Nociceptive, neuropathic, or nociplastic low back pain? The low back pain phenotyping (BACPAP) consortium's international and multidisciplinary consensus recommendations.

The potential to classify low back pain as being characterised by dominant nociceptive, neuropathic, or nociplastic mechanisms is a clinically relevant issue. Preliminary evidence suggests that these low back pain phenotypes might respond differently to treatments; however, more research must be done before making specific recommendations. Accordingly, the low back pain phenotyping (BACPAP) consortium was established as a group of 36 clinicians and researchers from 13 countries (five continents) and 29 institutions, to apply a modified Nominal Group Technique methodology to develop international and multidisciplinary consensus recommendations to provide guidance for identifying the dominant pain phenotype in patients with low back pain, and potentially adapt pain management strategies. The BACPAP consortium's recommendations are also intended to provide direction for future clinical research by building on the established clinical criteria for neuropathic and nociplastic pain. The BACPAP consortium's consensus recommendations are a necessary early step in the process to determine if personalised pain medicine based on pain phenotypes is feasible for low back pain management. Therefore, these recommendations are not ready to be implemented in clinical practice until additional evidence is generated that is specific to these low back pain phenotypes.

Posterior muscle chain activity during various extension exercises: an observational study.

BACKGROUND: Back extension exercises are often used in the rehabilitation of low back pain. However, at present it is not clear how the posterior muscles are recruited during different types of extension exercises. Therefore, the present study will evaluate the myoelectric activity of thoracic, lumbar and hip extensor muscles during different extension exercises in healthy persons. Based on these physiological observations we will make recommendations regarding the use of extensions exercises in clinical practice. METHODS: Fourteen healthy subjects performed four standardized extension exercises (dynamic trunk extension, dynamic-static trunk extension, dynamic leg extension, dynamic-static leg extension) in randomized order at an intensity of 60% of 1-RM (one repetition maximum). Surface EMG signals of Latissimus dorsi (LD), Longissimus thoracis pars thoracic (LTT) and lumborum (LTL), Iliocostalis lumborum pars thoracic (ILT) and lumborum (ILL), lumbar Multifidus (LM) and Gluteus Maximus (GM) were measured during the various exercises. Subsequently, EMG root mean square values were calculated and compared between trunk and leg extension exercises, as well as between a dynamic and dynamic-static performance using mixed model analysis. During the dynamic exercises a 2 second concentric contraction was followed by a 2 second eccentric contraction, whereas in the dynamic-static performance, a 5 second isometric interval was added in between the concentric and eccentric contraction phase. RESULTS: In general, the muscles of the posterior chain were recruited on a higher level during trunk extension (mean ± SD, 56.6 ± 30.8%MVC) compared to leg extension (47.4 ± 30.3%MVC) (p ≤ 0.001). No significant differences were found in mean muscle activity between dynamic and dynamic-static performances (p = 0.053). The thoracic muscles (LTT and ILT) were recruited more during trunk extension (64.9 ± 27.1%MVC) than during leg extension (54.2 ± 22.1%MVC) (p = 0.045) without significant differences in activity between both muscles (p = 0.138). There was no significant differences in thoracic muscle usage between the dynamic or dynamic-static performance of the extension exercises (p = 0.574). CONCLUSION: The recruitment of the posterior muscle chain during different types of extension exercises was influenced by the moving body part, but not by the type of contraction. All muscle groups were activated at a higher degree during trunk extension compared to leg extension. Based on the recruitment level of the different muscles, all exercises can be used to improve the endurance capacity of thoracic muscles, however for improvement of lumbar muscle endurance leg extension exercises seem to be more appropriate. To train the endurance capacity of the LD and GM extension exercises are not appropriate.

Conscious correction of scapular orientation in overhead athletes performing selected shoulder rehabilitation exercises: the effect on trapezius muscle activation measured by surface electromyography.

STUDY DESIGN: Controlled laboratory study. OBJECTIVES: To assess the effect of conscious correction of scapular orientation on the activation of the 3 sections of the trapezius muscle during shoulder exercises in overhead athletes with scapular dyskinesis. BACKGROUND: Previous research has led to the recommendation of 4 exercises for training of the trapezius muscle: prone extension, sidelying external rotation, sidelying forward flexion, and prone horizontal abduction with external rotation. However, the extent to which conscious correction of scapular orientation impacts trapezius muscle activation levels during these exercises is unknown. METHODS: Absolute (upper trapezius [UT], middle trapezius [MT], lower trapezius [LT]) and relative (UT/MT and UT/LT) muscle activation levels were determined with surface electromyography in 30 asymptomatic overhead athletes with scapular dyskinesis, during 4 selected exercises performed with and without conscious correction of scapular orientation. Repeated-measures analyses of variance were used to determine if a voluntary scapular orientation correction strategy influenced the activation levels of the different sections of the trapezius during each exercise. RESULTS: With conscious correction of scapular orientation, activation levels of the 3 sections of the trapezius muscle significantly increased during prone extension (mean ± SD difference: UT, 5.9% ± 8.6% maximal voluntary isometric contraction [MVIC]; MT, 13.8% ± 11.0% MVIC; LT, 9.8% ± 10.8% MVIC; P<.05) and sidelying external rotation (UT, 2.2% ± 4.4% MVIC; MT, 6.7% ± 10.6% MVIC; LT, 13.3% ± 24.4% MVIC; P<.05). There was no difference between conditions for sidelying forward flexion and prone horizontal abduction with external rotation. The UT/MT and UT/LT ratios were similar between conditions for all 4 exercises. CONCLUSION: Conscious correction of scapular orientation during the prone extension and sidelying external rotation exercises can be used to increase the activation level in the 3 sections of the trapezius in overhead athletes with scapular dyskinesis. Although lack of kinematic data limits the interpretation of the results, this study suggests that conscious correction of scapular orientation can be performed without altering the favorable UT/MT and UT/LT ratios that have been previously reported for these exercises.

Velocity of isokinetic trunk exercises influences back muscle recruitment patterns in healthy subjects.

Isokinetic exercises at different angular velocities on Cybex devices are often used for assessment and therapy in chronic low back pain patients. Little is known about the effect of velocity of movement on the muscle activity during these exercises. The purpose of this study was to investigate both relative muscle activity and ratios of local to global muscle activity at the different velocities of isokinetic movements on a Cybex dynamometer. Fifty-three healthy employees of Belgian Defence (26 male and 27 female) aged between 20 and 57 years old voluntarily performed isometric and isokinetic exercises at four different velocities. Surface electromyographic signals of different abdominal and back muscles were recorded on both sides. Both the relative muscle activity and the local to global muscle activity ratio of the back muscles were affected by changes in velocities of isokinetic exercises. The global muscle system was more influenced by changes in velocity, than the local muscle system. Abdominal relative muscle activity and ratios were not influenced by velocity of movement. This study revealed that the velocity of isokinetic extension exercises influences the recruitment of the back muscles, meaning that protocols of training programs should be adapted in function of the focus of the therapy.

Muscle functional MRI as an imaging tool to evaluate muscle activity.

Muscle functional magnetic resonance imaging (mfMRI) is an innovative technique that offers a noninvasive method to quantify changes in muscle physiology following the performance of exercise. The mfMRI technique is based on signal intensity changes due to increases in the relaxation time of tissue water. In contemporary practice, mfMRI has proven to be an excellent tool for assessing the extent of muscle activation following the performance of a task and for the evaluation of neuromuscular adaptations as a result of therapeutic interventions. This article focuses on the underlying mechanisms and methods of mfMRI, discusses the validity and advantages of the method, and provides an overview of studies in which mfMRI is used to evaluate the effect of exercise and exercise training on muscle activity in both experimental and clinical studies.

Computed tomographic analysis of the quality of trunk muscles in asymptomatic and symptomatic lumbar discectomy patients.

BACKGROUND: No consensus exists on how rehabilitation programs for lumbar discectomy patients with persistent complaints after surgery should be composed. A better understanding of normal and abnormal postoperative trunk muscle condition might help direct the treatment goals. METHODS: A three-dimensional CT scan of the lumbar spine was obtained in 18 symptomatic and 18 asymptomatic patients who had undergone a lumbar discectomy 42 months to 83 months (median 63 months) previously. The psoas muscle (PS), the paraspinal muscle mass (PA) and the multifidus muscle (MF) were outlined at the L3, L4 and L5 level. Of these muscles, fat free Cross Sectional Area (CSA) and fat CSA were determined. CSA of the lumbar erector spinae (LES = longissimus thoracis + iliocostalis lumborum) was calculated by subtracting MF CSA from PA CSA. Mean muscle CSA of the left and right sides was calculated at each level. To normalize the data for interpersonal comparison, the mean CSA was divided by the CSA of the L3 vertebral body (mCSA = normalized fat-free muscle CSA; fCSA = normalized fat CSA). Differences in CSA between the pain group and the pain free group were examined using a General Linear Model (GLM). Three levels were examined to investigate the possible role of the level of operation. RESULTS: In lumbar discectomy patients with pain, the mCSA of the MF was significantly smaller than in pain-free subjects (p = 0.009) independently of the level. The mCSA of the LES was significantly smaller in pain patients, but only on the L3 slice (p = 0.018). No significant difference in mCSA of the PS was found between pain patients and pain-free patients (p = 0.462). The fCSA of the MF (p = 0.186) and of the LES (p = 0.256) were not significantly different between both populations. However, the fCSA of the PS was significantly larger in pain patients than in pain-free patients. (p = 0.012).The level of operation was never a significant factor. CONCLUSIONS: CT comparison of MF, LES and PS muscle condition between lumbar discectomy patients without pain and patients with protracted postoperative pain showed a smaller fat-free muscle CSA of the MF at all levels examined, a smaller fat- free muscle CSA of the LES at the L3 level, and more fat in the PS in patients with pain. The level of operation was not found to be of importance. The present results suggest a general lumbar muscle dysfunction in the pain group, in particular of the deep stabilizing muscle system.

ISSLS prize winner: Smudging the motor brain in young adults with recurrent low back pain.

STUDY DESIGN: Cross-sectional design. OBJECTIVE: To investigate whether recurrent low back pain (LBP) is associated with changes in motor cortical representation of different paraspinal muscle fascicles. SUMMARY OF BACKGROUND DATA: Fascicles of the lumbar paraspinal muscles are differentially activated during function. Human studies indicate this may be associated with a spatially separate array of neuronal networks at the motor cortex. Loss of discrete control of paraspinal muscle fascicles in LBP may be because of changes in cortical organization. METHODS: Data were collected from 9 individuals with recurrent unilateral LBP and compared with 11 healthy participants from an earlier study. Fine-wire electrodes selectively recorded myoelectric activity from short/deep fascicles of deep multifidus (DM) and long/superficial fascicles of longissimus erector spinae (LES), bilaterally. Motor cortical organization was investigated using transcranial magnetic stimulation at different scalp sites to evoke responses in paraspinal muscles. Location of cortical representation (center of gravity; CoG) and motor excitability (map volume) were compared between healthy and LBP groups. RESULTS: Individuals with LBP had a more posterior location of LES center of gravity, which overlapped with that for DM on both hemispheres. In healthy individuals, LES center of gravity was located separately at a more anterior location to that for DM. Map volume was reduced in LBP compared to healthy individual across muscles. CONCLUSION: The findings highlight that LBP is associated with a loss of discrete cortical organization of inputs to back muscles. Increased overlap in motor cortical representation of DM and LES may underpin loss of differential activation in this group. The results further unravel the neurophysiological mechanisms of motor changes in recurrent LBP and suggest motor rehabilitation that includes training of differential activation of the paraspinal muscles may be required to restore optimal control in LBP.

Trapezius muscle timing during selected shoulder rehabilitation exercises.

STUDY DESIGN: Controlled laboratory study. OBJECTIVES: To examine the timing of the 3 portions of the trapezius muscle in relation to the posterior deltoid (PD) muscle and in relation to one another during 4 selected shoulder exercises: (1) prone extension, (2) forward flexion in side lying, (3) external rotation in side lying, and (4) prone horizontal abduction with external rotation. BACKGROUND: Deficiencies in trapezius muscle recruitment have been identified in patients with shoulder pain. Alterations in the trapezius muscle activation level and timing have been identified in previous research. Scapular muscle exercises in which the middle trapezius (MT) and lower trapezius (LT) muscle showed optimal activity with minimal upper trapezius (UT) muscle participation have been recently identified. However, it is currently unknown if these exercises also promote early activation of the scapular stabilizing musculature. METHODS: The intermuscular and intramuscular timing of muscle activation (based on an activation level of greater than 10% maximum voluntary contraction beyond basic activity) of the 3 portions of the trapezius muscle during 4 exercises were examined by surface EMG in 30 healthy subjects on the dominant side (14 males, 16 females). A 1-sample t test was used to determine which portions of the trapezius muscle were activated significantly earlier or later than the PD (intermuscular timing). An analysis of variance for repeated measures (3 levels) was used for each exercise to determine possible timing differences among the 3 portions of the trapezius muscle (intramuscular timing). RESULTS: Intermuscular and intramuscular differences in timing of the portions of the trapezius muscle were found. The UT was activated significantly later than the PD (P<.01), and the MT was activated significantly earlier than the PD (P<.01), during the prone extension exercise. During the horizontal abduction with external rotation exercise, the MT (P<.01) and the LT (P = .01) were activated significantly earlier than the PD. During prone extension, side-lying external rotation, and prone horizontal abduction with external rotation, significant differences were found between the UT and MT, between the UT and LT, but not between the MT and LT. In these exercises the MT and LT were activated significantly earlier than the UT. During forward flexion in side lying, no significant timing differences were found between the activation of the portions of the trapezius. CONCLUSIONS: With the exception of the LT during prone extension, the prone extension exercise and the prone horizontal abduction with external rotation exercise promote early activation of the MT and LT in relation to the scapular and glenohumeral prime mover. Taking into account the limited generalizability of the results due to a narrow age range, these exercises are potentially promising for the treatment of intermuscular and intramuscular timing disorders of the trapezius muscle.

The effect of increasing resistance on trunk muscle activity during extension and flexion exercises on training devices.

Although progressive resistance training of trunk muscles on devices is very common, today, the effects of increasing resistance on trunk muscle activity during dynamic extension and flexion movements on training devices have not been reported yet. Thirty healthy subjects participated in maximal isometric and submaximal dynamic (at 30%, 50% and 70% of maximum mean torque (MMT)) extension and flexion exercises on Tergumed lumbar training devices. The normalized (as a percentage of maximal voluntary isometric contractions (MVIC)) electromyographic activity of 16 abdominal and back muscles was investigated. The results of the present study indicated that in general, with increasing resistance from 30% MMT to 50% MMT and 70% MMT, the activity of all back muscles during the extension exercises and the activity of all abdominal muscles during the flexion exercises increased significantly. To train strength (>60% of MVIC), low intensities (30% and 50% MMT) appeared sufficient to affect the back muscles, but for the abdominals higher resistance (70% MMT) was required. In contrast to the other back muscles, the lumbar multifidus demonstrated high activity levels during both the extension and the flexion exercises. As the lumbar multifidus is demonstrated to be an important muscle in segmental stabilization of the lumbar spine, this finding may help in understanding the efficacy of rehabilitation programs using specific training devices.

Electromyographic activity of trunk and hip muscles during stabilization exercises in four-point kneeling in healthy volunteers.

Stabilization exercises are intended to optimize function of the muscles that are believed to govern trunk stability. Debate exists whether certain muscles are more important than others in optimally performing these exercises. Thirty healthy volunteers were asked to perform three frequently prescribed stabilization exercises in four-point kneeling. The electromyographic activity of different trunk and hip muscles was evaluated. Average amplitudes obtained during the exercises were normalized to the amplitude in maximal voluntary contraction (% MVIC). During all three exercises, the highest relative muscle activity levels (> 20% MVIC) were consistently found in the ipsilateral lumbar multifidus and gluteus maximus. During both the single leg extension (exercise 1) and the leg and arm extension exercise (exercise 2) the contralateral internal oblique and ipsilateral external oblique reached high levels (> 20%MVIC). During exercise 2 there were also high relative activity levels of the ipsilateral lumbar part and the contralateral thoracic part of the iliocostalis lumborum and the contralateral lumbar multifidus. During the leg and arm extension exercise with contralateral hip flexion (exercise 3) there were high relative muscle activity levels of all back muscles, except for the latissimus dorsi muscle. The lowest relative muscle activity levels (< 10% MVIC) were found in the rectus abdominis and the ipsilateral internal oblique during all exercises, and in the contralateral gluteus maximus during exercises 1 and 2. The results of this study show that in exercises in four-point kneeling performed by healthy subjects, hip and trunk muscles seem to work together in a harmonious way. This shows that when relative activity of muscles is measured, both "global and local" muscles function together in order to stabilize the spine.

The influence of specific training on trunk muscle recruitment patterns in healthy subjects during stabilization exercises.

Low back pain is a major problem involving high medical costs, therefore effective prevention strategies are essential. Stabilization exercises seem to facilitate the neuromuscular control of the lumbar spine and may be useful in prevention programs. To investigate whether specific lumbar stabilization training has an effect on muscle recruitment patterns in a healthy population, in the present study 30 subjects were recruited to perform two types of testing exercises, i.e. bridging exercises and exercises in four-point kneeling, both before and after training. Surface electromyographic data of different abdominal and back muscles were obtained. After training, analysis of the relative muscle activity levels (percentage of maximal voluntary isometric contraction) showed a higher activity of the local (segmental-stabilizing) abdominal muscles, but not of the local back muscles; minimal changes in global (torque-producing) muscle activity also occurred. Analysis of the local/global relative muscle activity ratios revealed higher ratios during all exercises after training, although not all differences were significant. These results indicate that muscle recruitment patterns can be changed in healthy subjects by means of a training program that focuses on neuromuscular control. Additional studies are needed to evaluate this type of training as a prevention strategy.

Trunk muscle activity in healthy subjects during bridging stabilization exercises.

BACKGROUND: Trunk bridging exercises are often used as therapeutic exercises for lumbopelvic stabilization. These exercises focus on the retraining of muscle coordination patterns in which optimal ratios between local segmental stabilizing and global torque producing muscle activity are assumed to be essential. However, a description of such ratios is lacking. The purpose of this study was to investigate both relative (as a percentage of maximal voluntary isometric contraction) muscle activity levels and ratios of local to global muscle activity, during bridging stabilization exercises. METHODS: Thirty healthy university students (15 men, 15 women) with a mean age of 19.6 year volunteered to perform 3 bridging exercises (single bridging, ball bridge and unilateral bridging). The surface electromyographic activity of different trunk muscles was evaluated on both sides. RESULTS: During all bridging exercises, the ratio of the internal oblique to the rectus abdominis was very high due to minimal relative activity of the rectus abdominis. In general, the ratio of the internal/external abdominal oblique activity was about 1. However, during the unilateral bridging exercise, the ipsilateral internal/external abdominal oblique activity ratio was 2.79 as a consequence of the significant higher relative activity of the internal oblique compared to the external oblique. The relative muscle activity and the ratios of the back muscles demonstrated similar activity levels for all back muscles, resulting in ratios about 1. CONCLUSION: Both the minimal relative activity of the rectus abdominis and the high internal oblique to the rectus abdominis activity ratio reported in the present study are in accordance with results of other trunk stabilization exercises. The relative muscle activity and the ratio of the abdominal obliques seem to alter depending on the task and the presumable need for stability. The findings concerning the relative muscle activity and the ratios of the back muscles support the assumption that during these bridging exercises, all back muscles contribute in a similar way to control spine positions and movements in a healthy population.

Reliability of a functional clinical test battery evaluating postural control, proprioception and trunk muscle activity.

OBJECTIVE: The purpose of this study was to examine the repeatability and reproducibility of the different tests of a clinical test battery evaluating the components of functional spinal stability: postural control (sway velocity data), proprioception (repositioning error), and muscle activation (electromyographic data). DESIGN: A total of 28 healthy volunteers participated in this study: 14 in the repeatability study and 14 in the reproducibility study. Each subject was tested three times, with an interval of 1 wk between the test sessions. The intraclass correlation coefficients and the standard error of the measurements as a percentage of the grand mean were calculated. RESULTS: The intraclass correlation coefficients for both the repeatability and the reproducibility evaluation showed good to excellent reliability for all variables (intraclass correlation coefficient, 0.60-0.98). The standard error of the measurements as a percentage of the grand mean ranged from 0.004 to 19.94. CONCLUSIONS: The functional clinical test battery investigated in this study proved to be a reliable tool in the assessment of healthy subjects. The evaluation of postural control, proprioception, and muscle activity (coordination, stabilization, maximal voluntary isometric contraction, endurance, and flexion-relaxation) showed good to excellent repeatability and reproducibility. Further analysis of the reliability of these variables in a clinical setting, particularly in patients with low back pain, seems appropriate.

Isokinetic Scapular Muscle Performance in Overhead Athletes With and Without Impingement Symptoms.

Context: Overhead activities such as throwing, tennis, or volleyball place athletes at considerable risk for overuse injuries. A relationship between scapulothoracic muscle imbalance and shoulder pain has been suggested.Objective: To compare the isokinetic muscle performance of the scapular muscles between overhead athletes with impingement symptoms and uninjured overhead athletes and to identify strength deficits in the patient population.Design: A repeated-measures analysis of variance with 1 within-subjects factor (side) and 1 between-subjects factor (group) was used to compare strength values and agonist:antagonist ratios across sides and across groups.Setting: University laboratory.Patients or Other Participants: Thirty overhead athletes with chronic shoulder impingement symptoms and 30 overhead athletes without a history of shoulder pain.Intervention(s): A linear protraction-retraction movement in the scapular plane at 2 velocities (12.2 cm/s and 36.6 cm/s).Main Outcome Measure(s): Isokinetic strength values and protraction:retraction ratios for both velocities.Results: Overhead athletes with impingement symptoms showed decreased force output:body weight at both velocities in the protractor muscles on the injured side compared with the uninjured side (-13.7% at slow velocity, -15.5% at high velocity) and compared with the control group at high velocity (-20.7%). On both sides, the patient group had significantly lower protraction:retraction ratios than the control group, measured at slow velocity (nondominant = -11%, dominant = -13.7%).Conclusions: Overhead athletes with impingement symptoms demonstrated strength deficits and muscular imbalance in the scapular muscles compared with uninjured athletes.

Scapular muscle recruitment patterns: trapezius muscle latency with and without impingement symptoms.

BACKGROUND: Altered muscle activity in the scapular muscles is commonly believed to be a factor contributing to shoulder impingement syndrome. However, one important measure of the muscular coordination in the scapular muscles, the timing of the temporal recruitment pattern, is undetermined. PURPOSE: To evaluate the timing of trapezius muscle activity in response to an unexpected arm movement in athletes with impingement and in normal control subjects. STUDY DESIGN: Prospective cohort study. METHODS: Muscle latency times were measured in all three parts of the trapezius muscle and in the middle deltoid muscle of 39 "overhand athletes" with shoulder impingement and compared with that of 30 overhand athletes with no impingement during a sudden downward falling movement of the arm. RESULTS: There were significant differences in the relative muscle latency times between the impingement and the control group subjects. Those with impingement showed a delay in muscle activation of the middle and lower trapezius muscle. CONCLUSION: The results of this study indicate that overhand athletes with impingement symptoms show abnormal muscle recruitment timing in the trapezius muscle. The findings support the theory that impingement of the shoulder may be related to delayed onset of contraction in the middle and lower parts of the trapezius muscle.

Scapular muscle recruitment pattern: electromyographic response of the trapezius muscle to sudden shoulder movement before and after a fatiguing exercise.

STUDY DESIGN: Test-retest reliability study and single-group repeated measures design. OBJECTIVES: To evaluate the muscle latency times of the 3 portions of the trapezius muscle to a sudden arm movement in normal shoulders and to determine if this recruitment pattern is altered as a result of fatigue. BACKGROUND: It has been suggested that shoulder impingement may be related to altered muscle activity and muscle fatigue in the scapular stabilizers. Fatigue-induced changes in latency times of the trapezius might influence scapular stability. METHODS AND MEASURES: Muscle latency times were investigated in 30 healthy shoulders with surface electromyography. Muscle activity was measured in all 3 sections of the trapezius and the middle deltoid muscle during a sudden downward falling movement of the arm. Subsequently the shoulder was fatigued on an isokinetic dynamometer, after which muscle latency time measurement was repeated. RESULTS: ANOVA for repeated measures revealed significant differences in latency times (P < 0.05) among the 4 muscles of interest. Although there were no significant differences among the 3 sections of the trapezius muscle, they all were recruited after the initialization of the deltoid muscle. The recruitment order of the shoulder muscles did not change with muscle fatigue. However, after fatigue, muscle responses were significantly slower in all muscles except for the lower trapezius (P < 0.05). CONCLUSIONS: There is a specific recruitment sequence in the shoulder muscles in response to a sudden arm movement characterized by initial activation of the middle deltoid muscle and followed by simultaneous contraction of all 3 sections of the trapezius. This muscle activation pattern is delayed but not altered with fatigue.