r = 0.70, P < 0.05).

CONCLUSION

Low muscle endurance in chronic polymyositis or dermatomyositis may be related to a low proportion of oxidative, slow-twitch type I fibers. Change in fiber type composition and increased muscle fiber area may contribute to improved muscle endurance and decreased muscle fatigue after a moderate physical training program.

Rehabilitative ultrasound imaging of the abdominal muscles

Rehabilitative ultrasound imaging (RUSI) of the abdominal muscles is increasingly being used in the management of conditions involving musculoskeletal dysfunctions associated with the abdominal muscles, including certain types of low back and pelvic pain. This commentary provides an overview of current concepts and evidence related to RUSI of the abdominal musculature, including issues addressing the potential role of ultrasound imaging in the assessment and training of these muscles. Both quantitative and qualitative aspects associated with clinical and research applications are considered, as are the possible limitations related to the interpretation of measurements made with RUSI. Research to date has utilized a range of methodological approaches, including different transducer placements and imaging techniques. The pros and cons of the various methods are discussed, and guidelines for future investigations are presented. Potential implications and opportunities for clinical use of RUSI to enhance evidence-based practice are outlined, as are suggestions for future research to further clarify the possible role of RUSI in the evaluation and treatment of abdominal muscular morphology and function.

Muscle fibre type shifting in the vastus lateralis of patients with COPD is associated with disease severity: a systematic review and meta-analysis

BACKGROUND

Skeletal muscle dysfunction is a common feature in chronic obstructive pulmonary disease (COPD) which is associated with intrinsic muscular abnormalities. One of the most consistently reported alterations is a shift from fibre type I to II in the vastus lateralis of these patients. Surprisingly, the relationship between this shift and the severity and phenotype of COPD remains unclear. A study was conducted to determine whether vastus lateralis muscle fibre type proportions are associated with COPD disease severity and to provide reference values for the proportions of fibre types in the vastus lateralis in COPD.

METHODS

A systematic review and a meta-analysis were conducted in which muscle fibre type data and markers of disease severity were collected from the literature.

RESULTS

The forced expiratory volume in 1 s (FEV(1)), the ratio of FEV(1) to forced vital capacity (FVC) and body mass index were positively associated with the proportion of type I fibres in COPD. A proportion of 51% for vastus lateralis fibre type I and 13% for fibre type IIX were calculated from the combined data as normal values for patients with typical GOLD stage 3-4 COPD aged 60-70 years. Based on these reference values, a proportion of fibre type I <27% and of fibre type IIX >29% were defined as pathologically abnormal.

CONCLUSIONS

This review sheds new light on the relationship between skeletal muscle abnormalities and important hallmarks of the disease in severe COPD, and identifies absence of data in GOLD stages 1-2. This review also provides reference values on fibre type composition for diagnostic purposes in COPD.

Gender specific activation patterns of trunk muscles during whole body tilt

Gender specific differences as evidenced in both anthropometric data and physical performance of healthy persons have been broadly demonstrated. Recently advancements in surface electromyography (SEMG) have shown possible differences in men's and women's muscle coordination patterns. However, quantitative information about gender related muscle co-ordination patterns are rare. This investigation was carried out to both verify if trunk muscle SEMG amplitude-force relationship differs between men and women and refine techniques of measurement and data analysis using SEMG. Thirty-one healthy volunteers (16 women, 15 men) were investigated during whole body tilt at angles from 5 degrees to 90 degrees (from quasi vertical to horizontal position). Subjects had to maintain body in body axis while their lower body was fixed and the upper body remained unsupported. SEMG was taken from five different trunk muscles of both sides simultaneously. At corresponding tilt angles women exhibited higher amplitude levels of their abdominal muscles in comparison to men, who were characterized by higher back muscle amplitudes. Abdominal muscles showed a non-linear SEMG amplitude-force relationship but differed between genders with more linearity in women. Back muscles showed a linear amplitude-force relationship with no differences between genders. Women were characterized by higher levels of co-contraction of all investigated muscles. The data are in accordance with histological investigations, which already proved specific fiber distribution patterns in both abdominal and back muscles and gender related differences in relative area of Type 1 fibers of back muscles. The observed differences in SEMG-force relationship for the abdominal muscles remain hypothetical because of lack of histological information.

The Adaptations to Strength Training

High-resistance strength training (HRST) is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations. This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions. The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors (e.g. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density) are also likely to contribute to this phenomenon. Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases. Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability. The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.

Mechanisms of fatigue differ after low- and high-force fatiguing contractions in men and women

The magnitude of failure in voluntary drive after fatiguing contractions of different intensities in men and women is not known. The purpose of this study was to compare the time to task failure and voluntary activation of men and women for a sustained isometric contraction performed at a low and high intensity with the elbow flexor muscles. Nine men and nine women sustained an isometric contraction at 20% and 80% of maximal voluntary contraction (MVC) force until task failure during separate sessions. The men had a shorter time to failure than women for the 20% but not the 80% MVC task. Voluntary activation was reduced to similar levels for the men and women at the end of the fatiguing contractions but was reduced less after the 80% MVC task than the 20% MVC contraction. Twitch amplitude was reduced similarly at task failure for both sexes and to similar levels at termination of the 20% and 80% MVC tasks. The rate of change in mean arterial pressure was the main predictor of time to failure for the low-force sustained contraction. These results suggest that women experienced greater muscle perfusion, less peripheral fatigue, and a longer time to task failure than men during the low-force fatiguing contraction. However, the low-force task induced greater central fatigue than the high-force contraction for both men and women. Thus, low-force, long-duration fatiguing contractions can be used in rehabilitation to induce significant fatigue within the central nervous system and potentially greater neural adaptations in men and women.

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.

Relationships among lateral abdominal muscles, gender, body mass index, and hand dominance

STUDY DESIGN

Exploratory.

OBJECTIVES

To explore whether hand dominance, gender, and body mass index (BMI) influence the thickness of the lateral abdominal muscles as measured by ultrasound imaging. To document the extent of improvement in response stability when an average of multiple measures was utilized.

BACKGROUND

Ultrasound imaging is a relatively new tool used to assess the lateral abdominal muscles. A better understanding of how these muscles contract in a healthy population can provide a reference for comparison to patients with low back pain (LBP).

METHODS AND MEASURES

Thirty-two healthy participants (17 males, 15 females) aged 18 to 45 years (mean +/- SD, 31.9 +/- 7.8 years) were studied. Measurements of muscular thickness of the lateral abdominal muscles were obtained bilaterally while the subjects were at rest, and while they performed the abdominal drawing-in maneuver. To determine the possible influence of hand dominance and gender on muscle thickness, t tests were used. Correlation coefficients were used to assess the relationship between BMI and muscle thickness. Standard error of the measurement was used to assess response stability of the ultrasound imaging technique.

RESULTS

No differences in the thicknesses of the transversus abdominis (TrA) muscle were measured during rest or while contracted, based on hand dominance (P > or = .73). Men had greater muscular thickness (P<.01), while the TrA in women represented a greater proportion of the total lateral abdominal muscles (P<.01). BMI was positively associated with muscle thickness (r> or =.66). Compared to a singular measurement, response stability improved by greater than 50% when an average of 3 measurements was used.

CONCLUSIONS

Future researchers should assess the need to control for gender and BMI as potential covariates in ultrasound imaging studies of the lateral abdominal muscles. Asymmetry in the lateral abdominal muscles in those with LBP would be in direct contrast to the bilateral symmetry measured in those without LBP.

Active hyperemia and vascular conductance differ between men and women for an isometric fatiguing contraction

To understand the role of muscle perfusion in the sex differences of muscle fatigue, we compared the time to task failure, postcontraction (active) hyperemia, and vascular conductance for an isometric fatiguing contraction performed by young men and women with the handgrip muscles at 20% of maximal voluntary contraction (MVC) force. In study 1, the men (n = 16) were stronger than the women (n = 18), and study 2, the men (n = 7) and women (n = 7) were matched for strength. Isometric contractions were sustained during two sessions: 1) until the target force could no longer be achieved or 2) for 4 min. For both studies, blood flow and vascular conductance were similar for the men and women at rest and after 10 min of occlusion, and at task failure for the fatiguing contraction estimated using forearm venous occlusion plethysmography. In study 1, the time to task failure was longer for the women (11.4 +/- 2.8 min) than for the men (8.4 +/- 2.4 min; P = 0.003). However, at the end of the 4-min contraction, active hyperemia and vascular conductance were greater for the men than the women (99 vs. 70% peak blood flow; P < 0.001). In study 2, the men and women had similar strength and a similar time to failure (8.4 +/- 1.6 vs. 8.6 +/- 2.3 min). Active hyperemia was greater for the men than the women (86 vs. 64% peak flow; P = 0.038) after the 4-min contraction, as was vascular conductance (80 vs. 57% peak conductance; P = 0.02). Thus the briefer time to failure of men than women for an isometric fatiguing contraction is a function of the greater strength of men but is not dependent on differences in the active hyperemia and vascular conductance.

Gender differences in pattern of hip and lumbopelvic rotation in people with low back pain

BACKGROUND

Findings from previous studies suggest gender may affect the pattern of hip and lumbopelvic motion during a multi-segmental movement. To date, no studies have examined movement patterns and low back pain symptom behavior during hip lateral rotation.

METHODS

Forty-six people (27 males and 19 females) with low back pain were examined. Three-dimensional kinematic data and low back pain symptoms were recorded during active hip lateral rotation. Percent of maximum lumbopelvic rotation was calculated for each 10% increment of maximum active hip lateral rotation.

FINDINGS

Men exhibited a greater percent of maximum lumbopelvic rotation (mean 49.3, SD 13.3) during the first 60% of hip lateral rotation than women (mean 36.2, SD 16.4) (P < 0.01). Nineteen (70.4%) of the men and seven (36.8%) of the women had pain with the hip lateral rotation test (P = 0.02).

INTERPRETATION

Men exhibited more lumbopelvic rotation in the early part of hip lateral rotation than women, and hip lateral rotation was more likely to be associated with symptoms in men than women. Greater lumbopelvic motion, earlier in hip lateral rotation, may make men more vulnerable to low back pain associated with hip lateral rotation. Factors that contribute to these gender differences should be investigated further.

A diffusion tensor imaging analysis of gender differences in water diffusivity within human skeletal muscle

The diffusive properties of adjacent muscles at rest were evaluated in male (n = 12) and female (n = 12) subjects using diffusion tensor imaging (DTI). The principle, second and third eigenvalues, trace of the diffusion tensor [Tr(D)], and two anisotropic parameters, ellipsoid eccentricity (e) and fractional anisotropy (FA), of various muscles in the human calf were calculated from the diffusion tensor. Seven muscles were investigated in this study from images acquired of the left calf: the soleus, lateral gastrocnemius, medial gastrocnemius, posterior tibialis, anterior tibialis, extensor digitorum longus and peroneus longus. A mathematical model was also derived that relates the eigenvalues of the diffusion tensor to the muscle fiber volume fraction, which is defined as the volume of muscle fibers within a well-defined arbitrary muscle volume. Females on average had higher eigenvalues and Tr(D) compared with males, with the majority of muscles being statistically different between the sexes. In contrast, males on average had higher e and FA than females, with the large plantar flexors--soleus, lateral gastrocnemius, and medial gastrocnemius--producing statistically different results. The behavior of the mathematical model for variations in fiber volume fraction produced similar trends to those seen when the experimental data were fit to the model. The model predicts that a larger volume fraction of skeletal muscle in males is devoted to fibers than in females, but the true underlying source of the gender discrepancy remains unclear. Although the model does not fully account for other transport processes, it does provide some insight into the limiting factors that affect the diffusion of water in skeletal muscle measured by DTI.