Participants
Young males were recruited following authorization by and the guidelines of the Ethical Committee of the Northern Metropolitan Health Service of Santiago, Chile. The study complied with the ethical principles of the Declaration of Helsinki.
Before the measurement and the beginning of the study, each participant was given all information about the test and the purpose of the study. Participants had to sign an informed consent form in order to participate in the study. Participation in the study was voluntary and participants could discontinue their involvement at any time. The inclusion criteria for sedentary subjects considered less than 150 minutes per week of moderate physical activity [20]. Subjects were excluded if they presented a history of any musculoskeletal pain in the last six months, pathological conditions of the vertebral column, neurological diseases, respiratory diseases, a systemic rheumatic condition, heritable connective tissue disorders, and/or any previous abdominal surgery.
Procedure
A 5–10 MHz linear array transducer was used SonoSite TITAN® (Sonosite Inc., Bothell, WA, USA). Ultrasound images were taken at a depth of 39 mm, and video was recorded (Pinnacle Dazzle® DVD Recorder HD, Corel Corp., Ottawa, ON, Canada) at a rate of 30 frames per second and a resolution of 720 × 480 pixels.
Video-photogrammetric analysis was performed using the GoPro Hero3 camera (GoPro Inc, San Mateo, CA, USA) at a rate of 60 frames per second and a resolution of 1440 × 1440 pixels. Offline pressure sensors were used to synchronize the 2D kinematic data from cervical flexion and deep fascia displacement of the MG. To determine cervical flexion, an established 2D marker protocol was implemented (Fig. 1a) [21].
The transducer of the ultrasound system was placed on the extreme dominant belly of the MG by attaching a clamping device composed of a thermoplastic polymer and two elastic bands with velcro (Fig. 1b).
All signals and video processing was analyzed with a programing language in Matlab software (MathWorks Inc, Natick, Massachusetts, USA). Lucas–Kanade affine template tracking was used to automatically track the markers of interests [2]. Prior to applying the algorithms, an exploratory consistency assay was performed to evaluate angular variation against the Gold Standard, which is based on the infrared SMART-D 140® recoding system (BTS BioEngineering, Milan, Italy). A 2D angular variation curve of movement determined by a goniometer with three markers was compared at 60 frames per second on a frontal plane with the GoPro. Lin’s concordance correlation coefficient was 0.999, and the average difference between the two methods was 0.360°, with threshold values of 1.0736–1.7946 (95 % confidence interval). The algorithm presented high precision and alignment with the Gold Standard. Finally, the angular variation curves of cervical flexion were processed using a 6 Hz low-pass filter [2].
A previously published methodology for automatic tracking was used to detect deep fascia displacement in the MG (Fig. 1c) [2]. From the automatically selected pixels to be used in tracking, the Euclidean distance (d) was calculated between the positions in time (t), (ptx, pty) and in the initial frame (p0x, p0y).
$$ {d}^t=\sqrt{{\left({p}_x^0-{p}_x^t\right)}^2+{\left({p}_y^0-{p}_y^t\right)}^2} $$
Formula 1. Calculation for Euclidean distance (dt).
The initial position on the axis (x) defined displacement in such a way that the distal end was positive and the proximal end negative (Fig. 1d). Since there are no prior reports for a time-related accumulation error for LKP tracking, an additional control experiment was performed in subjects that remained static during an ultrasound video recording of the deep fascia in the MG. This allowed for an evaluation of the basal tracking error in a temporal series. To ensure that subjects remained static, they were asked to suspend their breathing and hold a prone position for 4 s. This length of time was chosen as it was the average time recorded in experimental subjects from the start of movement to the end of the task. The basal tracking error was determined in two healthy men within the set ranges of the experimental groups. From the nine automatically selected tracking points, the Euclidean distance (dt) was calculated. There was an average difference of 0.0063 mm in dt (0.0664 pixels) between two consecutive frames, and the maximum standard deviation (max) (σt) was 0.0211 mm (0.2230 pixels) during the 4 s assay (total of 120 frames). This final error was defined as the basal tracking error (Fig. 2)
$$ \max \left({\sigma}^t\right)= \max \Big(\sqrt{{\displaystyle \sum_{k=1}^9}{\left({d}_k^t-{\displaystyle \sum_{p=1}^9}{d}_p^t*\frac{1}{9}\ \right)}^2*\frac{1}{9}\Big)} $$
Formula 2. Calculation for the basal tracking error.
To establish a reflex muscle contraction in the MG, the amplitude of muscle activity was recorded by surface electromyography at a sampling rate of 1000 Hz (Artoficio®, EMG VIII, Santiago, Chile). The skin was cleaned and the electrodes were positioned according to SENIAM recommendations [22]. To obtain a signal, a pass-band (20–450 Hz; Butterworth fourth order) and a band-stop filter (50 Hz; Butterworth fourth order) were used. Following this, the root mean square was applied with a 250 ms window. To assess electrical activity during the task, basal muscular activity was determined based on the average between a second before initiating the task and a second at the end of cervical spine flexion.
The task was practiced for 5 min prior to measurements, starting with the neck in a neutral position, as achieved by maintaining horizontal sight and keeping the other corporal segments relaxed and without any movement, and moving towards cervical spine flexion. Each subject was asked to perform each task three times. Each execution was standardized with a metronome so that the subject would finish each execution within 4 s. Maximal cervical flexion was performed in a sitting position with the lumbar and dorsal spine locked in maximal kyphosis and the knee fixed at full extension by a strap across the patella, which was adjusted as tightly as possible without causing pain or irritation for the subject. Ankles were locked at 90° by resting the foot on a static platform. Cervical flexion started from a neutral position to one of maximal flexion (Fig. 3). The task was considered successful if movement was only recorded for the cervical spine and, if this condition was not met, the test was nulled.
Data management and statistical analysis
Preliminary values were obtained from a pilot study in six subjects. The values of fascia displacement in the first 10 % of the cycle and those at the end of the cervical spine flexion were considered for sample size analysis. Since a statistical power of 90 %, an alpha of 5 %, and a loss rate 30 % were used to detect differences equal to or above 0.037 mm and with a standard deviation of 0.027 mm, the calculated sample size for the experimental condition was a minimum of 10 subjects.
Final average displacement was determined such that it would be above the basal LKP tracking error, thus determining the existence of deep fascia displacement of the MG. All data were analyzed with the SPSS for Windows 22.0 software package (IBM Corp, Armonk, New York, USA). A value of p < 0.05 was considered statistically significant. To determine normal distribution, the Shapiro-Wilk test was used. All data were normally distributed. To assess average MG fascial displacement, ANOVA testing was used, and readings from every 10 % of the cycle were compared against the first 10 % of the cycle.
