Dimensiones de Participación Política

Thirty healthy individuals were recruited from a university population at The University of North Carolina at Chapel Hill. Participants included both males and females between the ages of 18-30 who participated in at least 30 minutes of physical activity for at least 3 days a week over the past 6 months. All participants read and signed a consent form approved by the university’s Institutional Review Board.

Procedures

Determination of the maximal strength

Prior to data collection, participants’ maximal humeral elevation strength in both the sagittal and scapular planes and elbow flexion strength was assessed using a handheld dynamometer (HHD) (Lafayette Instrument, Lafayette, IN). HHD’s have

previously been shown to be both a reliable and valid assessment tool for measuring isometric shoulder strength, given that the researcher’s strength is greater than that of the

subject (Bohannon, 1988; Stratford & Balsor, 1994). These procedures were repeated three times each, and the average peak force output from the dynamometer was determined as the subject’s maximal strength.

Preparation for the Assessment of Muscle Activity

The locations of the electrode placement were determined through palpation and the location was marked on skin using a marker (Figures 1-3)(Cram, et al., 1997). The skin was abraded and cleaned using a 70% isopropyl alcohol solution to reduce electrical impedance. Once the EMG set up was complete, the participants proceeded to complete a 5-minute warm up on an Airdyne bike.

Assessment of the Muscle Activity during the Maximal Voluntary Isometric Contraction Following the warm up, maximum voluntary isometric contraction (MVIC) testing was completed in order to normalize the EMG data for the biceps brachii, anterior deltoid, middle deltoid, posterior deltoid, and infraspinatus.

MVIC testing consisted of 3 5-second trials, with the data from the middle 3 seconds used to calculate the MVIC

Preparation for the measurement of the humerothoracic kinematics

Following the assessment of the muscle activity during MVIC, humerothoracic kinematic data was measured using the Motion Star electromagnetic tracking device (Ascension Technologies, Burlington, VT) with the Motion Monitor software (Innovative Sports Training, Inc., Chicago IL). Five electromagnetic receivers were used for data collection. For each participant, a receiver was placed on the sternal notch, over the flat portion of the acromion, the mid-shaft of the humerus, and the ulnar aspect of the wrist. All of the receivers were secured with double sided tape, pre-wrap, and white athletic tape to

limit movement of the receiver on the skin. The 5th receiver was attached to a stylus and used to digitize landmarks on the participants. The digitization of thoracic, humeral, and scapular landmarks allowed for transformation of the receiver data from a global coordinate system to anatomically based local coordinate systems (Karduna, et al., 2001; Laudner, et al., 2006; J. B. Myers, et al., 2005; J. B. Myers, et al., 2007).

Assessment of the Muscle Activity during the Humeral Elevation Tasks

The muscle activities were collected while the participants performed humeral elevation tasks in the sagittal plane, scapular plane, and performed internal/external rotation at 90° of shoulder abduction. The order of tasks was counterbalanced. Participants stood in anatomical position facing the direction of the positive x-axis. During the humeral elevation task in the scapular plane (30° anterior to the frontal plane) and sagittal plane, the plane of the movement was guided by tape marked on the floor. During the internal/external rotation task, participants’ arms were positioned at 90° of shoulder abduction, with the elbow fixed in 90° of flexion by a custom made fiberglass brace. The investigator used a guide to help keep the participant’s shoulder in 90° of abduction during the duration of the trial. Participants were given the opportunity to practice the task until they felt comfortable. Elevation and descent of the participants arm during the elevation tasks was performed on a 2 second up, 2 second down time period, as controlled by a metronome set at 60 beats per minute. The elevation task in the sagittal and scapular planes were completed while the participants held weights that were 10% of the maximal elevation strength. Ten repetitions were completed for each movement task.

Following the completion of the humeral elevation tasks, fatigue group participants completed a biceps brachii fatigue protocol on an isokinetic dynamometer (Biodex Pro 3, Biodex Medical Systems Inc., Shirley, NY). The EMG electrodes and electromagnetic receivers remained attached to the participant during the fatigue protocol. Participants were positioned supine in the Biodex with their arm and elbow positioned for the biceps brachii elbow flexion fatigue protocol (figure 4). The arm was positioned at rest next to the participant, resting on the seat. The forearm was supinated and strapped to the flexion attachment. Participants were strapped across their lap, chest, and humerus to help limit any movement of the body.

The participants performed 3 maximal concentric contractions at a speed of

180°/second to determine the maximal torque output. The peak torque value from the three repetitions was used as the value for maximal torque output. Fatigue was defined as when torque output drops below 50% of the maximal torque output for 3 consecutive contractions (Voight, et al., 1996).

After 1-minute rest, participants performed concentric elbow flexion against the dynamometer moving at 180°/second until they reached fatigue. During the fatigue protocol, the investigator had real time feedback of torque output to determine when the participant reached fatigue. Once fatigue was achieved, the cycle of determination of the maximum torque output and the fatigue protocol was repeated twice more. During the second and the third cycles, the fatigue protocol was repeated until the subject reached fatigue based on the maximal torque determined immediately prior to the respective fatigue protocol.

Immediately following the fatigue protocol, participants repeated the humeral elevation tasks. All post-testing procedures were completed within 5 minutes of completing

the fatigue protocol. Additionally, the isometric elbow flexion task was performed after the completion of the post fatigue data collection to validate that the fatigue effect was still present at the end of the data collection.