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Muscular atrophy is a common occurrence post ACL injury and even more-so post ACLR, this morbidity is a primary cause for ongoing disability however the mechanisms behind it are less clear.
Arthrogenic muscle inhibition (AMI) is a presynaptic, ongoing reflex inhibition of musculature surrounding a joint after physical insult to structures of that joint. It is a natural response designed to protect the joint from further damage (Hopkins, J. T., & Ingersoll, C. D. 2000). Up regulation of inhibitory interneurons occurs throughout the sensory pathways of the injured area of the body which blunts the stimulating effect of mechanoreceptors (Rice, D. A., & McNair, P. J. 2010). Contrastingly, muscle atrophy is defined as a decrease in the size of a tissue or organ due to cellular shrinkage; the decrease in cell size is caused by the loss of organelles, cytoplasm and proteins (Bonaldo & Sandri 2013). This process is commonly caused by immobilisation and / or disuse of a particular area of the body
Disuse Muscle Atrophy (Jackman, R. W., & Kandarian, S. C. 2004)
Arthrogenic Muscle Inhibition (Rice, D. A., & McNair, P. J. 2010)
Understanding the key differences between the two is important in understanding your choice of intervention in overcoming the deficit. Whilst muscle atrophy may respond to traditional re-use strategies, this may effectively under load patients who have disability following AMI as the central signalling is depressed from each attempted contraction.
Interventions for the rehab professional that are useful in mitigating the effects of AMI include:
Cryotherapy
Neuromuscular Electrical Stimulation (NMES)
Transcutaneous Electrical Nerve Stimulation (TENS)
Blood Flow Restriction Training (BFRT)
Maximal Voluntary Isometric Contraction Training (MVIC)
Whilst AMI is a complex process that acts both locally and centrally to the injured area of the body and this is just a snippet of information, it is important for the sports rehabilitation professional to not necessarily have a physiologist level of understanding in the area, but to be cognisant of the differences between AMI induced atrophy and traditional muscular atrophy.
More to come...
References:
Hopkins, J. T., & Ingersoll, C. D. (2000). Arthrogenic Muscle inhibition: A Limiting Factor in Joint Rehabilitation, Journal of Sport Rehabilitation, 9(2), 135-159. Retrieved Aug 31, 2021, from https://journals.humankinetics.com/view/journals/jsr/9/2/article-p135.xml
Jackman, R. W., & Kandarian, S. C. (2004). The molecular basis of skeletal muscle atrophy. American journal of physiology. Cell physiology, 287(4), C834–C843. https://doi.org/10.1152/ajpcell.00579.2003
Paolo Bonaldo, Marco Sandri; Cellular and molecular mechanisms of muscle atrophy. Dis Model Mech 1 January 2013; 6 (1): 25–39. doi: https://doi.org/10.1242/dmm.010389
Rice, D. A., & McNair, P. J. (2010). Quadriceps Arthrogenic Muscle Inhibition: Neural Mechanisms and Treatment Perspectives. Seminars in Arthritis and Rheumatism, 40(3), 250–266. doi:10.1016/j.semarthrit.2009.10.001