Frequency-dependent changes in neuromuscular responses to cyclic lumbar flexion
Introduction
Repetitive motion in the occupational environment has been identified as a risk factor for neuromuscular disorders in epidemiological studies (Silverstein et al., 1986; NIOSH (1996), NIOSH (1999); Department of Labor, 1995; Pope, 1996). Specifically, workers engaged in activities which require repetitive flexion/extension of the lumbar spine represent one of the most costly components of general muscoloskeletal disorders based on lost wages, medical expenses, disability payments, etc. (Department of Labor, 1995; NIOSH (1996), NIOSH (1999); NAS, 2001). Silverstein et al. (1986) pointed out in an epidemiological study that the risk factors associated with repetitive motion work are multifactorial, being dependent on the magnitude of forces generated in each repetition, the number of repetitions for a given period and the duration of the work period as the primary factors.
Often, workers engaged in repetitive lumbar flexion/extension (loading/unloading boxes, assembly line workers, etc.) present with a low back disorder which cannot be diagnosed with routine medical procedures such as X-rays, MRI's, etc., due to the absence of obvious deficits as vertebral fractures, prolapsed disc, impingement, spinal cord stenosis, etc. A common denominator to this class of idiopathic low back disorders is the subjective report of pain from the patient and stiff lumbar musculature as evidenced from spasms or elevated EMG (Fisher and Chang, 1985; Haig et al., 1993; Hoyt et al., 1981; Miller, 1985; Roland, 1986; Shivonen et al., 1991). With the lack of evidence regarding the source of the symptoms, treatment of these conditions is often sub-optimal and lengthy. Our attempts to identify the source and the development of such a common idiopathic low back disorder revolved around the fact that the lumbar viscoelastic tissues (e.g., ligaments, discs and capsules) develop creep when subjected to prolonged cyclic loading (Adams et al., 1990; Ekstrom et al., 1996; Keller et al., 1989; Claude et al., 2003). Creep developed by the cyclic loading of ligaments was shown to result in micro-damage within the collagen fibers and modifications of their functional properties (Frank et al., 1985). Since the viscoelastic tissues of the spine are endowed with a variety of mechanoreceptors (Hirsch et al., 1963; Pedersen et al., 1956; Yahia and Newman, 1991) which give rise to reflexive activation of paraspinal muscles (Indahl et al (1995), Indahl et al (1997); Stubbs et al., 1998; Solomonow et al., 1998), micro-damage in these tissues can elicit a neuromuscular disorder (Williams et al., 2000; Jackson et al., 2001; Claude et al., 2003). In fact, Claude et al. (2003) demonstrated that 20-min of continuous cyclic flexion/extension of the feline spine results in substantial creep in the viscoelastic tissues as well as spasms as was evidenced from the EMG recorded from the multifidus muscles. The disorder associated with creep consisted of initial hyperexcitability of the multifidus muscles in the first hour of rest following the cyclic flexion/extension and delayed hyperexcitability several hours into the rest. It was further shown in that work that light load magnitudes resulted in significantly less severe disorder when compared to cyclic flexion/extension with larger peak loads.
Overall, the epidemiological data (Silverstein et al., 1986) is confirmed by experimental work (Williams et al., 2000; Jackson et al., 2001; Claude et al., 2003) that prolonged cyclic flexion/extension elicits a musculoskeletal disorder the extent of which is dependent on the load applied. Experimental work which determines the impact of the repetition rate (e.g., frequency) of flexion/extension on the resulting disorder is lacking. Some indirect data suggests that increase in the velocity or frequency of motion results in increased force development in the musculature (Marras et al., 1984; Marras and Mirka, 1993; Marras et al., 1995; Marras and Granata, 1997; Solomonow et al., 2001) and that may exceed the load levels which could be safely sustained by various structures (Marras and Granata, 1997). It is also known that higher rates of stretch applied to viscoelastic tissues may cause structural damage (Newman et al., 1994; Panjabi and Courtney, 2001).
In light of these findings, one can propose the hypothesis that cyclic flexion/extension of the lumbar spine at high frequencies may be a significant contributing factor to the development of neuromuscular disorders. Hence, the objective of this report was to determine the impact of the frequency of cyclic lumbar flexion/extension on the neuromuscular responses of paraspinal muscles. Specifically, the impact of the frequency of flexion/extension on the creep developed in the viscoelastic tissues and the hyperexcitabilities and spasms in the multifidus muscles were of interest.
Section snippets
Preparations
Nineteen adult cats weighing 4.1±0.48 kg were anaesthetized with a single injection of Chloralose (60 mg/kg) in a protocol approved by the Institutional Animal Care and Use Committee. The skin directly over the lumbar spine was dissected from the thoracic level to the sacral level and allowed to retract laterally, exposing the intact dorsolumbar fascia. The preparation was then placed in a rigid stainless steel frame that allowed the isolation of the lumbar spine by external fixation (details in
Results
Typical responses of the L-3/4 to L-5/6 multifidus EMG as well as the associated displacement and the cyclic load from two preparations (one at 0.1 Hz and the second at 0.5 Hz) is shown in Fig. 2. In general, the reflexive EMG from the multifidi of the three levels decreased with time during the 20-min of cyclic loading. Spontaneous spasms were superimposed on the decreasing EMG. The timing, duration, intensity and lumbar level of the spasms was random, and unpredictable as is the definition of
Discussion
The major findings of this study revolve around the fact that cyclic loading of the lumbar spine results in a transient neuromuscular disorder of intensity and duration which are dependent on the frequency at which the flexion/extension is performed. Cyclic loading at low frequency is associated with muscular hyperexcitability which is lower in intensity and shorter in duration during the 7 h of rest. Furthermore, lower frequencies result in the development of less creep in the viscoelastic
Acknowledgements
This work was supported by the National Institute of Occupational Safety & Health with Grants R01-OH-04079; and by the Occupational Medicine Research Center (Grant 2000–5)—07, from the Louisiana Board of Regents. Dawei Lu was a research medical student supported by the OMRC.
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