Recruitment of the Diaphragm and Sternocleidomastoid Muscle During Loaded Inspiration on Varying Sitting Support in Healthy Adults

Participant information sheets and consent form will be given and explained to participants prior to experiment. To ensure measurement consistency, one standardized assessor will be employed. Demographic data, including age, sex, exercise habit, and smoking history will be recorded. Health-related quality of life will be quantified by SF-12. Participants who score below 50 in PCS or MCS will be excluded from our study.

All eligible participants will undertake a standard spirometry test to obtain forced vital capacity (FVC), forced expiratory volume in one second (FEV1). Spirometry test will be performed 3 times, in accordance with guidelines recommended by the American Thoracic Society. Participants with findings of obstructive or restrictive disorders will be excluded. MIP will be obtained by a threshold device (S1breathe K5) under standard instruction. Before measuring MIP, specific instructions guiding participants to focus on diaphragm breathing will be instructed (one hand positioned on the subcostal region and allow air to go to the lower part of the chest where your hand is placed) to allow participants aware of the use of diaphragm during trials, therefore they can subconsciously deploy the technique in the subsequent trials. Participants will be allowed to perform 3 times (with nose-clip applied), with the best MIP will be recorded. The thickness of right diaphragm and SCM activity at maximum inspiration, maximum expiration, end-tidal inspiration, end-tidal expiration, and MIP were measured by ultrasonography and sEMG respectively.

Participants will then be instructed to perform IMT in different conditions in a randomized order, completing 4 sets 15 breaths, with a rest period of 5 mins between sets. 50% MIP will be calibrated at the POWERbreathe device with respect to each participant prior IMT. During each inspiratory pressure-targeted breath, simultaneous sEMG recording on the right SCM and ultrasonography of right hemi-diaphragm will be conducted. Participants's subjective exertion level will be recorded by Borg's Rate of Perceived Exertion Scale at the end of each set of recordings.

The POWERbreathe device requires 5 breaths to meet the targeted load. Following the manufacturer's instructions, participants will be instructed to perform with a sharp inspiration lasting 1-2s at each breath for reaching the required load. Each participant will be requested to complete 15 breaths, but only the data from 6th to 15th breaths will be included for analysis. The whole duration will take around 90 minutes. Password code will be used to guarantee anonymity and confidentiality for the data obtained.

Inclusion criteria

• aged 18-35 years - No recent thoracic surgery in the past 6 months
• Absence in history of chronic respiratory illness
• Able to breathe spontaneously
• Conscious, able to follow instructions on the use of IMT device

Exclusion criteria

• with known cardiovascular, pulmonary or musculoskeletal disorders that would limit their functional capacity (E.g. Asthma, COPD)
• presence with respiratory symptoms over past 2 weeks
• pregnancy
• malignancy
• presence of neurological disorders (e.g. ALS, stroke)
• PCS/ MCS in SF-12 scores below 50
• Findings in spirometry suggesting presence of obstructive or restrictive disorders