Reproducibility of electromyographic patterns in stretch-shortening type contractions

Summary Within a set of repeated measurements, 12 male subjects performed the following types of muscle contraction: (1) running at constant velocity (12 km· h^–1); (2) one-leg hopping at the frequency preferred; (3) both-legs hopping at the frequency preferred; (4) drop jumps from a height of 0.40 m. The surface electromyographs (EMG) from the left and right legs were recorded, together with the averaged signals from vertical ground reaction forces and angular displacement of the ankle joint. Reliability coefficients for the day to day and the week to week comparison were calculated. In addition, on any one test day the positions of the electrodes were systematically changed. The analysis of various EMG parameters (integrated-EMG) revealed high coefficients for both comparisons as well as for the total set of measurements. Qualitative comparisons of the EMG showed a high reproducibility of the shape of the patterns. If the relative position of the electrodes with respect to the belly of the muscle is changed, the amplitudes of the EMG recordings are markedly reduced. It is concluded that surface EMG is a reliable method for studies of the neuromuscular system. On the basis of the data presented, it is suggested that the term reliability should be replaced by the term reproducibility in order to cover both the aspect of the relative stability plus the aspects of linear changes and the scattering of the data in repeated measurements.     

Effect of mechanical compression due to load carrying on shoulder muscle fatigue during sustained isometric arm abduction: an electromyographic study

The use of surface electromyography (EMG) for studying the effect of mechanical compression of occupational origin on muscle fatigue has been the subject of poor attention in ergonomic research. This study examined the effect of backpack carrying on fatigue of two shoulder muscles during sustained low force static contraction: the middle deltoid (MD) muscle and the upper trapezius (UT) muscle on which the backpack strap exerted direct compressive force. EMG activities of MD and UT muscles, of the dominant and non-dominant sides, were studied on eight subjects during two tasks, a maximal and an exhausting submaximal bilateral isometric 90° arm abduction, which were performed while carrying a backpack load of 0, 10, and 20 kg, respectively. EMG amplitude (root mean square, RMS) and spectral (mean power frequency, MPF) parameters were computed from the recorded signals. No significant differences between the dominant and non-dominant sides were found for none of the parameters whatever the load-carrying mass. Load-carrying masses which were tested did not influence significantly the maximal bilateral arm abduction performance contrary to the time to exhaustion during the submaximal task, which decreased significantly with increasing load-carrying mass. A significant increase in fatigability, defined by the slope of MPF decrease, was observed for both muscles when load-carrying mass increased; but only UT presented a significant increase in muscle fatigue level, defined by the MPF value with respect to its initial value, at the end of the exhausting submaximal task. Furthermore, the increase in muscle activation of UT, quantified by RMS, during the exhausting task was not significantly higher with increasing load-carrying mass. So, the increased signs of local fatigue of UT may be interpreted by a localised blood flow impairment resulting from the direct compressive force exerted by backpack on this muscle.     

Single motor unit and spectral surface EMG analysis during low-force, sustained contractions of the upper trapezius muscle

Intramuscular and surface electromyographic (EMG) activities were recorded from the left and right upper trapezius muscle of eight healthy male subjects during 5-min long static contractions at 2% and 5% of the maximal voluntary contraction (MVC) force. Intramuscular signals were detected by wire electrodes while surface EMG signals were recorded with linear adhesive electrode arrays. The surface EMG signals were averaged using the potentials extracted from the intramuscular EMG decomposition as triggers. The conduction velocity of single motor units (MUs) was estimated over time from the averaged surface potentials while average rectified value and mean power spectral frequency were computed over time from 0.5 s epochs of surface EMG signal. It was found that (1) MUs were progressively recruited after the beginning of sustained contractions of the upper trapezius muscle at 2% and 5% MVC, (2) the conduction velocity of the MUs active since the beginning of the contraction significantly decreased over time, and (3) although the CV of single MUs significantly decreased, the mean power spectral frequency of the surface EMG did not show a consistent trend over time. It was concluded that spectral surface EMG analysis, being affected by many physiological mechanisms, may show limitations for the objective assessment of localized muscle fatigue during low force, sustained contractions. On the contrary, single motor unit conduction velocity may provide an early indication of changes in muscle fiber membrane properties with sustained activity.     

The influence of electrode position on bipolar surface electromyogram recordings of the upper trapezius muscle

Summary The effect of electrode position on the upper trapezius muscle on the myo-electric signal amplitude was investigated with special reference to arm position and estimate of force output. Previously, a depression of the electromyogram (EMG) signal has been reported midway between the seventh cervical vertebrae (C7) and acromion (Veiersted 1991, Eur J Appl Physiol 62:91–98) although this electrode position has been recommended (Zipp 1982, Eur J Appl Physiol 50:41–54). Ten healthy subjects performed maximal shoulder elevations with the arm in vertical, abducted and flexed positions and they performed a dynamic movement test. The myo-electric signal was recorded along the length of the right upper trapezius muscle by a 16-channel bipolar array electrode and was integrated with a 0.2-s time resolution. A region just lateral to the midpoint between C7 and the lateral edge of acromion was found with high and stable amplitudes (% coefficient of variation equalled 5.6). At the midpoint a dip in the amplitude profile appeared which was slightly displaced by arm abduction or flexion probably due to sliding, of the skin relative to the muscle. A linear EMG-force relationship was found in the region with high signal amplitudes, whereas the more lateral and the dip region showed highly variable EMG-force relationships. Thus, it was found that when using bipolar surface electrodes with an interelectrode distance of 2 cm a centre position 2 cm lateral to the midpoint between C7 and acromion provided good repeatability and high signal yield.     

Recommendations for the standardization of lead positions in surface electromyography

Summary Standardized lead positions for recording myoelectric signals are given for various muscles. The positions are given in terms of (1) the lead line connecting two anatomical landmarks and (2) the central lead point about which the electrodes are placed symmetrically on the lead line.This research was financially supported by the Sonderforschungsbereich 152 of the Deutsche Forschungsgemeinschaft     

Effects of electrode dislocation on electromyographic activity and relative rest time: Effectiveness of compensation by a normalisation procedure

Myofeedback seems a promising technique to treat myalgia related to computer work. A garment with embedded electrodes allows independent use by the subject. However, this inevitably results in electrode dislocations that can cause such variability in the electromyogram (EMG) parameters that the treatment fails. The objective was to investigate the variability introduced by electrode displacement on EMG root mean square (RMS) and relative rest time (RRT) and the effect of a normalisation procedure using a reference contraction to compensate for this. Dislocation was simulated using simultaneous recordings of pairs of electrodes situated 15 mm apart on the trapezius muscle, surrounding a central electrode pair at the standard position. To assess sensitivity for electrode displacement, intra-subject variability was used. To assess the effects of normalisation, both intra and inter-subject variability were used. Results indicated a large sensitivity of RMS to electrode displacements, with no differences between typing (42%) and the reference task (43%). No systematic changes were found relative to a specific electrode dislocation. Normalisation of RMS resulted in a decrease in the sensitivity for electrode displacement (33%), but did not decrease inter-subject variability (27%). The median value of the intra-subject variability of RRT, using a fixed threshold, was 4.9%, whereas the normalised threshold did not decrease the intra-subject variability (7%). The results suggest that RRT is suitable for individual follow-up measurements and applications such as myofeedback, without the need for a normalisation procedure. The use of RMS requires considerably more care, owing to its large sensitivity to electrode displacement and lack of substantial effects of normalisation.     

A method for electromyographic analysis of muscular contraction frequencies

Summary A method for evaluation of muscular contraction frequencies during prolonged work is presented. It is based upon computer-aided electromyographic (EMG) analyses. Variations in muscular tension, expressed as the relative force of contraction or torque in the joint, are followed using previously described EMG techniques. The number of times muscular tension crosses an amplitude band from below is then estimated. The band represents a predetermined range of amplitude. It is moved in steps from the lowest to the highest force level which occurred during the work task, and the frequency of band-crossings is computed for each band position. The frequencies are then plotted against the force levels at which the upper limit of the band was placed. From this relationship a frequency/force index is computed. The analysis is repeated several times using different band-widths. Thus, the method considers the contraction frequencies according to the load levels. It was developed to study the function of the musculo-venous pump in the legs during prolonged sedentary work, but might also be used to analyse such factors as muscular strain during work.     

Zero crossing rate of electromyograms during occupational work and endurance tests as predictors for work related myalgia in the shoulder/neck region

Summary The relationship between electromyographic signs of fatigue (ESF) during work and occupation-related myalgia in the shoulder/neck region was investigated in a longitudinal study. Forty-three healthy female assembly workers were studied over 2 years. Measurements were performed at the start of the study with follow-up measurements after 1 and 2 years. The ESF were estimated as the zero crossing rate of electromyograms (EMG) detected during short test contractions performed during short breaks in normal work. As a complement, an endurance test using EMG records was performed and analysed with the zero crossing technique. The occurrence of shoulder/neck disorders was assessed by a clinical investigation and a questionnaire. No significant relationship between ESF during work in year 0 and deterioration of the disorder was seen. On the other hand, the absolute zero crossing rate and the time constant of the zero crossing decline from the endurance test showed a significant relationship with deterioration of the disorder. The ESF during work year 2, showed a significant relationship with disorder year 2, while the endurance test parameters year 2 did not. It was concluded that ESF during work was not a predictor of muscle injury, whereas it could be useful as a diagnostic tool.     

The effect of hypoxia on shortening contractions in rat diaphragm muscle

Hypoxia is known to reduce isometric contractile properties of isolated rat diaphragm bundles. Its effect on isotonic contractile properties (i.e. force–velocity relationship and power output) has not been studied. We hypothesized that hypoxia reduces velocity of shortening and consequently power output of the unfatigued muscle, and shortens endurance time during isotonic contractions. Force–velocity relationship, power output, and fatigue resistance of rat diaphragm muscle bundles were measured during hypoxia (PO₂: 6.6 ± 0.2 kPa) and compared with hyperoxia (PO₂: 91.8 ± 0.7 kPa). Force was clamped from 1 to 100% of maximal tetanic force (P_o). Fatigue during isotonic contractions was induced by repeated stimulation every 2 s at a clamp level of 33% of P_o. Hypoxia did not affect isometric force generation compared with hyperoxia, nor contraction or relaxation time. In contrast, maximum shortening velocity decreased significantly (hypoxia: 4.2 ± 0.3, hyperoxia: 6.0 ± 0.2 L_o/s, P < 0.05). The force–velocity curve shifted downwards (P < 0.05). Hypoxia lowered power output at each load compared with hyperoxia (P < 0.05). The isotonic endurance time was shorter during hypoxia compared with hyperoxia (80 ± 2 vs. 130 ± 3 s, P < 0.05). These data show that hypoxia depresses isotonic contractile properties and power output, and reduces endurance time during repeated isotonic contractions.     

A simple technique for on-line measurement of contractions of single smooth muscle fibers under current or voltage clamp

This paper describes a simple technique for routine on-line measurement of length and unloaded, isotonic contractions of single smooth muscle fibers during electrophysiological experiments. The fiber is held by the recording electrode itself, stretched straight in a fastflowing stream of solution. The video image of the fiber is measured on-line by a simple computer program. Unlike other optical methods of tracking fiber length, the technique does not require a rigid one-dimensional contraction of the fiber. The technique is reliable and easy to use, and readily compatible with current clamp, voltage clamp, and rapid reversible application of neurotransmitters and drugs.     

声图像法的骨骼肌多参数测量

通过自行开发研制的超声运动与测量系统,连续、同步获取了肱二头肌等长收缩活动时的B超图像力矩、肌电信号,并将超声扫描骨骼肌得到的有关骨骼肌结构的空间形态变化信息定义为“声肌图（sonomyography,SMG）。通过采集到的信号研究并讨论了肌肉厚度变化率SMG和表面肌电信号、力矩之间的关系,并给出了经验公式,它对了解骨骼肌功能、对骨骼肌建模以及对康复设备的研发等有重要的作用和意义。     

The reproducibility of the oral glucose tolerance test over long (5 years) and short periods (1 week)

Summary Three oral glucose tolerance tests (oGTT) have been performed in 312 non-diabetic relatives of diabetics over a period of 10 years. In a second study 6 identical oGTT's have been performed at weekly intervals in 55 individuals. In this study the variance, calculated from the logarithmic values, increased in the following order: fasting (0.026), 1 h (0.035), 2 h (0.044) and 3 h values (0.047). The sum of the 1 h and 2 h values showed the lowest variance (0.024). No significant difference of the variances was found in the 43 individuals in whom both the long-term and the short-term studies have been performed. Thus, a great proportion of the total variance of glucose observed over longer periods only represents a random variation. This random variation is much higher than most other factors which might influence the result of an oGTT. A diagnosis based on a single oGTT is of only limited value.Supported by a grant from Deutsche Forschungsgemeinschaft (Ko 457/8)     

The course of the greater occipital nerve in the suboccipital region: a proposal for setting landmarks for local anesthesia in patients with occipital neuralgia

The anatomical relationships of the greater occipital nerve (GON) to the semispinalis capitis muscle (SCM) and the trapezius muscle aponeurosis (TMA) were examined to identify topographic landmarks for use in anesthetic blockade of the GON in occipital neuralgia. The course and the diameter of the GON were studied in 40 cadavers (29 females, 11 males), and the points where it pierced the SCM and the TMA were identified. The course of the GON did not differ between males and females. A left-right difference was detected in the site of the GON in the TMA region but not in the SCM region. The nerve became wider towards the periphery. This may be relevant to entrapment of the nerve in the development of occipital neuralgia. In three cases, the GON split into two branches before piercing the TMA and reunited after having passed the TMA, and it pierced the obliquus capitis inferior muscle in another three cases. The GON and the lesser occipital nerve reunited at the level of the occiput in 80% of the specimens. The occiput and the nuchal midline are useful topographic landmarks to guide anesthetic blockade of the GON for diagnosis and therapy of occipital neuralgia. The infiltration is probably best aimed at the site where the SCM is pierced by the GON.     

A negative expiratory pressure test during wakefulness for evaluating the risk of obstructive sleep apnea in patients referred for sleep studies

OBJECTIVE:

Obstructive sleep apnea is characterized by increased upper airway collapsibility during sleep. The present study investigated the use of the negative expiratory pressure test as a method to rule out obstructive sleep apnea.

METHODS:

Flow limitation was evaluated in 155 subjects. All subjects underwent a diurnal negative expiratory pressure test and a nocturnal sleep study. The severity of sleep apnea was determined based on the apnea-hypopnea index. Flow limitation was assessed by computing the exhaled volume at 0.2, 0.5, and 1.0 s (V_0.2, V_0.5, and V_1.0, respectively) during the application of a negative expiratory pressure and expressed as a percentage of the previous exhaled volume. Receiver-operating characteristic curves were constructed to identify the optimal threshold volume at 0.2, 0.5, and 1.0 s for obstructive sleep apnea detection.

RESULTS:

Mean expiratory volumes at 0.2 and 0.5 s were statistically higher (p<0.01) in healthy subjects than in all obstructive sleep apneic groups. Increasing disease severity was associated with lower expiratory volumes. The V_0.2 (%) predictive parameters for the detection of sleep apnea were sensitivity (81.1%), specificity (93.1%), PPV (98.1%), and NPV (52.9%). Sensitivity and NPV were 96.9% and 93.2%, respectively, for moderate-to-severe obstructive sleep apnea, and both were 100% for severe obstructive sleep apnea.

CONCLUSION:

Flow limitation measurement by V_0.2 (%) during wakefulness may be a very reliable method to identify obstructive sleep apnea when the test is positive and could reliably exclude moderate and severe obstructive sleep apnea when the test is negative. The negative expiratory pressure test appears to be a useful screening test for suspected obstructive sleep apnea.     

The importance of frequency and amount of electrical stimulation for contractile properties of denervated rat muscles

Soleus (SOL) and extensor digitorum longus (EDL) muscles were denervated and directly stimulated for 23–69 days through implanted electrodes employing three different patterns. The stimulation was delivered in impulse trains where the pulse frequency differed (20, 75, and 150 Hz), while the train duration (0.3 s) and train repetition rate (1 min^-1) were identical. Consequently, the number of pulses varied such that higher frequency was combined with a higher amount of stimulation. In both SOL and EDL the high-frequency pattern resulted in shorter twitch time-to-peak, greater post-tetanic potentiation, and greater tetanic force than the low frequency. Isotonic shortening velocity was increased to the same extent by all the patterns in SOL whereas in EDL fast intrinsic shortening velocity was maintained by the low-frequency pattern while it was decreased by the high-frequency pattern. We attribute this unexpected effect on the EDL to the larger number of pulses in the high-frequency pattern. By combining the present findings with previous data on directly stimulated rat muscles we conclude: in SOL the twitch duration is influenced by both the frequency and the amount of impulse activity, higher frequencies and smaller amounts leading to faster twitches. The EDL twitch duration is similarly dependent on the amount of activity, but the role of frequency is more unclear. In both SOL and EDL the isotonic shortening velocity is reduced by increasing amounts of activity and there is no evidence that impulse frequency plays a role. In EDL force output is strongly influenced by the impulse frequency, low frequencies resulting in low force outputs irrespective of the amount of activity.     

The effect of insulin on skeletal muscle contractions and its relation to the effect produced by β-adrenoceptor stimulation

The soleus, a slow-contracting, and the extensor digitorum longus (EDL), a fast-contracting muscle, from the guinea-pig were prepared for measurement of isometric contractions in vitro. Insulin, 2.5–55 mu/ml, caused a dose-dependent depression of twitches and subtetanic concentrations of the soleus muscle similar to and additive with that produced by the β₂-adrenoceptor agonist, terbutaline. The effect of terbutaline but not that of insulin was blocked by propranolol. Insulin had no apparent effect on the contractions of the EDL, whereas terbutaline increased the force of contraction. When depressed by KC1, however, insulin partially restored the twitch tension in both muscles. The possible role of effects on the Na⁺-K⁺ transport is discussed.     

The slow inward calcium current is responsible for a part of the contraction of patch-clamped rat myoballs

The slow inward calcium current and the contractile response were simultaneously recorded in voltage clamped (whole cell patch clamp recording) rat myoballs in primary culture.The shape of the contraction(T)/potential(V) relationship and the application of the inorganic calcium channel blocker cadmium (1.5 mM), which suppresses a part of the contractile activity, demonstrate the existence of two components of contraction. One of them is related to the slow calcium current.