Richard E. Hughes

Roles of deltoid and rotator cuff muscles in shoulder elevation

OBJECTIVE: The objective of this study was to measure abduction moment arms of the supraspinatus, subscapularis, infraspinatus, and deltoid (anterior, middle, and posterior portions) muscles during humeral elevation in the scapular plane (abduction). DESIGN: Moment arms were measured by conducting an in vitro experiment. BACKGROUND: The moment arm of a muscle represents its mechanical advantage, which is an important determinant of muscle function. METHODS: Measurements were made on 10 fresh frozen cadaveric specimens. Tendon excursions were measured as the humerus was elevated in the plane of the scapula. The principle of virtual work was used to estimate the muscle moment arm of each muscle by computing the slope of the tendon excursion versus joint angle relationship. RESULTS: Moment arms were affected by joint angle in a non-linear fashion. The anterior deltoid, middle deltoid, subscapularis, and infraspinatus muscles had abduction moment arms throughout most of the range of motion studied. The posterior deltoid had an adduction moment arm. Internal and external humeral rotation affected the elevation moment arms of all six muscles. CONCLUSIONS: Abduction moment arm magnitudes of the muscles studies vary throughout the arc of elevation. This study was limited by considering broad muscles to have a single line of action. RELEVANCE: The positive elevation moment arms of the infraspinatus and subscapularis muscles indicate that they can elevate the arm in addition to acting as stabilizers. Thus this study suggests a biomechanical explanation for the clinical success of conservative treatment for rotator cuff tears.

Force analysis of rotator cuff muscles

A 3-dimensional static biomechanical model of the glenohumeral joint was used to investigate rotator cuff muscle forces during maximal isometric exertions (abduction, adduction, internal rotation, and external rotation) and static arm elevation. Muscle moment arms and cross sectional areas were determined from studies of cadaveric specimens, and maximal isometric strength data were collected using a Cybex II dynamometer. Predicted posterior deltoid forces were very low during abduction in the scapular plane. The model predicted the highest rotator cuff muscle forces during maximal internal rotation (subscapularis) and external rotation (infraspinatus, teres minor, and supraspinatus) exertions. The results indicate that abduction exertions may not produce the greatest loads on the supraspinatus tendon, and that analyses of arm elevation may underestimate the potential loads on the rotator cuff. The strong effect of external rotation exertions on supraspinatus and infraspinatus forces suggest that ergonomic efforts to prevent rotator cuff disease should include reduction of internal rotation loading on the arm. Moreover, it may be important to include warnings about external rotation exertions, in addition to arm elevation, in patient education.

Mechanical advantage of the thumb muscles

The purpose of this study was to measure the moment arms of four extrinsic muscles (flexor pollicis longus, extensor pollicis longus, extensor pollicis brevis, and abductor pollicis longus) and four intrinsic muscles (flexor pollicis brevis, abductor pollicis brevis, adductor pollicis, and opponents pollicis) of the thumb at the interphalangeal, the metacarpophalangeal, and the carpometacarpal joints in the same cadaver specimens and to examine the specific role of each muscle. Measurements were made on seven fresh frozen cadaver hands. The moment arms were measured during flexion/extension of the interphalangeal joint, flexion/extension and adduction/abduction of the metacarpophalangeal joint, and flexion/extension and adduction/abduction of the carpometacarpal joint. Moment arms were computed using the slope of the tendon excursion joint angle relationship. The specific function of each muscle was determined by multiplying the measured moment arms by the maximum force that each muscle can generate. It was found that the flexor pollicis longus was a pure flexor while flexor pollicis brevis was an adductor as well as a flexor, the extensor pollicis longus was an extensor and an adductor, extensor pollicis brevis was an extensor and a mild abductor, the abductor pollicis longus was an extensor as well as an abductor, the abductor pollicis brevis was mainly an abductor, the adductor pollicis was a major flexor as well as an adductor, and the opponents pollicis was a flexor and an abductor.

Age-related changes in normal isometric shoulder strength

Normative data are useful for assessing isometric shoulder strength in patients with bilateral shoulder abnormalities. The purpose of this study was to develop a normative database for shoulder strength in the clinically relevant positions and movements. Twenty combinations of exertions and postures—including flexion, extension, abduction, adduction, internal rotation, and external rotation—were tested for both the dominant and nondominant sides. Strength was measured isometrically using a modified Cybex II dynamometer. The cross-sectional study design included 120 subjects (60 women and 60 men) ranging in age from 20 to 78 years from southern Minnesota. Tables of normative strength data were constructed. Multivariate analyses were performed to assess the effect of age, sex, and weight on strength. Age was negatively associated with all strength measures, and weight was positively associated with them. Men were stronger than women when controlling for age and weight. Statistically significant differences between dominant and nondominant shoulders were found for only some of the strength measures taken. These normal data will be useful to the clinician, as they permit a standard against which to compare shoulder strength.

Comparison of two methods for computing abduction moment arms of the rotator cuff

Biomechanical models of the shoulder mechanism require estimates of muscle moment arm magnitude. Some shoulder models have estimated muscle moment arms by assuming an idealized minimum distance path from the origin to insertion that passes around the bony geometry. Alternatively, the principle of virtual work can be used to estimate moment arms from tendon excursion and joint-angle data. The purpose of this study was to determine if these two methods give different estimates of abduction moment arms for the supraspinatus, infraspinatus, and subscapularis muscles. Muscle moment arms were estimated for these muscles on ten fresh frozen cadaver specimens. The results showed a significant difference between the two estimation methods. Average differences were 3.1 mm (10.6%), 3.9 mm (43.9%), and 7.2 mm (70.3%) for the supraspinatus, subscapularis, and infraspinatus muscles, respectively. These results suggest that shoulder models based on the origin-insertion method may give higher rotator cuff muscle force estimates than methods using the slope of the tendon excursion vs joint angle relationship.

Biomechanical effect of medial advancement of the supraspinatus tendon: A study in cadavera

During the repair of some rotator-cuff tears, the torn tendon cannot be freed up adequately to permit reattachment at its original anatomical site of insertion. An option is to advance the site of insertion medially and reattach the tendon to a trough in the sulcus or to the humeral head. The biomechanical effects of such medial advancement on the moment arm of the supraspinatus muscle during glenohumeral elevation were studied in ten fresh-frozen shoulders from cadavera. Medial advancement of the site of insertion of the supraspinatus tendon was simulated by the placement of suture anchors in the sulcus of the proximal part of the humerus at points three, ten, and seventeen millimeters medial to the junction of the supraspinatus tendon and the bone. These distances were chosen not because they represent clinical options but because the large range allowed biomechanical study of medial advancement. Nylon lines were attached to the suture anchors and were passed back through an eyehook at the midpoint of the supraspinatus muscle. The excursion of each line was measured as the humerus was elevated, and the moment arm was estimated from the joint angle and excursion data with use of the principle of virtual work. Three and ten millimeters of medial advancement of the tendon (attachment in the sulcus) had a minimum (non-significant) effect on the moment arm during elevation compared with the value determined for the intact condition. However, seventeen millimeters of medial advancement was found to reduce the moment arm significantly (p < 0.05). CLINICAL RELEVANCE: Our study of cadavera indicates that a limited amount of medial advancement (as much as ten millimeters) is acceptable from a biomechanical point of view, although the clinical maximum is dictated by other clinical factors.

Normative values of agonist-antagonist shoulder strength ratios of adults aged 20 to 78 years☆☆☆

OBJECTIVEnTo determine normative values for isometric flexion/extension, abduction/adduction, and external/internal rotation strength ratios about the shoulder and to determine if these ratios are affected by age or gender.nnnSTUDY DESIGNnA cross-sectional study of 120 healthy volunteers (60 men, 60 women) aged 20 to 78 years.nnnSETTINGnOrthopedic research laboratory.nnnMETHODSnFlexion and extension strengths were measured isometrically using a Cybex II dynamometer at arm flexion angles of 30 degrees, 60 degrees, and 90 degrees. Abduction and adduction strengths were measured at 30 degrees, 60 degrees, and 90 degrees abduction. Internal and external rotation strengths were measured (1) with the arm abducted 15 degrees and neutral external/internal rotation and (2) with the arm abducted 90 degrees and externally rotated 30 degrees above the transverse plane.nnnOUTCOME MEASURESnIsometric strength ratios for flexion/extension, abduction/adduction, and external/internal rotation.nnnRESULTSnNo statistically significant differences in agonist/antagonist strength ratios were found between dominant and nondominant sides or between genders. Age was associated with changes in strength ratios for measurements taken with the arm flexed or abducted 90 degrees. Posture was found to affect strength ratios.nnnCONCLUSIONSnThese data can serve as a normative reference for clinical use.

Abduction moment arm of transposed subscapularis tendon

OBJECTIVEnThe purpose of this study was to analyze the effects of the procedure of superior transposition of the subscapularis on the biomechanics of glenohumeral abduction.nnnDESIGNnThe abduction moment arms of the subscapularis muscle for the normal attachment and transposed tendon were measured on 10 cadaver shoulders and compared to that for the normal supraspinatus tendon for which it is intended to substitute.nnnBACKGROUNDnSuperior transposition of the subscapularis tendon has been recommended for surgical repair of massive tears of the rotator cuff, but the effect of this procedure on shoulder biomechanics has not been reported.nnnMETHODSnThe moment arm about an instantaneous center of rotation was derived, based on the slope of tendon excursion-glenohumeral angle curve. To simulate the insertion of the transposed subscapularis tendon, pseudo-insertion sites were created.nnnRESULTSnSuperior transposition of the subscapularis tendon significantly increased its abduction moment arm. The effect was optimal when the simulated insertion site was lateral rather than medial and, to a lesser extent, anterior versus posterior.nnnCONCLUSIONSnThe results provided a biomechanical rationale for subscapularis tendon transposition in restoring the loss of abduction strength of the shoulder in a massive cuff tear.

Reliability of a simple shoulder endurance test

Reduced shoulder endurance in rotator cuff tear patients has been observed clinically. A simple and inexpensive shoulder endurance test protocol was developed. This study reports the test-retest reliability of the protocol. Twenty healthy volunteers without a history of shoulder pathology participated in the study. Each subject was tested twice, each on a separate day. Test-retest correlation coefficients of the arm endurance times were 0.59 and 0.60 for the dominant and nondominant sides, respectively. The protocol appears promising for assessing shoulder function.

Torso loading via a harness method activates trunk muscles less than a hand loading method

There is a need for data comparing muscle activity under equivalent moments at the torso using different loading methods. In torso biomechanics, it is often necessary to apply moments to people and then measure or predict the muscle activity in response to the external load. Alternative methods exist to generate moments including applying forces by a chest harness and loads held in the hands. The limitation of previous research has been to use only one of the loading methods, generally harness loading, assuming it produces muscle activity equal to when loads are held in the hands. The objective of the this study was to test if equivalent moments at L3/L4 applied to subjects through two loading methods resulted in the same muscle activity. Subjects maintained a static posture while attempting a combination of flexion/extension, lateral bending, and torsion to counter the external loads applied via hand and harness loading methods. Hand and harness loading did not result in equal muscle activity as measured by electromyographic techniques. The left and right latissimus dorsi had increases in activity (16-25%) when loads were held with the hands instead of applied via the harness. Increased activity for hand loading was also found for the left and right rectus abdominis (13-17%) and left external oblique (24%). The left and right erector spinae and right external oblique exhibited the same activity for both loading methods. The current findings indicate that studies of torso loading using a shoulder harness to create torso moments may be lowering activity of some torso muscles.