Effects of Volitional Preemptive Abdominal Contraction on Shoulder Muscle Function Following Shoulder Muscle Fatigue



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The volitional preemptive abdominal contraction (VPAC) strategy termed abdominal bracing maneuver (ABM), is commonly used during shoulder exercises. It is unknown how VPAC use affects shoulder muscle functions and shoulder proprioception when shoulder muscles are fatigued. Shoulder muscle fatigue is a common result of upper extremity dominant (UED) sports movements and discovering methods that reduce its effects is vital for the practicing orthopedic rehabilitation clinician. Objectives: The objectives of this study are to identify VPAC and muscle fatigue effects on (1) selected parascapular and glenohumeral muscles, (2) movement patterns (trajectories), (3) shoulder proprioception abilities, and (4) shoulder muscle amplitudes during seated, proprioception trials, during movements that are common to UED sports movements, with and without shoulder muscle fatigue presence. Design: A within-subjects, multifactorial, repeated measures design. Setting: A clinical biomechanics research laboratory. Patients or Other Participants: Thirty-six asymptomatic volunteers between 18 and 40 years of age.Interventions: Subjects performed arm elevation in flexion and scaption movement patterns with and without VPAC and muscle fatigue presence. In addition, subjects attempted to reproduce pre-measured flexion angles before and after a shoulder muscle fatigue protocol. Main Outcome Measures: Electromyography was used to examine muscle contraction amplitudes and onset timing of the anterior deltoid (AD), posterior deltoid (PD), upper trapezius (UT), lower trapezius (LT), serratus anterior (SA), and the infraspinatus (IF) muscles. Muscle response amplitudes were quantified by calculating root mean squared electromyography. Shoulder muscle activation timing was determined by the difference between initial muscle activation time and initial arm movement. Arm movement trajectories were observed by the elbow movement path. Shoulder proprioception was measured by shoulder flexion reproduction angles. Kinematic data was collected during the shoulder flexion trials, showing the accuracy in the subjects’ ability to reproduce a reference angle. All data were collected before, and after, a shoulder muscle fatigue protocol and with and without VPAC. Electromyographic data from the AD, PD, UT, LT, SA, and the IF muscles were collected to test muscle contraction amplitudes during the angle reproduction trials. Results: The VPAC increased the IF muscle amplitudes during flexion (Mdn = .028, p = .005, r = -.33) and scaption (Mdn = .028, p = .007, r = -.32), (p<.008), and the LT muscle amplitudes during flexion (Mdn = 0.036(0.076), p = .004) (p<.008). Shoulder muscle fatigue significantly increased the UT muscle amplitudes during flexion (Mdn = .082, p = .003, r = -.35) and scaption (Mdn = .086, p = <.001, r = -.43) and the PD muscle amplitudes during the flexion movement (Mdn = .079, p = <.001, r = -.46) (p<.008). Muscle fatigue decreased the PD muscle onsets during flexion (Mdn = 285.83, p = .003, r = -.36) and scaption (Mdn = 188.50, p = .006, r = -.33) and the AD (Mdn = -59.83, p = .001, r = -.39), LT (Mdn = 104.33, p = .001, r = -.40), and IF (Mdn = -4.17, p = .007, r = -.32) muscle onsets during flexion (p<.008). The VPAC increased movement toward the body during flexion and scaption, (p<.05). Muscle fatigue increased movement away from the body during flexion and scaption, (p<.05). The VPAC nor muscle fatigue significantly affected active shoulder flexion angle reproduction (p<.008). During the active shoulder flexion angle reproduction trials, the VPAC significantly decreased the IF muscle amplitudes (Mdn = .019, p = .002, r = -.37) and muscle fatigue significantly increased the UT (Mdn = .059, p = .001, r = -.39) and LT muscle amplitudes (Mdn = .023, p = .004, r = -.34). Conclusions: The limited effects of VPAC on parascapular and glenohumeral muscle function have beneficial results, such as increased stabilization at the shoulder joint. In addition, VPAC effects have the potential to counteract shoulder muscle fatigue effects during flexion. The VPAC nor muscle fatigue significantly affected shoulder proprioception as measured by the maximum angle obtained during attempted active shoulder flexion angle reproductions. Muscle fatigue and VPAC effects on muscle amplitudes did not change during proprioception exercise, as observed in the results that support previous literature.