Muscle function and fatigability of trunk flexors in males and females

Does pain influence control of muscle force? A systematic review and meta-analysis

BACKGROUND AND OBJECTIVE

In the presence of pain, whether clinical or experimentally induced, individuals commonly show impairments in the control of muscle force (commonly known as force steadiness). In this systematic review and meta-analysis, we synthesized the available evidence on the influence of clinical and experimental pain on force steadiness.

DATABASES AND DATA TREATMENT

MEDLINE, EMBASE, PubMed, CINAHL Plus and Web of Science databases were searched from their inception to 19 December 2023, using MeSH terms and pre-selected keywords related to pain and force steadiness. Two independent reviewers screened studies for inclusion and assessed their methodological quality using a modified Newcastle-Ottawa risk of bias tool.

RESULTS

In total, 32 studies (19 clinical pain and 13 experimental pain) were included. Meta-analyses revealed reduced force steadiness in the presence of clinical pain as measured by the coefficient of variation (CoV) and standard deviation (SD) of force (standardized mean difference; SMD = 0.80, 95% CI = 0.31-1.28 and SMD = 0.61, 95% CI = 0.11-1.11). These findings were supported by moderate and low strength of evidence respectively. In the presence of experimental pain, meta-analyses revealed reductions in force steadiness when measured by the CoV of force but not by the SD of force (SMD = 0.50, 95% CI = 0.01-0.99; and SMD = 0.44, 95% CI = -0.04 to 0.92), each supported by very low strength of evidence.

CONCLUSIONS

This work demonstrates that pain, particularly clinical pain, impairs force steadiness. Such impairments likely have clinical relevance and could become targets for treatment when managing people experiencing musculoskeletal pain.

SIGNIFICANCE STATEMENT

This systematic review and meta-analyses enhances our understanding of motor impairments observed in people experiencing musculoskeletal pain. It underscores the significance of incorporating force steadiness assessment when managing individuals experiencing musculoskeletal pain. Additionally, it suggests that future research should explore the potential benefits of force steadiness training in alleviating patients' symptoms and enhancing their functional performance. This could potentially lead to the development of innovative therapeutic approaches for individuals suffering from musculoskeletal pain.

A comparison of individual force decline profiles during a fatiguing eccentric trunk flexion and extension protocol: a pilot study

Introduction

Muscle fatigue, characterized by diminished force production and contraction sustainability, can impair muscle coordination and increase joint instability. Differing force profiles used in fatiguing tasks, such as prolonged eccentric trunk protocols, might provide insights into individualized strategies and resulting spinal stability. Thus, this study assessed individual differences in fatigue characteristics during an eccentric trunk flexion-extension protocol in a population of asymptomatic individuals.

Methods

Twelve participants (2 f/10 m, 29 ± 4 years, 78.4 ± 16.9 kg, 1.76 ± 0.10 m) performed an eccentric trunk flexion and extension protocol on an isokinetic dynamometer (45° flexion to 10° extension; 60°/s), with final analysis on 8 participants for trunk flexion and 11 for trunk extension due to data exclusions. Participants engaged in a maximal all-out (AO) task for 2 min. Each participant's torque output (Nm) was assessed on a repetition-by-repetition basis, and smoothened by a moving average of 5 repetitions. Individual time profiles for reaching fatigue thresholds (10%, 15%, 20% and 30% reduction of initial torque output), and inter subject variability (by coefficient of variation, CV in %) were assessed throughout the AO task. Further, percentage torque reduction and variability were assessed at mid (1-minute) and end (2-minute) of task.

Results

On average, for flexor and extensor muscles combined, participants reached a force reduction of 10% within 23.2 ± 19.1 s, of 15% within 44.9 ± 19.6 s, of 20% in 62.4 ± 26.3 s, and of 30% within 79.2 ± 21.8 s. The variability between individuals for the timepoint of reaching the defined torque thresholds was assessed by CV ranged between 23.4% and 103.8% for trunk flexor muscles, and between 28.4% and 56.5% for trunk extensor muscles.

Discussion

A reduction of up to 20% was seen on average for all participants within 1-minute of eccentric trunk flexion and extension. Different inter-individual force output profiles were seen throughout the AO protocol, potentially related to physiological, skill-based, technical, adaptational, and/or motivational factors. The increase in fatigue resulted in a reduction in variability among individuals. A 2-minute protocol effectively induced pronounced fatigue, offering insights into individual force profiles and strategies.

Differences in Injury Profiles Between Female and Male Athletes Across the Participant Classification Framework: A Systematic Review and Meta-Analysis

BACKGROUND

Female sex is a significant determinant of anterior cruciate ligament (ACL) injury. It is not understood if sex is a key determinant of other sports-related injuries.

OBJECTIVE

The aim of this systematic review was to identify where differences in injury profiles are most apparent between the sexes in all sports across the six-tiered participant classification framework.

METHODS

This systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement and the 'implementing PRISMA in Exercise, Rehabilitation, Sport medicine and SporTs science'(PERSiST) guidance. The databases PubMed, CINAHL, Web of Science, SPORTDiscus, Medline, Scopus, Cochrane Library and EBSCO were searched from database inception to 24 April 2023. Longitudinal, prospective and retrospective cohort studies and cross-sectional and descriptive epidemiology studies that used standard injury data collection were included. Studies were excluded if injuries were not medically diagnosed and if injuries were not reported and/or analysed by sex. Two reviewers independently extracted data and assessed study quality using the Downs and Black checklist.

RESULTS

Overall, 180 studies were included (8 tier-5, 40 tier-4, 98 tier-3, 30 tier-2, 5 tier-1 studies; one study included data in two tiers). Of those, 174 studies were of moderate quality and six studies were of limited quality. In sex-comparable sports, there was moderate evidence that female athletes had greater risk of knee injury (relative risk (RR) 2.7; 95% CI 1.4-5.5), foot/ankle injuries (RR 1.25; 95% CI 1.17-1.34), bone stress injury (RR 3.4; 95% CI 2.1-5.4) and concussion (RR 8.46; 95% CI 1.04-68.77) than male athletes. Male athletes were at increased risk of hip/groin injuries (RR 2.26; 95% CI 1.31-3.88) and hamstring injuries (RR 2.4; 95% CI 1.8-3.2) compared with females, particularly in dynamic sports. Male athletes were 1.8 (1.37-2.7) to 2.8 (2.45-3.24) times more likely to sustain acute fractures than female athletes, with the highest risk in competition.

DISCUSSION

Most studies in all cohorts were of moderate quality (mean/range of scores tier-5: 17 ± 2.2 [14-20], tier-4: 16.9 ± 1.9 [11-21], tier-3: 16.9 ± 1.5 [11-20], tier-2: 16.3 ± 2.2 [11-20], tier-1 studies: 15.6 ± 1.3 [14-17] out of 28 on the Downs and Black checklist), with only six studies of limited quality. Female athletes' propensity for bone stress injuries highlights opportunities to reinforce development of optimal bone health during adolescence and to outline the effects of energy availability. Earlier strength development and exposure to neuromuscular training programmes and modification of skill development in female athletes may be effective strategies for reducing lower limb injury risk. Key components of neuromuscular training programmes could be beneficial for reducing hip/groin and hamstring injury risk in male athletes. There may be a need for sex-specific prevention and return-to-sport protocols for sports-related concussion in female athletes.

CONCLUSIONS

Female sex was a key determinant of sports-related injuries beyond ACL injury including foot/ankle injury, bone stress injury and sports-related concussion. Male sex was a key determinant of hip/groin, hamstring injury and upper limb injury.

TRIAL REGISTRY

PROSPERO registration number: CRD42017058806 (last updated on 7th June 2023).

Core Function in Adults With Severe Asthma and Its Relationship With Breathing Symptoms

BACKGROUND

People with asthma may have skeletal muscle dysfunction but data describing core function in severe asthma are limited.

OBJECTIVE

To compare core function between people with severe asthma and healthy controls and to determine the difference between males and females. Furthermore, we aimed to investigate the association between core function and breathing symptoms.

METHOD

Adults with a diagnosis of severe asthma and healthy controls undertook an assessment that included 3 core function tests: partial sit-up, Biering-Sorensen, and side bridge. Breathing symptoms were assessed by the modified Medical Research Council dyspnea scale, modified Borg scale, and Nijmegen questionnaire.

RESULTS

People with severe asthma (n = 136) (38% male, age median [Q1-Q3] 59 y [45-68], body mass index 30 kg/m2 [26-37]) were compared with 66 people without respiratory disease (47% male, age 55 y [34-65], body mass index 25 kg/m2 [22-28]). There was no difference between groups in the partial sit-up (P = .09). However, participants with severe asthma performed worse with the Biering-Sorensen (P < .001), and the left and right side bridge test (P < .001 for both) than the healthy comparison group. Similar results were found when comparing males and females separately. Males with severe asthma had increased function compared with their female counterparts in the left side bridge test. Core function tests correlated with the breathing symptom measures, the modified Medical Research Council, modified Borg scale, and Nijmegen questionnaire (-0.51 > r > -0.19; P ≤ .03).

CONCLUSIONS

Adults with severe asthma have worse core function than their control counterparts, independent of sex. Furthermore, as core function decreases, breathing symptoms increase.

Cognitive challenge as a probe to expose sex- and age-related differences during static contractions

Despite activities of daily living being frequently performed simultaneously with a cognitive task, motor function is often investigated in isolation, which can hinder the applicability of findings. This brief review presents evidence that 1) performing a cognitive challenge simultaneously with a motor task can negatively impact force steadiness and fatigability of limb muscles during a static contraction, 2) the negative impact on old adults (>65 years old), particularly older women is greater than young when a cognitive challenge is simultaneously performed with a static motor task, 3) age-related mechanisms potentially explain impairments in motor performance in the presence of a cognitive challenge, and 4) the mechanisms for the age-related decrements in motor performance can be distinct between men and women. These observations are highly relevant to the older adults, given the increased risk of accidents and injury when a motor task is performed with a high cognitive-demand task, especially in light of the expanding reliance on an aging workforce.

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.

Effect of 12 Weeks Core Training on Core Muscle Performance in Rhythmic Gymnastics

BACKGROUND

Rhythmic gymnastics performance is characterized by technical elements involving flexibility, aerobic capacity and strength. Increased core strength in rhythmic gymnastics could lead to improved sporting performance.

OBJECTIVE

The aim of this study was to analyze the effect of 12 weeks of core muscle training on core muscle performance in rhythmic gymnasts.

METHODS

A randomized controlled study involving 24 rhythmic gymnastics was conducted. Participants were randomly assigned to a control group (CG; n = 12; age 13.50 ± 3.17 years) or a training group (TG; n = 12; age 14.41 ± 2.35 years). Body composition, isometric strength of trunk, core endurance and core muscle electromyographic activity were measured (EMG) after 12 weeks of core training. Independent sample t-tests were carried out to compare baseline values between groups. A two-way repeated-measures analysis of variance (ANOVA) (time × group) was applied.

RESULTS

The TG improved body composition, trunk lean mass (mean differences MD = -0.31; p = 0.040), lean mass (MD = 0.43; p = 0.037) and bone mass (MD = -0.06; p < 0.001) after training. Core training increased isometric strength of trunk, flexion test (MD = -21.53; p = 0.019) and extension test (MD = 22.7; p = 0.049), as well as the prone bridge core endurance test (MD = -11.27; p = 0.040). The EMG values also increased in the TG in prone bridge for front trunk (MD = -58.58; p = 0.026).

CONCLUSIONS

Core strength training leads to improvements in body composition, as well as improvements in trunk strength and increases in muscle electromyographic activity. These improvements could therefore improve performance during competitive rhythmic gymnastics exercises.

Does pain influence force steadiness? A protocol for a systematic review

INTRODUCTION

Performing contractions with minimum force fluctuations is essential for everyday life as reduced force steadiness impacts on the precision of voluntary movements and functional ability. Several studies have investigated the effect of experimental or clinical musculoskeletal pain on force steadiness but with conflicting findings. The aim of this systematic review is to summarise the current literature to determine whether pain, whether it be clinical or experimental, influences force steadiness.

METHODS AND ANALYSIS

This protocol for a systematic review was informed and reported in line with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Protocols and the Cochrane Handbook for Systematic Reviews of Interventions. Key databases will be searched from inception to 31 August 2020, including MEDLINE, EMBASE, PubMed, CINAHL Plus, ZETOC and Web of Science. Grey literature and key journals will be also reviewed. Risk of bias will be assessed with the Newcastle-Ottawa tool, and the quality of the cumulative evidence assessed with the Grading of Recommendations, Assessment, Development and Evaluation guidelines. If homogeneity exists between groups of studies, meta-analysis will be conducted. Otherwise, a narrative synthesis approach and a vote-counting method will be used, while the results will be presented as net increases or decreases of force steadiness.

ETHICS AND DISSEMINATION

The findings will be presented at conferences and the review will be also submitted for publication in a refereed journal. No ethical approval was required.

PROSPERO REGISTRATION NUMBER

CRD42020196479.

Effect of Aging on Trunk Muscle Function and Its Influence on Falls Among Older Adults

The objective was to determine whether trunk muscle function is influenced by the aging process and to identify if the trunk can be an important factor in older people's falls over a period of 1 year. The peak torque, rate of torque development, and torque steadiness of the trunk extensors and flexors were compared between a young group, older group (older adults with no episodes of falls), and older faller group (older adults who had suffered at least one fall episode over a period of 1 year) by one-way analysis of variance, followed by the post hoc Tukey test. The adjusted multivariate linear regression was applied to verify the association between the number of falls and the trunk parameters in older adults. The young group showed higher extensors and flexors peak torque and rate of torque development, and lower extensor torque steadiness at 10% when compared with older groups. Only trunk flexor peak torque showed a negative association with the number of future falls (p = .042), but there was no difference in trunk muscle function between the older group and the older faller group.

The Effect of Repeated Shoulder Motion on Scapular Dyskinesis in Army ROTC Cadets

INTRODUCTION

Altered or abnormal scapular motion has been associated with increased shoulder pain and dysfunction. Fatigue of the scapular stabilizing muscles resulting from repeated arm motion has been reported to alter scapular kinematics, which could result in the clinical assessment of scapular dyskinesis. The purpose of this study was to characterize the effect of repeated arm motion on the development of scapular dyskinesis. The hypothesis was that repeated arm motion will induce a subtle or obvious scapular dyskinesis.

MATERIALS AND METHODS

About 30 army Reserve Officer Training Corps (ROTC) cadets participated in the research study. The cadets level of shoulder function was determined using the Quick Disabilities of the Arm Shoulder and Hand and Pennsylvania Shoulder Score shoulder scores. Cadets performed an exercise protocol of 30 repetitions of weighted shoulder motion in the frontal plane. Shoulder and scapular musculature strength measurements were recorded prior to and immediately following the exercise protocol using hand-held dynamometry. The scapular dyskinesis test was performed prior to the exercise protocol and during the last five repetitions of the exercise protocol.

RESULTS

Prior to the exercise, protocol 3/30 were categorized with scapular dyskinesis on the left side and 3/30 were categorized with scapular dyskinesis on their right side. Following the exercise, protocol 8/30 were categorized with left-side scapular dyskinesis (χ2 = 9.167, P = 0.002) and 6/30 with right-side dyskinesis (χ2 = 4.537, P = 0.033). Shoulder strength decreased following the exercise protocol for all participants. The participants that developed scapular dyskinesis were weaker than those with normal scapular motion. However, the difference did not reach statistical significance. Participants graded with obvious scapular dyskinesis following the exercise protocol reported higher Quick Disabilities of the Arm Shoulder and Hand scores (P = 0.04) and lower scores on the Pennsylvania Shoulder Score (P = 0.005).

CONCLUSIONS

Repeated shoulder motion increased the frequency of scapular dyskinesis in army ROTC cadets. The cadets that developed scapular dyskinesis also reported greater disability and lower function of the upper extremity. The results provide a link between scapular dyskinesis, upper extremity function, and the strength of the scapular stabilizing muscles. Improving the strength of the scapular stabilizing musculature might reduce the effects of repeated arm motions.

Association between the trunk muscle function performance and the presence of vertebral fracture in older women with low bone mass

BACKGROUND

Despite the clinical importance, it has remained unclear which changes in the trunk muscle function parameters are more associated with the presence of vertebral fracture (VF).

AIMS

The aim of this study was to verify the association between the trunk muscle function performance and the presence of VF in older women with low bone mass. The secondary aim was to evaluate the correlation between trunk muscle function and both fall history and muscle mass.

METHODS

This cross-sectional study was composed by 94 women over 60 years within value of T-Score lumbar spine BMD  <- 1.0 DP. Multidimensional evaluations were performed: appendicular skeletal muscle mass index (ASMI) was determined by the total body DEXA; the radiographic evaluations measured the degree of thoracic kyphosis and classification of VF. The trunk muscle function parameters, such as peak torque (PT), rate of torque development (RTD) and torque steadiness (TS) were evaluated by isokinetic dynamometer. The trunk muscle endurance was evaluated by the timed loaded standing test. The adjusted multivariate logistic regression model and multivariate linear regression were performed to verify the association between the variables studied.

RESULTS

The results showed that the trunk muscle function parameter with greater association with the presence of VF is TS extensors (OR = 1.70; p < 0.001). The other two significant muscle parameters were: RTD₃₀ flexors (OR = 0.31; p = 0.033) and PT extensors (OR = 0.13; p = 0.009). No statistical association was found between the presence of VF and the ASMI and trunk muscle endurance. No correlation between trunk muscle function and fall history was observed. There was a weak correlation between ASMI and extensor PT (R² = 0.21; p = 0.027) and extensor RTD₃₀ (R² = 0.21; p = 0.026).

CONCLUSIONS

This study demonstrated that deficit in trunk muscle function has shown a strong association with the presence of VF, highlighting issues heretofore unexplored regarding the association between VF with muscle power and motor control.

Patients With Spinal Muscular Atrophy Use High Percentages of Trunk Muscle Capacity to Perform Seated Tasks

OBJECTIVE

The aim of the study was to investigate trunk function during seated upper limb tasks in patients with spinal muscular atrophy types 2 and 3.

DESIGN

Seventeen persons with spinal muscular atrophy and 15 healthy controls performed several tasks when sitting unsupported, such as reaching (and placing) forward and sideward. Joint torque and muscle activity were measured during maximum voluntary isometric contractions. Three-dimensional kinematics and normalized muscle activity were analyzed when performing tasks.

RESULTS

Trunk joint torques were significantly decreased, approximately 45%, in patients with spinal muscular atrophy compared with healthy controls. Active range of trunk motion was also significantly decreased in all directions. When performing tasks, the average back muscle activity was 27% and 56% of maximum voluntary isometric contractions for healthy controls and spinal muscular atrophy and for abdominal muscles 10% and 44% of maximum voluntary isometric contractions, respectively. Trunk range of motion did not differ when performing daily tasks.

CONCLUSIONS

The trunk of patients with spinal muscular atrophy is weaker compared with healthy controls, reflected by reduced trunk torques and decreased active range of motion. In addition, patients with spinal muscular atrophy use high percentages of their trunk muscle capacity to perform tasks. Clinicians should take this into account for intervention development, because using high percentages of the maximum muscle capacity results in fatigue and muscle overloading.

Don't forget the trunk in Duchenne muscular dystrophy patients: more muscle weakness and compensation than expected

BACKGROUND

Performing daily activities independently becomes more difficult in time for patients with Duchenne muscular dystrophy (DMD) due to muscle weakness. When performing seated daily activities, the trunk plays an indispensable role besides the upper extremities. However, knowledge is lacking on the interaction between trunk and upper extremities. Therefore the aim was to investigate whether patients with DMD use trunk movement to compensate for reduced arm function when performing seated tasks, and whether this is related to increased muscle activity.

METHODS

Eighteen boys with DMD and twenty-five healthy controls (HC) performed several tasks when sitting unsupported, like reaching (and placing) forward and sideward, drinking and displacing a dinner plate. Maximum joint torque and maximum surface electromyography (sEMG) were measured during maximum voluntary isometric contractions. Three-dimensional movements and normalized sEMG when performing tasks were analyzed.

RESULTS

Significantly decreased maximum joint torque was found in DMD patients compared to HC. Trunk and shoulder torques were already decreased in early disease stages. However, only maximum trunk rotation and shoulder abduction torque showed a significant association with Brooke scale. In all reaching and daily tasks, the range of motion in lateral bending and/or flexion-extension was significantly larger in DMD patients compared to HC. The trunk movements did not significantly increase with task difficulty (e.g. increasing object weight) or Brooke scale. Normalized muscle activity was significantly higher in DMD patients for all tasks and muscles.

CONCLUSIONS

Boys with DMD use increased trunk movements to compensate for reduced arm function, even when performing relatively simple tasks. This was combined with significantly increased normalized muscle activity. Clinicians should take the trunk into account when assessing function and for intervention development, because DMD patients may appear to have a good trunk function, but percentage of muscle capacity used to perform tasks is increased.

Fatiguing Trunk Flexor Exercise Decreases Pain Sensitivity in Postpartum Women

Background

Low back pain (LBP) is common in the general population and among postpartum women. Abdominal muscle exercise is often used to treat LBP, but it is unknown if fatiguing abdominal muscle exercise can produce exercise-induced hypoalgesia (EIH).

Objectives

To assess pressure pain thresholds (PPTs) at rest and following fatiguing trunk flexor exercise (EIH) in (1) nulligravid and postpartum women to evaluate the impact of pregnancy and childbirth and (2) nulligravid women and men to examine sex differences.

Methods

Seventy healthy adults (31 postpartum women, 23 nulligravid women, 16 men) participated. Postpartum and nulligravid women were tested twice (16-18 weeks apart) to identify changes in EIH with postpartum recovery. PPTs were measured at the nailbed and superior rectus abdominis before and after exercise to investigate systemic and local EIH, respectively. Rectus abdominis muscle thickness was assessed with ultrasound.

Results

Postpartum women reported lower PPTs than nulligravid women at the abdomen (p < 0.05) whereas postpartum women had lower PPTs at the nailbed during the first session only. Men reported higher nailbed PPTs (p = 0.047) and similar PPTs at the abdomen than women (p = 0.294). All groups demonstrated EIH at the abdomen (p < 0.05). Systemic EIH was absent in postpartum and nulligravid women (p > 0.05), while men demonstrated hyperalgesia. Local EIH was positively associated with muscle thickness for men and women, which was not significant at the second timepoint.

Limitations

Acute exercise response may not reflect changes that occur with exercise training.

Conclusion

Fatiguing trunk flexor exercise produced local EIH for all groups including postpartum and nulligravid women. Clinically, trunk exercises may be useful for acute pain relief for clinical populations that are characterized by pain and/or weakness in the abdominal region muscles in populations with abdominal pain syndromes.

Fatigability of the Lumbopelvic Stabilizing Muscles in Women 8 and 26 Weeks Postpartum

Background

Pregnancy and childbirth are associated with lumbopelvic pain and instability. Fatigability of the lumbopelvic stabilizing muscles after childbirth is unknown, and no clinical tests exist to assess this important metric of muscle function.

Objectives

To compare fatigability of the lumbopelvic stabilizing muscles in postpartum and nulligravid (control) women using the Active Straight Leg Raise (ASLR) Fatigue Task, and to determine if fatigability is associated with inter-recti distance (IRD), physical function, and pain/disability.

Study design

Longitudinal case-control study.

Methods

Twenty-nine nulligravid (25.4 ± 9.1 years) and 31 postpartum women (31.4 ± 5.2 years; vaginal delivery n=18) were tested at two time points, 16 weeks apart (postpartum women tested at 8-10 and 24-26 weeks postpartum). Muscular function was assessed with manual muscle testing (MMT), the ASLR Test, and a new ASLR Fatigue Task. Other measures included IRD, rectus abdominis thickness, physical activity, and six-minute walk distance.

Results

Postpartum women were 23% more fatigable (p=0.028) and were weaker (MMT) (p<0.001) than controls up to 26 weeks postpartum. The ASLR fatigue task (time-to-failure) was associated with smaller IRD, greater rectus abdominis thickness, higher physical activity levels, greater MMT strength, and further distance walked in six minutes (p<0.05).

Conclusion

Postpartum women (up to 6 months) had greater fatigability of the lumbopelvic stabilizing muscles and lower physical function than nulligravid women, suggesting core muscle function and fatigability should be assessed after pregnancy and childbirth. The ASLR Fatigue Task could be a clinically useful tool to determine fatigability of the lumbopelvic stabilizing muscles in women postpartum.

Age and sex related differences in shoulder abduction fatigue

BACKGROUND

Injury prevalence data commonly indicate trends of higher rates of work-related musculoskeletal disorders in older workers over their younger counterparts, and for females more than males. The purpose of this study was to investigate age and sex-related differences in manifestations of shoulder muscle fatigue in a cohort of young and older working age males and females, in a single experiment design allowing for direct comparison of the fatigue effects between the target groups.

METHODS

We report upper trapezius muscle fibre Conduction Velocity (CV) as an indicative measure of muscle fatigability, and isometric endurance time, at three levels of shoulder abduction lifting force set relative to participants' maximal strength.

RESULTS

Upper trapezius conduction velocity was significantly different between the young and old groups (p = 0.002) as well as between males and females (p = 0.016). Shoulder abduction endurance time was affected by age (P = 0.024) but not sex (p = 0.170).

CONCLUSIONS

The study identified age-related improvement in muscle fatigue resistance and increased resistance for females over males, contrary to injury prevalence trends. The muscle fatigue effects are most likely explained by muscle fibre type composition. Experimental fatigue treatments of the upper trapezius were tested at exposures relative to the participants' strength. Absolute strength is higher when young and is generally higher for males. The findings of this study point towards age and sex-related differences in strength rather than in muscle fatigue resistance as a primary cause for the differences in the injury trends.

Impaired Trunk Flexor Strength, Fatigability, and Steadiness in Postpartum Women

PURPOSE

To determine whether postpartum women (vaginal and cesarean delivery) have deficits in trunk flexor strength, fatigability and steadiness, compared with nulligravid women, up to 26 wk postpartum. We hypothesized that postpartum women would be weaker, more fatigable, and have greater torque fluctuations than controls, with cesarean delivery showing greater deficits than vaginal delivery.

METHODS

Twenty-two control women (nulligravid) and 29 postpartum women (20-40 yr, 19 who delivered via vaginal birth, 13 via Caesarian section) participated. Postpartum women were tested 8 to 10 wk and 24 to 26 wk postpartum. Control women were tested 16 to 18 wk apart. Maximal voluntary isometric contractions (MVC) were performed at multiple trunk positions with the trunk flexor muscles. To determine trunk flexor fatigability, subjects performed intermittent isometric contractions at 50% MVC (6-s contraction, 4-s rest) in upright sitting until task failure. An MVC was performed during the fatiguing task (one per minute) and at 10 and 20 min of recovery.

RESULTS

At 8 and 26 wk, postpartum women (groups pooled) were weaker at all trunk angles (38% and 44% respectively, P < 0.05) than controls despite no differences in handgrip strength. Postpartum women were more fatigable (71% and 52% respectively) and had greater torque fluctuations than controls (P < 0.05). At 8 wk postpartum, women who had a cesarean delivery, were 59% more fatigable (P = 0.004) than the vaginal delivery group, with no difference between delivery types at 26 wk postpartum.

CONCLUSIONS

Musculoskeletal recovery, including trunk flexor muscle strength and fatigability, is incomplete at 26 wk postpartum. These findings provide a rationale for future studies to address outcomes of rehabilitation programs specifically targeted at improving strength and fatigability of the trunk flexor muscles after pregnancy and childbirth.

Performance Fatigability: Mechanisms and Task Specificity

Performance fatigability is characterized as an acute decline in motor performance caused by an exercise-induced reduction in force or power of the involved muscles. Multiple mechanisms contribute to performance fatigability and originate from neural and muscular processes, with the task demands dictating the mechanisms. This review highlights that (1) inadequate activation of the motoneuron pool can contribute to performance fatigability, and (2) the demands of the task and the physiological characteristics of the population assessed, dictate fatigability and the involved mechanisms. Examples of task and population differences in fatigability highlighted in this review include contraction intensity and velocity, stability and support provided to the fatiguing limb, sex differences, and aging. A future challenge is to define specific mechanisms of fatigability and to translate these findings to real-world performance and exercise training in healthy and clinical populations across the life span.