Patient Characteristics and Inclusion Criteria
An all male study population (n = 11, average weight = 85.4 kg (13.1), average height = 179 cm (11) and age 27.6 (3.2) with greater than six months of weight training experience, without back pain or upper limb injuries, was recruited from a convenience sample of College students. Each subject signed an information and consent form, approved by the Research Ethics Board (Canadian Memorial Chiropractic College) explaining the procedures and risks involved with study participation.
The subjects performed five different trunk muscle exercises on two different surfaces (stability ball and floor) and two separate normalization tasks.
EMG Data Collection Hardware Characteristics
Disposable bipolar Ag-AgCl disc surface electrodes with a diameter of 1.0 cm were adhered bilaterally over the muscle groups studied with a centre to centre spacing of 1.5 cm. EMG electrodes were placed parallel with the muscle fibres on the skin above the rectus abdominus, external oblique, internal oblique and lower erector spinae (L3) on the right side of each subject. The raw EMG was amplified between 1000 and 20,000 times depending on the subject. The amplifier had a CMRR of 10,000:1 (Bortec EMG, Calgary AB, Canada). Raw EMG was banned pass filtered (10 and 1000 Hz) and A/D converted at 2000 Hz using a National Instruments data acquisition system.
EMG Normalization Procedure
In order to compare values of muscle activity across subjects it was necessary to normalize the EMG data. This required that all EMG values be expressed as a percentage of the maximum EMG activity that can be produced voluntarily by a muscle. Subjects performed two repetitions of two different maximal voluntary contractions (MVC). The subjects were first required to perform a three second maximal isometric trunk curl up and twist against an immovable resistance to maximally recruit the rectus abdominis, internal oblique and external oblique muscles. Second, the subjects performed an isometric prone trunk extension against a fixed resistance to recruit the erector spine and multifidus musculature. The muscle activity during all subsequent experimental tasks was expressed as a percent of the peak activity found during the normalization procedure (MVC exercises). Subjects were allowed to familiarize themselves with the movements before muscle activity was recorded.
Description of Exercise Tasks
Feedback from instructors was given in order to achieve a consistent spine and lower limb posture during the following tasks. Subjects aimed to keep their spines in a neutral position with their legs parallel to their trunk during the bridging exercises. The following tasks were chosen because they are common exercises performed in rehabilitation and exercise programs. No attempt was made to control for the different body position relative to gravity between the different exercises. It is recognized that the body's position relative to gravity and the influence of gravity is different between exercises using a Swiss ball and those on the ground. Therefore, conclusions regarding the influence of an unstable surface on trunk muscle cannot be made as the body's position confounds this. The side bridge was added to give the reader a frame of reference for the muscle activity found during the other exercises. It was not performed on the Swiss ball as this exercise is not commonly performed on a Swiss ball and the participants were not familiar with the exercise. Figures , , , , illustrate the exercises investigated. Two trials of each of these tasks were recorded. EMG data was collected for 5 seconds during the isometric portion each task. The tasks the participants were required to complete were as follows:
1. Supine Bridge – Subjects began by lying supine on the floor with their feet flat on ground, knees bent 90 degrees, toes facing forward and hands on the floor by their sides, palms facing down. Pushing through the heels, subjects lifted their pelvis off the ground to form a plank.
2. Supine Bridge with Stability Ball – The same procedure was applied as in task #1, however, in this variation the individuals placed their feet flat on a stability ball.
3. Prone Bridge – Subjects assumed a prone position on the floor, and when instructed established a prone plank position with elbows placed beneath the shoulders and upper arms perpendicular to the floor. In this position only the feet and the forearms were touching the floor.
4. Prone Bridge with Stability Ball – The same procedure was applied as in task #3, however, in this variation the individual's forearms were placed on a stability ball.
5. Side Bridge – Subjects assume a side plank position with elbow under shoulder and upper arm perpendicular to the ground.
The normalization tasks and the exercise tasks for both studies were processed in an identical manner. Raw EMG from each trial was smoothed using an RMS averaging (window of 100 ms, 50 ms overlap) technique. The average activity, expressed as a percent of the normalization contraction, was found for the exertion portion of each exercise and repetition. The average of two repetitions for each exercise and for each muscle was then calculated.
A repeated measures ANOVA with a post hoc Tukey test was used to determine activation level differences within each muscle across bridging exercise tasks.
All statistical tests were performed at the 5% level of significance.