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Improving the performance of underwater undulatory swimming (UUS) improves swimming time, so it is important to identify the pattern of muscle coordination in swimmers with fast UUS. This study aimed to identify muscular coordination in the trunk and lower limb during UUS in elite swimmers. Nine swimmers (aged 20 ± 2 years; height, 1.74 ± 0.03 m; weight, 73.0 ± 4.4 kg) participated in this study. Measurements were taken by electromyography of eight muscles: rectus abdominis (RA), internal abdominal muscle (IO), rectus femoris (RF), erector spinae (ES), multifidus (MF), tibialis anterior (TA), and thigh biceps (BF), and gastrocnemius (GS). For evaluation of muscle coordination, “muscle synergy” and “activation coefficient” were calculated using non-negative matrix factorization from electromyographic data. Kick frequency, kick amplitude, swim velocity, and kinematics of the pelvis were also calculated. Kick cycle was divided into two kick phases: downward kick (from the highest toe vertical coordinate to the lowest point) and upward kick (from the lowest point to the highest point). Kick frequency, kick amplitude, and swimming velocity were 1.9 ± 0.3 Hz, 0.45 ± 0.6 m, and 1.8 ± 0.2 m·s −1 , respectively. The maximum backward pelvic tilt was 94.4 ± 4.5° and the minimum (forward) was 90.8 ± 5.7°. Three muscle synergy values were extracted from each swimmer during UUS: those involved in the transition from upward kick to downward kick (Synergy 1), downward kick (Synergy 2), and upward kick (Synergy 3). Synergy 1 involved mainly the RF, IO, and RA, which were activated during the turn from the upward to the downward phase. Synergy 2 involved mainly the MF, ES, and TA in the downward kick. Synergy 3 corresponded to the coordination of the BF and GS, which were active in the upward kick. In UUS by elite swimmers, both the upward kick and downward kick followed the trunk muscles involved in the pelvic forward–backward tilt movement, and lower limb muscles were activated. Muscle coordination based on pelvic forward-backward tilt during UUS is expected to contribute to the coaching field for elite swimmer development.

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ObjectivesThis study aimed to clarify the trends of injury occurrence in the Japan national swim team for 15 years and to evaluate the effectiveness of the lumbar injury prevention project. It also aimed to verify the incidence of swimming-related injuries among swimmers by sex, age and swimming style.MethodsThe target group comprised 488 swimmers who participated in the Olympics, Asian Games and Universiade from 2002 to 2016; we compiled data for the total number of injuries in each body part. The lumbar injury prevention project started in 2008 and included two components (deep trunk muscle exercises and evaluation of lumbar disc degeneration using MRI). We analysed the prevalence of lumbar injury before (2002–2008) and after (2009–2016) implementation of the lumbar injury prevention project by χ2 test. We compared age, sex and swim strokes between the injured and non-injured groups by χ2 test and unpaired t-test.ResultsThe most common injury site was the lower back, followed by the shoulder and knee. The lumbar injury prevalence was significantly lower after implementation of the prevention project (23.5% vs 14.8%; p<0.05). Shoulder injuries were common in backstroke swimmers. The injury rate was significantly higher in female than in male swimmers. The injured group was significantly older than the non-injured group.ConclusionsLumbar injury prevention intervention might be effective to prevent lower back injury in swimmers. Injury risk factors included female and old age; younger female athletes should prevent the development of injuries as they mature.

Abdominal bracing exercise is used to enhance all abdominal muscle activity. There are two methods of abdominal bracing-"activate the abdominal muscles without hollowing the lower abdomen" and "activate the abdominal muscles while inflating the lower abdomen". Although the latter method is widely used in athletic fitness coaching, the electromyographic activities of the abdominal muscles in this method have not been measured yet. This study aimed to clarify the electromyographic activity of all abdominal muscles during abdominal bracing using the latter method of "activating the abdominal muscles while inflating the lower abdomen". Thirteen healthy men (age: 22 ± 2 years) participated in this study. Transversus abdominis (TrA), internal oblique (IO), external oblique (EO), and rectus abdominis (RA) electromyography was recorded using an intramuscular fine wire and surface electrodes. The participants performed abdominal bracing and hollowing for 5 s (seconds) in six different positions. The electromyographic data during trials were calculated as a percentage of maximal voluntary isometric contraction (%MVIC). A two-way ANOVA was used to compare the %MVIC of each trunk muscle between abdominal bracing and hollowing among the six different positions. In TrA electromyography, there was no difference between abdominal bracing and hollowing. On the other hand, the activities during abdominal hollowing were significantly higher than those during abdominal bracing in IO and EO (p < 0.05). In RA, the activities during abdominal bracing were significantly higher than those during abdominal hollowing (p < 0.05). Abdominal bracing under the method of inflating the lower abdomen is not recommended for high activity of the IO and EO.