Augmented Feedback Effects on Shoulder Rotator Performance

All participants will complete a standardized warm-up protocol of approximately 10 minutes prior to the testing sessions. The warm-up will begin with arm circles to promote active mobilization of the shoulder joint. This will be followed by one set of 10 repetitions of shoulder internal and external rotation exercises performed at 90° of shoulder abduction and 90° of elbow flexion using a resistance band providing submaximal resistance. Following the warm-up, a familiarization session will be conducted to increase participants' familiarity with the ForceFrame device used in the testing and training sessions. During this session, participants will perform five repetitions of isometric contractions, each lasting 5 seconds, at a self-selected submaximal effort level. The familiarization session is intended to ensure participants' adaptation to the testing procedure, learning of the correct movement sequence, and execution of the exercise with proper technique. Following completion of the warm-up protocol, isometric muscle strength and rate of force development of the dominant shoulder internal and external rotator muscles will be assessed. All measurements will be performed using an isometric dynamometer (ForceFrame). During testing, participants will be positioned in a supine position with the knees flexed and feet flat on the ground, while the non-tested arm will be placed across the chest.

The tested shoulder will be positioned at 90° of abduction and the elbow at 90° of flexion, with the hand placed at a comfortable height appropriate for pushing against the sensors. The testing protocol will consist of three repetitions of maximal isometric contractions, each lasting 5 seconds, with 20 seconds of rest between repetitions. Maximum force and rate of force development values will be recorded during each trial. Throughout testing, participants will be instructed to maintain the standardized position and keep their arms stable. Prior to each test, standardized verbal instructions will be provided, instructing participants to perform the contractions "by producing maximal force as quickly as possible." The test protocol will be administered at three time points: before the isometric training (pre-test), immediately after the training protocol (post-test), and 15 minutes after training to assess immediate retention (immediate retention test).

Following the pre-test, participants will perform the isometric training protocol targeting the shoulder internal and external rotator muscles. The training protocol will also be conducted using the ForceFrame device. During training, participants will be positioned identically to the testing protocol, in a supine position with the knees flexed and feet flat on the ground, while the non-tested arm will be placed across the chest. The tested shoulder will be positioned at 90° of abduction and the elbow at 90° of flexion. The training protocol will consist of one set of three repetitions, each involving a 5-second isometric contraction, with 20 seconds of rest between repetitions. Throughout the training, participants will be instructed to maintain the standardized position and keep their arms stable. Standardized verbal instructions will be provided to all participants, instructing them to perform the isometric contractions "by producing maximal force as quickly as possible."

Knowledge of results will be provided between repetitions during the training session. Following each repetition, participants will receive immediate quantitative feedback regarding their maximum force (peak force) and rate of force development. The feedback will be delivered by the same physiotherapist to all participants, using a clear and emphatic tone, slightly louder than normal conversational volume to ensure it is distinguishable from background noise.

Inclusion criteria

• Healthy individuals aged between 18 and 30 years,
• Body Mass Index (BMI) below 30 kg/m²,
• An activity level of 5 or higher according to the Tegner Activity Scale,
• Full active range of motion (ROM) in shoulder flexion, abduction, internal, and external rotation.

Exclusion criteria

• Resence of current pain complaints,
• Provocation of symptoms during at least one of the specific shoulder clinical tests (Neer Test, Hawkins-Kennedy Test, Jobe Test, Resisted External Rotation Test, and Apprehension Test),
• History of any upper extremity injury, pain, or surgical intervention within the past 12 months,
• Presence or history of hypertension, cardiovascular diseases, peripheral vascular diseases, deep vein thrombosis, neurological disorders, systemic inflammation, diabetes, cancer, or obesity,
• Unwillingness to participate in the study.