Sixteen asymptomatic individuals (7 m/9 f; 28 ± 5 years; 69 ± 11 kg; 174 ± 9 cm) were recruited in a university setting and included in the study after fulfilling inclusion/exclusion criteria. Inclusion criteria were: age (≥18 to 45 years) and weekly sport activity (≥ 3 sport-session/week). Only sport activities demanding a large involvement of the shoulder were included. Exclusion criteria were: pregnancy, infection, cancer, neurological or metabolic diseases, previous shoulder injury (<2 months) or surgery (<6 months), pain (shoulder and general) and not-overhead sports (e.g. running, cycling). Before measurements, all participants were clearly (verbally and in written form) informed about all procedure details and signed a written informed consent. The institutional research committee approved the study, according to the Declaration of Helsinki and its amendment in 2008.
2.2 Measurement Protocol
First, anthropometrical data, sport activity and training sessions per week were collected using a self-developed questionnaire. History of injuries, level of mobility, strength and actual pain of the dominant shoulder were investigated using a paper version of the Costant&Murley test31,32. Each participant subjectively rated their current shoulder pain (dominant arm) using a visual analog scale (VAS; 0 = no pain to 10 = maximum imaginable pain). Pain threshold for study inclusion was set at £ 1 (no pain or very mild). Shoulder strength was tested at the end of the questionnaire, consisting of a short functional test31,32 that checked if participants were able to hold a 4.5 kg weight in shoulder abduction 90° in the scapular plane with the arm extended for 5-sec. This was followed by preparation of the dominant shoulder for electromyographic (EMG) assessment. In detail, six pairs of EMG-electrodes were positioned over six main shoulder muscles as reported in Table 1.
Next, a standardized shoulder warm-up and a maximal isometric voluntary contraction (MVIC) were performed by all individuals. MVICs were performed on an isokinetic dynamometer (Contrex MJ; Physiomed Elektromedizin AG, Germany) for the purpose of EMG-normalization. Participants laid supine on the device’s backrest holding a handle with the hand in a supinated position. The trunk was fixed with an additional diagonal belt. The dominant arm was placed in measurement position by the principal investigator, fixed with an additional brace at the proximal part of the elbow. The first MVIC test position was (i) internal/external rotation shoulder abducted to 90° in the scapula plane, neutral humeral rotation, elbow flexed 90° (0° MVIC angle). After a 1-min rest, this was followed by (ii) frontal flexion/extension arm stretched, 90° flexion in the frontal plane (90° MVIC angle). The employed positions demonstrated optimal shoulder muscles voluntary activation during isometric contraction30,34,35 and EMG-MVIC assessment using an isokinetic device reported high reliability33. All individuals initially performed a 5-sec isometric contraction attempt (familiarization: sub-max. effort). After a 1-min rest, a 5-sec MVIC (maximal effort) contraction was produced whilst EMG-data was recorded. This procedure was applied for each movement direction (internal and external rotation, frontal flexion and extension). Afterwards, each participant performed three dynamic shoulder exercises with the dominant shoulder, randomly ordered in a standing position. Exercises were performed holding different custom-built tubes for a total of four conditions (four tubes). EMG of the shoulder muscles was assessed during all three exercises under all four conditions.
2.3 Exercise protocol
The three performed exercises were selected from previous published studies3,13,14 assessing specific exercise programs for the shoulder: (I) internal/external rotation, shoulder abducted to 90° in the scapula plane, neutral humeral rotation, elbow flexed 90° (In/Ex), (II) abduction/adduction, arm straightened, oriented at 30° of scapula plane (scaption) till 90° arm-elevation in frontal plane (Ab/Ad) and (III) flexion/extension, arm outstretched, following a diagonal pattern (start position counter-lateral hip) till 180° shoulder-extension (F/E). During the movements, the hand was rotated continuously, in every repetition, from prone to neutral position in exercises I and III, as well as vice versa in exercise II. Each exercise consisted of five repetitions with a 30-sec recovery pause between conditions and was performed under all four conditions (pipes): empty-pipe (P), water-filled-pipe (PW), weight-pipe (PG) and weight + water-filled-pipe (PWG). Except for the initial condition (P) all other conditions were randomly ordered. Between each exercise, a 1-min rest was performed and the order of the exercises was randomized. Participants were instructed to grab the tube in the middle portion and keep a constant-moderate exercise velocity, ≈3-sec for each contraction mode (concentric, eccentric). Individuals stood in front of a mirror for visual feedback. The principal investigator constantly checked proper exercise execution (speed, technique) to correct the participant if necessary. All included participants were able to perform all exercises with proper execution on first attempt.
2.4 Exercise tool & conditions
Four pipes with similar basic characteristics (Table 2) were custom-built in order to represent four different exercise conditions. The empty-pipe (P) consisted of a stable mass of low weight (baseline characteristics). The water-pipe (PW) was filled with 550 ml of normal water, representing a condition of low weight with unstable mass behavior and consequential increased instability, due to the water shaking inside the tube. The weight-pipe (PG) had two additional weight plates (2 kg each + 0.5 kg fastening screws) fixed at the bottom of each side, representing a condition of higher weight of stable mass. Last, the water + weight pipe (PWG) consisted of a 400 ml of water and two additional weight plates (1.25 kg each + 1.1 kg fastening/seal up materials) fixed at the pipe’s sides, representative of a combined condition of unstable (water) and stable (weight plate) high weight mass.
2.5 EMG analysis
Neuromuscular activity of the following shoulder muscles was assessed with a 6-lead surface EMG setup: Mm. upper trapezius (U.TA), lower trapezius (L.TA), lateral deltoid (DE), latissimus dorsi (LD), serratus anterior (SA) and pectoralis major (PE). EMG-data were recorded using a wireless surface capture system (Myon 320, RFTD-32, sampling frequency 4000Hz, myon AG, Switzerland). Electrode (bipolar pre-gelled Ag/AgCl; Ambu, Medicotest, Denmark, type P-00-S) placement was carefully determined (Table 1). The skin was shaved, slightly roughened with sandpaper, to remove surface epithelial layers and disinfected. Inter-electrode impedance was checked to be <5 kΩ23,36. Wireless transmitters (m320TXA), forwarding the signal to a central receiver unit (m320RX, bandwidth: 5-500 Hz, butterworth filter 4th order, digitized), were placed at the skin and connected to EMG electrodes by short cables. To include a synchronized trigger signal for start- and end-point detection of each movement direction, an additional telemetric accelerometer was used. After detecting the start- and end-point of the movement cycle, signals were A/D-converted (NI PCI 6229, 250 kS/s, 16-Bit, National Instruments®, Austin, TX, USA) and stored on a personal computer (IMAGO record master, LabView®-based, pfitec, biomedical systems, Endingen, Germany). Post-processing of the EMG and ACC data was done using a customized software solution (IMAGO process master, LabView®-based, pfitec, biomedical systems, Endingen, Germany). The collected EMG-data was visually screened for detection of possible artefacts in each contraction mode (concentric, eccentric) for all 5 repetitions. After that, the signals were rectified and the root mean square [RMS (V/s)] was calculated. Maximal voluntary isometric contraction values (MVIC) were obtained calculating 1 sec of the highest activity plateau (out of the 5-sec measured), using visual inspection by the same principal investigator, for all EMG data.
2.6 Data analysis
All non-digital data were paper-pencil collected in the questionnaire and case report form (CRF) and later transferred to a digital Excel data-sheet (Microsoft, Redmond, WA, USA, Version 15.18). A plausibility check was performed by screening all data-sets for implausible or extreme values. Abnormal values were recalculated or revised in relation to the hand-written CRF information and raw-data. Raw RMS (V/s) and MVIC-normalized RMS (%MVIC) values were averaged across the 5 repetitions for both contraction modes (concentric, eccentric) in all conditions (pipes). Scapular stabilizer (SR) and contraction (CR) ratios for the normalized amplitudes were calculated for all exercises and conditions. SR were calculated by dividing the RMS (%MVIC) of U.TA with RMS (%MVIC) of L.TA and SA [e.g. RMS (%MVIC)-U.TA/RMS (%MVIC)-L.TA]. CR were assessed by dividing the mean RMS (%MVIC) of the concentric to the mean RMS (%MVIC) of the eccentric phase [e.g. In/Ex: RMS (%MVIC)-Ex/RMS (%MVIC)-In]. Statistical analysis was performed using the SPSS software (SPSS 21.0, IBM Corp., Armonk, NY, USA). Normal distribution of the data was tested (Shapiro-Wilk) and descriptively analyzed (mean ± SD). Analysis of differences between conditions (pipes) for each muscle was performed for the RMS (%MVIC) values, contraction ratios (CR) and scapula ratio (SR) using paired t-tests with a level set at p < 0.05 and subsequent manual Bonferroni correction (a=0.008) to account for multiple testing. Pearson’s correlation coefficient (r) was calculated for each muscle and reported as mean value, representing consistency of the measurements protocol between conditions. Differences between RMS means (D%RMS) and percentage of the difference between conditions were calculated.