Shah KM1, Diehl I1, Hyer M1, Schur J1, McClure P1
1Arcadia University, Physical Therapy, Glenside, United States
Background: Shoulder muscle force is commonly assessed during clinical examination using both an isometric “make” test against a fixed resistance or a “break” test where the examiner exerts enough force to break the isometric contraction.
Purpose: The purpose of this study was to compare the force produced during these two forms of testing and to explore biomechanical mechanisms involved with each form of testing.
Methods: Data were collected on 25 subjects (12M) with a mean age of 36.5 (15.6). Subjects had no current or recent history of shoulder problems and had varied activity levels. After providing informed consent and demographic data, subjects were seated in a custom chair for shoulder muscle external rotation (ER) and elevation(EL) force testing. A hand-held force transducer connected to a computer was used to measure force using MotionMonitor software and sampling at 1000Hz. For external rotation the force was measured on the forearm just proximal to the ulnar head with the elbow flexed and the arm by the side. For elevation the force was measured just proximal to the ulnar head with the elbow straight and the arm elevated to 90 degrees in the scapular plane with thumb up. For both tests, the force was measured isometrically for approximately 3 seconds while the examiner held the transducer stationary, after which the examiner exerted enough force to break the contraction and move the arm. Shoulder angle was measured continuously using an arm-mounted inertial measurement unit (IMU). Differences between peak isometric and break forces were analyzed with a paired t-test.
Results: For ER, peak isometric force was 86.4(50.2)N while peak break force was 117.4(58.1)N (p 0.01). Peak ER break force occurred after an average of 1.8 (1.4) degrees of motion. Increase in ER break force ranged from -3.7% to 108.6%. For EL, peak isometric force was 76.6(50.5)N while peak break force was 110.1(37.6)N (p 0.01). Peak EL break force occurred after an average of 7.9(3.8) degrees of motion. Increase in EL break force ranged from -3.5% to 145.9%.
Conclusion(s): Peak forces measured during a break test are significantly higher than purely isometric forces. The amount of change associated with a break test compared to an isometric make test was also quite variable. Peak force during a break test occurred after angular motion of the arm, suggesting that an eccentric contraction was occurring and is a likely explanation for higher forces measured. Electromyography may also provide insight into understanding the mechanism of additional force during a break test. Further, examining the differences in make and break forces based on activity levels may help understand how “trained” muscles perform.
Implications: Clinicians measuring shoulder force should consistently use either a pure isometric make test or a break test for serial measurements with patients. Even a small angular change during testing associated with “breaking” the patient's force will lead to substantially different values compared to a purely isometric test.
Keywords: shoulder, force, isometric, eccentric
Funding acknowledgements: Not applicable
Purpose: The purpose of this study was to compare the force produced during these two forms of testing and to explore biomechanical mechanisms involved with each form of testing.
Methods: Data were collected on 25 subjects (12M) with a mean age of 36.5 (15.6). Subjects had no current or recent history of shoulder problems and had varied activity levels. After providing informed consent and demographic data, subjects were seated in a custom chair for shoulder muscle external rotation (ER) and elevation(EL) force testing. A hand-held force transducer connected to a computer was used to measure force using MotionMonitor software and sampling at 1000Hz. For external rotation the force was measured on the forearm just proximal to the ulnar head with the elbow flexed and the arm by the side. For elevation the force was measured just proximal to the ulnar head with the elbow straight and the arm elevated to 90 degrees in the scapular plane with thumb up. For both tests, the force was measured isometrically for approximately 3 seconds while the examiner held the transducer stationary, after which the examiner exerted enough force to break the contraction and move the arm. Shoulder angle was measured continuously using an arm-mounted inertial measurement unit (IMU). Differences between peak isometric and break forces were analyzed with a paired t-test.
Results: For ER, peak isometric force was 86.4(50.2)N while peak break force was 117.4(58.1)N (p 0.01). Peak ER break force occurred after an average of 1.8 (1.4) degrees of motion. Increase in ER break force ranged from -3.7% to 108.6%. For EL, peak isometric force was 76.6(50.5)N while peak break force was 110.1(37.6)N (p 0.01). Peak EL break force occurred after an average of 7.9(3.8) degrees of motion. Increase in EL break force ranged from -3.5% to 145.9%.
Conclusion(s): Peak forces measured during a break test are significantly higher than purely isometric forces. The amount of change associated with a break test compared to an isometric make test was also quite variable. Peak force during a break test occurred after angular motion of the arm, suggesting that an eccentric contraction was occurring and is a likely explanation for higher forces measured. Electromyography may also provide insight into understanding the mechanism of additional force during a break test. Further, examining the differences in make and break forces based on activity levels may help understand how “trained” muscles perform.
Implications: Clinicians measuring shoulder force should consistently use either a pure isometric make test or a break test for serial measurements with patients. Even a small angular change during testing associated with “breaking” the patient's force will lead to substantially different values compared to a purely isometric test.
Keywords: shoulder, force, isometric, eccentric
Funding acknowledgements: Not applicable
Topic: Musculoskeletal: upper limb
Ethics approval required: Yes
Institution: Arcadia University
Ethics committee: Institutional Review Board
Ethics number: 13105
All authors, affiliations and abstracts have been published as submitted.
