The aim of the study is to investigate test-retest reliability and agreement of trunk muscle activation during functional tasks and clinical sensorimotor control tests in pain-free persons.

Optimal muscle function is essential for daily life functioning. The current literature suggests that alterations in trunk muscle activation might be one of the contributing factors to the persistence and recurrence of musculoskeletal disorders, including low back pain and lower limb injuries. It is crucial that trunk muscle activation during functional tasks has sufficient reliability and agreement to interpret (longitudinal) studies.

A limited number of studies have investigated the reliability and agreement of trunk muscle activation during dynamic (functional) tasks. Moreover, all these studies used simple discrete outcomes such as mean or peak amplitude. An important limitation is that these single zero-dimensional outcome values ignore the temporal aspects of dynamic tasks. As such, potentially relevant time-specific characteristics of muscle activation might be missed. A comprehensive reliability analysis of curve data (i.e., one-dimensional data) that considers the temporal timeframe is therefore required. An overview of reliability analyses of curve data can be found in Pini et al. This paper concluded that integrated pointwise indices can be recommended. Recently, this innovative analysis was used in biomechanical studies investigating the kinematics and kinetics during jump landing. However, to the best of our knowledge, integrated pointwise indices have not been used to determine reliability and agreement of muscle activation during functional tasks.

The lumbar multifidus has received extensive attention over the past 25 years. The multifidus muscle consists of deep and short fibers that overlay up to 2 segments (i.e., deep multifidus) and more superficial and longer fibers that cross up to 5 segments (i.e., superficial multifidus). As a result of their anatomy, the deep multifidus (DM) predominantly provides segmental stabilization through compressive forces, while the superficial multifidus (SM) mainly generate lumbar movement (i.e., extension, lateral bending, and rotation). Previous studies showed that the distinction between DM and SM activation is especially important for persons with low back pain. Although the results in persons with low back pain vary, the current literature shows a trend towards an overall reduced activation of the DM, while the activation of the SM is often augmented. As such, a clinical test that assesses voluntary DM activation has been proposed and specific motor control therapy that targets the DM has been shown to reduce pain and disability in patients with low back pain. Fine-wire electromyography (EMG) is the only technique that is able to record selective deep muscle activation during dynamic functional tasks, as opposed to other techniques such as surface EMG. However, reliability studies that use fine-wire EMG to measure selective DM and SM activation during functional tasks are currently lacking.

Therefore, this study aims to investigate test-retest reliability and agreement of trunk muscle activation measured with surface and fine-wire EMG during functional tasks in pain-free persons.

Inclusion Criteria:

- Pain-free persons between 18 and 65 years old

Exclusion Criteria:

• Persons experiencing low back pain in the past year
• Patients with serious underlying conditions (e.g., multiple sclerosis)
• Previous spinal surgery
• Persons with any type of blood clotting disorder
• Persons with upper-limb complaints that prevent them from exerting (maximum) force with their arms or hands.

Pain-free persons will be recruited through online advertisement, flyers and posters.