The influence of age on isometric endurance and fatigue is muscle dependent: a study of shoulder abduction and torso extension

The present study examined differences in isometric muscle capacity between older (55–65 years) and younger (18 – 25 years) individuals. A total of 24 younger and 24 older participants (gender balanced within each group) performed sustained shoulder abductions and torso extensions to exhaustion at 30%, 50% and 70% of individual maximal voluntary contraction (MVC). Along with endurance time, manifestations of localized fatigue were determined based on changes in surface electromyographic signals obtained from the shoulder (middle deltoid) and the torso (multifidus and longissimus thoracis) muscles. Strength recovery was monitored using post-fatigue MVCs over a 15-min period. Compared to the younger group, older individuals exhibited lower muscular strength, longer endurance time and slower development of local fatigue. Age effects on fatigue were typically moderated by effort level, while effects of gender appeared to be marginal. Non-linear relationships between target joint torque and endurance time were observed, with effects of age differing between shoulder abduction and torso extension. Overall, the effects of age on endurance and fatigue were more substantial and more consistent for the shoulder muscle than for the torso muscles and were likely related to differences in muscle fibre type composition. For strength recovery rates, no significant age or gender effects were found in either experiment. In summary, this study suggests that differences in isometric work capacity do exist between older and younger individuals, but that this effect is influenced by effort level and the muscle tested.     

Muscle fatigue during intermittent isokinetic shoulder abduction: age effects and utility of electromyographic measures

Most existing evidence regarding the effects of age on muscular fatigue has focused on prolonged isometric contractions, repeated maximum dynamic contractions and individuals beyond traditional retirement age (>65 years). In the present study, age-related differences in muscle fatigue during submaximal dynamic efforts were examined. There were 24 younger (18-25 years) and 24 older (55-65 years) participants, all of whom were healthy and active, with equal numbers of each gender within each age group. Participants performed repetitive, intermittent shoulder abductions until exhaustion, at peak moments of 30% and 40% of individual maximum voluntary isokinetic contraction (MVIC) and with cycle durations of 20 and 40 s. Fatigue development was determined based on changes in MVIC, electromyographic (EMG) signals and ratings of perceived discomfort (RPD). Following the exhaustive exercises, strength recovery was monitored using a series of MVICs over a 15-min period. Results indicated the existence of an age-related fatigue resistance, with the older group demonstrating significantly slower rates of MVIC decline and RPD increase and smaller modifications in EMG-based fatigue measures. These age effects were generally more pronounced at the higher effort level. Main effects of effort level and cycle duration were also significant, while gender effects appeared to be marginal. Rates of strength recovery were not significantly influenced by age. In addition, the utility of standard EMG-based fatigue measures was assessed. Findings indicated that time-dependent changes in static and dynamic EMG-based measures were roughly comparable in terms of sensitivity and variability, supporting the use of standard EMG analyses for fatigue monitoring during intermittent dynamic contractions.     

An exponential model of isometric muscular fatigue as a function of age and muscle groups

Despite the recommendations that an important design criterion is not to exceed 15% of an operator's maximum voluntary contraction (MVC) with any muscle that must be used for a long period of time, many tasks involve the exertion of much higher forces coupled with long-term contractions. Many studies have investigated the force-time relationship of isometric muscle contractions to determine the endurance time of a given relative force. To date, however, direct studies of muscle performance throughout fatiguing tasks have not been conducted to the same degree. This research was concerned with studying the effects of different muscle groups (biceps vs quadriceps) of subjects with different age groups (20·29 vs 50·59 years of age) on long-term muscular isometric contractions at different levels of %MVC (20%, 40%, 60%, 80%, and 100% MVC), and modelling the functional data to describe the time course of strength decrement. The data revealed that the time course of strength decrement was best modelled by the function:

An experiment, using 20 subjects with each subject performing 10 conditions (two muscle groups × five levels of %MVC), showed that this function accounted for over 95% of the variance of strength decrement. AH parameter estimates were statistically significant.     

An exponential model of isometric muscular fatigue as a function of age and muscle groups.

Despite the recommendations that an important design criterion is not to exceed 15% of an operator's maximum voluntary contraction (MVC) with any muscle that must be used for a long period of time, many tasks involve the exertion of much higher forces coupled with long-term contractions. Many studies have investigated the force-time relationship of isometric muscle contractions to determine the endurance time of a given relative force. To date, however, direct studies of muscle performance throughout fatiguing tasks have not been conducted to the same degree. This research was concerned with studying the effects of different muscle groups (biceps vs quadriceps) of subjects with different age groups (20-29 vs 50-59 years of age) on long-term muscular isometric contractions at different levels of %MVC (20%, 40%, 60%, 80%, and 100% MVC), and modelling the functional data to describe the time course of strength decrement. The data revealed that the time course of strength decrement was best modelled by the function: [formula: see text] An experiment, using 20 subjects with each subject performing 10 conditions (two muscle groups x five levels of %MVC), showed that this function accounted for over 95% of the variance of strength decrement. All parameter estimates were statistically significant.     

Muscle fatigue during intermittent isokinetic shoulder abduction: Age effects and utility of electromyographic measures

Most existing evidence regarding the effects of age on muscular fatigue has focused on prolonged isometric contractions, repeated maximum dynamic contractions and individuals beyond traditional retirement age (>65 years). In the present study, age-related differences in muscle fatigue during submaximal dynamic efforts were examined. There were 24 younger (18–25 years) and 24 older (55–65 years) participants, all of whom were healthy and active, with equal numbers of each gender within each age group. Participants performed repetitive, intermittent shoulder abductions until exhaustion, at peak moments of 30% and 40% of individual maximum voluntary isokinetic contraction (MVIC) and with cycle durations of 20 and 40 s. Fatigue development was determined based on changes in MVIC, electromyographic (EMG) signals and ratings of perceived discomfort (RPD). Following the exhaustive exercises, strength recovery was monitored using a series of MVICs over a 15-min period. Results indicated the existence of an age-related fatigue resistance, with the older group demonstrating significantly slower rates of MVIC decline and RPD increase and smaller modifications in EMG-based fatigue measures. These age effects were generally more pronounced at the higher effort level. Main effects of effort level and cycle duration were also significant, while gender effects appeared to be marginal. Rates of strength recovery were not significantly influenced by age. In addition, the utility of standard EMG-based fatigue measures was assessed. Findings indicated that time-dependent changes in static and dynamic EMG-based measures were roughly comparable in terms of sensitivity and variability, supporting the use of standard EMG analyses for fatigue monitoring during intermittent dynamic contractions.     

Strength,endurance and fatigue response of rotator cuff muscles during isometric exertions

Among the shoulder musculoskeletal disorders (MSDs), rotator cuff disorders are prevalent and known to substantially limit an individual's strength and ability to work. Despite the frequency and cost of rotator cuff injuries, very little research has been done on understanding the load response relationship of rotator cuff muscles. Therefore, this study was aimed at evaluating the strength, endurance and fatigue response of rotator cuff muscles. As a first step, strengths of Supraspinatus, Infraspinatus, and Teres Minor were measured for ten healthy male participants using muscle specific Maximum Voluntary Contractions (MVC). Then, endurance time and surface Electromyography (EMG) data were recorded during 15%, 30%, 45%, and 60% MVC exertions. Infraspinatus exhibited the highest strength followed by Teres Minor and Supraspinatus. Despite the strength differences, small variations were observed in the endurance time between the muscles. The effect of %MVC exertions on endurance time, average muscle activity and muscle fatigue were statistically significant. Median frequency decreased and muscle activation increased with an increase in force exertion levels; however, the changes observed for an increase in the exertion level from 15% to 30% MVC were much higher than the changes observed for an increase in the exertion level from 45% to 60% MVC.Relevance to industryResults of this study indicate that the rotator cuff muscles have different strengths but exhibit very similar endurance and fatigue behavior. These findings can assist ergonomic practitioners with the design and/or evaluation of workplace upper extremity exertions to reduce/manage stress on the rotator cuff muscles and shoulder.     

Subjective and objective evaluation of shoulder muscle fatigue

In order to relate EMG signs of localized muscle fatigue to subjectively perceived muscle fatigue, a study was undertaken on 20 healthy volunteers exposing their right trapezius muscle by raising the arm to 90° of abduction. Every person performed two contractions: 0 kg hand load during 5 min and 2 kg hand load during 2.5 min. Surface EMG was recorded and analysed with respect to RMS amplitude and mean power frequency (MPF). Subjective muscle fatigue was estimated with a psychophysical rating scale (Borg's CR-10 Scale). At high load level we found a significant correlation between MPF and the CR-scores (r = ? 0.46), but at low load level there was no correlation. At high load level there was a linear decrease of MPF with increasing load dose, but at low load level the MPF did not change, despite significant subjective fatigue. There was a linear rise of the CR-score with increasing load dose, more pronounced at high load level. It was concluded, that at low load levels common in working life, MPF and subjective scores seem to provide different fatigue information. Moreover, the MPF did not seem to work as a valid estimator of muscle fatigue at this low load level. Caution is recommended if it is to be used in static low-load situations.     

The effect of shoulder posture on performance,discomfort and muscle fatigue whilst working on a visual display unit

An investigation was conducted to determine the relationship between posture and performance while working on a VDU. Twenty-one female subjects performed a 20 min choice reaction time task in a within-subject design with shoulder flexion (0° versus 30°) and noise (55 dB versus 75 dB) as factors. Discomfort measures, electromyography (EMG) measures and measures of performance including reaction time and percentage of errors were collected. The results suggested poorer performance in the 30° shoulder flexion posture compared to the 0° shoulder flexion posture (performance index F_1,20 = 3.95, p = 0.061). There was also significantly greater discomfort and fatigue, as indicated by EMG, in the 30° shoulder flexion compared to 0° shoulder flexion (discomfort: F_1,20 = 99.30; p = 0.0001 EMG: F_1,20 = 4.25, p = 0.052). The results of this study provide the most direct evidence that the posture of VDU users can affect their performance at a task. A number of possible mechanisms for posture to affect performance are explored using the data. The utility of data showing the effect of posture on performance data is discussed in terms of financial modelling to justify the cost of ergonomic intervention. Relevance to industryPoor posture whilst working at a VDU has been previously linked with the development of poor health but has not been directly linked with poor performance. This paper provides evidence to show posture can effect VDU performance. This enables organisations to use estimates of performance changes to model the costs of investment in ergonomic interventions.     

Interfering effects of the task demands of grip force and mental processing on isometric shoulder strength and muscle activity

The purpose of this study was to examine the interfering effects of physical and mental tasks on shoulder isometric strength in different postures. Fifteen volunteers (seven women, eight men) performed a series of isometric shoulder exertions at 30 degrees , 60 degrees and 90 degrees of both shoulder flexion and abduction alone and with the addition of a 30% grip force, a mental task (Stroop test) and both additional tasks simultaneously. The shoulder tasks were completed either at maximal intensity, or while maintaining a shoulder posture without any additional effort. Surface electromyography (EMG) from seven muscles of the shoulder girdle and shoulder moment were collected for each 6 s shoulder exertion. When normalized to maximum exertion, no differences were found between genders and no differences existed between conditions when subjects maintained each posture without exerted force. In the maximal shoulder exertion trials, an increase in shoulder angle (in either plane) resulted in an increase in EMG in most muscles, while shoulder moment decreased in flexion and remained constant in abduction. Shoulder moments and muscle activation were greatest in the shoulder exertion alone condition followed by adding a 30% grip and the Stroop test, with the addition of both tasks further reducing the exerted shoulder moment and EMG. However, muscle activity did not always decrease with shoulder strength and remained elevated, indicating a complex coactivation pattern produced by an interfering role of the tasks. Overall, it was found that a mental task can have the same or greater effect as a concurrent grip and should be considered when assessing muscular loading in the workplace, as typical biomechanical modelling may underestimate internal loads. The results not only provide valuable shoulder strength data but also practical strength values, depending on additional tasks.     

Interfering effects of the task demands of grip force and mental processing on isometric shoulder strength and muscle activity

The purpose of this study was to examine the interfering effects of physical and mental tasks on shoulder isometric strength in different postures. Fifteen volunteers (seven women, eight men) performed a series of isometric shoulder exertions at 30°, 60° and 90° of both shoulder flexion and abduction alone and with the addition of a 30% grip force, a mental task (Stroop test) and both additional tasks simultaneously. The shoulder tasks were completed either at maximal intensity, or while maintaining a shoulder posture without any additional effort. Surface electromyography (EMG) from seven muscles of the shoulder girdle and shoulder moment were collected for each 6 s shoulder exertion. When normalized to maximum exertion, no differences were found between genders and no differences existed between conditions when subjects maintained each posture without exerted force. In the maximal shoulder exertion trials, an increase in shoulder angle (in either plane) resulted in an increase in EMG in most muscles, while shoulder moment decreased in flexion and remained constant in abduction. Shoulder moments and muscle activation were greatest in the shoulder exertion alone condition followed by adding a 30% grip and the Stroop test, with the addition of both tasks further reducing the exerted shoulder moment and EMG. However, muscle activity did not always decrease with shoulder strength and remained elevated, indicating a complex coactivation pattern produced by an interfering role of the tasks. Overall, it was found that a mental task can have the same or greater effect as a concurrent grip and should be considered when assessing muscular loading in the workplace, as typical biomechanical modelling may underestimate internal loads. The results not only provide valuable shoulder strength data but also practical strength values, depending on additional tasks.     

Real-time indicators and influence factors of muscle fatigue in push-type work

Muscle fatigue is a significant cause of musculoskeletal injury and can easily induce unsafe behaviour. Push-type work is a common type of physical work, and if not designed appropriately, may lead to muscle fatigue. Previous studies on muscle fatigue mainly focus on investigating continuous force exertion, and in most of them, a constant muscle force is assumed, thereby ignoring the fluctuations present in exertion. In this study, bolt hole drilling was chosen to represent typical push-type work, and the muscle fatigue from this work was examined. The experimental system designed in this study monitored the muscle force in real time. In the experiments, different thrust angles (15°, 45°, and 75°), different relative force values (20% MVC, 40% MVC, and 60% MVC; MVC: maximum voluntary contraction) and different working time intervals (0 s, 30 s, 60 s, and 90 s) were considered. The results demonstrate that there is a significant positive correlation between the rating of perceived exertion (RPE) and muscle force attenuation (r = 0.786, p = 0). The cubic regression model (Y = − 0.00071x³ − 0.024x² − 0.334x + 1.146, R² = 0.639) fits the data most closely. Therefore, force attenuation can be used as a real-time indicator of muscle fatigue. In addition, the relative force value and thrust angle have a significant impact on the RPE score, whereas the working time interval has no major effect on it. This study provides a new method for evaluating muscle fatigue and a basis for the design of push-type work to reduce fatigue-induced accidents and musculoskeletal injuries.     

The specificity of endurance training on muscular power and muscle fibre size

In an effort to further examine the specificity of training concept, six female student volunteers (mean age=25.3 years) were endurance trained three times a week for a period of twelve weeks. The training consisted of thirty minutes of continuous running per workout. Identical test batteries were administered prior to (pre) and immediately following (post) the treatment period. Each battery of tests consisted of three measures of muscular leg power (vertical jump test, 18-29 m sprint, and the Margaria Step Test), and two measures of cardiovascular endurance (Cooper's 12 minute run, and V?O₂ max during treadmill running). A muscle biopsy from the lateral aspect of the gastrocnemius muscle was also included. The biopsy specimens were stained for myosin ATPase. Percentages of fast twitch (FT) and slow twitch (ST) muscle fibres as well as mean individual FT and ST fibre areas were determined. The training produced significant changes in the exercising subjects for thel2-minute run (p=0.006)and VO₂max(p=0.003). No changes were found in the tests of muscular power or individual fibre areas. These data indicate that the endurance training program was of sufficient intensity to effect an increase in cardiovascular fitness. However, these improvements in endurance were not accompanied by any significant changes in muscular leg power or individual muscle fibre size.     

The effect of a resistance-training program on muscle strength, physical workload, muscle fatigue and musculoskeletal discomfort: An experiment

The aim of the study was to investigate the effectiveness of a resistance-training program on muscle strength of the back and neck/shoulder muscles, relative physical workload, muscle fatigue and musculoskeletal discomfort during a simulated assembly and lifting task. Twenty-two workers were randomized over an 8-week resistance-training group, and a control group. Isokinetic muscle strength was assessed using the Cybex dynamometer, muscle fatigue was measured using EMG, and perceived discomfort was measured using a 10-point scale. At the follow-up, we found no effect of the resistance-training program on isokinetic muscle strength of the back and shoulder muscles. Furthermore, we did not find any effect on EMG data, nor on musculoskeletal discomfort during the simulated work tasks. However, trained workers performed the lifting tasks for a longer time before reporting considerable discomfort than those in the control group.     

Facebook use depending on age: The influence of social comparisons

Facebook use is analyzed depending on age and occurrence of social comparisons. The hypotheses state that age is negatively associated with Facebook use and that this association is mediated by social comparison orientation. Data collection was realized via the Internet. The online-questionnaire included information on Facebook use on the basis of a behavioral report (cf., McAndrew & Jeong, 2012), an inventory for measuring social comparison orientation by Gibbons and Buunk (1999), and demographic questions. Results are based on 335 participants. They confirm both the negative association between age and frequency of Facebook activities and the mediation of this association by comparison orientation. These results are interpreted on the basis of evolutionary theory. This study offers new insights on the mediating role of social comparisons in the relationship between age and Facebook use. In the discussion suggestions for further routes of research on the link between age, comparison orientation, and Facebook use are outlined.     

The effect of phone design on upper extremity discomfort and muscle fatigue

OBJECTIVE: To compare a small cellular clamshell phone with a traditional office phone in the development of discomfort and muscle fatigue over time during phone use. BACKGROUND: Phone use involves low-level static exertions that may be influenced by phone design. Phone design and its interactions with anthropometry may change shoulder and hand postures assumed during use, which in turn may modify the length-strength relationship and moment arms of the muscles. METHOD: Ten adults participated in a study that simulated phone use using a small clamshell and a traditional office phone. Discomfort information and electromyographic (EMG) muscle activity were monitored on four upper extremity muscles. Discomfort and fatigue data (EMG median frequency shifts) were analyzed to assess differences between phones as well as differing effects attributable to anthropometry. RESULTS: Median frequency shifts supported discomfort claims and indicated muscle fatigue in the deltoid and thenar muscles. Biomechanical measures demonstrated that participants with short limb lengths developed more severe signs of thenar fatigue. Participants with longer arms developed greater discomfort in the neck, shoulder, and back. The deltoid confirmed this occurrence, showing signs of muscle fatigue. CONCLUSION: Phone design and anthropometry influenced the development of discomfort and fatigue during phone use. Phone design dictated grip style, resulting in differing discomfort and fatigue levels. Anthropometry influenced the severity of the discomfort and fatigue present in the shoulder and hand. APPLICATION: Use of small clamshell phones may contribute to a lack of rest and recovery from typical workday exposures. It should be explored from an ergonomic perspective.     

Development and validation of a cumulative exposure shoulder risk assessment tool based on fatigue failure theory

Abstract

Objective

To present a new risk assessment tool for shoulder intensive occupational tasks based on fatigue failure theory.     

The influence of mental load on muscle tension

We examined the differences in muscle tension and in physiological measures depending on the type of mental task. Fifteen participants performed tests for sustained attention, vigilance and maintaining posture only. We analysed electromyogram (EMG) measures of extensor digitorum (ED), flexor carpi ulnaris (FU), deltoideus (DE) and trapezius (TR), and heart rate (HR) and respiratory frequency (RF). Measures indicated higher values for mental tasks than for maintained posture only with significant differences in all measures. The following relationships were also significant: between DE and physiological measures (HR and RF), between ED and the amplitude of EMG of the other three muscles, between FU and TR and between HR and RF. The results of this study showed that the relationship between mental demands and muscle tension was mostly reflected by tension in the arm and shoulder girdle muscles and, to a lesser degree, in forearm muscles.

Practitioner Summary: We focused on physiological and muscle tension measures differentiating work according to the level of mental demands. Differences in sustained attention, vigilance and maintaining posture only proved that mental demands were mostly reflected by tension in arm and shoulder girdle muscles and, to a lesser degree, in forearm muscles.     

The influence of controllable task-lighting on productivity: a field study in a factory

This study examines whether or not a controllable task-lighting system that allows people to select high lighting levels will enhance productivity under real working conditions. For a period of 16 months a study was carried out in a luminaire factory in Finland in which such a task-lighting system was installed above 10 individual workstations. The illuminances selected by the users were recorded and productivity was monitored. Enhancing productivity can be relevant in industrial processes. The increase of productivity for the test group was +4.5% compared to a reference group, and statistically significant. The mechanism for this increase can be improved visual performance, biological effects of light, or psychological effects. Different dimming speeds were used to see whether the subjects' choices were based on illuminance or on the response of the control system. Decreasing the dimming speed of the system decreased the illuminance chosen by 13%. However, at slower dimming speeds the subjects took 55% longer to reach a given level, which suggests that they were aiming to set the lighting to their preferred level and not just pushing the button for a certain time.