Ergonomic evaluation of the effects of forearm conditions and body postures on trapezius muscle activity during smartphone texting

Neck-shoulder and upper back musculoskeletal symptoms in smartphone users have gained increasing attention. We evaluated trapezius muscle activity and fatigue using an objective method (surface electromyography, sEMG), and discomfort using a subjective method (questionnaire), in smartphone users during the performance of different text-entry tasks. Fifteen participants were recruited to perform six text-entry tasks under different forearm conditions (floating and supported) and body postures (sitting, standing, lying, and walking). We collected upper trapezius (UT) and lower trapezius (LT) sEMG data, and recorded muscle discomfort scores after each task. We found that static postures (especially sitting) during smartphone use predisposed to lower muscle activity with higher fatigue level and discomfort scores than dynamic posture; there was a significant main effect of body posture on average sEMG amplitude (aEMG) and discomfort scores of both muscles (all p < 0.05). Moreover, using a smartphone with the forearm supported can reduce muscle activity, fatigue level, and discomfort scores; there was a significant main effect of the forearm condition on aEMG and discomfort scores of both muscles (all p < 0.05). Our study indicates that smartphone use with the forearm floating for a long time in a static posture should be avoided (especially while sitting).     

Accuracy and validity of observational estimates of shoulder and elbow posture

This study investigated the accuracy of video-based observational posture analysis for the elbow and shoulder. Posture analyses were conducted by 28 ergonomists for four jobs presented on a traditional VHS format video recording. Estimates of posture from the observational-based methods were compared with values measured directly with an optical motion capture system. Ergonomists used categorical posture scales and a continuous visual analog scale to estimate the peak and most frequently occurring or average posture for each job. Use of a three-category scale resulted in misclassifications of peak and most frequently occurring elbow and shoulder posture with a probability averaging 30.1%. With the six-category posture scale this average probability of misclassification increased to 64.9%. Using a continuous visual analog scale peak shoulder elevation was the only posture for which the average error among ergonomists' estimates was significantly different from zero (p<0.05). Correlations between the estimated postures and measured postures were higher and statistically significant (p<0.05) for elbow flexion and shoulder elevation (r2 between 0.45 and 0.66) but were considerably lower and not significant (r2 between 0.03 and 0.18) for the peak and average horizontal shoulder abduction. Ergonomists' estimates of the temporal distribution of shoulder posture, indicating the duration severity of the posture, appeared to be biased such that the percentage of the cycle time in each posture category was estimated as more uniformly distributed than the measured values indicated.     

Linking head and neck posture with muscular activity and perceived discomfort during prolonged smartphone texting

Prolonged use of the smartphone for texting was studied on a sample of 17 healthy young adult participants (11 females), who were asked to type interactively on a handheld device while standing and sitting. Upper body kinematics ad upper trapezius surface EMG were recorded to capture parameters of joint kinematics and muscular activity, which were then compared with self-reported indicators of discomfort through the Borg's CR 10 scale questionnaire. The results indicated that, while showing a similar postural strategy in both conditions that is assumed to be controlled by the visual system to maintain a constant viewing distance from the device, the participants adopted a “stiffer” posture of the head and neck during standing than when sitting, with a direct correlation between upper trapezius muscle activity, neck angle and perceived discomfort. The evidence obtained in this study highlights the importance of monitoring muscle activity and head and neck kinematics, to assess the biomechanical risk factors of neuromuscular disorders associated with smartphone overuse.     

Modelling smartphone addiction: The role of smartphone usage,self-regulation,general self-efficacy and cyberloafing in university students

The present study investigates the roles of smartphone usage, self-regulation, general self-efficacy and cyberloafing in smartphone addiction. We conducted an online survey which received responses from 598 participants attending a public university in Ankara, Turkey. The results showed that both the duration of smartphone usage and cyberloafing positively affected smartphone addiction. The effect of self-regulation on smartphone addiction was negative and significant. In addition, neither self-regulation nor general self-efficacy had an effect on cyberloafing. Research results are discussed within the context of the effect of smartphone addiction on learning environments and individuals.     

Naturalistic data collection of head posture during smartphone use

Association between smartphone use and head-down tilt posture has not yet been quantitatively evaluated in natural settings. This study aimed to objectively assess the angle and duration of head-down tilt posture of smartphone users during a typical working day via naturalistic data collection. Thirty-one college students conducted their typical school activities while their head posture and smartphone-app usage records were collected simultaneously for 8?hours. Participants spent 125.9?minutes (median usage duration) on their smartphones with significantly larger head-down tilt (p?<?.05) than when they were not using the phone. Head tilt angle greater than 30° was found to be more common when using the phone, while head tilt less than 20° was more common when they were not using the phone. Study findings provide empirical evidence that supports an association between the duration of smartphone use and the intensity of head-down tilt posture.

Practitioner Summary: Head postures of young smartphone users were quantified for 8?hours continuously during a typical workday using a wearable sensor. Participants spent more time in larger head-down tilt postures (greater than 30°) when they were using their smartphones as compared to when they were not using them.     

Effects of changes in sitting work posture on static neck and shoulder muscle activity

Abstract

In order to analyse the effect of changing the sitting posture on the level of neck and shoulder muscular activity, an electromyographic (EMG) study of ten healthy experienced female workers from an electronics plant was undertaken. A standardized, simulated task was performed in eight different sitting work postures. Using surface electrodes, the level of muscular activity was recorded as normalized, full-wave rectified low-pass filtered EMG. The results showed that the whole spine flexed sitting posture gave higher levels of static activity in several neck and shoulder muscles than the posture with a straight and vertical spine, which in turn gave higher levels than the posture with slightly backward-inclined thoraco-lumbar spine.     

Neck loads and posture exposure of helicopter pilots during simulated day and night flights

This study investigated neck loads and neck postures experienced by Canadian Forces (CF) helicopter pilots during routine simulator day and night flights. During the night flights pilots’ helmets were equipped with night vision goggles (NVG) which have been implicated in the increase of neck pain amongst pilots. Pilot’s postures were determined from video recordings of their flight missions to: (1) identify the neck postures assumed during day and night flight missions, and (2) determine the amount of biomechanical cervical spine loading experienced during these flights. Neck postures and peak and cumulative cervical kinetic loading were significant different (P < 0.05) between day and night. During night flights the percentage of work cycle spent in mildly flexed posture increased significantly from 43% during day flights to 74% during night flights. As well, cumulative reaction compression values significantly increased from 583 kN s to 694 kN s. The biomechanical results lend support to previous physiological investigations of NVG use and further justify the need for guidelines associated with NVG use.

Relevance to industry

The research highlights the biomechanical demands on the neck resulting from wearing a helmet with added weight from night vision equipment. The results have direct implication to military and commercial operators required to wear night vision equipment or added weight to the head.     

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.     

Vibrations transmitted from human hands to upper arm,shoulder, back,neck, and head

Some powered hand tools can generate significant vibration at frequencies below 25 Hz. It is not clear whether such vibration can be effectively transmitted to the upper arm, shoulder, neck, and head and cause adverse effects in these substructures. The objective of this study is to investigate the vibration transmission from the human hands to these substructures. Eight human subjects participated in the experiment, which was conducted on a 1-D vibration test system. Unlike many vibration transmission studies, both the right and left hand-arm systems were simultaneously exposed to the vibration to simulate a working posture in the experiment. A laser vibrometer and three accelerometers were used to measure the vibration transmitted to the substructures. The apparent mass at the palm of each hand was also measured to help in understanding the transmitted vibration and biodynamic response. This study found that the upper arm resonance frequency was 7–12 Hz, the shoulder resonance was 7–9 Hz, and the back and neck resonances were 6–7 Hz. The responses were affected by the hand-arm posture, applied hand force, and vibration magnitude. The transmissibility measured on the upper arm had a trend similar to that of the apparent mass measured at the palm in their major resonant frequency ranges. The implications of the results are discussed.Relevance to industryMusculoskeletal disorders (MSDs) of the shoulder and neck are important issues among many workers. Many of these workers use heavy-duty powered hand tools. The combined mechanical loads and vibration exposures are among the major factors contributing to the development of MSDs. The vibration characteristics of the body segments examined in this study can be used to help understand MSDs and to help develop more effective intervention methods.     

The effects of posture on forearm muscle loading during gripping

The purpose of this study was to quantify the response of the forearm musculature to combinations of wrist and forearm posture and grip force. Ten healthy individuals performed five relative handgrip efforts (5%, 50%, 70% and 100% of maximum, and 50 N) for combinations of three wrist postures (flexed, neutral and extended) and three forearm postures (pronated, neutral and supinated). ‘Baseline’ extensor muscle activity (associated with holding the dynamometer without exerting grip force) was greatest with the forearm pronated and the wrist extended, while flexor activity was largest in supination when the wrist was flexed. Extensor activity was generally larger than that of flexors during low to mid-range target force levels, and was always greater when the forearm was pronated. Flexor activation only exceeded the extensor activation at the 70% and 100% target force levels in some postures. A flexed wrist reduced maximum grip force by 40-50%, but EMG amplitude remained elevated. Women produced 60-65% of the grip strength of men, and required 5-10% more of both relative force and extensor activation to produce a 50 N grip. However, this appeared to be due to strength rather than gender. Forearm rotation affected grip force generation only when the wrist was flexed, with force decreasing from supination to pronation (p<0.005). The levels of extensor activation observed, especially during baseline and low level grip exertions, suggest a possible contributing mechanism to the development of lateral forearm muscle pain in the workplace.     

The effects of posture on forearm muscle loading during gripping

The purpose of this study was to quantify the response of the forearm musculature to combinations of wrist and forearm posture and grip force. Ten healthy individuals performed five relative handgrip efforts (5%, 50%, 70% and 100% of maximum, and 50 N) for combinations of three wrist postures (flexed, neutral and extended) and three forearm postures (pronated, neutral and supinated). 'Baseline' extensor muscle activity (associated with holding the dynamometer without exerting grip force) was greatest with the forearm pronated and the wrist extended, while flexor activity was largest in supination when the wrist was flexed. Extensor activity was generally larger than that of flexors during low to mid-range target force levels, and was always greater when the forearm was pronated. Flexor activation only exceeded the extensor activation at the 70% and 100% target force levels in some postures. A flexed wrist reduced maximum grip force by 40-50%, but EMG amplitude remained elevated. Women produced 60-65% of the grip strength of men, and required 5-10% more of both relative force and extensor activation to produce a 50 N grip. However, this appeared to be due to strength rather than gender. Forearm rotation affected grip force generation only when the wrist was flexed, with force decreasing from supination to pronation (p < 0.005). The levels of extensor activation observed, especially during baseline and low level grip exertions, suggest a possible contributing mechanism to the development of lateral forearm muscle pain in the workplace.     

Long-duration assessment of upper arm posture and motion and their association with perceived symptoms among bakery workers

Awkward shoulder postures have been suggested to be a cause of shoulder discomfort among bakery workers. This study aimed at long-duration assessment of upper arms posture and motion and their association with perceived symptoms among bakery workers. Among workers in three types of bread systems, fifty-seven bakers were randomly selected. The self-reported rates for the perceived severity and frequency of shoulder discomfort were collected through questionnaires. Working postures and movements of the shoulders during work were continuously recorded with inclinometry measurements for three hours. Percentage of time spent with the upper arm right elevated more than 60° was significantly correlated with the perceived discomfort rate in the right shoulder for all bakery workers (r = 0.48 to 0.63, p ≤ 0.05). A significant correlation was found between angular velocity with the perceived discomfort intensity for right upper arm.(r = 0.60 to 0.87, ≤0.005) of all workers in the three baking systems. Physical exposure in bakers was characterized by significantly more awkward postures and the percentage of time spent with the upper arms elevated more than 60°. The current findings can facilitate more informed decisions with respect to both engineering (e.g.ergonomic workstation and tool design) and administration (e.g. work organization) control strategies.     

Further studies of shoulder and neck pain and exposures in customer service work with low biomechanical demands

The aim of the study was to establish insight into work exposures that cause shoulder and neck pain among occupational groups that have low biomechanical exposure and experience work stress from client/customer contact, among other exposures. Four occupational groups were studied, in health care (n = 20), retail (n = 22), banking (n = 26), and university secretaries (n = 26), a total of 94 volunteers. Thirty-nine were classified as pain-afflicted in the shoulder and neck, while 55 were pain-free. The subjects' perceptions of biomechanical and psychosocial exposures were established by use of quantitative questionnaires and by explorative interviews with open-ended questions, covering the same themes. Heart rate and trapezius EMG were recorded over a full workday and the following leisure period. Trapezius median and static activity during work were 3.3% and 0.3% EMG_max, only marginally higher than trapezius activity in the leisure period (2.7% and 0.2% EMG_max). The quantitative questionnaire did not identify any variable that correlated with shoulder and neck pain except perceived general tension. The interviews established that the interaction with clients or customers was an important source of work stress. Such stress appeared to be a complex entity not easily characterized by established psychosocial questionnaires. The physiological variables were at most weakly elevated in periods with high stress as compared to periods with low stress. The authors caution against relying on standardized quantitative questionnaires and/or physiological recordings to characterize work stress in occupations with emotional stress through client/customer service work.     

Evaluation of the loading of neck and shoulder musculature in overhead pushing and pulling exertions

Despite substantial epidemiological evidence relating overhead exertions with work‐related musculoskeletal disorders (WMSD) of the neck, effects of such exertions on the loading of neck or cervical spine musculature are not well understood. In this study, the effects of overhead pushing and pulling exertions on the loading of the cervical spine were evaluated using electromyography (EMG) and subjective discomfort ratings. Additionally, the role of gender as well as individual strength on the loading of neck musculature during such exertions was evaluated. Twenty‐four healthy individuals (12 men and 12 women) participated in this study. Each participant performed overhead pushing and pulling exertions, exerting 25%, 50%, and 75% of their respective maximum strengths. Overhead pushing exertions were found to be significantly more strenuous to the neck musculature than were the pulling exertions. Gender had no significant effect on the activities of the neck muscles. Participants with high strength, however, were able to exert more force at comparatively low muscle activation levels. Subjective discomfort ratings were strongly correlated with the EMG data. At various workplaces, avoiding overhead exertions is rather impossible due to material, interface, and site constraints. Based on the results of this study, however, during such exertions, an interchange between directions of force application could prevent sustained loading of the neck muscles, fatigue, and consequently the probability of neck WMSD incidents. © 2011 Wiley Periodicals, Inc.     

Biomechanical loading of the shoulder complex and lumbosacral joints during dynamic cart pushing task

The primary objective of this study was to quantify the effect of dynamic cart pushing exertions on the biomechanical loading of shoulder and low back. Ten participants performed cart pushing tasks on flat (0°), 5°, and 10° ramped walkways at 20 kg, 30 kg, and 40 kg weight conditions. An optoelectronic motion capturing system configured with two force plates was used for the kinematic and ground reaction force data collection. The experimental data was modeled using AnyBody modeling system to compute three-dimensional peak reaction forces at the shoulder complex (sternoclavicular, acromioclavicular, and glenohumeral) and low back (lumbosacral) joints. The main effect of walkway gradient and cart weight, and gradient by weight interaction on the biomechanical loading of shoulder complex and low back joints was statistically significant (all p < 0.001). At the lumbosacral joint, negligible loading in the mediolateral direction was observed compared to the anterioposterior and compression directions. Among the shoulder complex joints, the peak reaction forces at the acromioclavicular and glenohumeral joints were comparable and much higher than the sternoclavicular joint. Increased shear loading of the lumbosacral joint, distraction loading of glenohumeral joint and inferosuperior loading of the acromioclavicular joint may contribute to the risk of work-related low back and shoulder musculoskeletal disorder with prolonged and repetitive use of carts.     

Variation in upper limb posture and movement during word processing with and without mouse use

Work postures and movements of the upper limb were analysed for 12 ‘mouse’ operators and 12 ‘non-mouse’ computer operators employed in word-processing work. Measurements were carried out during correction of a given text. ‘Mouse’ operators spent 64% of the working time with the operative wrist deviating more than 15° towards the ulnar side, while ‘non-mouse’ operators spent 96% of the time with the corresponding wrist in neutral position towards radial deviation. The rotation in the shoulder was at all times in neutral position towards inward rotation for ‘non-mouse’ operators, while ‘mouse’ operators worked 81% of the time with the shoulder rotated outward more than 30°. ‘Mouse’ operators corrected a longer text during the given time. Our observations showed long periods of strenuous working postures for ‘mouse’ operators compared to ‘non-mouse’ operators. We believe that further investigations need to be carried out on the effects of word-processing techniques and to develop ergonomic work station designs for the ‘mouse’ and other non-keyboard input devices.     

Evaluation of document location during computer use in terms of neck muscle activity and neck movement

This study evaluated the impact on neck movement and muscle activity of placing documents in three commonly used locations: in-line, flat desktop left of the keyboard and laterally placed level with the computer screen. Neck excursion during three standard head movements between the computer monitor and each document location and neck extensor and upper trapezius muscle activity during a 5 min typing task for each of the document locations was measured in 20 healthy participants. Results indicated that muscle activity and neck flexion were least when documents were placed laterally suggesting it may be the optimal location. The desktop option produced both the greatest neck movement and muscle activity in all muscle groups. The in-line document location required significantly more neck flexion but less lateral flexion and rotation than the laterally placed document. Evaluation of other holders is needed to guide decision making for this commonly used office equipment.     

Predictors of perceived effort in the shoulder during load transfer tasks

The mechanism of muscular effort perception in the shoulder was examined in this experiment. Two shoulder biomechanical models and experimental muscle activity data were used to assess physical exposure for a series of reaching tasks. Effort perception was quantitatively correlated to these measures of physical loading, both at the resultant torque (r² = 0.50) and muscle activity model-based muscle force predictions (MFPs): r² = 0.42, electromyography (EMG): r² = 0.26) levels. Muscle data did not explain variation in effort perception more fully than torque data. The inclusion of subject and task variables improved the ability of each model to explain variability in effort perception (torque: r² = 0.74; MFP: r² = 0.67, EMG: r² = 0.64). These results suggest that effort perception may not be fully explained by only an image of the motor command, but is rather a complex integrative quantity that is affected by other factors, such as posture and task goals, which may be dependent on sensory feedback.     

The effects of multifocal refractive lenses on occipital extension and forward head posture during a visual task

The purpose of this study was to determine if a change in forward head posture and occipital extension occurred in participants who wore multifocal lenses vs. those persons with non-multifocal lenses while performing an 8-min visual reading task on a visual display unit (VDU). Forty-two healthy human participants were recruited for this study. Thirty-three participants completed the study. Fourteen participants wore multifocal lenses and 19 wore frames with non-multifocal lenses. To evaluate the degree of change of forward head posture and occipital extension digital photographs of cervical posture were taken at four different time intervals: prior to performing the reading task and at 3, 5 and 8 min during the reading task. The digital photographs were analysed utilizing a computer program. Two one-way ANOVA were utilized to determine the degree of change of forward head posture and occipital extension between groups. A significant difference was identified between groups for changes in degrees of forward head posture while performing a visual reading task on a VDU. However, no significant difference between groups was found for occipital extension while performing the same task. Multifocal wearers exhibit greater degrees of change in forward head posture and occipital extension than non-multifocal wearers. These postural changes may place them at a greater risk for musculoskeletal disorders and headaches.     

Impact of pulling direction and magnitude of force exertion on the activation of shoulder muscles

Shoulder musculoskeletal disorders (MSD) are frequently associated with the work activities that demand forceful arm exertions in pushing and pulling directions. Considering the ability of shoulder joint to exert forces in nearly any direction, our understanding of the shoulder muscles activation as affected by pushing and pulling exertions is limited. In this study the activation of seven shoulder muscles were studied for 10 male participants during pulling exertions performed in five directions (pull right, pull left, pull back, pull down and pull up) using three force levels (22.24 N, 33.36 N and 44.48 N). Exertions performed in pulling right and pulling up directions produced higher activation and received higher perceived exertion ratings than the exertions performed in the other directions. Rotator cuff and middle deltoid muscles activation were consistently higher during pulling up and pulling right exertions compared to the other muscles. A high correlation was found between the activation of rotator cuff and deltoid muscles and the perceived exertion ratings. The rotator cuff and middle deltoid muscles activation observed during the pulling up and pulling right exertions can be explained by the concavity compression mechanism which stabilizes the glenohumeral joint of shoulder.Relevance to industryThe muscle activation data expressed in terms of Maximum Voluntary Contraction (MVC) and perceived exertion ratings are widely used by the ergonomic practitioners to design and/or evaluate workplace exertions. This study provides such data for several shoulder muscles during pulling exertions performed under different conditions.