Analysis of muscle fatigue in helicopter pilots

How Does the Use of an Intraoral Scanner Affect Muscle Fatigue? A Preliminary In Vivo Study

The purpose of this study was to evaluate muscle activation and fatigue in the operator during tooth preparation and intraoral scanning by simulating these tasks in two types of dental unit chair systems (UCS). Six participants were recruited, and the above tasks were simulated. Electrodes were placed on the skin over five types of muscles (arm, neck, and shoulder muscles), and the maximal voluntary contraction (MVC) was measured. Electromyography (EMG) was assessed during the simulation, and EMG values were normalized using MVC. The root mean square (RMS) EMG (%MVC) and muscle fatigue (%) were calculated. Owing to a lack of normal distribution of the data, Mann−Whitney U test and Kruskal−Wallis H test were performed for statistical comparison, and Bonferroni adjustment was performed for multiple comparisons (α = 0.05). There was no significant difference in RMS EMG between the two types of dental UCS (intraoral scanning, p = 0.237; tooth preparation, p = 0.543). Moreover, the RMS EMG and muscle fatigue were not significantly different between the two tasks (p > 0.05). There was significant muscle fatigue after the intraoral scanner use was simulated thrice (p < 0.001). It is necessary to refrain from performing continuous intraoral scanning and tooth preparation and to take appropriate rest to reduce the incidence of musculoskeletal disorders in dentists in clinical settings.

Mechanisms of Cervical Spine Disc Injury under Cyclic Loading

STUDY DESIGN

Determination of human cervical spine disc response under cyclic loading.

PURPOSE

To explain the potential mechanisms of intervertebral disc injury caused by cyclic loading.

OVERVIEW OF LITERATURE

Certain occupational environments in civilian and military populations may affect the cervical spine of individuals by cyclic loading. Research on this mechanism is scarce.

METHODS

Here, we developed a finite element model of the human C4-C5 disc. It comprised endplates, five layers of fibers, a nucleus, and an annulus ground substance. The endplates, ground substance, and annular fibers were modeled with elastic, hyperviscoelastic, and hyper-elastic materials, respectively. We subjected the disc to compressive loading (150 N) for 10,000 cycles at frequencies of 2 Hz (low) and 4 Hz (high). We measured disc displacements over the entire loading period. We obtained maximum and minimum principal stress and strain and von Mises stress distributions at both frequencies for all components. Further, we used contours to infer potential mechanisms of internal load transfer within the disc components.

RESULTS

The points of the model disc displacement versus the loading cycles were within the experimental corridors for both frequencies. The principal stresses were higher in the ground matrix, maximum stress was higher in the anterior and posterior annular regions, and minimum stress was higher along the superior and inferior peripheries. The maximum principal strains were radially directed, whereas the minimum principal strains were axially/obliquely directed. The stresses in the fibers were greater and concentrated in the posterolateral regions in the innermost layer.

CONCLUSIONS

Disc displacement was lower at high frequency, thus exhibiting strain rate stiffening and explaining stress accumulation at superior and interior peripheries. Greater stresses and strains at the boundaries explain disc injuries, such as delamination. The greater development of stresses in the innermost annular fiber layer (migrating toward the posterolateral regions) explains disc prolapse.

Muscle-specific endurance of the trapezius muscles using electrical twitch mechanomyography

BACKGROUND

Symptoms of fatigue and pain are often reported for the trapezius muscle in the shoulder. The present study evaluated endurance in the trapezius muscles of healthy participants using electric twitch mechanomyography (ETM).

METHODS

Surface electrodes and a tri-axial accelerometer were placed over the left trapezius muscle. Muscles were stimulated for 3 min each at 2 Hz, 4 Hz and 6 Hz. Maintenance of acceleration during muscle twitches was used to calculate an endurance index (EI). Subjects (n = 9) were tested on two separate days to assess reproducibility of the trapezius EI measurements. The endurance measurements were made on the wrist flexor and vastus lateralis muscles for comparison. Near infrared spectroscopy was used to measure muscle oxygenation (HbO₂) during the stimulation protocol (n = 8).

RESULTS

Mean (SD) EI was 84.9% (8.7%), 63.3% (19.1%) and 41.7% (20.0%) for 2 Hz, 4 Hz and 6 Hz, respectively. The coefficients of variation were 7.4%, 11.3% and 24.0% for 2 Hz, 4 Hz and 6 Hz, respectively. EI values were significantly lower in the trapezius compared to arm and leg muscles (p < 0.05). HbO₂ values were unchanged from resting values with electrical stimulation.

CONCLUSIONS

The EI as measured by ETM may provide a reproducible method of evaluating function in trapezius muscles that is not influenced by oxygen saturation.

Long-term effects of exercise programs among helicopter pilots with flying related LBP

BACKGROUND

Flying related transient Low Back Pain (LBP) among helicopter pilots is considered an occupational distress.

OBJECTIVE

To examine if exercise programs can alleviate transient LBP.

METHODS

Sixty-five helicopter pilots (92% males), all reporting flying related LBP, responded to an epidemiological survey and a long-term follow-up, 44.8 months later, comprising questions regarding transient LBP and number of sick leaves. Data from 37 pilots participating in two exercise programs, A; general for LBP, B; focused for lumbar trunk (LT), included information from clinical examinations and muscular endurance tests of the LT before and after intervention. Twenty-eight pilots did not participate in any intervention.

RESULTS

At long-term follow-up 42% of the pilots still reported flying related transient LBP. Among participants in program B 26% had persistent pain, 70% in program A and 46% among pilots without intervention. Sick-leave reduction was only observed among participants in program B (30% to 4%). Upon re-occurrence of LBP symptoms, half of the pilots in program B again performed exercises to improve their pain.

CONCLUSION

This study indicates that exercise programs focused towards lumbar trunk muscular endurance reduces flying related transient LBP and sick-leave among helicopter pilots. These findings may have implications for the pilots' working conditions.

Vehicle Exposure and Spinal Musculature Fatigue in Military Warfighters: A Meta-Analysis

CONTEXT

 Spinal musculature fatigue from vehicle exposure may place warfighters at risk for spinal injuries and pain. Research on the relationship between vehicle exposure and spinal musculature fatigue is conflicting. A better understanding of the effect of military duty on musculoskeletal function is needed before sports medicine teams can develop injury-prevention programs.

OBJECTIVE

 To determine if the literature supports a definite effect of vehicle exposure on spinal musculature fatigue.

DATA SOURCES

 We searched the MEDLINE, Military & Government Collection (EBSCO), National Institute for Occupational Safety and Health Technical Information Center, PubMed, and Web of Science databases for articles published between January 1990 and September 2015.

STUDY SELECTION

 To be included, a study required a clear sampling method, preexposure and postexposure assessments of fatigue, a defined objective measurement of fatigue, a defined exposure time, and a study goal of exposing participants to forces related to vehicle exposure.

DATA EXTRACTION

 Sample size, mean preexposure and postexposure measures of fatigue, vehicle type, and exposure time.

DATA SYNTHESIS

 Six studies met the inclusion criteria. We used the Scottish Intercollegiate Guidelines Network algorithm to determine the appropriate tool for quality appraisal of each article. Unweighted random-effects model meta-analyses were conducted, and a natural log response ratio was used as the effect metric. The overall meta-analysis demonstrated that vehicle exposure increased fatigue of the spinal musculature (P = .03; natural log response ratio = -0.22, 95% confidence interval = -0.42, -0.02). Using the spinal region as a moderator, we observed that vehicle ride exposure significantly increased fatigue at the lumbar musculature (P = .02; natural log response ratio = -0.27, 95% confidence interval = -0.50, -0.04) but not at the cervical or thoracic region.

CONCLUSIONS

 Vehicle exposure increased fatigue at the lumbar region.

Fatigue responses of the human cervical spine intervertebral discs

Numerous studies have been conducted since more than fifty years to understand the behavior of the human lumbar spine under fatigue loading. Applications have been largely driven by low back pain and human body vibration problems. The human neck also sustains fatigue loading in certain type of civilian occupational and military operational activities, and research is very limited in this area. Being a visco-elastic structure, it is important to determine the stress-relaxation properties of the human cervical spine intervertebral discs to enable accurate simulations of these structures in stress-analysis models. While finite element models have the ability to incorporate viscoelastic material definitions, data specific to the cervical spine are limited. The present study was conducted to determine these properties and understand the responses of the human lower cervical spine discs under large number of cyclic loads in the axial compression mode. Eight disc segments consisting of the adjacent vertebral bodies along with the longitudinal ligaments were subjected to compression, followed by 10,000 cycles of loading at 2 or 4Hz frequency by limiting the axial load to approximately 150 N, and subsequent to resting period, subjected to compression to extract the stress-relaxation properties using the quasi-linear viscoelastic (QLV) material model. The coefficients of the model and disc displacements as a function of cycles and loading frequency are presented. The disc responses demonstrated a plateauing effect after the first 2000 to 4000 cycles, which were highly nonlinear. The paper compares these responses with the "work hardening" phenomenon proposed in clinical literature for the lumbar spine to explain the fatigue behavior of the discs. The quantitative results in terms of QLV coefficients can serve as inputs to complex finite element models of the cervical spine to delineate the local and internal load-sharing responses of the disc segment.

Assessment of early onset of driver fatigue using multimodal fatigue measures in a static simulator

Driver fatigue is an important contributor to road accidents. This paper reports a study that evaluated driver fatigue using multimodal fatigue measures, i.e., surface electromyography (sEMG), electroencephalography (EEG), seat interface pressure, blood pressure, heart rate and oxygen saturation level. Twenty male participants volunteered in this study by performing 60 min of driving on a static simulator. Results from sEMG showed significant physical fatigue (ρ < 0.05) in back and shoulder muscle groups. EEG showed significant (ρ < 0.05) increase of alpha and theta activities and a significant decrease of beta activity during monotonous driving. Results also showed significant change in bilateral pressure distribution on thigh and buttocks region during the study. These findings demonstrate the use of multimodal measures to assess early onset of fatigue. This will help us understand the influence of physical and mental fatigue on driver during monotonous driving.

Firefighters muscular recovery after a heavy work bout in the heat

Occasionally firefighters need to perform very heavy bouts of work, such as smoke diving or clearing an accident site, which induce significant muscle fatigue. The time span for muscular recovery from such heavy work is not known. The purpose of this study was to evaluate firefighters' force-, neural-, metabolic-, and structural-related recovery after task-specific heavy work in the heat. Fifteen healthy firefighters (14 males and 1 female) performed a 20-min heavy work bout that simulated smoke diving and the clearance of an accident site at 35 °C. After the work, muscular recovery was evaluated by wrist flexion maximal voluntary contraction (MVC), average electromyography during MVC and during 10%MVC, rate of force production, motor response and stretch reflex responses, muscle oxygen consumption and oxygenation level, and wrist flexor muscle pennation angle. Recovery was followed for 4 h. Each of the 12 measured parameters changed significantly (p < 0.05) from those at baseline during the follow-up. Muscle oxygen consumption and the wrist flexor pennation angle remained elevated throughout the follow-up (oxygen consumption baseline, 12.9 ± 1.7 mL O2·min(-1)·(100 g)(-1); 4-h value, 17.5 ± 1.6 mL O2·min(-1)·(100 g)(-1); p < 0.05 and pennation angle baseline, 15.7 ± 0.8°; 4-h value, 17.8 ± 0.8°; p < 0.05). Muscle reoxygenation rate was elevated for up to 2 h (baseline, 2.3 ± 0.4 μmol·L(-1)·min(-1); 2-h value, 3.4 ± 0.4 μmol·L(-1)·min(-1); p < 0.05). The other 9 parameters recovered (were no longer significantly different from baseline) after 20 to 60 min. We concluded that the recovery order in main components of muscle function from fastest to slowest was force, neural, metabolic, and structural.

Back pain in Polish military helicopter pilots

OBJECTIVES

Low back pain in pilots of military helicopters is widely discussed in the world literature. Vibrations and improper seat configuration cause that cervical, thoracic and lumbar spine might be exposed to overloading. The aim of the study was to determine the incidence and intensity of pain in the spine as well as to identify subjective risk factors of back pain and its effect on the pilot's actions while flying a helicopter.

MATERIALS AND METHODS

112 pilots, aged 25-56 years (mean age: 34.8 ± 6.3 years), actively flying helicopters, participated in the questionnaire survey. The questionnaire containing five groups of questions, elaborated by the authors, was used.

RESULTS

70% of the pilots reported pain complaints. Pain was localized in different parts of the spine. Uncomfortable body posture during prolonged flights resulting from the lack of lumbar support exerted the highest effect on pain provocation.

CONCLUSIONS

This questionnaire survey enabled to determine the prevalence of pain, its type and intensity. Knowledge on low back pain imposes the necessity to eliminate the negative environmental effect on helicopter pilotage. Elimination of risk factors is possible through appropriate physical exercises and adjustment of pilots' seats. The authors plan to conduct--in the future--detailed annual examinations among pilots with back pain and, based on the results, to elaborate a prophylactic program containing simple forms of isometric and isotonic training involving paravertebral and deep abdominal muscles designed specifically for pilots of military helicopters.