Explorer Kinematic and Electromyographic Changes During 200 m Front Crawl at Race Pace

▼ The purpose of this study was to analyse eventual kinematic and electromyographic changes during a maximal 200 m front crawl at race pace. 10 male international level swimmers performed a 200 m maximal front crawl test. Images were recorded by 2 above and 4 under water cameras, and electromyographic signals (EMG) of 7 upper and lower limbs muscles were analysed for 1 stroke cycle in each 50 m lap. Capillary blood lactate concentrations were collected before and after the test. The variables of interest were: swimming speed, stroke length, stroke and kick frequency, hand angular velocity, upper limb and foot displacement, elbow angle, shoulder and roll angle, duration of stroke phases, and EMG for each muscle in each stroke phase. Generally, the kinematic parameters decreased, and a relative duration increased for the entry and pull phases and decreased for the recovery phase. Muscle activation of fl exor carpi radialis, biceps brachii, triceps brachii, peitoral major and upper trapezius increased during specifi c stroke phases over the test. Blood lactate concentration increased signifi cantly after the test. These fi ndings suggest the occurrence of fatigue, characterised by changes in kinematic parameters and selective changes in upper limbs muscle activation according to muscle action. D ow nl oa de d by : U ni ve rs id ad e do P or to . C op yr ig ht ed m at er ia l. 50 Training & Testing Figueiredo P et al. Kinematic and Electromyographic Changes ... Int J Sports Med 2013; 34: 49–55 [ 38 ] evaluated muscle fatigue in upper body muscles during a 100 m all-out front crawl; although no changes in EMG amplitude of the pectoralis major, latissimus dorsi and triceps brachii muscles were found, it increased for the triceps brachii and the lower part of the latissimus dorsi. Although the contribution of the lower limbs to the total propulsion in front crawl is known to be low [ 16 , 23 ] , the kick plays an important role in providing stability for the whole stroke by facilitation of body position, optimizing propulsion and minimizing resistance [ 24 ] , as well as in assisting an economical body roll, as it is linked to the hydrodynamic forces [ 45 ] , implying physiological changes (e. g., oxygen consumption) [ 28 ] . In addition, proper kicking is required as a foundation for development of good coordination in the global front crawl technique [ 36 ] . In this way, the changes that might occur in the lower limbs actions during high-intensity swim should be taken into account. In spite of the above-mentioned fi ndings, to the best of our knowledge, no studies have been conducted to analyse the changes of such kinematic and electromyographic parameters during a high intensity swim competition distance. The present study aimed to investigate kinematic and electromyographic changes during a maximal 200 m front crawl at race pace, as it is described that fatigue evolves during this event. Changes on general stroke parameters, with the decrease of the velocity and stroke length and increase of the stroke frequency [ 2 , 13 ] , on bioenergetics, with the increase of energy cost [ 19 ] , along the eff ort were reported. Methods ▼ Participants 10 international level male front crawl swimmers (mean ± SD of 21.6 ± 2.4 years old; 76.4 ± 6.1 kg of body mass; 1.85 ± 0.07 m of height; 1.89 ± 0.08 m of arm span), 200 m specialists (91.6 ± 2.1 % of the 25 m pool world record average speed), volunteered to participate in the study. All procedures were performed in accordance with the ethical standards proposed by Harriss and Atkinson [ 21 ] and the local ethics committee approved the study. All swimmers provided written informed consent. Swimmers were marked with white half spheres attached to a black complete swimsuit, allowing manual digitisation for further 3-dimensional reconstruction. 21 anatomical landmarks were used: the vertex of the head, 7 th cervical vertebra, mandible (mental protuberance), and the right and left tip of the third distal phalanx of the fi nger, wrist axis, elbow axis, shoulder axis, hip axis, knee axis, ankle axis, fi fth metatarsophalangeal joint, and the tip of the fi rst phalanx. Data collection After a moderate intensity individual warm-up totalling 1000 m, subjects underwent a 200 m maximal front crawl test, using a push off start and open turns to eliminate the infl uence of the dive in the analysis of the fi rst stroke cycle and due to the EMG apparatus. One complete non-breathing stroke cycle, at midpool, was recorded for each 50 m of the 200 m front crawl. The stroke cycle was defi ned as the period between 2 consecutive hand entries of the right hand. Swimmers were instructed to avoid breathing while swimming through the calibrated space. The swimming test was recorded by 6 synchronised video cameras (Sony ® DCR-HC42E) 4 under and 2 above the water. All cameras recorded the motion of the swimmer at a sampling frequency set at 50 fi elds per second and an electronic shutter speed of 1/250 s. The recorded swimming space was calibrated using a frame of the following dimensions: 3.0 m length (x), 2.0 m height (y), and 3.0 m (z) width (accuracy and reliability established by Figueiredo et al. [ 18 ] ). 3-dimensional reconstruction of the 21 body landmarks, digitised manually and frame by frame using APAS (Ariel Dynamics Inc), was computed using DLT [ 1 ] , Zatsiorsky anatomical model adapted by de Leva [ 15 ] , and a 6 Hz low pass digital fi lter. Tests were conducted in a 25-m indoor swimming pool (27.5 °C). The EMG activity was measured at a sampling frequency of 1 000 Hz, with a 16-bit analogue to digital conversion (BIOPAC System, Inc). The EMG signal of 7 muscles (fl exor carpi radialis, FCR; biceps brachii, BB; triceps brachii, TB; pectoralis major, PM; upper trapezius, UT; biceps femoris, BF; and rectus femoris, RF), which have been shown to have high activity during front crawl swimming (for review see Clarys and Cabri [ 11 ] ), was recorded from the right side of the body using bipolar (inter-electrode distance of 2 cm) Ag–AgCl circular surface electrodes, with preamplifi ers (AD 621 BN). The electrodes were placed parallel to the direction of muscle fi bres on the surface of the muscle belly according to international standards [ 21 ] . Before electrode fi xation, the skin surface was shaved, abraded, and cleaned with alcohol. Afterwards, electrodes were covered with an adhesive bandage (Opsite Flexifi x ® ) to avoid contact with water [ 14 , 33 ] . All cables were fi xed to the skin by adhesive tape to minimise the perturbation of the natural movement and interference with the signal. In addition, swimmers wore a complete swimsuit (Fastskin, Speedo ® ) with a cable entrance opened in the medium-dorsal position; over the water, a steel cable was extended with a sheave to which the cables corresponding to each one of the electrodes were fi xed. A reference electrode was attached to the skin of the patella. The total gain of the amplifi er was set at 1 100 with a common mode rejection ratio of 110 dB [ 14 ] . To synchronise EMG and video, an electronic fl ashlight signal/ electronic trigger was marked simultaneously on the video and EMG recordings. Capillary blood samples (5 μl) were collected from the ear lobe to assess rest and post exercise blood lactate by means of a portable lactate analyser (Lactate Pro, Arkray, Inc.). Blood lactate was measured before and at 1, 3, 5, and 7 min after the test; the peak value was used for further analysis as an indicator of exercise intensity. Data analysis Kinematical data analysis was done using APAS (Arial Dynamics, Inc.). The mean horizontal speed was calculated by dividing the swimmer’s mean whole-body centre of mass (CM) horizontal displacement by the time spent to complete 1 stroke cycle. Stroke frequency (SF) was the inverse of the time to complete 1 stroke cycle. Stroke length (SL) was the horizontal displacement of the CM during 1 stroke cycle. Angular speed of the hand was derived through a digitising procedure and considered to be the mean angular speed of the line from the midpoint of the wrist to the fi ngertip projected onto the xy plane of the external reference system. Kick frequency (KF) was the inverse of the time to complete 1 kick cycle, defi ned as the period between 2 consecutive maximum vertical coordinates of the right foot. The backward displacement amplitude and slip amplitude were calculated through the diff erence between the coordinates of the most forward point and the most backward position of the D ow nl oa de d by : U ni ve rs id ad e do P or to . C op yr ig ht ed m at er ia l. 51 Training & Testing Figueiredo P et al. Kinematic and Electromyographic Changes ... Int J Sports Med 2013; 34: 49–55 fi ngertip, and of the entry and exit of the fi ngertip, respectively ( ● ▶ Fig. 1 ). The vertical motion of the upper limb was represented by displacements of the fi ngertip, wrist, and elbow. The y displacement of the fi rst phalanx tip was representative of the foot’s vertical motion. Both the y direction motion of the upper limb and foot were referenced to an external point. The lateral motion of the upper limb was calculated as the absolute z displacement, referenced to the swimmer’s CM. Shoulder roll angle was determined by the arc-tangent (Sy/Sz), where Sy and Sz are the y and z components of the shoulder unit vector, i. e., the angle between the unit vector of the line joining the shoulders, projected onto the yz plane (the plane perpendicular to the swimming direction) and the horizontal. The 3-dimensional elbow angle was calculated in 4 time moments within the underwater stroke cycle ( ● ▶ Fig. 1 ): (i) entry of the hand in the water (A – entry); (ii) beginning of fi nger backward movement (B – fi rst back); (iii) fi nger vertically aligned with the shoulder (C – shoulder x); (iv) end of backward movement (D – end back). These time moments were calculated based on the horizontal displacement of the fi nger and shoulder during the stroke cycle. The elbow angle range during the pull and push phases was calculated as: C–B and D–C, respectively. 4 separate phases were identifi ed within every stroke cycle ( ● ▶ Fig. 1 ), from the swimmer’s (3-dimensional model) horizontal (x) and vertical displacement (y) of the digitized fi nger coordinates and noting the time corresponding to these displacements using APAS (Arial Dynamics, Inc.): entry, pull, push, and recovery [ 10 , 36 ] . Time was expressed in seconds and as a percentage of the stroke cycle. Kick cycle phases were calculated based on the vertical displacement of the foot: the downbeat, from the maximal to minimum vertical coordinates; and, the upbeat, from the minimum to maximal vertical coordinates. The EMG data analysis was performed using the MATLAB 2008a software environment (MathWorks Inc., Natick, Massachusetts, USA). The raw EMG signals were band-passed fi ltered (8–500 Hz), full-wave rectifi ed and smoothed with a 4 th order Butterworth fi lter (10 Hz) for the linear envelope. The integration of the rectifi ed EMG (iEMG) was calculated, per unit of time, to eliminate the stroke phases duration eff ect (iEMG/t). The signal was partitioned in 40 ms windows to fi nd the maximal iEMG value for each swimmer over every stroke cycle in the mid-pool. To normalise the results, iEMG/t was expressed as a percentage of iEMG maximum value obtained during the 200 m [ 9 ] . Statistical analysis All data were checked for normality and expressed as means and SD. The compound symmetry, or sphericity, was checked using the Mauchley test [ 44 ] . A 1-way repeated measures analysis of variance (ANOVA) was used to assess changes in the measured variables over the 4 laps during the race. When signifi cance was determined, post-hoc comparisons were conducted with Bonferroni analysis. A repeated measures t-test was used to compare blood lactate concentration between the beginning and end of the eff ort. Statistical signifi cance was set at p ≤ 0.05. All statistical tests were performed using STATA 10.1 software (StataCorp, Inc.). Eff ect size between laps and between beginning and end of the 200 m were computed with Cohen’s f and Cohen’s d, respectively. The criteria for interpreting the eff ect size were based on Cohen’s [ 12 ] suggestion that f eff ect sizes of 0.1 are small, 0.25 moderate, and 0.4 large and d eff ect sizes of 0.2 are small, 0.5 moderate, and 0.8 large. Results ▼ Mean ± SD, p, f and F values of the repeated measures ANOVA are displayed in ● ▶ Table 1 for the general biomechanical parameters, hand angular v and KF, in ● ▶ Table 2 for the arm and foot lengths, and in ● ▶ Table 3 for the angle variables computed. Changes in race parameters were observed as denoted by the signifi cance level and large eff ect sizes. Swimming speed, hand angular v, SL, SF, maximal depths of fi nger, wrist and elbow, elbow angle at the end back, shoulder roll and KF signifi cantly diminished along the 200 m. Whereas, maximal elbow width showed a signifi cant increase. Mean ± SD relative and absolute durations of stroke phases for each lap, as well as the propulsion/non-propulsion ratio, are presented in ● ▶ Fig. 2 (left and right panel, respectively). The entry and pull phases presented an increase of the relative duration along the 200 m (F 3,27 = 5.25, p = 0.005, f = 0.23; F 3,27 = 3.37, p = 0.03, f = 0.36, respectively), while a decrease was observed for the recovery phase (F 3,27 = 9.08, p < 0.001, f = 0.37). Absolute durations of the entry phase increased progressively from the 0.1