Electromyographic effects of elbow joint flexors and extensors during induced muscle pain.

Sample<br>Thirteen volunteers, all males (mean age 27±5 years, 178±11 cm, and 78 ±17 kg) participated in the experiment.<i>Apparatus and movement</i>Two minutes of continuous horizontal elbow flexion/extension movements (115º range) aiming at a 22º-wide target positioned at each end of the arc of motion were performed on a manipulandum. The average number of movements (flexion and extension of the elbow joint) performed was 95,23 ± 14.24. A load of 10 kg was added to the manipulandum to increase the mechanical strength of the task to be performed. Volunteers were seated comfortably with the dominant arm in semi-prone position, strapped at an adjustable support and fixed at 45º of shoulder horizontal flexion in 90º of abduction. Shoulder angles were defined relative to the coronal plane with 0º corresponding to the arm aligned with this plane. The forearm was strapped to a light manipulandum positioned horizontally aligned with the arm support (Fig. 1). The elbow joint was positioned just above the fulcrum of the manipulandum so that only horizontal movements were permitted. The volunteers were instructed to grasp a handle and to do the movements as fast and as accurately as possible. Visual feedback of the elbow position, the target width, and the range of motion were shown to the volunteer on a computer screen. Verbal encouragement was provided in order to obtain the maximum possible speed of the cyclic contractions.<i>Experimental muscle pain</i>An infusion of 1.5 ml of sterile hypertonic saline (5.8%) was injected intramuscularly into the biceps brachialis muscle (lateral head) at a rate of 90 ml/h, using a disposable stainless needle (27 g, 40 mm) connected via a tube (IVAC, G30303) to an infusion pump (ALARIS medical systems, Asena, UK). Pain intensity was assessed using a 10-cm electronic visual analogue scale (VAS), where 0 cm represented "no pain" and 10 cm represented "intolerable pain." The VAS signal was continuously recorded. Participants were allowed to adjust the values using their non-exercising hand and were instructed to focus on the VAS during breaks between individual trials. Mean VAS scores obtained during and between the trials were calculated.<i>General procedures</i>The volunteers attended the laboratory in two sessions, separated by at least one week. In each session, two minutes of flexion and extension movements were performed in three conditions, namely, pre-infusion, immediately after infusion of hypertonic (or isotonic) saline solution and 30 minutes after the pain, eventually caused by the infusion, was vanished.<i>Pain assessment</i>To assess pain perception, a 10-centimeter electronic visual analog scale (VAS) was used. On the scale, 0 cm represented "no pain" and 10 cm indicated "intolerable pain." Subjects could continuously adjust the VAS score using their hand not involved in the exercise to reflect their pain intensity throughout the experiment. They were instructed to focus on the VAS during rest periods between trials. The mean VAS scores during both exercise and rest periods were then calculated. The angular position of flexion and extension of the elbow joint, as well as the electromyographic activity of the muscles analyzed were synchronously acquired.<i>Kinematic and EMG recordings</i>Electromyography signal (EMG) of biceps brachii (long head), triceps brachii (lateral head), trapezius, and brachioradialis muscle were recorded. The EMG intensity of each muscle was normalized by the respective peak (average of two trials) of maximal voluntary isometric contraction recorded before the first set of trials. An electrogoniometer (Biometrics SG110, Ladysmith, Vt.) was used to measure elbow angular position. A pair of surface electrodes (Medicotest 72001-k, ØLstykke, Denmark) was placed in the direction of the muscle fibers (2 cm apart) on shaved, abraded cleaned skin. On biceps brachii (long head) the electrodes were placed on the line between the medial acromion and the fossa cubit at 1/3 from the fossa cubit. For the triceps brachii (lateral head) the electrodes were placed at 50 % on the line between the posterior crista of the acromion and the olecranon at 2 finger widths lateral to the line. For upper trapezius the electrodes were placed at 50% on the line from the acromion to the spine on vertebra C7. On the brachioradialis belly, the electrodes were placed in the distal part of the upper arm where the muscle becomes superficial, lateral to the line between acromion and cubital fossa. All the electrode placements were done according to SENIAM recommendation. The EMG signals were bandpass filtered (second order, 20–500 Hz), amplified (1,000–10,000 times; CounterPoint MK2, Dantec, Skovlunde, Denmark) and sampled at 2 kHz. Pain intensity (VAS), electrogoniometer, and electromyographic signals were acquired in parallel by an analog/digital converter and stored on a personal computer.The dataThe data are stored in a .csv format that can be read using excel software. The data set comprises files with the sEMG signal from thirteen volunteers plus one file named “EMG_pain_info” in .txt format. The data files are separated in columns by comma. The files are organized by columns in the following structure: Visual analog scale, electronic goniometer biceps brachii, trapezius, triceps brachii, brachioradialis. These files are named as S01BBHa, where the S01 stands for subject 01, and BB stands for biceps brachialis (the muscle where pain was induced), H or I stands for hypertonic or isotonic saline solution, and “a, b and c” stands for the epoch of the protocol, being a the epoch pre infusion, b the epoch immediately after infusion, and c is the epoch containing data acquired immediately after the pain has completely vanished. Altogether 7279 contractions data, of 13 subjects in three different conditions and three epochs. The EMG_pain_info file contains details about the data collection, and anthropometric data of subjects evaluated in the study.