2. Plan de marketing
2.3. Presupuesto anual
Normalising data is important to compare the quantified surface EMG signal. The surface EMG signal may be changed from one to the next session by several factors, including a slight change in electrode site, change in skin impedance, or a change in skin temperature (Cram et al., 1998c; LeVeau & Andersson, 1992). Normalisation can minimise the effects of these changes between different sessions. Normally one reference muscle contraction is
chosen in order to normalise EMG data. The most common reference contraction is a maximum voluntary isometric muscle contraction (MVC) (Cram et al., 1998c; LeVeau &
Andersson, 1992; Soderberg & Knutson, 2000). Once the MVC is recorded, the value of the EMG signal is divided by the value of the MVC with respect to the amplitude of the signal.
As a result, the normalised EMG signal is relative to the MVC and it can always be shown as a percentage of the MVC, however, there is no evidence in the literature that chiropractors use this MVC technique in their clinical practice. In this study, the experimental protocol was designed to approximate the protocol used in the clinical setting, therefore the MVC was not measured. In the present study, normalising the EMG signal was used to assess the reproducibility of the surface EMG signal to measure the low back muscle activity during static posture without the MVC technique.
a) Sitting with no arms raised to 90 degrees (SITN) for 10 subjects
After normalising, during sitting with no arms raised (SITN) ICC value was improved for all normalised RMS ratios with respect to the lumbar superficial multifidus muscle group (Table 3 – 6). This improvement in ICC values after normalising the data was similar to the results of a previous study (Lehman, 2002), however, only one ICC value was greater than 0.75 after normalisation which is an indication of acceptable reliability (Fleiss, 1986).
b) EMG signal cleaned by the manual and semi-automatic ECG cleaning techniques
When the signal was cleaned by the manual ECG cleaning technique, four point kneeling posture had poorer reliability than that of sitting and standing posture with respect to medial muscle group. Standing was more reliable posture than sitting posture. In other words, standing posture had less repositioning error than the other two postures. This order was similar with a previous study (Preuss et al., 2003). However, the reliability of the EMG signal was changed when the signal was cleaned by the semi-automatic ECG cleaning technique. In
other words, standing posture had more repositioning error than the other postures when the raw data was cleaned by the semi-automatic ECG cleaning technique. Semi-automatic ECG cleaning technique could take into account more variability in the amplitude of the signal than manual ECG cleaning technique although the normalised technique was used to minimise several factors which affects the quality of the signal.
There were two reasons to consider as to why the surface EMG is still not reproducible after normalising and cleaning the signal, even though great care was taken to minimise those factors that introduce variability. First, the motor unit recruitment strategy used by the central nervous system (CNS) may be different between different recording sessions. The number of motor units recruited for a specific muscle contraction depends on the level of exertion required. As the force of muscle contraction increases, more motor units are recruited.
Different motor units contain muscle fibres that are either fast-twitch or slow-twitch. Motor units containing slow-twitch muscle fibres are smaller than those motor units containing fast-twitch muscle fibres. Also motor units containing slow-fast-twitch muscle fibres are easier to excite than motor units containing fast-twitch muscle fibres. In other words, during a muscle contraction small motor units are recruited first. As the muscle contraction increases, the larger motor units that contain fast-twitch muscle fibres are recruited. The static posture with no arms raised is associated with a low level of muscle contraction. During such low level contractions, the small motor units are much more likely to be recruited rather than the big motor units (Edgerton et al., 1983). Although the number of motor units recruited for a given contraction will be similar, the CNS could recruit different motor units to carry out the same task. If during each recording session different small motor units are recruited, it is likely that the amplitude of the surface EMG signal may be influenced by this difference.
The second reason why the surface EMG signal recorded from the lumbar paraspinal muscles may be associated with poor to moderate reliability is related to lumbo-pelvic alignment.
Static posture requires the different segments of the spine and pelvis to align themselves in order to keep the trunk in a vertical or horizontal axis. Maintaining the static posture consists of many combinations of alignment, and each alignment is associated with a slightly different position of the vertical projection of the centre of gravity of the body. O’Sullivan et al. (2006) investigated trunk muscle activation during three different types of unsupported sitting posture (thoracic upright sitting, slump sitting, and lumbo-pelvic upright sitting) using surface EMG. In their study, there was a significant difference in spinal-pelvic curvature among the three different sitting postures in the sagittal plane. The different curvatures were associated with different levels of trunk muscle activation. In present study, three customised mechanical pointers (the mastoid process, acromion process, and iliac crest) were used in order to reproduce the sitting posture. It is possible that spinal-pelvic curvature in sagittal plane was not the same at each recording session despite the use of these mechanical pointers to guide the position of the different body segments. The study by Womersley and May (2006) found there was a difference lumbar angle in relaxed sitting between non-low back pain and low back pain groups. Therefore, in clinical practice, it could be possible to record the EMG signal from low back muscles during different relaxed sitting posture without any mechanical pointers. Mechanical markers including hands and knees position were used to guide of reproducing four point kneeling posture. Therefore, shoulder joint and hip joint angles could be reproduced as 90 degrees flexion for next session. However, spinal-pelvic curvature in sagittal plane for four point kneeling posture may not be reproduced as well as sitting posture.
As a result, the magnitude of the surface EMG signal may have been different at each recording session.
3.5. Conclusion
This study suggests that the ECG artefact remains a significant impediment to on-line analysis of a low level of the amplitude of the signals when the surface EMG signal was recorded from the lumbar paraspinal muscles during static postures. Therefore, it is important to inspect visually the raw signal before processing. As a result, without visual inspection for the raw signal, chiropractors may use the contaminated EMG signal (the EMG signal + the ECG artefact) as a diagnostic tool when the signal is recorded from paraspinal muscles during the maintenance of a static posture. The contaminated signal was not a valid measure of the level of exertion of the lumbar paraspinal muscles during static postures, mostly because the ECG artefact was a larger component than the EMG signal. In addition, the waveform of the ECG artefact was not consistent, because heart rate is variable. When the EMG signal is contaminated by the ECG artefact, the ECG artefact should be removed by the ECG cleaning techniques.
With regard to the static posture investigated, such static postures are associated with a low level of lumbar paraspinal muscle contraction. Although moderate muscle contraction may improve the reliability of the surface EMG signal recorded from the lumbar paraspinal muscles between different recording sessions and reduce the frequency of the ECG artefact, the reliability was still poor to moderate (ICC < 0.75) for non-normalised and normalised data.
This occurred despite removal of the ECG artefact and considerable effort to minimise differences in the adopted static postures and electrode sites. Additionally, this study employed strict skin-cleaning protocols to ensure optimal and reproducible recording conditions.
When the surface EMG signal is recorded from the lumbar paraspinal muscles during maintenance of a static posture, there are a number of steps which are necessary to carry out.
Visual inspection is the first step to identify whether the raw EMG signal is contaminated by the ECG artefact. After that, the second step is the ECG cleaning when the raw signal is contaminated by the ECG artefact. Such signal is required to be cleaned by the ECG cleaning technique before calculating the amplitude and frequency of the signal. Then, the cleaned data (only EMG signal) is required to be normalised to compare between subjects and between different days. This is the third step. When the reliability of the normalised cleaned data between different recording sessions is good in a healthy population, this signal can be used.
However, this study concludes that reliability of surface EMG is associated with a level of low back muscle contraction is poor when the signal is recorded from the lumbar paraspinal muscles during static posture.
If the surface EMG signal is still used to measure the functional status of the lumbar paraspinal muscles in clinical practice, alternative activity which is associated with greater than a moderate level of lumbar paraspinal muscle contraction will be recommended instead of a static posture. This activity will bring two advantages. One advantage is that the EMG signal may be less contaminated by the ECG artefact. Therefore, it may not be necessary to clean the EMG signal. Another advantage is that the reliability of the cleaned signal (only EMG signal) may be better than the results of our study. In future studies, firstly it is necessary to define the activity which is associated with greater than a moderate level of the lumbar paraspinal muscle contraction.