Short rest between cyclic flexion periods is a risk factor for a lumbar disorder
Introduction
Epidemiologic data identifies repetitive (cyclic) flexion to be a risk factor for cumulative trauma disorder (CTD) in the lumbar spine (Andersson, 1999, Bernard et al., 1997, Manchikanti, 2000, Melhorn, 2003). Up to 42% of the costs associated with worker’s compensation claims are associated with low back pain, many of which are diagnosed as idiopathic (Manchikanti, 2000). Identifying risk factors and mechanisms of injury could help lessen the large financial burden and safety issue that CTD presents in the workplace. In the case of cyclic loading, a cumulative effect is observed by the presence of creep in the viscoelastic tissues caused by microdamage to the collagen fibers present in the tissue (Williams et al., 2000). Because of the cumulative effect, even moderate loads are sufficient to cause an acute neuromuscular disorder. Microdamage to the collagen fibers elicits an acute inflammatory response in an attempt to promote healing (Solomonow et al., 2003b, Solomonow, 2004). Given inadequate rest time and further exposure to load, acute inflammation can become chronic and can lead to a debilitating, cumulative neuromuscular disorder (Fransen et al., 2002; Solomonow et al., 2004). The symptoms of CTD include pain, weakness, spasms, and decreased range of motion (Barbe et al., 2003, Bernard et al., 1997, Fryer et al., 2004, Kang et al., 2002, Halbertsma et al., 2001). More insight is needed to understand the mechanisms and risk factors of CTD so as to prevent and treat the disorder more effectively.
Previous work with a feline model using static flexion has identified load magnitude (Sbriccoli et al., 2004a, Sbriccoli et al., 2004b), load duration (LaBry et al., 2004), number of repetitions (Sbriccoli et al., 2004a, Sbriccoli et al., 2004b), short rest periods (Courville et al., 2005), and work to rest ratios (Sbriccoli et al., 2006) to be risk factors in the development of an acute neuromuscular disorder. This disorder was manifested by EMG spasms and decreased EMG during the loading periods and by initial and delayed hyperexcitability during the rest period. Recent studies with cyclic flexion have examined the effect of load frequency (Lu et al., 2004), load magnitude (Claude et al., 2003), and number of repetitions (Navar et al., 2006) on a feline model. Furthermore, cyclic flexion has been shown to have a more harmful effect than static flexion for similar protocols investigating loading repetitions and load magnitude. Further exploration into the effect of rest durations on cyclic flexion and its relative effect to static flexion is needed and that is the focus of this study.
The objective is to investigate the effect of short rest durations on cyclic loading in order to provide biomechanical and physiological evidence to support and validate previous epidemiological findings. In a previous static loading study examining the effect of rest periods it was found that work to rest ratios of 1:1 or lower prevented the development of an acute neuromuscular disorder (Courville et al., 2005). We hypothesize that short rest durations between cyclic loading periods will cause an acute neuromuscular disorder. Also, we predict that cyclic loading will elicit a more intense neuromuscular disorder than static loading for identical load magnitudes, load durations and rest durations. Specifically, a work to rest ratio of 1:1 with cyclic flexion may not allow enough rest to prevent the development of an acute neuromuscular disorder.
Section snippets
Preparation
Twenty-one adult cats weighing 3.53 kg ± 0.34 were used in this study. Cats were anesthetized intravenously with 60 mg/kg chloralose with the pre-anesthetic intramuscular injection of xylazine, according to a protocol approved by the Institutional Animal Care and Use Committee (IACUC). The skin overlying the lumbar spine was dissected to expose the lumbar fascia, and an S-shaped stainless-steel hook was applied around the supraspinous ligament between L-4 and L-5. The preparation was then
Results
Fig. 1a, Fig. 1b depicts a typical response of one preparation subjected to six 10-min working periods separated by 5-min rest periods. The top three traces depict the raw EMG signal from electrodes placed between L-3/4, L-4/5, L-5/6, respectively, the fourth trace shows the displacement and the bottom trace the applied load. The EMG amplitude decreases over time during the working periods, and random spasms are present throughout. Creep is evident in the displacement traces and accumulates
Discussion
The major outcomes of this study show that short rest between periods of cyclic loading elicits an acute neuromuscular disorder. While a neuromuscular disorder is not present in the group subjected to 20-min rest, an acute neuromuscular disorder is present in the groups subjected to 10-min rest and 5-min rest. Furthermore, the intensity of the acute neuromuscular disorder increased with shorter rest periods. Caused by the accumulation of creep and microdamage to the soft tissues in the lumbar
Acknowledgement
This work was supported by Grant R01-OH-007622 from the National Institute of Occupational Safety and Health.
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2013, Applied ErgonomicsCitation Excerpt :Hyper-excitability may improve trunk stiffness and serve to protect against injury (Solomonow, 2012). Solomonow (2012) suggests that initial hyper-excitability occurs within an hour following 120 min of cyclic flexion exposure, and Hoops et al. (2007) reported initiation during the first 10 min of recovery. Here, small initial muscle hyper-excitability was indicated by increased reflex gain, appearing in the first 5-min of recovery (for the case of 50% duty cycle; Fig. 4).
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2012, Journal of Electromyography and KinesiologyTime dependent spine stability: The wise old man and the six blind elephants
2011, Clinical BiomechanicsCitation Excerpt :Under such conditions it is assumed that in this case the EMG-Force relationships are linear (Solomonow et al., 1987a) after normalization. Furthermore, the equation for the intensity of muscular activity taken from previously published reports (Navar et al., 2006; Hoops et al., 2007; Le et al., 2007) is divided into two categories; low-risk work and high-risk work. Low-risk work is defined as a dose–duration (load magnitudes, number of repetitions, in between work sessions rest duration, frequency of cyclic loading etc.) of flexion–extension which does not cause a neuromuscular disorder associated with an acute inflammation of viscoelastic tissues several hours post work.
High-frequency loading of lumbar ligaments increases proinflammatory cytokines expression in a feline model of repetitive musculoskeletal disorder
2010, Spine JournalCitation Excerpt :Recent research supports the hypothesis that repetitive or cyclic activity induces creep [6–10] and microdamage [11–13] in viscoelastic tissues (ligaments, discs, facet capsule, and dorsolumbar fascia). As the microdamage becomes severe enough, an acute inflammation [14–17] sets in the viscoelastic tissues, triggering spasms, pain, and sustained hyperexcitability of muscles that support and protect the joint [18–21]. Further exposure to cyclic activity may convert the acute inflammation into chronic inflammation, viscoelastic tissue remodeling, and eventual degeneration [14].
High magnitude cyclic load triggers inflammatory response in lumbar ligaments
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