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

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. At microscopic level, it was realized that office workers with trapezius myalgia have more type II fibers (fast twitch) and less type I fiber (slow twitch), when compared to healthy workers. It was proposed that insufficiency of blood supply and overloading of type I fibers cause metabolic stress. The reliance on increased anaerobic metabolism and low-load prolonged activity of upper trapezius in working leads to development of trapezius myalgia eventually. It is reasonable to deduce that increasing aerobic capacity from a higher proportion of slow oxidative type I fibers may be able to cope with the prolonged low-level activity of upper trapezius. Study has indicated that pain and inflammation might activate type III and IV sensory endings. This could stimulate gamma-motor neurons, and potentially alter muscle spindle excitability, leading to increased muscle tone. Two previous studies have demonstrated that eccentric training can increase the pressure pain threshold and decrease muscle stiffness of upper trapezius muscle in chronic neck-shoulder pain patients. It is possible that increased fatigue threshold might alleviate the increased resting tone and thus muscle stiffness. Therefore, investigators hypothesize that a low-load endurance training of upper trapezius, compared to stretching exercise, might yield better improvement in neck-shoulder pain and muscle stiffness. To the extent of comprehension of investigators, there is no randomized controlled trial (RCT) comparing the effect of endurance training with stretching exercise of upper trapezius on neck-shoulder pain. Therefore, the objective of this study is to investigate the effectiveness of low load endurance exercise as compared to stretching exercise on upper trapezius on pain, pressure pain and muscle stiffness in patients with chronic neck-shoulder pain. The hypothesis is that endurance exercise of the upper trapezius should result in significantly better improvement in terms of neck-shoulder pain, pressure pain threshold and muscle stiffness than stretching exercise. 2. Study Design: This study is a two-arm, parallel, assessor-blinded, randomized controlled trial. A two-group pretest-posttest design is implemented. This study will be conducted between November 2024 and March 2025, in accordance with the Consolidated Standards of Reporting Trials (CONSORT) guidelines. 3. Stratification and Randomization: Participants will be recruited through flyer or email in The Hong Kong Polytechnic University and social media. After the initial screening for eligibility, randomization will be stratified using the 'Minimize' software to ensure an almost even distribution of key parameters across each group. Stratification factors will include age, gender (male or female), hand dominance (left or right), symptomatic side of unilateral neck-shoulder pain (left or right), body mass index (kg/m square), working hours (smaller or equal to 40 hours/week, or larger than 40 hours/week), and Neck Disability Index (NDI) Score (0-24 with no to moderate disability, 25-34 with severe disability, 35-50 with complete disability). Randomization will be conducted prior to baseline measurements, with eligible participants assigned to one of the two groups: endurance exercise (EE) group or stretching exercise (SE) group for the affected side of neck-shoulder region. 4. Study Groups: Exercises will be instructed by two physiotherapists who have 3 years of experience in musculoskeletal physiotherapy. Video demonstrations will be provided to participants to facilitate the exercise compliance. All participants with neck-shoulder pain will complete a 5-week home-based intervention according to group allocation. Message reminders will be sent to participants through a mobile communication application to ensure compliance with the assigned exercises. A video call will be made through the mobile communication application every week to monitor each participant's rehabilitation progression. 4.1 Experimental Group: Upper trapezius low-load endurance exercise (EE) group The upper trapezius low-load endurance exercise, namely the overhead shrug exercise will be prescribed and practiced by the participants allocated to the EE group. Overhead shrug movement is chosen instead of shoulder shrug in neutral position for its more specific and precise activation of upper trapezius with minimal activation of levator scapulae, for which overactivity of levator scapulae is commonly found in individuals with neck-shoulder pain. Participants in the EE group will perform the overhead shrug exercise for the affected side using a green Thera-Band Elastic Band 4.2 Comparison Group: Upper trapezius stretching exercise (SE) group Participants in the SE group will perform static upper trapezius stretching exercise for the affected side. Load management principles will be taught to all participants to avoid excessive loading and adverse events. Participants should report to the two physiotherapists through the mobile communication application immediately in case of adverse conditions, such as neck-shoulder soreness or pain lasting for more than 2 days after exercise, an increase in neck-shoulder pain by equal to or more than 2 out of 10 cm on the VAS score during or after the practice of the exercise, or the appearance of neurological symptoms, such as numbness or pins and needles. These conditions will be reassessed by the physiotherapists in order to determine whether modification of the exercise dosage is required, or termination of the exercise programme is needed. 5. Data Collection Procedure: After obtaining written informed consent from the participants, two principal investigators (physiotherapists who are blinded to group allocation and are responsible to conduct all the outcome assessments) will then carry out the initial screening of the recruited participants. The participants will be assessed at two different time points for all outcome measures: (1) baseline assessment (before treatment); (2) after the end of treatment (week 5). 6. Demographic, Primary and Secondary Outcome Variables: Prior to randomization, demographic data will be recorded for each participant, including age, gender, body mass index, hand dominance, symptomatic side of unilateral neck-shoulder pain, duration of neck-shoulder pain, occupation, and duration of DSE usage per week (working hours). The intervention group allocation will then be completed by an independent research personnel. Primary outcomes include pain assessed by VAS scale, muscle stiffness of upper trapezius assessed by MyotonPRO, pressure pain theraband assessed by electronic pressure algometer over upper trapezius, and upper trapezius endurance measured by 5-minute electrical stimulation protocol. Secondary outcomes include two questionnaires, which comprise of Neck Disability Index (NDI) and Disabilities of Shoulder, Arm and Hand (DASH) Questionnaire. 7. Data Analysis: All statistical analyses will be conducted using IBM SPSS software (IBM, Armonk, NY, USA) with a 5% significance level (two-sided). For validation, the data will be entered twice. Outcome measures will be analyzed based on the intention-to-treat principle. Baseline characteristics and demographic data between the two study groups will be compared using one-way ANOVA, chi-square test, or Fisher's exact test, as appropriate. Two-way repeated measures ANOVA will be adopted to compare changes in primary and secondary outcomes at the two time points between the study groups. 8. Clinical Significance and Implications: This RCT will provide high-quality evidence to verify the efficacy of the specific exercise therapy on management for neck-shoulder pain. This research is the first RCT comparing the effect of upper trapezius endurance training with traditionally used stretching exercise on neck-shoulder pain. If endurance training is found to be more effective in decreasing pain, pressure pain, and muscle stiffness, it could inform the clinical management by refining the exercise prescription for promoting the clinical outcome for individuals suffering from neck-shoulder pain. In addition, it could also inform physiotherapists of the possible mechanisms underlying the improvement of neck-shoulder pain by examination of the relationship between biomechanical properties of muscles, such as muscle endurance or muscle stiffness, and neck-shoulder pain. Low-load endurance training may also enhance self-management of neck-shoulder pain. Endurance training of the upper trapezius is a non-invasive and easily accessible exercise with minimal equipment required. It is one of the practical choices of exercise that can be performed both in clinical and home settings, which may improve patients' adherence to exercise. In addition, it is a cost-effective intervention due to the simplicity and low cost of implementing this training. This is particularly important and beneficial in the healthcare system with limited resources. In conclusion, this study has the potential to improve the understanding of effective treatments for neck-shoulder pain. By providing evidence on the benefits of low-load endurance training of the upper trapezius, it could influence clinical practice and improve the quality of care for patients suffering from this chronic musculoskeletal condition of the spine.