Human trunk extensor muscles physiology and ergonomics.

Abstract

The paravertebral muscles (PM) act together with the hamstrings and calf muscles as important postural muscles. Both the histochemistry, biochemistry, strength and endurance of the PM were studied. Moreover attention was drawn to the exposure of PM, in particular the internal exposure level but to a certain degree repetitiveness and duration, in various job elements, and their various physiological acute responses. The thesis also deal with possible relations between the function of the PM and the magnitude of low-back trouble (LBP), and if PM muscle fatigue may play a role as a mediating factor for the occurrence of work related LBP. The lumbar PM is dominated by relatively small ST fibers with a well-developed network of capillaries, especially distinct in the central sections of PM (lumbar longissimus muscle) in females. It is remarkable that ST fibers are of the same size or larger compared to the FT fibers even in well-trained subjects. Further on PM is characterized by high activity levels of enzymes, oxidative as well as non-oxidative, important for the resynthesis of ATP and CrP. Also the level of muscles glycogen concentration is high. Altogether the PM have seemingly a potential for different metabolic pathways which may be selectively activated for a given activity. The average trunk extensor MVC varies in the different studies from 194-342 Nm and 252-450 Nm in females and males respectively. This is in accordance with predictions based specific strength, muscle cross sections and lever arms from the literature. The large range in strength due to dimension, age and training have to be considered when such data are used for e.g. ergonomic standardization and biomechanical modelling. The sex difference is smaller (female:male trunk extensor strength ratio = 0.7-0.8) than reported earlier. The small age reduction (25-60 year) of the trunk extensor strength, 0.5% per year, is probably caused by the fact that the ST dominated musculature is less sensitive to atrophy. The results indicate that a secular reduction of the trunk extensor strength has occurred during the last 2-3 decades possibly caused by a more sedentary lifestyle, both in working life due to mechanization introduced gradually during that period, and in leisure time activity. An important finding is that the static endurance time is significantly larger in the trunk extensors compared to other muscles, and larger in females than males. Possible explanations for the findings include 1) the reported histo- and biochemical results, 2) favorable blood flow conditions and 3) specific activation strategies of the muscles constituting the PM. In the vocational studies manifest signs of fatigue in the lumbar paravertebral muscles, are seen, including changes in both the energy spectra of the surface EMG towards lower frequencies and increases in the RMS amplitude, reduction of the static endurance time, and increase in the rate of perceived exertion during a working day in bricklayers performing highly repetitive work with their trunk extensors (1000:1200 bricks per day). Similar events do also occur during standing letter sorting with a very low close to static exposure of PM. Ergonomic arrangements may have a marked reducing effect on the internal exposure of PM especially if the job include manual handling operations (e.g. forest work, aircraft loading), but it is not possible to compensate for repetitiveness, and long task duration by so-called "good" ergonomics (e.g. introducing mats or shoes in upright working operations). The chronic effects (i.e.LBP) are also studied. It is found that trunk extensor static endurance time in 77 postmen was related to the degree of LPB, but this was not the case for strength of the trunk-extensors and flexors. Thus, it is shown that a broad variety of work related exposures may cause muscle fatigue in PM. The possible aetiological role of muscle fatigue is however not explained. (ABSTRACT TRUNCATED)