were 21.38 ± 1.64 years, 167.71 ± 7.55 cm, and 60.04 ± 9.22 kg, respectively. The subjects were students at the department of physical therapy of Y University. None of the subjects had complained of pain in the musculoskel- etal region during the previous six months, and none had orthopedic or neurological disease. Prior to participation, all participants were required to read and sign an informed consent form, in accordance with the ethical standards of the Declaration of Helsinki. The protocol for this study was approved by the local ethics committee.
Four kinds of bridging exercise were conducted in the experiment.
Exercise 1 (supine bridging): In the starting position of the bridging exercise in the supine position, the subjects bent their knee joints at 90 degrees and spread both arms at about 30 degrees, with both hands on the ground. They kept their head and neck in a straight position, with their eyes looking at the ceiling (Fig. 1).
Exercise 2 (supine bridging on balance pads): The sub- jects adopted the same position as that of exercise 1, but they placed their feet on TOGU balance pads (Fig. 2).
Exercise 3 (unilateral bridging): Again, adopting the same position as that of exercise 1, the subjects raised their dominant-side leg (Fig. 3).
Exercise 4 (prone bridging on the elbows and toes): In a prone position, the subjects bent their elbows at 90 degrees and supported their bodies with their forearms and toes, with their neck slightly extended and their eyes looking to the front (Fig. 4).
To reduce errors while performing the different exercis- es, the subjects adopted a neutral position, with the pelvis raised. The researcher stood at the side to ensure that the subjects adopted the correct position. Prior to the exercises, the subjects were also advised how to conduct the exercises. A dynamic air cushion (TOGU, Germany) was used for the unstable support surface. The subjects performed the ex- ercises in a random manner by selecting a closed envelope containing the name of the exercise to be performed. The subjects started each exercise on the command of “Start” and took a rest of 5 minutes after each exercise to prevent
Fig. 2. Supine bridging exercise on TOGU
Fig. 4. Prone bridging exercise on the el- bows and toes
muscle fatigue.
A surface electromyography system (Telemyo 2400T-
G2, Noraxon, USA) was used to measure the activities of the trunk muscles. To reduce the skin’s resistance prior to the application of Ag/Ag-Cl (Biopac, diameter 2 cm) elec- trodes to the subjects’ bodies, their skin was shaved to remove hair from the area where the electrodes would be attached. The surface of their skin was then cleaned with absorbent cotton, which was sterilized by soaking in alco- hol, before the electrodes were attached along the direction of the muscle fibers. The ground electrode was placed on the anterior superior iliac spine (ASIS) of the dominant side.
To measure the muscle activities of the subjects in the different exercise positions, the electrodes were attached to four muscles: 3 cm lateral to the umbilicus for the rectus abdominis (RA); midway between the anterior iliac spine and symphysis pubis and above the inguinal ligament for the internal oblique muscle (IO); 15 cm lateral to the umbi- licus for the external oblique muscle (EO); and 2 cm lateral to the L2 spinous process for the erector spinae (ES)13). The electrodes were attached to the dominant side muscles of the subjects. The surface electromyography signals were digitized and processed using the MR-XP program on a personal computer. The sampling rate of the surface elec- tromyography signals was 1024 Hz. Signals were band-pass filtered between 20 and 500 Hz, and full-wave rectified. The root mean square (RMS) of the values was calculated.
To standardize the action potential of each muscle, the maximal voluntary isometric contraction (MVIC) was used. MVIC measurement positions were adopted follow- ing a previous study14). Each position was held for 7 sec- onds to reduce measurement variation at the start and end points of the exercise. The activity of the muscles was mea- sured for 5 seconds, excluding the first and last second. The collected data were statistically processed using SPSS 18 PASW Statistics, and the averages and standard deviation of the general characteristics of the subjects were calculated. Repeated one-way analysis of variance was used to deter- mine changes in the activities of the muscles in the di