Awareness of Visual Offset Reduces but Does Not Eliminate Joint Repositioning Errors in Virtual Reality.

The present study investigated the effect of visual offset (visuo-proprioceptive mismatch) in joint repositioning task in a three-dimensional virtual reality (VR) environment when participants were instructed to ignore vision. Twenty-five physically healthy young individuals performed shoulder joint position sense test. Repositioning accuracy was tested under two visual conditions, accurate and offset visions, and two instructions, no guidance or ignore vision. In accurate vision trials, the virtual hand of the tested limb seen in VR was congruent with where the participant placed their hand. In the offset vision condition, the virtual hand was seen 8° above or below their actual hand in the vertical plane. Repositioning error (i.e. constant error) in offset vision trials was lower when the participants were instructed to ignore vision compared to when no instruction about the visual offset was given (p < 0.001). However, constant error in offset vision trials was larger than accurate vision trials when the participants tried to ignore vision in both visual conditions (p < 0.001). Our results suggest that humans may be able to down-weight vision to some extent by conscious effort, while the influence of vision is difficult to eliminate when vision is present.

Usage and acceptability of data normalization in baseball pitching.

In baseball pitching biomechanics, kinetic values are commonly ratio 'normalised' by dividing by mass or mass*height to allow for comparison between athletes of different sizes. However, creating a normalised ratio variable should meet certain statistical assumptions. Our purpose was to determine if elbow valgus torque predicted by pitching velocity is influenced by normalisation using regression model comparison with and without normalised torque values. Motion capture data for youth to professional pitchers (n = 1988) were retrospectively analysed. Normalisation assumptions were tested by comparing linear regression models to analogous models with an intercept fixed at zero and by examining remaining correlations between the confounding variable and new, normalised variable. Both mass (p < 0.001) and mass*height (p < 0.001) normalisation did not remove their respective relationship with torque. After accounting for mass or mass and height, velocity predicted 10% of variance in elbow valgus torque, whereas velocity predicted 59% of mass normalised torque and 45% of mass*height normalised torque. Ratio normalisation does not fully account for anthropometric variables that differ across pitchers and leads to different conclusions in the magnitude of velocity's predictive effect on elbow valgus torque. Therefore, we recommend using regression model comparison to account for anthropometric variables in baseball pitching kinetic data.

Experimentally Induced Pain Results in Reduced Activity of the Rotator Cuff Muscles in Healthy Subjects.

Shoulder pain is a complex, prevalent problem that is multifactorial in nature. While there are many potential causes, one common suspect is the rotator cuff musculature. The purpose of the present study was to induce pain in the supraspinatus muscle of healthy subjects and observe the resulting changes in muscle activity. Eight muscles on 23 subjects were assessed using electromyography: anterior, middle, and posterior deltoid; pectoralis major; upper trapezius; latissimus dorsi; serratus anterior; supraspinatus; and infraspinatus. It was hypothesized that the rotator cuff muscles would display reduced activity during pain, and that reductions in activity would remain after the pain had dissipated. Both of the rotator cuff muscles measured did indeed display reduced activity in a majority of the dynamic, isometric, and maximal contractions. Many of those reductions remained after the pain had subsided.

Peripheral sensitization is demonstrated in subacromial pain syndrome, with central sensitization found only in females.

The objective of this study was to explore whether hypersensitivity in patients with subacromial pain syndrome (SPS) manifests purely as localized peripheral sensitization or central sensitization, is influenced by the presence of subacromial pain, and presents similarly in male and female patients. Pressure pain threshold was assessed in both a patient cohort with unilateral SPS and an uninjured matched control group. Control subjects were assessed twice, with a 15 minute rest period between testing, while patients were assessed at baseline and after an almost instantaneous reduction in pain arising from an anesthetic injection in patients. Patients received a subacromial injection consisting of both anesthetics (3 cc of 2% lidocaine and 6 cc 0.5% Marcaine with Epinephrine) and a corticosteroid agent (1 cc DepoMedrol). Patients demonstrated hypersensitivity across the involved shoulder only, providing evidence for peripheral sensitization. There were trends for hypersensitivity across remote joints, however when separated by sex, only female patients demonstrated both peripheral and central sensitization. Immediate pain reduction had no influence on hypersensitivity in the short-term. Clinical Significance: Neuropathic components are likely present in some patients with subacromial pain syndrome, and female patients may be particularly at risk for presenting with neuropathic pain. These findings are applicable towards understanding the heterogeneous etiology underlying subacromial pain syndrome and informing clinical management.

Weakness in patients with subacromial pain syndrome is local and more pronounced in females.

BACKGROUND: Subacromial pain syndrome is the predominant cause of shoulder pain, accounting for approximately half of all shoulder complaints. This population presents with weakness of the involved shoulder. However, there is a gap in our understanding of how pain contributes to this weakness, and whether there are sex related differences. METHODS: Regional and global isometric strength was tested at the involved shoulder joint and remote joints (uninvolved shoulder and both knees) in patients with subacromial pain syndrome. Data were collected before and after acute pain reduction with a subacromial injection. FINDINGS: Patients demonstrated weakness at the involved shoulder while remote joints demonstrated normal strength. When compared to healthy controls, male patients were shown to exhibit greater levels of weakness than female patients at the involved shoulder, based on comparisons with sex-matched controls using z-scores. Pain reduction (through an anesthetic injection) had no influence on strength in the short-term. INTERPRETATION: Weakness in patients appears to be sex dependent and is not resolved with reduction of pain. This calls into question the assumptions of the physiological causes of this weakness.

Submaximal contractions can serve as a reliable technique for shoulder electromyography normalization.

There are a number of ways to normalize electromyographical data, the most common of which is using a maximal contraction as a reference. However, this technique is not always practical. The purpose of the present study was to assess the reliability of an electromyographical data normalization technique using standardized submaximal contractions. Twenty healthy subjects (ten male, ten female) were used for testing, which was performed using both surface and fine-wire electromyography over two sessions at 15, 30, 45, and 60 percent of the day 1 maximum force. There was a mean of 5.3 days between test days. Data were compared between days, and the resulting ICC and standard error of the measurement values indicate varying levels of reliability at each submaximal percent. All heads of the deltoid, the upper trapezius and the serratus anterior demonstrated good reliability for at least one submaximal condition. The latissimus dorsi and supraspinatus demonstrated moderate reliability for at least one submaximal condition. Finally, the infraspinatus demonstrated poor reliability under all conditions. For situations in which MVC is impractical or anticipated to change, EMG amplitude normalization to one of these submaximal percentages appears to be a viable technique, at least for most muscles.

Joint Position Accuracy Is Influenced by Visuoproprioceptive Congruency in Virtual Reality.

Weighted integration of visual and proprioceptive information is important in movement planning and execution. The present study used a virtual reality system to determine how upper limb movement consistency and accuracy are altered when (a) vision of the limb is removed and (b) proprioception and vision of the limb are misaligned. A one degree of freedom upper limb movement task was performed under three visual conditions of the limb; accurate vision, no vision, and offset vision. Movement consistency was unaltered by the change in visual condition. Compared to the accurate vision condition, movement accuracy was unchanged in the no vision condition but decreased with a visual offset. When available, vision was relied upon more heavily than proprioception for task completion.

Muscle Activity Before and After Subacromial Injection.

CONTEXT: Shoulder muscle activation in patients with subacromial impingement is highly cited and variable in the literature. Differences between studies could be due to artifacts introduced by normalization practices in the presence of pain. Ultimately, this lack of knowledge pertaining to pathogenesis limits the clinical treatment and restoration of muscular function. DESIGN: A total of 21 patients with stage 2 subacromial impingement and 21 matched controls were recruited for EMG testing of their affected shoulder during an arm elevation task. The patients were tested before and after receiving an injection to their subacromial bursa. METHODS: The EMG from 7 shoulder muscles were measured before and after treatment during humeral motion in the scapular plane. RESULTS: Our findings indicate an increase in anterior deltoid, middle deltoid, and upper trapezius activity following the injection; further, this trend extended to the controls. The control subjects had a greater activation of the latissimus dorsi at peak arm elevation when compared with the patient group postinjection. CONCLUSIONS: Our results indicate that a reduction in subacromial pain is associated with changes in shoulder muscle recruitment, primarily of the deltoid. This change in deltoid activity may lend evidence to rotator cuff function in patients without rotator cuff tears.

Shoulder Joint Position Sense Can Be Reduced by Sensory Reference Frame Transformations.

Joint position sense (JPS) is commonly evaluated using an angle replication protocol with vision occluded. However, multiple sources of sensory information are integrated when moving limbs accurately, not just proprioception. The purpose of this study was to examine different availability of vision during an active JPS protocol at the shoulder. Specifically, the effects of four conditions of vision availability were examined for three target shoulder elevation angles (50°, 70° & 90°): vision occluded continuously (P-P); vision available continuously (VP-VP); vision occluded only during target memorization (P-VP); and vision occluded only during target position replication (VP-P). There were 18 participants (M age = 21, SD = 1 years). We used separate repeated ANOVAs to examine the effect of condition and target angle on participants' absolute error (AE, a measure of accuracy) and constant error (CE, a measure of directional bias). We found a significant main effect for condition and angle for both dependent variables (p < 0.01), and follow-up analysis indicated that participants were most accurate in the VP-VP condition and least accurate in the P-VP condition. Further follow-up analysis showed that accuracy improved with higher target elevation angles, consistent with previous research findings. Constant error results were similar, as there was a prominent tendency to overshoot the target. Unsurprisingly, participants performed best at the angle replication protocol with their eyes open. However, while accuracy was reduced when vision was occluded during target memorization, it was restored during target replication. This finding may have indicated an accuracy cost due to introduced noise when transforming sensory information from a proprioceptive reference frame into a visual reference frame.

Clinical Outcomes and Shoulder Kinematics for the "Gray Zone" Extra-articular Scapula Fracture in 5 Patients.

AIMS: There is a subset of scapula fractures, which can be considered in the "gray zone," where treatment guidelines are not clear-cut, based on published literature. Our paper presents the outcomes of five such scapula fractures treated non-operatively. METHODS: Adult patients who had been treated non-operatively at our institution for an isolated scapula fracture from 2003-2012 were found using Current Procedural Terminology (CPT) codes. Based on injury imaging, these five patients had scapula fractures in the "gray zone."Subjects completed questionnaires [Simple Shoulder Test (SST), PROMIS Global Health Scale vs 1.1, PROMIS SF vs 1.0 Physical Function 12a, and the American Shoulder and Elbow Surgeons Score (ASES)] and physical exams were performed to assess range of motion and strength. Glenohumeral kinematics were obtained via motion analysis using the Trackstar 6 Degree of Freedom (DOF) motion tracking system by Northern Digital Incorporated. RESULTS: All subjects were right hand dominant. 3/5 fractures involved left, non-dominant, scapulae. Motion analysis demonstrated similar recruitment of the scapula during the glenohumeral rhythm for the fractured shoulders compared with the same arm of age matched control subjects. No significant differences occurred in either range of motion (ROM) or scapula-humeral coordination when comparing uninjured scapulae to the same arm of age matched control subjects. CONCLUSIONS: All subjects' demonstrated acceptable clinical outcomes when treated non-operatively. Minor differences were seen in subjective surveys. However, the kinematic analysis showed no differences in measured scapula-humeral rhythm or range of motion. It is proposed that immediate controlled range of motion and rehabilitation be considered in these patients and could be the focus of a larger prospective study. LEVEL OF EVIDENCE: Level IV (Case Series).

Shoulder and elbow joint position sense assessment using a mobile app in subjects with and without shoulder pain - between-days reliability.

OBJECTIVES: To determine between-days reliability and the minimal detectable change for shoulder and elbow joint position sense assessment using a validated mobile app, in subjects with and without shoulder pain. DESIGN: Reliability study. SETTING: Clinical measurement. PARTICIPANTS: Subjects with (n = 25) and without shoulder pain (n = 29). MAIN OUTCOME MEASURES: Subjects were assessed by the same examiner in two sessions, with one-week interval. Active joint repositioning tests of shoulder flexion and scaption and elbow flexion were assessed at the target-angles of 50°, 70°, 90° and 110°. Intra-class correlation coefficient, standard error of measurement and minimal detectable change were calculated for constant, absolute, total and variable errors. RESULTS: Good to excellent reliability was found for constant, absolute and total errors at the target-angle of 50° of scaption for healthy subjects; at 110° of shoulder flexion and all target-angles for elbow for both groups. CONCLUSIONS: The mobile app is a reliable tool and may be useful for assessing shoulder joint position sense mainly at 110° of flexion and for elbow between 50° and 110° of flexion in subjects with and without shoulder pain. Minimal detectable changes were demonstrated and may help clinicians to follow-up rehabilitation and researchers to interpret findings of studies.

Force perception at the shoulder after a unilateral suprascapular nerve block.

There are two key sources of information that can be used to match forces-the centrally generated sense of effort and afferent signals from mechanical receptors located in peripheral tissues. There is currently no consensus on which source of information is more important for matching forces. The corollary discharge hypothesis argues that subjects match forces using the centrally generated sense of effort. The purpose of this study was to investigate force matching at the shoulder before and after a suprascapular nerve block. The nerve block creates a sensory and muscle force mismatch between sides when matching loads. The torque matching accuracy did not change after the nerve block was administered. Directionally, the torque error was in the direction proposed by the corollary discharge hypothesis. However, the mismatch between deltoid EMG was substantially greater compared to the changes in the torque matching error after the block. The results support that sensory information is used during force matching tasks. However, since the nerve block also created a sensory disruption between sides, it is not clear how sensory information is reweighted following the nerve block and a role for sense of effort is still implicated.

Feasibility of using a fully immersive virtual reality system for kinematic data collection.

Commercially-available Virtual Reality (VR) systems have the potential to be effective tools for simultaneous visual manipulation and kinematic data collection. Previously, these systems have been integrated with research-grade motion capture systems to provide both functionalities; however, they are yet to be used as stand-alone systems for kinematic data collection. The present study aimed to validate the HTC VIVE VR system for kinematic data collection by evaluating the accuracy of its position and orientation signals. The VIVE controller and tracker were each compared to a Polhemus Liberty magnetic tracking system sensor for angular and translational measurement error and signal drift. A sensor from each system was mounted to opposite ends of a rigid segment which was driven through fifty rotations and fifty translations. Mean angular errors for both the VIVE tracker and controller were below 0.4°. Mean translational error for both sensors was below 3 mm. Drift in the Liberty signal components was consistently lower than drift in VIVE components. However, all mean rotational drift measures were below 0.1° and all mean translational measures were below 0.35 mm. These data indicate that the HTC VIVE system has the potential to be a valid and reliable means of kinematic data collection. However, further investigation is necessary to determine the VIVE's suitability for capturing extremely minute or high-volume movements.

An Investigation Into Force Sense at the Shoulder.

Proprioception is assessed more often through joint position sense and kinesthesia than force sense. The purpose of this study is to investigate force sense at the shoulder. A total of 12 subjects were recruited. An ipsilateral force reproduction protocol at the shoulder at 50°, 70°, and 90° and 120%, 140%, and 160% baseline torque. Dependent variables were constant error (CE) and root mean square error. An effect was found for load on absolute (p = .001) and normalized CE (p < .001). CE decreased with increased load. An effect for angle was found for absolute root mean square error (p = .002), more accurate at 50° (p = .01), but no effect when normalized (p = .19). With increased loads, subjects undershot the target and CE approached zero. Because of the differing behavior in CE and root mean square error, and absolute and normalized data, force sense studies should examine error from these perspectives.

No Relationship Between Joint Position Sense and Force Sense at the Shoulder.

In practice, a single test is used to quantify an individual's proprioception. Previous studies have not found a correlation between joint position sense (JPS) and force sense (FS), which are submodalities of proprioception. The purpose of the present study is to determine if root mean square (RMS) error in JPS and FS are related at the shoulder, controlling for external load and elevation angle. Active shoulder angle and force reproduction protocols were performed. No correlation was found between JPS and FS (r = -.019, p = .941) nor were any individual angle and load combinations significant. The main effect for angle in JPS was significant (p < .001). Follow-up contrast demonstrated a significant (p < .001) decrease in RMS error with increased elevation. A significant load by angle interaction was found for FS (p = .014). Follow-up simple effects tests by angle demonstrated RMS error decreased with load at 50° and 70° but not at 90°. By load, RMS error only decreased for 120% between 50° and 90°. JPS and FS demonstrate different behavior with load and angle. This differing behavior is more likely responsible for the lack of correlation than angle and load differences in JPS and FS protocols.

The contribution of the supraspinatus muscle at sub-maximal contractions.

During maximum effort, the supraspinatus muscle contributes approximately 50% of the torque need to elevate the arm, but this has not been examined at sub-maximal levels. The purpose of this study was to determine the contribution of the supraspinatus muscle to shoulder elevation at sub-maximal levels. Seven healthy subjects (four males, three females) performed isometric ramp contractions at the shoulder. Middle deltoid electromyography (EMG) and force applied at the wrist were collected before and after a suprascapular nerve block. For the same level of deltoid EMG, less external force will be measured after the nerve block as the supraspinatus muscle no longer contributes. The difference between the EMG/force curve was the contribution of the supraspinatus muscle. The supraspinatus contributed 40%, 95% CI [32%-48%], to shoulder elevation. The effect of angle (p = .67) and % maximal voluntary contraction (p = .13) on supraspinatus contribution were not significant. The maximum is slightly less than reported in a previous suprascapular nerve block study using maximal contractions. The results from this study can be used to assess supraspinatus contribution in rotator cuff tears, after rehabilitation interventions, and as a restraint in computation modelling.

College Pitchers Demonstrate Directional Differences in Shoulder Joint Position Sense Compared With Controls.

CONTEXT: The relationship between overhead throwing and its effect on proprioception is not well understood. It is important to gain a better understanding of how these are related, to protect overhead athletes from an increased risk of injury. OBJECTIVE: To investigate proprioceptive alterations in the overhead thrower's shoulder. DESIGN: Cross-sectional study. Independent variables are limb (dominant and nondominant), group (thrower or control), and target angle. Dependent variables are joint position sense and range of motion. SETTING: An orthopedic biomechanics lab and university athletic training facility. PARTICIPANTS: Twelve Division I baseball pitchers and 13 nonthrowing control subjects. INTERVENTION: Shoulder proprioception was assessed using an active joint repositioning task administered with an iPod Touch. MAIN OUTCOME MEASURE: Root mean square error and constant error of repositioning angles were used to assess accuracy and directional patterns, respectively. RESULTS: Both groups demonstrated significantly higher joint acuity at the 80° external rotation target angle compared with 60° (1.5° [0.5°], P = .01). There were no differences in accuracy between groups. Constant error revealed differing repositioning patterns between limbs for the pitchers and also between groups for the dominant side. Although the throwing shoulder overshot the target angles by 0.4°, all nonthrowing shoulders undershot by an average of 2.7°. CONCLUSIONS: There is no difference in shoulder joint position sense accuracy between throwers and nonthrowers, although both groups display increased accuracy closer to their end range of external rotation. The throwing shoulder demonstrates a different repositioning pattern, overshooting the desired target angle, while all other shoulders undershoot.

Subacromial Anesthetics Increase Proprioceptive Deficit in the Shoulder and Elbow in Patients With Subacromial Impingement Syndrome.

Shoulder proprioception gives information regarding arm joint position and movement direction. Several studies have investigated shoulder proprioceptive acuity in patients with subacromial impingement syndrome (SIS); however, differences in protocols and between-subjects designs have limited scientific inferences regarding proprioception and SIS. We aimed to determine within-subject differences in shoulder and elbow proprioceptive acuity in 17 patients with stage 2 SIS following treatment of a local anesthetic injection. In addition, we used 17 healthy, age-, sex-, and arm dominance-matched controls to determine the magnitude of differences after treatment. Joint position sense (JPS) was measured before and after treatment in both groups in the sagittal plane for the shoulder and elbow. Our results indicate that patients with SIS have less sensitivity to angular position and tended to overshoot their targets with greater variability during angle-matching tasks for the shoulder (1.8° difference, P = .042) and elbow (5.6° difference, P = .001) than controls. The disparities in JPS found in patients with SIS were not resolved following subacromial injection; in fact, the magnitude of the errors increased after treatment where postinjection errors were significantly greater (P = .046) than controls, with an average difference of 2.4°. These findings suggest that patients with SIS have decrements in either the signaling or processing of proprioceptive information and may use pain to reduce these inequalities.

Errors in Shoulder Joint Position Sense Mainly Come from the Glenohumeral Joint.

While synchronous movement of the glenohumeral and scapulothoracic joints has been emphasized in previous kinematics studies, most investigations of shoulder joint position sense have treated the shoulder complex as a single joint. The purposes of this study were to investigate the joint position sense errors of the humerothoracic, glenohumeral, and scapulothoracic joints at different elevation angles and to examine whether the errors of the glenohumeral and scapulothoracic joints contribute to the errors of the humerothoracic joint. Fifty-one subjects with healthy shoulders were recruited. Active joint position sense of the humerothoracic, glenohumeral, and scapulothoracic joints was measured at 50°, 70°, and 90° of humerothoracic elevation in the scapular plane. The results showed that while scapulothoracic joint position sense errors were not affected by target angles, there was an angle effect on humerothoracic and glenohumeral errors, with errors decreasing as the target angles approached 90° of elevation. The results of a multiple regression analysis revealed that glenohumeral errors explained most of the variance of the humerothoracic errors and that scapulothoracic errors had a weaker predictive relationship with humerothoracic errors. Therefore, it may be necessary to test scapular joint position sense separately in addition to the assessment of the overall shoulder joint position sense.

Deltoid Electromyography is Reliable During Submaximal Isometric Ramp Contractions.

The EMG and load relationship is commonly measured with multiple submaximal isometric contractions. This method is both time consuming and may introduce fatigue. The purpose of this study was to determine if the electromyography (EMG) amplitude from the middle deltoid was reliable during isometric ramp contractions (IRCs) at different angles of elevation and rates of force application. Surface EMG was measured at 3 shoulder elevation angles during IRCs at 4 submaximal levels of maximum voluntary contraction (MVC). Data were reliable in all conditions except during the rate relative to the subjects' MVC at 90° for 30% and 40% MVC. The main effect for angle on EMG amplitude was found to be significant, p < .01. EMG at 90° was greater than at 60° (p < .017) and at 30° (p < .017). The main effect of force level on EMG amplitude was significant, p < .01 and follow-up contrast demonstrated a significant (p < .001) linear increase of EMG amplitude with force level. We conclude that EMG amplitude from IRCs are reliable across all shoulder elevation angles and up to 40% MVC. IRCs are a feasible method for recording EMG at the deltoid.