Muscle synergy is defined as a specific and consistent spatiotemporal pattern of muscle activations that leads to similar joint trajectories (Ting and McKay, 2007) and have been proposed as a neural strategy for simplifying the neuromuscular control. These synergies can be identified from electromyographic (EMG) patterns recorded from numerous muscle decomposition algorithms (e.g principal component analysis, PCA) based on two components, (i) "muscle synergy vectors" which corresponds to the relative loading of each muscle within each synergy; and (ii) "synergy activation coefficient" which represents the temporal activity of the muscle synergy (Frere and Hug, 2012). Some researchers observed that temporal recruitment patterns were robust across various mechanical constraints while the muscle weightings varied across subjects or test conditions (Cappellini et al., 2006; Ivanenko et al., 2004). These studies showed that muscle synergies were stable across tasks and yet flexible enough to allow inter-individual variability and accommodate errors or changes.
Working out on sculling ergometers, either on fixed (FE) or slides ergometer (SE), was a crucial training component for competitive rowers. The slides ergometer (SE) was an improvisation from fixed ergometer (FE) to bridge the gap of mechanics between ergometer rowing and on-water rowing. For Concept 2, the SE consists of a rail that was mounted underneath the fixed ergometer. Both types of ergometers were widely utilized by rowers for training (Colloud et al., 2006; Maestu et al., 2005; Secher, 1993), evaluation (Colloud et al., 2006) and team selection (Elliott et al., 2002; Maestu et al., 2005). Although rowing on slides ergometer (SE) was hypothesized to be less physiologically demanding than FE rowing (Mahony et al., 1999), recent findings indicated that physiological variables (i.e., maximal heart rate, peak lactate concentration and peak aerobic capacity) were not significantly different on both rowing ergometers except for anaerobic capacity (Holsgaard-Larsen and Jensen, 2010).
On the other hand, Colloud et al. (2006) reported significant difference in force curve profiles (i.e., handle and stretcher force) during SE and FE rowing. A large anterior-posterior force at the stretcher was produced by the rower to move his center of mass in the positive and negative directions when rowing on FE. This causes considerable amount of contact force and external power (i.e., the product of the force exerted on the handle by its velocity) during the catch and the finish phases. Conversely, low inertial force was necessary to accelerate the rower's center of mass on SE ergometer (Colloud et al., 2006). Hence, the differences between force profiles on FE and SE may have implications on the pattern of muscle recruitment, coordination (Colloud et al., 2006; Green and Wilson, 2000) and adaptation (Roth et al., 1993).
Muscle synergy is particularly important in rowing because as a power-endurance sport that recruits 70% of total muscle mass (Steinacker et al., 1998; Roberts et al., 2005), rowers need to have enhanced physiological capacity coupled with efficient muscle synergy. Despite the importance of muscle coordination on rowing performance (Rodriguez et al., 1990; Tachibana et al., 2007), no studies have been conducted comparing the muscle synergy during FE and SE rowing. As the muscle activity is a large determinate of metabolic rate during maximal effort activities (Wakeling et al., 2010) such as 6 min maximal rowing, and muscle synergy is a strategy to simplify neuromuscular control, it is thus compelling to explore the underlying relationships. Therefore, this study was undertaken in an attempt to evaluate the muscle synergy during 6 minutes maximal rowing of physically active untrained males. The subjects were not specifically trained in competitive rowing to exclude the effect of training bias on muscle synergy. Our second aim was to evaluate the laterality of muscle synergy between the right and the left sides of the body as the previous studies only assumed the symmetries of muscle synergy on sculling ergometer rowing (Turpin et al., 2011a; Nowicky et al., 2005; So et al., 2007).
There is no a-priori power analysis test for PCA analysis, however, based on previous studies of muscle synergy (Hug et al., 2011; Ivanenko et al., 2004; Turpin et al., 2011a; 2011b; 2011c; Wakeling and Horn, 2009) we decided to recruit nine physically active males (age: 26.78 [+ or -] 2 years, mass: 80.61 [+ or -] 11.48 kg, height: 1.81 [+ or -] 0.07 m). The group consisted of competitive triathletes, long distance runners, cyclists and rugby players who had never been involved in competitive rowing. A separate familiarization session was undertaken before the real experiment to ensure the safety of the subjects and to reduce potential risks. For each subject a written informed consent was obtained. All tests and scientific experiments comply with the ethical code of University of Delaware Internal Review Board.
Experiments were carried out on a Concept 2 model D ergometer (Morrisville, Vermont, USA). The slides system consists of a pair of rails that can be attached to the ergometer to simulate OW rowing mechanics. Drag factor was manually adjusted relative to the subjects' body weight which resembled the resistance effect during OW rowing (Kane et al., 2008). Simultaneous visual feedback was provided to subjects through an attached display that showed data on heart rate, stroke length, stroke rate, power output, distance covered and time. Stroke-to-stroke data were assessed using the RowPro v2.006 software (Digital Rowing) in conjunction with the Concept 2 interface. These data were averaged into 30s intervals.
Eight rowing-specific muscles were evaluated bilaterally: Gastrocnemius Lateralis (GL), long head of Biceps Femoris (BF), Rectus Femoris (RF), Erector Spinae (ES), Lattisimus Dorsi (LD), Brachioradialis (BR), Triceps Lateralis (TR) and Deltoid Medius (DM). The muscles activity was recorded using wireless Noraxon Telemyo DTS Desk Receiver (Noraxon, Scottsdale, AZ). Pairs of surface Ag/AgCl wet gel electrodes (Noraxon, Scottsdale, AZ) were attached to the skin with a fixed 20 mm inter-electrode distance. Before the electrodes were applied, the skin was shaved and cleaned with alcohol to minimize impedance. Electrode placement followed the recommendations by SENIAM (Hermens et al., 2000) for all muscles, except for LD and BR, which were not referenced by SENIAM. For LD, the electrode was placed on the muscular curve at T12 (de Seze and Cazalets, 2008) and for BR, the electrode was placed at 1/6 of the distance from the midpoint between the cubital fossa to the lateral epicondyle of the ulna (Muceli et al., 2010). Raw EMG signals were recorded at sampling rate of 1500 Hz.
The position and orientation of the wrist joint projected along the longitudinal axis of the ergometer (i.e., the rowing direction) was analyzed to define the rowing cycle. Their three-dimensional trajectories were captured using ten infrared cameras (Vicon MX, Oxford, UK). The spatial accuracy of the system is better than 1 mm (root mean square). The rowing cycle was defined as the time between two successive local maxima. The points of local maxima and minima indicated catch and finish positions, respectively...