Acute Effect of a Gluteal Activation Warm-up on Hip Muscle Activity and Kinematics During a Single Leg Squat

Muscle pre-activation is the application of exercises to specific muscle groups performed in the warm-up phase in order to increase the neuromuscular response. Among different modalities of warm-up, gluteal activation warm-up is one of the modalities being investigated recently. The hip joint perform movements in the three planes of movement (sagittal, frontal and transverse), being the gluteus maximus and gluteus medius the main muscles that perform triple actions in this joint. Not only about the joint itself but, the research about the hip joint kinematics is of interest due to its importance on pelvic stability and trunk control, playing a key role on motor control during dynamic tasks. This also claim for the increase emphasis that has been put on strategies that could help clinicians to better understand factors associated with hip muscle dysfunction and injury occurrence. The literature has been shown that hip muscle weakness are associated with the occurrence of knee injuries like ACL (anterior cruciate ligament) tears or patellofemoral dysfunction and interventions focused on better activate hip muscles, specifically gluteus maximus and medium, have been demonstrating to be key strategies to reduce undesirable movement patterns associated to lower limb injury risk factors. Although widely used in clinical and sports practice, it is unclear whether, in fact, performing gluteal activation exercises in warm-up phase is effective in increasing electromyographic activation (EMG), as well as the mechanisms that explain eventual gluteal activation improvement of neuromuscular function. Thus, the aim of this study is to verify the acute effect of a gluteal activation warm-up protocol on gluteus maximus (superior fibers) and gluteus medius EMG activation and kinematics during single leg squat. The hypotheses of this study are that after gluteal activation warm-up program, there will be an increase in the muscle recruitment (mean) levels of the gluteus maximus (superior fibers) and gluteus medium. About kinematics, the hypotheses that a decrease on hip adduction absolute angle and hip and knee frontal plane ROM (range of motion) on maximum knee flexion during single leg squat will occur. Participants will perform the test procedures during two visits, with a minimum interval of 24 hours and maximum of one week. The study will be conducted in two stages. In the first stage, anthropometric data collection, intervention protocol application and biomechanical data collection will be performed. The intervention application order will be random and counterbalanced which half of the participants will receive the gluteal gluteal activation warm-up protocol and the other half a standard warm-up protocol at the first visit. In the second stage, the intervention will be applied and biomechanical data will be collected. In this context, the following design will be adopted on both days: (1) Preparation and familiarization (2) intervention protocol application (standard warm-up or gluteal activation warm-up) (3) Immediately after the protocol, three (3) repetitions of the single leg squats will be performed and (4) data collection of maximal voluntary isometric contraction will be extracted. Since the first visit, the participant's routine will be performed as follow: (1) Anamnesis; (2) Anthropometric assessment; (3) electromyography electrodes fixation; (4) retro-reflective markers fixation for kinematic evaluation. (5) intervention protocol execution; (6) Assessment of movement pattern during single leg squats and (7) evaluation of muscle strength by handheld dynamometry and muscle electrical activity by electromyography via maximal voluntary isometric contraction (MIVC). The participants will be invited to make a second visit to the laboratory, to be scheduled in advance, to repeat the procedures for assessing muscle strength, muscle electrical activity, performing another intervention protocol and assessing kinematic during the single leg squat. At first, participants will receive all information about the procedures. For all tests, participants will receive specific positioning and execution guidance. For the anamnesis, participants will answer general questions about past and current health history, dominant lower limb and physical exercise through a questionnaire. For anthropometric data, measurements of height and body mass will be performed. For EMG and kinematic, electromyography electrodes will be fixed to the region of the gluteus maximus and gluteus medius muscles in the hip for assessment of electrical activity and retro-reflexive markers will be placed on anatomical marks of interest (lower limbs and pelvis) to evaluate the kinematic on single leg squat. A intervention protocol will be decided by draw and familiarization will be given informing position and execution of the exercises. Immediately after the intervention protocol, a single leg squat test will be performed only on the dominant limb. For such, three consecutive trials will be performed, without intervals between them. Familiarization and information will be given about the performance of the test. Kinematic data will be processed using KINOVEA software (Creative Commons 3.0, France). From the reference points attached only on the dominant limb, four body segments will be virtually reconstructed: pelvis, thigh, leg and foot. From these segments, the angles will be calculated at a static position, representing the start position and deepest knee flexion degree representing the end of the movement. The absolute values of hip adduction at maximum knee flexion angle (primary outcome) and differences between the two moments (ROM) in each test will be calculated to obtain the following ranges of motion (secondary outcome): knee on sagittal plane (flexion / extension), knee on frontal plane (adduction / abduction), hip on frontal plane (adduction / abduction) and pelvis on frontal plane (pelvic drop / contralateral elevation). All the three squat repetitions will be analyzed. The average of the three repetitions will be used for statistical analysis. EMG raw signals will be assessed using an TeleMyo DTS Desk Receiver® electromyograph (Noraxon® USA, Inc., Scottsdale, AZ, USA). Data will be sent in real time to a computer via Bluetooth, recorded and analyzed by MyoResearch 3.6 Applications Clinical software (Noraxon USA, Inc., Scottsdale, AZ, USA). A 10-500 Hz bandpass filter will be applied to EMG data. The MIVC with maximum amplitude of the EMG signal during the MIVC of each muscle will be recorded and will represent 100% of muscle activity. Muscle activities recorded during squats will then be expressed as a percentage of MVIC.

For statistical analysis, a IBM SPSS Statistics software, version 20.0 (IBM Corporation, Armonk, New York) will be utilized to process and calculate data. A descriptive analysis of anthropometric variables will be performed to characterize the subjects, mean and standard deviation will be adopted for numerical variables. For inferential statistics, tests for normality will be employed. From kinematic data, only absolute values of hip frontal angle and ROM for hip and knee on frontal plane (hip and knee adduction/abduction) and knee on sagittal plane (knee flexion/extension) will be calculated. For EMG, mean EMG activity (Gluteus maximus and medius) over the three single leg squat repetitions will be considered for statistical analysis. Data reliability between conditions will be based on the intraclass correlation coefficient (ICC).