Participants
Twenty-two healthy subjects (11 men, 11 women; mean age ± SD: 24.55 ± 5.00) participated in this study. All participants were volunteers and gave their informed, written consent according to the protocol approved by the Université du Québec à Trois-Rivières (Canada) Ethics Committee.
Experimental protocol
The one hour experimental session included maximal voluntary contractions and a trunk muscle endurance task in a lateral isometric hold position. Participants’ height and weight were measured prior to the experimental task. The Baecke-f questionnaire was also completed by all participants in order to assess their physical activity levels (daily participation in sports and leisure activity) [14]. The experimental task was thoroughly explained and demonstrated by the experimenter before any data were recorded.
Maximal voluntary contraction
The endurance task was preceded and followed by three maximal voluntary contractions (MVC) in the same position, and participants were allowed a 2-minute rest period between the MVC pre-endurance and endurance task, and 15 minutes between post-endurance MVC and the other side pre-endurance MVC. MVC assessments, conducted 2 minutes prior to, and immediately after each endurance protocol, were performed against a force transducer that measured trunk muscle strength in the lateral position in accordance with the procedure presented by Ledoux et al. [7]. Participants were tested in the same position they held in the endurance tasks. Verbal encouragements were provided to maximize voluntary contraction. MVCs were performed against a fixed harness around the shoulders connected inline to a uni-axial force transducer on the floor (NTEP-87-057A3 class III, Artech, Riverside, CA, USA). For each MVC trial, force data were recorded at a sampling rate of 1,000 Hz and filtered digitally with a fifth-order Butterworth filter (10-Hz low-pass cutoff frequency). The higher force value obtained in 3 consecutive 10-s trials was used as the reference for MVC.
Trunk muscle endurance task
Participants were asked to perform sustained isometric contractions of the trunk muscles in a lateral position for both left and right sides named according to the side up (Figure 1). The endurance tasks (left and right) were counterbalanced across participants to control for sequence order effects. The protocol was explained and demonstrated before any experimental task was undertaken. During the left lateral isometric hold test, participants were positioned on the right side on a 45 degrees Roman chair. Ledoux et al. [7] suggested that this protocol could be used as an alternative to the side bridge test. When used to assess muscle fatigability, it yielded endurance time slightly longer than to those reported by McGill[10]. The test was developed as an alternative to the side bridge test to evaluate endurance time in older adults and adults with upper limb injuries (who could not attain support off the floor). This test also creates the possibility to assess maximal voluntary contraction in the same position. The trunk, from the anterior superior iliac spine and up, was unsupported. Arm support was allowed prior to the endurance task. On the researcher’s cue, the participants removed their arms from the support and folded them across the chest with hands placed on the opposite shoulder. During the endurance task, participants were asked to keep their trunk and head in line with their lower limbs which were one above the other. The goal for all participants was to hold this position as long as they could. In order to ensure that participants abide by instructions, the same assistant observed the entire task for every participant and gave them verbal feedback to ensure proper position based on thorough observation. Failure to comply with instructions resulted in a warning by the assistant and the task was ended if the participant failed to follow instructions three times. Verbalized encouragement was provided throughout the test.
Electromyography (EMG)
Surface EMG data were collected with BiPole disposable surface Ag-AgCl electrodes (Bortec biomedical, Calgary, Alberta, Canada) applied bilaterally on specific muscles in line with fiber direction. Inter-electrode distance was fixed at 2 cm and electrode diameter was 1 cm. Muscle activity of the external oblique, rectus abdominis, lumbar erector spinae (L2 and L5 level) was recorded according to McGill et al. [15]. A ground electrode was placed on the left olecranon of each participant. Skin impedance was reduced by (1) shaving body hair, (2) gently abrading the skin with fine-grade sandpaper (Red Dot Trace Prep, 3 M, St. Paul, MN, USA), and (3) wiping the skin with alcohol swabs. EMG activity was recorded using a single differential Delsys Surface EMG sensor with a common mode rejection ratio of 92 dB at 60 Hz, a noise level of 1.2 μV, a gain of 10 v/v ± 1%, an input impedance of 1015 Ω (Model DE2.1, Delsys Inc., Boston, MA, USA) and sampled at 1,000 Hz with a 12-bit A/D converter (PCI 6024E, National Instruments, Austin, TX, USA). The EMG data were filtered digitally by a 10- to 450-Hz band-pass, zero-lag, fifth-order Butterworth filter. They were collected by LabView (National Instruments) and processed by Matlab (MathWorks, Natick, MA, USA).
Data analysis
Maximal voluntary contraction (Newton) and maximal EMG root mean square values (RMS) (for each muscle) were obtained for every MVC trial to assess muscle fatigue. Muscular fatigue was assessed during the fatigue protocol through power spectral analysis. Median power frequency (MedF) was calculated from successive non-overlapping windows of 250 ms by Fast-Fourier transformation. Least square linear regression analysis was applied to MedF time series (MedF as a function of time) to estimate the rate of decline (MedFslope). In order to express the decay rate of MedF as the percent change from the initial value (NMFslope in%s-1), MedFslope were divided by the initial MedF [16, 17] . The equation below was used to obtained individual holding time ratios:
(1)
Statistical analysis
Right and left maximal voluntary contractions (Newtons) and RMS were compared between pre and post endurance tasks with 2-tailed t-tests for dependent samples. 2-tailed t-tests for dependant samples were also used to compare holding times between left and right lateral isometric hold tests. In order to test for our main hypothesis, the NMFslope of antagonist muscles in both conditions (e.g. left external oblique vs. right external oblique during left lateral isometric hold test) were compared using 2-tailed t-tests for independent samples. T-tests for independent samples (1-tailed) comparing NMFslope to 0 were conducted for every muscle in both conditions to determine if statistically significant decay of NMF were induced by the fatigue protocols. Simple correlation tests were performed separately for each baseline characteristic and holding times in order to evaluated the possible correlations. Statistical significance was set at p ≪ 0.05 for all analyses.