Hypertonic muscle refers to muscle tissue exhibiting excessive resting tone. This tone is regulated by the central nervous system, and while damage to this system can contribute to hypertonicity, more common factors include suboptimal movement patterns. For instance, a dog engaging in strengthening exercises with improper form or "a tough bout of exercise." (Burton, 2002). 

Burton (2002) explains that when one muscle becomes hypertonic, its opposing muscle tends to exhibit weakness. For example, individuals who are sedentary for extended periods, such as those seated at desks for eight hours daily, often experience hypertonicity in muscles such as the trapezius or deltoids, which contract to prevent excessive forward bending. In contrast, the pectoralis major may weaken due to its antagonistic position, driven by the overactivity of the shoulder musculature. 

 Hypertonic muscles are characterised by rigidity, resulting in stiffness, discomfort, and pain during movement. 

The efficacy of massage therapy in addressing hypertonic muscle is notable, often yielding significant improvements within a limited number of sessions. Through massage, the muscle can be restored to its optimal functionality, allowing fibres to lengthen and contract effectively, facilitating fascia responsiveness to muscular stresses, and ensuring normal sensory feedback from the associated neurons. Consequently, the result is a more supple and flexible musculature, alleviating pain and enabling canines to resume regular activities. While complete resolution may not always be attainable, massage therapy is likely to enhance overall comfort levels and mobility in affected dogs. 

Information sourced from

 Burton, W. (2022) 'Muscle hypertonicity: 3 proven ways to relieve tight muscles', The Smart Chiropractor Blog. Available at: https://blog.thesmartchiropractor.com/relief-from-muscle-hypertonicity/