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Myofascial Trigger Points, Sensitization, and Chronic Musculoskeletal Pain
Published in Sahar Swidan, Matthew Bennett, Advanced Therapeutics in Pain Medicine, 2020
Vy Phan, Jay P. Shah, Pamela Stratton
Remarkably, key tenets of Simons’ Integrated Trigger Point Hypothesis overlap with the role of muscle in MTrP development suggested by the Cinderella Hypothesis.32 Musculoskeletal disorder symptoms may arise from muscle recruitment patterns during sub-maximal level exertions with moderate or low physical load among office workers, musicians, and dentists, in which myalgia and MTrPs have been commonly reported.29 According to Henneman’s size principle, smaller type I muscle fibers are recruited first and de-recruited last during static muscle exertions. As a result, these “Cinderella” fibers are continuously activated and metabolically overloaded, in contrast to larger motor muscle fibers that spend less time being activated and do not work as hard. This property makes these fibers more susceptible to muscle damage and calcium dysregulation, key factors in the formation of MTrPs.33 Treaster et al. demonstrated that low-level static continuous muscle contractions during 30 minutes of typing induced the formation of MTrPs, supporting the Cinderella Hypothesis.34
Not just cousins
Published in Francesco E. Marino, Human Fatigue, 2019
Although comparison of skeletal muscle architecture provides some basis with which to understand human adaptation to bipedalism, further analysis of human neuromuscular adaptations shows distinct preferential characteristics for muscular endurance over power. The relationship between motor nerves and the variability of motor units is indicative of the preferential adaptation for the way in which muscular force is initiated, developed and maintained. The accepted Henneman size principle (Binder & Ruenzel 1983; Somjen & Carpenter 1965) posits that muscle contraction is always initiated by small motor units, followed in an orderly fashion by larger ones as greater force is produced. A key component of this principle establishes that the most fatigue-resistant muscle fibres (Type I) are recruited first, allowing for fine motor control, followed by the larger, less fatigue-resistant fibres (Type II) (Sale 1987). A fundamental aspect of the fibre type principle is the relationship that proportion and distribution of fibre types have with the needs of the species. For instance, in comparing the fibre type distribution of triceps surae between orangutan and the chimpanzee, the orangutan had a higher proportion of Type I fibres, which corresponds to the orangutan’s adapted slow and controlled movements in their tree-dwelling habitat. In contrast, the higher proportion of Type II fibres in the chimpanzee reflects their compromise between controlled movements and the need for speed and power when terrestrial (Myatt et al. 2011). In humans, the estimate is that soleus and gastrocnemius muscles are about 70% and 50% Type I, respectively (Edgerton et al. 1975) which indicates a fibre type distribution in these muscles corresponding to fatigue resistance, especially in the soleus muscle.
Pelvic floor muscle function and EMG in nulliparous women of different ages: a cross-sectional study
Published in Climacteric, 2018
D. A. S. Bocardi, V. S. Pereira-Baldon, C. H. J. Ferreira, M. A. Avila, A. C. S. Beleza, P. Driusso
Another possible explanation is that many women of different ages are unable to perform the maximal voluntary contraction of the PFM, even when asked to do so, due to lack of awareness and motor control of the region. Based on Henneman’s size principle27, these women recruit only type I fibers to perform contractions. Because aging primarily affects type II fibers, it is not possible to observe differences between age groups in levels of contraction of the PFM8. It should also be noted that, at any age, there are great differences between individuals25. In the present study, for example, among women 40–49 years old, there were women with pressures of 20 cmH2O and 140 cmH2O. The reasons for these great variations between individuals still need to be investigated, but show that there does not seem to be a standard of normal function and activation of the PFM in nulliparous Brazilian women aged 18–69 years old. It may be possible that a pressure of 20 cmH2O is capable of maintaining the function of the PFM in these women, and only when aging is associated with other risk factors is it responsible for dysfunctions that affect the pelvic floor.