Ian Cathers

Standard Maximum Isometric Voluntary Contraction Tests for Normalizing Shoulder Muscle EMG

A comparison of electromyographic (EMG) activity of muscles between and within subjects, and during separate occasions of testing, requires normalization. The most common way for generating the reference level used for normalizing shoulder EMG data is with a maximum isometric voluntary contraction (MVIC). The purpose of this study was to develop a parsimonious set of standardized tests that generate an MVIC in all the major muscle groups of the shoulder. Twelve muscles of the dominant shoulder of 15 subjects were examined using a combination of surface and intramuscular electrodes during 15 tests. The results indicated that many tests maximally activated more than one muscle simultaneously. Four tests were identified as being sufficient for generating an MVIC in the 12 muscles examined and are recommended as the standard set for normalizing shoulder muscle EMG: abduction 90° with internal rotation (“empty can”), internal rotation in 90° abduction (“internal rotation 90°”), flexion at 125° with scapula resistance (“flexion 125°”), and horizontal adduction at 90° flexion (“palm press”). The use of these shoulder normalization tests will make comparisons between shoulder EMG studies more reliable.

Otolith and canal reflexes in human standing

We used galvanic vestibular stimulation (GVS) to identify human balance reflexes of the semicircular canals and otolith organs. The experiment used a model of vestibular signals arising from GVS modulation of the net signal from vestibular afferents. With the head upright, the model predicts that the GVS‐evoked canal signal indicates lateral head rotation while the otolith signal indicates lateral tilt or acceleration. Both signify body sway transverse to the head. With the head bent forward, the model predicts that the canal signal indicates body spin about a vertical axis but the otolith signal still signifies lateral body motion. Thus, we compared electromyograms (EMG) in the leg muscles and body sway evoked by GVS when subjects stood with the head upright or bent forward. With the head upright, GVS evoked a large sway in the direction of the anodal electrode. This response was abolished with the head bent forward leaving only small, oppositely directed, transient responses at the start and end of the stimulus. With the head upright, GVS evoked short‐latency (60–70 ms), followed by medium‐latency (120 ms) EMG responses, of opposite polarity. Bending the head forward abolished the medium‐latency but preserved the short‐latency response. This is compatible with GVS evoking separate otolithic and canal reflexes, indicating that balance is controlled by independent canal and otolith reflexes, probably through different pathways. We propose that the short‐latency reflex and small transient sway are driven by the otolith organs and the medium‐latency response and the large sway are driven by the semicircular canals.

Neural network assisted cardiac auscultation

Traditional cardiac auscultation involves a great deal of interpretive skill. Neural networks were trained as phonocardiographic classifiers to determine their viability in this rôle. All networks had three layers and were trained by backpropagation using only the heart sound amplitude envelope as input. The main aspect of the study was to determine what topologies, gain and momentum factors lead to efficient training for this application. Neural networks which are trained with heart sound classes of greater similarity were found to be less likely to converge to a solution. A prototype normal/abnormal classifier was also developed which provided excellent classification accuracy despite the sparse nature of the training data. Future directions for the development of a full-scale computer-assisted phonocardiographic classifier are also considered.

A comprehensive analysis of muscle recruitment patterns during shoulder flexion: An electromyographic study

Although flexion is a common component of the routine clinical assessment of the shoulder the muscle recruitment patterns during this movement are not clearly understood making valid interpretation of potential muscle dysfunction problematic. The purpose of this study was to comprehensively examine shoulder muscle activity during flexion in order to compare the activity levels and recruitment patterns of shoulder flexor, scapular lateral rotator and rotator cuff muscles. Electromyographic (EMG) data were recorded from 12 shoulder muscles sites in 15 volunteers. Flexion was performed in standing in the sagittal plane at no load, 20%, and 60% of each subjects maximum load. EMG data were normalized to maximum values obtained during maximum voluntary contractions. Results indicated that anterior deltoid, pectoralis major, supraspinatus, infraspinatus, serratus anterior, upper, and lower trapezius were activated at similar moderate levels. However, subscapularis was activated at low levels and significantly lower than supraspinatus and infraspinatus. Similar activity patterns across time were demonstrated in the muscles that produce flexion torque, laterally rotate the scapula, as well as supraspinatus and infraspinatus, and did not change as flexion load increased. The onset of activity in supraspinatus and anterior deltoid occurred at the same time and prior to movement of the limb at all loads with infraspinatus activity also occurring prior to movement onset at the medium and high load conditions only. Posterior rotator cuff muscles appear to be counterbalancing anterior translational forces produced during flexion and it would appear that supraspinatus is one of the muscles that consistently “initiates” flexion. Clin. Anat. 24:619–626, 2011.

Associations among work-related stress, cortisol, inflammation, and metabolic syndrome.

This cross-sectional study examined the relationship between work-related stress, cortisol, and C-reactive protein (CRP) in predicting metabolic syndrome (MtS). Self-reported work stress measured by the effort reward imbalance ratio (ERI), anthropometric data, CRP, and saliva cortisol were collected from 204 healthy Jordanian male workers. ERI and cortisol were significantly associated with the presence of MtS (OR = 4.74, 95% CI: 2.13-10.55; OR = 3.03, 95% CI: 2.08-4.40; OR = 11.50, 95% CI: 2.16-59.14, respectively). The odds of MtS in men with high ERI and high cortisol were significantly higher than that of men with low ERI and low cortisol (OR = 11.50, 95% CI: 2.16-59.14). CRP was significantly associated with MtS (OR = 2.51, 95% CI: 1.50-4.20). The odds of MtS were significantly higher in centrally obese men with both high ERI and CRP level. Thus, high ERI along with high cortisol or high CRP increases the risk for MtS, especially among centrally obese men.

Voluntary modulation of human stretch reflexes

It has been postulated that the central nervous system (CNS) can tune the mechanical behavior of a joint by altering reflex stiffness in a task-dependant manner. However, most of the evidence supporting this hypothesis has come from the analysis of H-reflexes or electromyogram (EMG) responses. Changes in overall stiffness have been documented but, as yet, there is no direct evidence that the CNS can control reflex stiffness independently of the intrinsic stiffness. We have used a novel identification algorithm to estimate intrinsic and reflex stiffness and feed it back to subjects in real-time. Using this biofeedback, subjects could learn to control reflex stiffness independently of intrinsic stiffness. At low torque levels, subjects could vary their reflex stiffness gain by a factor of 4, while maintaining elastic stiffness and torque constant. EMG measurements confirmed that the contraction levels of the ankle muscles remained constant. Further experiments showed that subjects could change their reflexes rapidly on command. Thus, we conclude that the CNS can control reflex stiffness independently and so has great flexibility in adjusting the mechanical properties of a joint to meet functional requirements.

The rotator cuff muscles have a direction specific recruitment pattern during shoulder flexion and extension exercises

A recent study has shown that posterior rotator cuff (RC) muscles are recruited at significantly higher levels than the anterior RC during shoulder flexion. It was proposed that the mechanism whereby the posterior RC muscles were providing shoulder stability during flexion was to counterbalance potential anterior humeral head translation caused by flexion torque producing muscles. This hypothesis implies that anterior RC activity should be higher than posterior RC activity during extension to prevent posterior humeral head translation. As the normal recruitment pattern of the RC during extension has not been established, the purpose of this study was to examine this hypothesis by comparing shoulder muscle activation levels and recruitment patterns during flexion and extension exercises. Electromyographic (EMG) activity was recorded from 9 shoulder muscles in 15 volunteers. Flexion and extension exercises were performed in prone at 20%, 50%, and 70% of each participants maximal load. A repeated measures ANOVA was used to determine differences between exercises, muscles and loads, while Pearsons correlation analysis was used to relate mean EMG patterns. During extension subscapularis and latissimus dorsi were activated at higher levels than during flexion; during flexion, supraspinatus, infraspinatus, deltoid, trapezius, and serratus anterior were more highly activated than during extension. In addition, the pattern of activity in each muscle did not vary with load. These results support the hypothesis that during flexion and extension the RC muscles are recruited in a direction specific manner to prevent potential antero-posterior humeral head translation caused by torque producing muscles.

An Arabic version of the perceived stress scale: translation and validation study.

BACKGROUND The Perceived Stress Scale has been designed to measure the degree to which situations in a persons life are perceived as stressful. OBJECTIVE The paper describes the development of an Arabic version of the Perceived Stress Scale. DESIGN A translation process with cross-cultural considerations was employed to produce an Arabic version of the Perceived Stress Scale. SETTINGS Participants were asked to complete the Arabic version Perceived Stress Scale twice in their homes. PARTICIPANTS The Jordanian study population for the Arabic version Perceived Stress Scale validation consisted of 126 volunteers (74 male, 52 female). Ninety participants completed the scale twice (55 male, 35 female), of whom 58 were high schools teachers and 32 technical workers. Arabic was the first language of all participants and all gave informed consent. RESULTS The Arabic version Perceived Stress Scale reliability and validity were evaluated. Prior to an exploratory factor analysis, the suitability of data for factor analysis was assessed with acceptable results. The exploratory factor analysis showed two factors with eigenvalues greater than 1.0 (45.0% of variance). The Cronbachs alpha coefficients were 0.74 (Factor 1), 0.77 (Factor 2) and 0.80 for the Arabic version Perceived Stress Scale overall. The test-retest reliability had an intra-correlation coefficient of 0.90. CONCLUSIONS The Arabic version Perceived Stress Scale showed an adequate reliability and validity. Therefore, the Arabic Perceived Stress Scale is considered a suitable instrument to assess perceived stress in Arabic people.

Does passive mobilization of shoulder region joints provide additional benefit over advice and exercise alone for people who have shoulder pain and minimal movement restriction? A randomized controlled trial.

Background Passive mobilization of shoulder region joints, often in conjunction with other treatment modalities, is used for the treatment of people with shoulder pain and minimal movement restriction. However, there is only limited evidence supporting the efficacy of this treatment modality. Objective The purpose of this study was to determine whether passive mobilization of shoulder region joints adds treatment benefit over exercise and advice alone for people with shoulder pain and minimal movement restriction. Design This was a randomized controlled clinical trial with short-, medium- and longer-term follow-up. Setting The study was conducted in a metropolitan teaching hospital. Patients Ninety-eight patients with shoulder pain of local mechanical origin and minimal shoulder movement restriction were randomly allocated to either a control group (n=51) or an experimental group (n=47). Intervention Participants in both groups received advice and exercises designed to restore neuromuscular control at the shoulder. In addition, participants in the experimental group received passive mobilization specifically applied to shoulder region joints. Measurements Outcome measurements of shoulder pain and functional impairment, self-rated change in symptoms, and painful shoulder range of motion were obtained at 1, 3, and 6 months after entry into the trial. All data were analyzed using the intention-to-treat principle by repeated-measures analyses of covariance. Results No statistically significant differences were detected in any of the outcome measurements between the control and experimental groups at short-, medium-, or longer-term follow-up. Limitations Therapists and participants were not blinded to the treatment allocation. Conclusion This randomized controlled clinical trial does not provide evidence that the addition of passive mobilization, applied to shoulder region joints, to exercise and advice is more effective than exercise and advice alone in the treatment of people with shoulder pain and minimal movement restriction.

Tracking performance with sinusoidal and irregular targets under different conditions of peripheral feedback.

When studying muscle stretch reflexes with tonic stimuli or making a clinical assessment of muscle tone, it is imperative that the subject does not track the stretch stimulus either consciously or unconsciously. Such tracking contaminates reflex responses with voluntary ones and so invalidates any conclusions reached. Ideally, the stimuli used should be beyond the speed of a persons tracking ability. Both experiments on tonic stretch reflexes and clinical assessment of muscle tone of necessity involve the application of perturbations to the same limb from which a response is to be measured. These perturbations produce different peripheral feedback from the limb, including particularly cutaneous signals but also different Golgi tendon and muscle spindle afference than would occur for similar movements made voluntarily. This combination of peripheral signals resulting from perturbation of a limb is referred to here as perturbational feedback. There is evidence in the literature that subjects can generate voluntary responses to same-limb perturbations within latencies normally accepted for reflexes. Such fast responses might enable faster targets to be tracked voluntarily. In this study the tracking frequency response for the forearm was investigated using sinusoidal and irregular target signals. Perturbations were applied to a manipulandum and the subjects were required to voluntarily track these perturbations under two conditions: (1) where their arm was secured in the manipulandum and therefore they had perturbational feedback of tracking errors and (2) where their tracking arm was not in contact with the manipulandum and they had only visual or kinesthetic feedback of tracking errors. For sinusoidal target inputs, perturbational feedback allowed superior tracking performance. Many subjects could produce good tracking responses at 5 Hz and some as high as 7 Hz. This is a considerably higher frequency than was found when perturbational feedback was not present and greater than has been reported in the literature for all other types of tracking (typically about 2 Hz). In contrast, when irregular signals having power up to 4 Hz were used, perturbational feedback conferred only a marginal advantage on tracking performance. The enhancement of sinusoidal tracking performance by feedback may be due to the fact that cyclic movements can be internally generated by the subject and minimal reference to external cues provided by perturbational feedback can be used to synchronise such self-generated movements with a target. In contrast, this proposed mechanism cannot be used for tracking of irregular targets. Since perturbational feedback did not improve the maximum tracking frequency for irregular targets, there was no evidence for the operation of a shorter latency, same-limb displacement response.