Musculoskeletal Disorders. Sean Gallagher
Читать онлайн книгу.
with fatigue failure techniques, one can use biomechanical estimates of tissue stress to estimate injury probabilities associated with either mono‐task jobs or combinations of tasks. Chapter 5 discusses the benefits of applying fatigue failure principles to biomechanical analyses to help estimate the fatigue life of tissues experiencing repeated biomechanical stress.
Damage versus Healing. As indicated earlier, musculoskeletal health is expected to be dependent upon both damage development (due to repeated stress) and healing. As noted long ago by Nash, despite the additional complexity of the presence of a healing component, it is relatively easy to develop an injury model that incorporates both cumulative damage development and healing using a fatigue failure approach (Nash, 1966). This simply involves use of the Palmgren–Miner method (Chapter 9) to ascertain the cumulative damage associated with the experienced variable magnitude loading, and then subtracting out the damage healed over the same time frame. If the rate of cumulative damage is greater than the healing rate, damage will occur; in contrast, if the healing rate equals or exceeds the rate of damage development, no damage would be expected to occur. This model can also be employed to assess risk in conditions associated with impaired healing (such as psychological stress, aging, and obesity), as discussed in Chapter 10. Thus, in contrast to previous models, fatigue failure techniques are well positioned to address important complexities of the biological environment that are important in maintaining musculoskeletal health. This “damage and healing” model may be useful, for example, in assessing the MSD risk associated with factors known to impair the healing process, such as psychological stress, obesity, and aging (Guo & DiPietro, 2010). These and other implications are discussed further in Chapter 10.
Real‐Time Risk Assessment. The fatigue failure theory also has techniques that could be used as real‐time exposure assessment methods continue to develop and mature (Radwin, 2011). Due to the variable amplitude loading histories experienced by humans in the performance of physical work, a method will be needed to evaluate the risk associated with complex and irregular loading curves. Fortunately, this situation is also encountered in the loading of other materials and methods have been developed to deconstruct complex loading curves into cycles that can then be assessed by fatigue failure models. The consensus technique is known as rainflow analysis (Matsuishi & Endo, 1968). This technique evaluates stress “reversals” (half‐cycles) associated with complex loading histories and derives full cycles for analysis while accounting for the entire load history (Chapter 9). Thus, the fatigue failure approach is well positioned for the future of risk assessment in which complex loading curves derived from real‐time exposure assessment can be rapidly analyzed using validated techniques as discussed further in Chapter 13.
Cumulative Risk from Different Loading Modalities. Fatigue failure methods also hold the promise of combining the risks associated with diverse loading modalities. For example, imagine a delivery driver who experiences whole‐body vibration from driving across rough roads and then must carry heavy boxes to complete the delivery. Fatigue failure methods hold the promise to combine these two types of repetitive stress into a single measure of risk and provide an estimate of cumulative damage associated with both exposures. Whole‐body vibration is known to be an exposure that imposes repeated stress on the low back (Gallagher & Schall, 2017) as does repetitive compressive loading due to lifting (Brinckmann, Biggemann, & Hilweg, 1988). Since both can be expressed in terms of cumulative damage, fatigue failure techniques could be used to combine these disparate loading modalities to obtain a cumulative measure of overall risk not previously available (Chapter 9).
Combining Static and Cyclic Loading. MSDs may also be the result of both static and/or dynamic activities. Thus, it would be helpful to have a method of calculating the combined risk associated with a combination of creep (static) loading and cyclic (dynamic) loading. Fortunately, the fatigue failure theory also has a validated method of evaluating the cumulative effects associated with combined static and dynamic loading (Wright, Carroll, Sham, Lybeck, & Wright, 2016). This method allows estimating the proportion of risk associated with dynamic versus static loads overall, as well as the proportion of individual task risks for both types of loading (See Chapters 9 and 13).
Assessing Risks of Job Rotation. Job rotation is a technique sometimes used in industry in an attempt to balance the biomechanical demands associated with different jobs. However, a recent award‐winning article in the journal Ergonomics using fatigue failure techniques demonstrated that job rotation may be more harmful than helpful in terms of the overall MSD risk to a pool of workers (Mehdizadeh et al., 2020). Due to the nature of the fatigue failure risk function, any reduction in risk for higher risk jobs is guaranteed to be smaller than the increase in risk experienced by those in lower‐risk jobs (See Chapter 13). The magnitude of this effect will vary from extremely large, if a single high‐risk job is present in the pool, to small if all jobs are fairly low risk. Such an analysis would be difficult to achieve with some prior models but is straightforward using fatigue failure principles.
Influence of Personal Characteristics on MSD Risk. Fatigue failure also offers the potential to incorporate certain personal characteristics into risk assessment. As an example, fatigue failure theory stipulates that the ultimate strength of a material determines the number of cycles to failure at different percentages of that strength. If an individual performs a lifting task resulting in a 2,500 N compressive load on the spine, they will incur damage more quickly if they have a spine ultimate strength of 6,000 N (say at age 60) as opposed to 8,000 N (at age 30). Factors such as age, anthropometry, sex, bone mineral density, and others could be considered to develop “personalized” risk assessments that may be more protective for workers based on their individual characteristics. Risks associated with personal characteristics are discussed in Chapter 12.
The preceding examples provide a sampling of the opportunities that may be realized with the use of the fatigue failure approach to MSD risk, many of which would be difficult to realize with previous approaches. There are numerous other useful applications of this model as will be described in detail later in the book.
In summary, fatigue failure is a universally accepted causal mechanism of damage nucleation and propagation for nonbiological materials, and there is ample evidence to suggest that the same process occurs in musculoskeletal tissues. Much of what has been learned regarding the process of material damage resulting from repeated stress appears applicable to the assessment of musculoskeletal risk, and many techniques developed in this theory appear to provide ready solutions to challenging problems faced by musculoskeletal researchers. These include simple methods of estimating the cumulative impact of multiple tasks having highly variable loading conditions. Techniques are also available for assessing cumulative damage associated with complex loading curves that will be useful soon as real‐time exposure assessment methods for MSDs become available. Furthermore, models incorporating the effects of healing and other biological processes critical to musculoskeletal health have been put forth, thus allowing the complexity of the fatigue failure process in the biological environment to be more fully understood.
As indicated by the earlier discussion, there are many topics to be discussed and implications to be addressed when evaluating the effects of a fatigue failure process in a biological environment and the roles of damage and healing in overall musculoskeletal health. We have structured the 16 chapters in a logical order, and the chapters are grouped into four general themes. Chapters 2–5 provide detailed information regarding common MSDs and the components of the musculoskeletal system, including the structure and function of musculoskeletal structures, the material properties of these tissues, and the important role of nerves and the nervous system in the musculoskeletal system. Chapters 6–9 cover fundamental concepts of biomechanics, evidence of