The study explores the effect of proprioceptive neuromuscular facilitation (PNF) exercises combined with virtual reality (VR) motion capture system and concurrent feedback (CF), on early shoulder muscle activation in healthy individuals. Thirty healthy individuals performed three PNF D2 shoulder exercises sequentially: first PNF alone, then PNF with VR (using U-Ball game (BeCure Global Inc.) on Xbox Kinect (PNF+VR), and PNF with VR and CF (PNF+VR+CF), with the latter two in a randomized order. Using wireless surface electromyography (EMG) and 3D inertial measurement units (Noraxon USA, Inc.), the activation of the upper trapezius (UT), lower trapezius (LT), infraspinatus (INF), and serratus anterior muscles (SA), along with shoulder flexion, abduction, and external rotation range of motion were recorded during three shoulder exercises.

Study Design A one-way repeated measures design was employed to investigate the potential benefits of augmenting PNF exercises with VR and concurrent feedback for enhancing early shoulder muscle activation in healthy individuals.

Participants Thirty healthy volunteers (14 males and 16 females with an average age of 26.2 ± 4 years) were recruited through convenience sampling. The inclusion criteria required being 18-45 years old and capable of performing PNF D2 shoulder flexion exercise. Exclusion criteria, designed to ensure sample homogeneity and data reliability, included any shoulder injuries in the dominant arm within the last 3 months, such as pain in the scapular region, rotator cuff tear or reconstruction, shoulder dislocation, shoulder instability, frozen shoulder, fracture, and chronic neck pain. This study received ethical approval from the Institutional Review Board (IRB) as minimal risk research (IRB # 2023-0322-01). The study protocol conforms to the ethical guidelines of the 1975 Declaration of Helsinki (Helsinki Declaration as revised in 2013).

Researchers screened potential participants for inclusion criteria. Eligible individuals were given a detailed explanation of the study's purpose, interventions, and their role, followed by informed consent. Participants then completed a demographic information form. To protect confidentiality, participants were assigned numerical identifiers, and all identifying information was stored separately from research data in a secure location.

Outcome measures This study utilized a combination of technologies to measure shoulder muscle activation and ROM. The NORAXON 3D Motion Capture system, equipped with Ultium EMG sensors and Ultium Motion sensors, provided precise tracking of muscle activity and movement patterns (ICC ranging from 0.87-0.98). The Ultium surface EMG Sensors were utilized to detect the Maximum Voluntary Contraction (MVC) as well as muscle activation in the form of % MVC during three different interventions. Ultium Motion ROM sensors were utilized to analyze the PNF D2 shoulder flexion exercise in three different planes of motion via flexion, abduction, and external rotation. Data collection was conducted using MyoMUSCLE and MyoMOTION software.

Participants' subjective feedback was collected using the Borg Rating of Perceived Exertion (RPE) scale and a customized questionnaire including enjoyment, confidence, frustration, boredom, and level of difficulty.

Interventions The Xbox Kinect and BeCure game software were utilized for VR exercises. The U-Ball game challenged participants to hit balls coming from random targets on a screen using the PNF D2 shoulder flexion on their dominant arm. The game setting ensured 14 balls appeared within the intervention period. Visual feedback was provided through an on-screen avatar mirroring the participant's movements. While the game displayed score elements, these were excluded from the study analysis to focus on proper exercise form.

This study implemented three distinct shoulder muscle activation interventions. Intervention I (PNF alone): Foundational PNF D2 shoulder flexion exercise, incorporating shoulder flexion, abduction, and external rotation. This exercise was performed at a metronome-guided pace of 14 beats per minute to establish a consistent baseline and match the U-Ball game's tempo. Intervention II (PNF+VR): Combined the PNF D2 shoulder flexion exercise with a VR motion capture game (the U-Ball game). Intervention III (PNF+VR+CF): PNF D2 shoulder flexion exercise combined with the U-Ball game and the addition of auditory concurrent feedback ("Sword") which referenced the action of unsheathing a sword in a diagonal upward direction. Each intervention lasted 60 seconds, followed by a two-minute rest period which included the survey completion.

Procedures EMG and ROM Sensor Placement Surface EMG electrodes were placed on the identified motor points of the upper trapezius (UT), lower trapezius (LT), infraspinatus (INF), and serratus anterior (SA) muscles. ROM sensors were attached to the upper spine (C7/T1), lower spine (T12/L1), and deltoid tuberosity on the participants' dominant arm. Anatomical landmarks utilized for sensor placement were confirmed by the same two researchers for consistency. A male and female researcher assisted with sensor placement to ensure a respectful and comfortable environment.

Introduction to PNF D2 Flexion Exercise At a learning station, participants were introduced to the PNF D2 shoulder flexion exercise and auditory concurrent feedback via a pre-recorded instructional video. In the video the researcher explained how the auditory "Sword" concurrent feedback was utilized. Another researcher then provided a live demonstration, allowing participants to practice the pattern and confirm their understanding.

MVC Measurements and Calibration Researchers conducted MVC tests for baseline muscle activity. Participants exerted maximum effort against manual resistance applied by the researcher who trained to apply consistent resistance across sessions. ROM sensor calibration took place 1.5 meters from the Xbox Kinect camera, aligning participants with the U-Ball game's software requirements for optimal interaction. Subsequently, muscle activation from UT, LT, SA and INF muscles and shoulder ROM (flexion, abduction, and external rotation) were monitored and recorded for one minute using the MyoMUSCLE and MyoMOTION software (Noraxon Inc.) during the three exercises.

Intervention Administration Participants began with the PNF D2 shoulder exercise at 14 beats per minute paced by a metronome to match the tempo of the U-Ball game for 1 minute. After this initial intervention, participants were given a 2-minute break where they completed a survey (Borg and customized questionnaire). Participants were then randomized for interventions II and III. This randomization involved blindly selecting one of two papers marked II or III, designed to minimize carryover effects.

Inclusion Criteria:

• being 18-45 years old
• capable of performing PNF D2 shoulder flexion exercise

Exclusion Criteria:

• any shoulder injuries in the dominant arm within the last 3 months
• pain in the scapular region
• rotator cuff tear or reconstruction
• shoulder dislocation
• shoulder instability
• frozen shoulder
• fracture
• chronic neck pain