Fatigue in Modern Production: Implications for Musculoskeletal Health
We’ve all felt fatigue at one time or another….
Perhaps as the result of strenuous activity or immense concentration, or as a symptom of illness or simply from staying up too late. And yet, if you ask a group of people what they mean by fatigue, you’ll likely get a wide range of answers. Fatigue is complex, and perhaps as a result of this complexity, surprisingly little is known about muscle fatigue in modern production processes. This is despite the numerous laboratory studies on the detrimental effects of muscle fatigue and anecdotal evidence from many workplaces.
Modern Production
Neuro-muscular fatigue (known as muscle fatigue) involves the muscles and the central nervous system. Muscle fatigue in the trunk, shoulders and hand/arms can have multiple negative effects on a person, including reduced work capacity, increased discomfort, poorer motor control (with negative effects on product quality) and risk of acute injury. For these reasons alone, muscle fatigue should be minimized by the application of ergonomics. Additionally, if the fatigue is constant and of long duration, it is believed to be a precursor to work-related musculoskeletal disorders, such as shoulder injury or low-back pain. Existing research has already confirmed that musculoskeletal disorders and their personal, firm and societal costs are a substantial problem in the automotive sector.
Fatigue in the trunk, shoulders and hand/arms can have multiple negative effects on a person and their work performance. Negative effects include: poorer motor control (with potential negative effects on product quality), increased discomfort, reduced maximal capacity and increased risk of acute injury. As well, if a person experiences neuromuscular fatigue over a long duration, it is believed to be a precursor to the development of work-related musculoskeletal disorders (MSDs), such as shoulder injury, carpal tunnel syndrome or low back pain. Musculoskeletal disorders and their personal, firm and societal costs are a substantial problem in the automotive sector.
The Role of Ergonomics
To provide a link between ergonomics and product quality: The goal is to develop methods of linking product deficits/assembly mis-operations to poor workplace ergonomics and perform empirical studies in organizations to better understand these links. Development work required includes extending the scope of common ergonomics job and task assessments to include factors such as vision: a nut may be cross-threaded due to fatigue or to poor visual access.
Mounting evidence suggests that musculoskeletal disorders (MSDs) may be the result of a fatigue failure process in musculoskeletal tissues. Evaluations of MSD risk in epidemiological studies and current MSD risk assessment tools, however, have not yet incorporated important principles of fatigue failure analysis in their appraisals of MSD risk. This article examines the evidence suggesting that fatigue failure may play an important role in the aetiology of MSDs, assesses important implications with respect to MSD risk assessment and discusses research needs that may be required to advance the scientific community's ability to more effectively prevent the development of MSDs. Practitioner Summary: Evidence suggests that musculoskeletal disorders (MSDs) may result from a fatigue failure process. This article proposes a unifying framework that aims to explain why exposure to physical risk factors contributes to the development of work-related MSDs. Implications of that framework are discussed.
Gender & Obesity
Obesity rates in the geriatric population have emerged as a serious health concern in recent decades. Yet, obesity-related differences in neuromuscular performance and motor control during fatiguing tasks, and how they are modified by gender, specifically among older adults, are still largely unexplored. The first aim of this study was to understand obesity and gender-related differences in endurance time among older adults. Motor variability has been linked with inter-individual differences in the rate of fatigue development, and as potentially revealing underlying mechanisms of neuromuscular control. Hence, the second and third aims of this study were to investigate to what extent motor variability at baseline could predict inter-individual differences in endurance time, and whether systematic obesity and gender differences exist in motor variability among older adults. Fifty-nine older adults (65 years or older) were recruited into four groups: obese male, obese female, non-obese male, and non-obese female. Participants performed submaximal intermittent isometric knee extensions until exhaustion. Knee extension force and muscle activation signals (surface electromyography) of a primary agonist muscle, the Vastus Lateralis (VL), were collected. Endurance time and metrics quantifying both the size and structure of variability were computed for the force and EMG signals, using coefficient of variation (within cycles and between cycles) and sample entropy measures. While group differences in endurance time were primarily associated with gender, adding individual motor variability measures as predictor variables explained significantly more variance in endurance time, thus highlighting the relevance of motor variability in understanding neuromotor control strategies. Males exhibited longer endurance times, higher EMG CV, lower EMG SaEn, lower force CV, and higher force SaEn than females. These findings are interpreted to indicate males as using a motor strategy involving better "distribution" of the neural efforts across synergists and antagonists to achieve better performance during the knee extension task. No obesity-related changes in endurance time were found. However, obese individuals exhibited a greater cycle-to-cycle variability in muscle activation, indicating a larger alteration in the recruitment of motor units across successive contractions and potentially increased neural costs, which may have contributed to comparable endurance time and performance as non-obese older adults.
Key Findings
MSDs are usually not characterized by complete rupture of a tissue but are instead characterized by a lower-magnitude, localized tissue damage sufficient to trigger an inflammatory response. The point to bear in mind is that fatigue failure is a process of progressive and localized structural damage that occurs when a material is subjected to repeated loading and unloading. The process begins with exposure of healthy tissues to sufficient levels of loading and repetition that leads to development of microscopic fissures in affected tissues. Continued loading of the tissue will cause these microscopic fissures to expand. The rate of this expansion depends on both the magnitude of the load and the number of loading cycles. The damage that accumulates during this process need not approach that required for complete tissue failure to result in an MSD. What fatigue failure studies demonstrate is that even this sub ultimate failure damage can accumulate rapidly when loads are high and will accumulate more slowly (or not at all) when loads are more modest.