Methods
Subjects
A cross sectional study design was used to compare the muscle activity pattern during PHE in two groups of women: women with chronic non-specific LBP (N = 10,
average age: 33.6 (SD = 7.27) years old, average height: 163.1 (SD = 8.25) cm, average weight: 59.5 (SD = 10.34) kg) and women with no history of LBP (N = 10, average age: 29.8 (SD = 5.67) years old, average height: 161.2 (SD = 7.36) cm, average weight: 58.4 (SD = 5.44) kg). The LBP patients were referred by orthopedic specialist and physiotherapy clinics. The patients included if they have a history of non-specific LBP for more than 6 weeks duration before the study date, or intermittent LBP with at least three previous episodes lasting more than one week during the year before the study [18]. The healthy subjects were recruited from university students. The exclusion criteria in both groups were pregnancy,history of dyspnea, history of hip pain,dislocation or fracture, history of lumbar spine surgeries,history of anterior knee ligament injury or rupture, history of anterior knee pain,recent episodes of ankle sprain, leg length difference of more than 1 cm, inability to perform active PHE without pain, history of lower extremity injury in the past 3 months, shortness of hip flexors, those who participate in programs to prepare
for competitive sports (exercise more than 3 days a week), positive neurological symptoms and cardiopulmonary disorders. Each eligible subject was enrolled
after signing an informed consent form approved by the human subjects committee at the University of Social Welfare and Rehabilitation Sciences. Ethical approval for
this study was granted from the internal ethics committee at the University of Social Welfare and Rehabilitation Sciences.
The dominant leg was chosen for investigation. The muscle activity of IES, CES, GM and HAM during PHE was measured by the MIE-MT8 Telemetry EMG instrument
(MIE-Medical Research Ltd). A preamplifier with a gain of (4000×), band pass filtered (6-500 HZ), A-D converted (sampling rate = 1000 HZ) was used. The subjects were asked to lie prone with their arms at their side and head was in mid line. The skin was shaved,rubbed and cleaned with alcohol. To record muscle activity, disposable, bipolar, self adhesive Ag/Agcl electrodes were placed in pairs with distance of 1.5-2 cm from each other and parallel to the muscle fibers [19]. Electrodes placement to collect EMG signals were as follow: for the ES muscles, bilaterally at least 2 cm lateral to spinous process of L3 parallel to the vertebral column on the muscle belly; for the GM, at the mid point of a line running from S2 to the greater trochanter;and for the HAM, laterally on the mid distance between gluteal and popliteal fold.
The maximum voluntary electrical activity (MVE) for each muscle was firstly calculated for normalization procedure. Test methods to calculate MVE were similar to those described for manual muscle testing of the muscles,as described by Kendall et al [20]. The pelvis was secured to the bed with a sling to prevent pelvic motion
substitution only during MVE testing. For the ES muscles the subject was asked to bring up her trunk against the maximum resistance that entered bellow the scapula.
For the GM, hip joint was placed in extension position and knee flexed to 90 degrees, resistance applied to the distal aspect of posterior portion of thigh. The HAM was tested while hip joint was placing in extension position, the knee was flexed to nearly 70 degrees, and resistance was applied to the distal aspect of the posterior portion of the shank during knee flexion. Each contraction was repeated 2 times and held 5 seconds. One minute rest was given between contractions. Before testing,the subjects were familiarized with the standard position and movement. All subjects were asked to lift the chosen leg off the bed to 10 degrees whilst keeping the
knee straight, as soon as they heard the command “lift”. An adjustable bar was placed at this level and the subjects were asked to extend their hip until the calcaneous touched the bar. The subjects were instructed only to reach the adjustable bar and were not instructed to press against the bar with the distal segment of the lower extremity.
This was repeated 3 times for each individual. Figure 1 depicts an example of the raw EMG signals for tested muscles. The raw data were processed into the root
mean square (RMS). The EMG signals collected during hip extension were expressed as percentage of the calculated mean RMS of MVE (%MVE).
Time broadness is the time elapsed (in %) of the motion cycle between the peak of the first muscle to reach maximal activity and the peak of the last muscle to reach maximal activity. Time broadness can show to what extent the muscles are simultaneously involved in producing a motion during a motion cycle. Time broadness
provides indirect information on muscle coordination [21]. The muscle activity pattern was characterized by maximal amplitude of normalized voluntary electrical activity and by time broadness in the percent of the movement cycle. The pattern is different in case there is a difference in any of the parameters above.