Lower trapezius weakness and shoulder complex biomechanics during the tennis serve

Purpose This study aimed to assess the effect of lower trapezius (LT) weakness on humeral and scapular kinematics and shoulder muscle activity during the tennis serve. Methods Fifteen competitive male tennis players (age, 23.8 ± 3.4 yr; height, 182.8 ± 6.7 cm; mass: 76.6 ± 8.7 kg; tennis experience: 15.6 ± 4.9 yr) performed two tennis serves before and after selective fatigue of the LT (25-min electric muscle stimulation). During each tennis serve, racket, humeral and scapular kinematics and the activity of 13 shoulder muscles were recorded using an optoelectronic system synchronized with indwelling and surface electromyography. The serve was split into five phases, that is, early and late cocking, acceleration, early and late follow-through. Results Selective fatigue led to a 22.5% ± 10.4% strength decrease but did not alter maximum racket speed and humerothoracic joint kinematics. However, increased scapular upward rotation was observed in the acceleration (P = 0.02) and early follow-through (P = 0.01) phases. Decreased muscular activity was observed during the early cocking phase for the LT (P = 0.01), during the acceleration phase for the LT (P = 0.01), anterior deltoid (P = 0.03), pectoralis major (P = 0.04), and subscapularis (P = 0.03), and during the early follow-through phase for the anterior deltoid (P = 0.03) and LT (P = 0.04). Conclusions The LT weakness altered neither serve velocity nor humerothoracic joint kinematics, but impaired scapulothoracic kinematics and anterior shoulder muscle activation. Such alterations may reduce the subacromial space and jeopardize humeral head stability. These findings shed new light on the consequences of LT weakness, highlighting the importance of monitoring and strengthening this muscle in overhead athletes

An Electromyography Study of Muscular Endurance during the Posterior Shoulder Endurance Test

The primary purpose was to determine if there is a difference between the median frequency slopes of 5 posterior shoulder muscles during the initial portion of the Posterior Shoulder Endurance Test (PSET) at the 90⁰ and 135⁰ shoulder abduction positions. Fifty-five healthy volunteers (31 females) participated. The median frequency of the posterior deltoid (PD), upper trapezius (UT), middle trapezius (MT), lower trapezius (LT), and infraspinatus (INF) was measured during the PSET at 90⁰ and 135⁰ of shoulder abduction. External torque of 13±1 Nm was used for females and 21±1 Nm for males. A fixed effect multi-variable regression model was used to investigate the median frequency slopes. Males and females were analyzed separately. Median frequency slopes demonstrated fatigue in all 5 of the muscles. The PD fatigued greater than the UT in males (p=0.0215) and greater than the LT in females (p=0.008). The time to task failure (TTF) was greater at 90° than 135° for females and males (p=0.016; p=0.0193) respectively. The PSET causes fatigue in all of the muscles that were tested, with the PD fatiguing at a greater rate compared to one muscle for each sex. This investigation supports using TTF as a clinical measure of shoulder girdle endurance at 90° shoulder abduction

Effects of fatigue on pelvic and scapular stabilizers in overhead throwing

Human movement is based on proximal to distal sequencing of the kinetic chain. Efficiency of the shoulder is dependent upon the stability and function of the pelvis and scapula. PURPOSE: To determine if pelvic and scapular muscle activations are altered in overhead throwing following fatigue. METHODS: Eleven [19.2 ± 1.0 years; 168.9 ± 6.6 cm; 68.9 + 8.7 kg] softball players volunteered. Surface electromyographic [sEMG] electrodes were placed on bilateral gluteus medius, throwing arm latissimus dorsi, lower trapezius [LT], middle trapezius [MT], and serratus anterior [SA]. Participants had to catch a simulated hit ball and perform their positional throw. Infielders caught and threw to second base and outfielders crow hopped and threw to second, simulating a game setting where a runner was trying to steal. After 5 throws, participants threw a 2 kg ball into a rebounder until maximum perceived fatigue on 0-3 scale [3 = fatigue]. Following fatigue, 5 more throws were performed. The fastest throw pre and post fatigue were selected for sEMG analysis using a paired T-test. RESULTS: There were no significant differences in muscle activation pre and post fatigue during the acceleration phase of throwing. It was revealed that all muscles activation was increased post-fatigue excluding LT. CONCLUSION: The SA and LT provide a force couple to stabilize the scapula in arm elevation. Thus, while no statistical differences were observed, the increase in SA and decrease in LT could be an indicator of possible scapular instability following fatigue. Further research is needed to understand the effects of fatigue on pelvic and scapula stabilization during overhead throwing

An Electromyography Study of Muscular Endurance during the Posterior Shoulder Endurance Test

The primary purpose was to determine if there is a difference between the median frequency slopes of 5 posterior shoulder muscles during the initial portion of the Posterior Shoulder Endurance Test (PSET) at the 90° and 135° shoulder abduction positions. Fifty-five healthy volunteers (31 females) participated. The median frequency of the posterior deltoid (PD), upper trapezius (UT), middle trapezius (MT), lower trapezius (LT), and infraspinatus (INF) was measured during the PSET at 90° and 135° of shoulder abduction. External torque of 13 ± 1 Nm was used for females and 21 ± 1 Nm for males. A fixed effect multi-variable regression model was used to investigate the median frequency slopes. Males and females were analyzed separately. Median frequency slopes demonstrated fatigue in all 5 of the muscles. The PD fatigued greater than the UT in males (p = 0.0215) and greater than the LT in females (p = 0.008). The time to task failure (TTF) was greater at 90° than 135° for females and males (p = 0.016; p = 0.0193) respectively. The PSET causes fatigue in all of the muscles that were tested, with the PD fatiguing at a greater rate compared to one muscle for each sex. This investigation supports using TTF as a clinical measure of shoulder girdle endurance at 90° shoulder abduction

The Influence of the Lower Trapezius Muscle on Shoulder Impingement and Scapula Dyskinesis

This dissertation contains three experiments all conducted in an outpatient physical therapy setting. Shoulder impingement is a common problem seen in overhead athletes and other individuals and associated changes in muscle activity, biomechanics, and movement patterns have been observed in this condition. Differentially diagnosing impingement and specifically addressing the underlying causes is a vital component of any rehabilitation program and can facilitate the individuals return to normal function and daily living. Current rehabilitation attempts to facilitate healing while promoting proper movement patterns through therapeutic exercise and understanding each shoulder muscles contribution is vitally important to treatment of individuals with shoulder impingement. This dissertation consisted of two studies designed to understand how active the lower trapezius muscle will be during common rehabilitation exercises and the effect lower trapezius fatigue will have on scapula dyskinesis. Study one consisted of two phases and examined muscle activity in healthy individuals and individuals diagnosed with shoulder impingement. Muscle activity was recorded using an electromyographic (EMG) machine during 7 commonly used rehabilitation exercises performed in 3 different postures. EMG activity of the lower trapezius was recorded and analyzed to determine which rehabilitation exercise elicited the highest muscle activity and if a change in posture caused a change in EMG activity. The second study took the exercise with the highest EMG activity of the lower trapezius (prone horizontal abduction at 130¢ª) and attempted to compare a fatiguing resistance protocol and a stretching protocol and see if fatigue would elicit scapula dyskinesis. In this study, individuals who underwent the fatiguing protocol exhibited scapula dyskinesis while the stretching group had no change in scapula motion. Also of note, both groups exhibited a decrease in force production due to the treatment. The scapula dyskinesis in the fatiguing group implies that lower trapezius function is vitally important to maintain proper scapula movement patterns and fatigue of this muscle can contribute and even cause scapula dyskinesis. This abnormal scapula motions can cause or increase the risk of injury in overhead throwing. This dissertation provides novel insight about EMG activation during specific therapeutic exercises and the importance of lower trap function to proper biomechanics of the scapula

Implications of Variability of Electromyographic Measurements for Assessing Localized Muscle Fatigue

The impact of work-related musculoskeletal disorders (WMSDs) is enormous due to a combination of direct and indirect costs associated with healthcare, lost workdays and human suffering. Because of the established relationship between Localized Muscle Fatigue (LMF) development and WMSDs, and in order to reduce and/or prevent WMSDs in workplaces, different fatigue assessment methods have been developed. Surface Electromyography (SEMG) is a commonly used LMF assessment technique. The SEMG signals are typically analyzed in time and frequency domains to predict LMF based on a relative change with respect to initial, or under no-fatigue conditions. Quantifying such change, however, relies on the assumption that the SEMG measures without fatigue present, under different muscular demands, can serve as an appropriate reference within the joint range-of-motion. To our knowledge, the assumption that the electromyographic measures do not change/vary due to factors other than LMF has not been thoroughly tested. Therefore, the objective of this study was to quantify variability of various SEMG measures in non-fatigued shoulder muscles and its implication for assessing muscle fatigue. In the first Specific Aim, an experiment was performed to quantify variability of six EMG measures (RMS, MAV, ZC, MnPF, MdPF, and PFB11-22 Hz) in seven non-fatigued shoulder muscles. Twelve human participants performed 120 occupationally relevant static holding tasks. The variability in SEMG data was quantified using Mean Square Error (√MSE) obtained from ANOVA models. The SEMG measures were found to vary between 5.32% to 12.25% due to factors other than muscle fatigue. The narrowest range of variability was observed for ZC (10.20% to 11.00%), and the largest range of variability was observed for MdPF (8.72% to 12.25%). In the second Specific Aim, a relationship between SEMG variability and LMF based on perceived exertion ratings was studied. Twelve human participants performed 8 fatigue inducing exertions for 10-45 seconds. The data were analyzed to identify muscle fatigue onset based on the perceived exertion ratings and the corresponding relative changes in SEMG measures. A good agreement was observed between the definition of LMF based on perceived exertion ratings and the relative change in the SEMG measures (quantified in Aim 1) for ZC, MnPF, and MdPF. And the study concludes that for the shoulder muscles a change higher than 11.00%, 11.45%, and 12.25% in ZC, MnPF, and MdPF, respectively, can be an indication of LMF. In conclusion, the study findings suggest that a change higher than 11.00%, 11.45%, and 12.25% in ZC, MnPF, and MdPF, respectively, can be an indication of LMF. These findings could be useful in improving real-time fatigue predication models and/or methods to curtail the incidence of LMF based WMSDs in workplaces

Influence of a Single Swim Training on the Scapular Position and Isometric Muscle Strength in Young Swimmers

The purpose of our study was to investigate the influence of one swim training on isometric scapular muscle strength and the static position of the scapula in a sample of 30 healthy adolescent swimmers who were measured on two sessions at a local aquatic center. Fatigue as result of a single swim training may influence scapular position and strength. Measures included isometric evaluation of serratus anterior and upper trapezius peak isometric force using a push and shrug procedure, serratus anterior to upper trapezius strength ratios, and a semi-dynamic evaluation of lateral scapular displacement employing the Kibler method. When tested using a repeated measures ANOVA, none of the strength parameters changed significantly as a result of the single swim training. Adolescent swimmers did employ higher values for the serratus anterior than for the upper trapezius. In the three positions of the LSST, the lateral displacement of the scapula increased significantly after the training (p = .001, p \u3c .001, p \u3c .001) and a minor scapular asymmetry was found. (p = .037, p = .011, p = .005). The highest LSST values were present on the dominant side, at both test moments

Scapular Muscle Assessment in Patients with Lateral Epicondylalgia

The role rehabilitation plays in the management of patients with lateral epicondylalgia (LE) remains elusive secondary to high recurrence rates. Addressing scapular muscle deficits may be important in the rehabilitation of patients with LE. However, it is unknown if scapular muscle impairments exist in a working population of patients with LE. The purpose of this dissertation was to assess scapular muscle strength and endurance in a working population of patients with LE. Clinical scapular muscle assessment tools are limited in their ability to isolate specific muscles. Rehabilitative ultrasound imaging (RUSI) is a potentially useful tool but few studies have investigated its utility. Absolute muscle thickness measurements were obtained on healthy individuals for the lower trapezius (LT) and serratus anterior (SA) under three conditions (arm at rest, arm elevated with a low load, arm elevated with a high load). For both the LT and SA, a significant distinction could be made in muscle thickness between rest and a loaded condition but not between the two load conditions. Furthermore, excellent reliability was demonstrated for both muscles. It is unknown whether arm dominance plays a role in scapular muscle assessments. Therefore, healthy individuals between the ages of 30 and 65 were recruited to compare the effect of arm dominance on scapular muscle strength, endurance, and change in thickness measured by RUSI. Results indicate that arm dominance does significantly affect some measures of scapular muscle strength and endurance. However, the differences between the dominant and non-dominant limbs were not beyond measurement error. Scapular muscle strength, endurance, and change in muscle thickness of the LT and SA were assessed in 28 patients presenting with signs and symptoms consistent with LE. LT strength, SA strength, middle trapezius strength, endurance, and change in SA thickness were significantly less in patients with LE compared to matched controls. SA and LT strength were significantly less in the involved limb compared to the uninvolved limb in patients with LE. The results suggest that assessing scapular muscle endurance as well as LT and SA strength is indicated when evaluating patients with LE, and the results should be compared to normative data