The Effect of Low-load Endurance Training of Upper Trapezius on Pain, Pressure Pain and Muscle Stiffness in Chronic Neck-shoulder Pain Compared to Stretching Exercise: a Randomized Controlled Trial

1. Introduction:

Neck-shoulder pain, also known as the proxy for neck or shoulder pain in research of global burden of diseases, is one of the leading causes of disability and is a common work-related musculoskeletal disorder in workers performing static or repetitive work, leading to huge socio-economic burdens worldwide. It was revealed that about 203 million people suffered from neck-shoulder pain globally. Owing to factors such as aging population and increasing prevalence of sedentary lifestyle, the related burden of neck-shoulder pain on the individuals and health care system is expected to be increasing. Therefore, effective intervention, particularly the active and self-care approach, is needed to address this issue.

Exercise therapy is one of the widely used interventions for chronic neck-shoulder pain because exercise is relatively safe with temporary and benign side effects. Examples of active exercises for neck-shoulder pain include but are not limited to specific neck and shoulder stretching, strengthening and endurance training, cervico-scapulothoracic stabilization exercises, neuromuscular exercises with eye-neck coordination and proprioceptive training, and mindfulness exercises. Evidence has shown that strengthening exercises, when combined with either endurance or stretching exercise, has been shown to be beneficial for neck-shoulder pain. Nevertheless, when either stretching or endurance exercise is incorporated as the only intervention, it appears to have minimal effect on pain and function. Due to the lack of high quality evidence, there is still uncertainty concerning the effectiveness of these specified types of exercise for management of neck-shoulder pain.

Muscle stiffness refers to the tissue resistance during passive stretching and it would depend on the type of external force applied and the deformation of the structure caused. Thus, muscles can be stretched without tearing for flexibility and joint range of motion, demonstrating extensibility, and they can return to their original form after being stretched or contracted, demonstrating elasticity. These mechanical properties allow muscles to function effectively for generating movement, stabilizing joints, and maintaining postures. Some evidence has shown that neck-shoulder pain is related to increased stiffness of the upper trapezius muscle. Recent studies have also demonstrated the association between the myofascial stiffness, elasticity and chronic neck-shoulder pain. However, the precise association between upper trapezius 'tightness' and neck-shoulder pain remains unclear. A possible explanation is that altered tissue morphology of the myofascia leads to altered muscle imbalance between prevertebral and spinal extensor muscles. On the contrary, another possible mechanism is that increased muscle tone in upper trapezius is a protective strategy to limit painful neck movements, which mimics the phenomenon that increased stiffness in trunk extensor muscles among low back pain patients.

If muscle stiffness of upper trapezius is associated with neck-shoulder pain, it is expected that lengthening or releasing its muscle tone should induce long-term improvement in terms of pain. Yet, neck muscle stretching and acupuncture, addressing the myofascial stiffness and trigger point, has been associated with short-term relief from neck-shoulder pain only. Accordingly, active trigger points appear in the upper trapezius among 40% of mechanical neck-shoulder pain. Hence, stretching exercise might alleviate short-term pain only by targeting trigger points rather than addressing muscle stiffness by inhibiting dorsal horn of nociception. As a result, neck-shoulder pain will still be present in the long term due to the nociceptive input resulting in the formation of new trigger points.