2.1 Surface Electromyography
EMG data were collected from a Tele-Myo DTS EMG instrument with a wireless telemetry system (Noraxon, Inc., Scottsdale, AZ, U.S.A.). The sampling rate was 1000 Hz. A digital band-pass filter (Lancosh FIR), which filtered the raw signals, was between 20 and 450 Hz. Root-mean-square values were calculated with a moving window of 50 ms and Myo-Research Master Edition 1.06 XP software analyzed the EMG data.
An investigator prepared the electrode sites by shaving the subjects’ hair from the immediate vicinity of the muscle belly and cleaning the skin, using isopropyl alcohol with a sterile gauze pad to diminish impedance to the EMG signal; then, the electrodes were fixed on the proper sites (Hermens et al. 2000). Electrodes were positioned over the midsection of the muscle bellies, as in previous studies determining the sites of gluteal muscles (Ayotte et al. 2007; Bolgla and Uhl 2005) and detailed by Rainoldi et al. (2004). For the Gmax, two active electrodes were placed half the distance between the sacral vertebrae and the greater trochanter in the middle belly on an oblique angle at the level of the trochanter or slightly above it. For the Gmed muscle, electrodes were placed directly superior to the greater trochanter of the femur, one-third of the distance between the iliac crest and the greater trochanter of the femur. For the TFL muscle, electrodes were placed 2 cm inferior, and slightly
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lateral, to the anterosuperior iliac spine (Cram, Kasman, Holtz 1998). Proper placement of the electrodes was confirmed by viewing the subjects while completing five repetitions of SHA. Electrode contacts were checked before all contractions (Cram, Kasman, Holtz 1998).
Maximal voluntary isometric contraction (MVIC) in the standard manual muscle-test position was used to normalize the Gmax, Gmed, and TFL (Kendall, McCreary, Provance 2005). To obtain the MVIC for the Gmax, the investigator tested each participant’s resistance to hip extension by having the subject lie fully prone, with the knee flexed to 90°, and applying a downward force at the posterior thigh. To obtain the MVIC values for the Gmed, subjects assumed a side-lying position on the treatment table with the test leg up and the bottom hip and knee flexed for stability.
The test leg was abducted to approximately 50% of hip abduction, and the hip was placed in extension and slightly laterally rotated. An investigator applied downward force at the ankle while maintaining the hip with the other hand. To obtain the MVIC for the TFL, the subjects assumed a supine position on the treatment table with the hip flexed and slightly medially rotated with the knee extended. The investigator applied downward force at the ankle in the direction of the hip extension. Subjects performed the MVIC twice for the Gmax, Gmed, and TFL muscles. The mean value from the two trials was used for data analysis. Subjects performed for 5 seconds at MVIC with a 10 second rest between contractions. Subjects had a 3 minute rest between muscles tested (Soderberg and Knutson 2000). The EMG amplitudes collected during each exercise were expressed as a percentage of the average MVIC (%MVIC).
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The muscle activity of the Gmax, Gmed, and TFL was collected during the SHA exercises in the frontal plane (SHA-N, SHA-MR, and SHA-LR), in randomized order;
subjects drew lots to avoid learning effects or fatigue.
Every SHA exercise speed was comfortable to each participant. EMG data were collected for 5 seconds during the isometric phase and calculated from the middle 3 seconds of each exercise to avoid any skin-electrode connecting element and possible starting or ending effects (Ayotte et al. 2007; Soderberg and Knutson 2000). Subjects performed three trials under each SHA condition, with a 3 minute rest between exercises (Sykes and Wong 2003). The mean value was used for data analysis (De Luca 1997).
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2.2 Three-dimensional Ultrasonic Motion Analysis System
The three-dimensional ultrasonic motion analysis system (CHS-HS, Zebris Medizintechnik GmbH, Isny im Allgau, Germany) calculated the hip rotation range of motion (ROM) of each subject and monitored compensatory pelvic movement (pelvic tilting in the sagittal plane, pelvic rotation in the horizontal plane, pelvic obliquity in the frontal plane) during SHA exercises. One triple marker was placed above the lateral femoral epicondyle to measure amounts of hip rotation (Kiss and Illyés 2012).
The other was located on the midline of the pelvis by fastening a strap around the pelvis at the level of both posterior superior iliac spines to monitor pelvic movement (Oh et al. 2007; Park et al. 2011).
The measurement sensor, consisting of three microphones, was positioned to the back of the subject so that it faced the markers. The side-lying position with neutral hip rotation was calibrated to zero as a reference position. The sampling rate was 20 Hz.
SHA-MR and SHA-LR were calculated using 50% of the maximal ROM for each MR and LR. If the angle of hip rotation and pelvic movement exceeded 5°, the data were regarded as deviations and discarded.
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The isometric strength of the Gmed was evaluated with a hand held dynamometer (hand held dynamometer, Lafayette instrument company, North Lafayette, USA), which measures static force from 0-199.9kg, with reported accuracy to 0.1kg ± 2%
(Bohannon 1988; Marino, Nicholas, Gleim 1982). Several researchers have reported that the hand held dynamometer has good to excellent test-retest reliability for muscle testing in the lower extremities (Intraclass correlation coefficient = 0.82 to 0.98) (Bohannon 1999; Fenter et al. 2003; Heinert et al. 2008; Kimura et al. 1996; Nadler et al. 2000; Wang, Olson, Protas 2002).
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