One of the fundamental principles of swimming is to minimise resistance by maintaining good alignment between the longitudinal axis of the body and the intended line of progression (Counsilman, 1973; Maglischo, 2003). Thus, the prone swimmer seeks to progress in the swimming direction and avoid rotations of the body about a vertical axis of the external reference frame (yaw) so that the body remains aligned with the swimming direction. While many studies have looked at the effect of body roll on performance in front crawl, for example, Lui et al. (1993), Payton et al. (1997), Cappaert et al. (1998), Castro et al. (2003), Yanai (2004), Seifert et al. (2005), Castro et al. (2006), Psycharakis and Sanders (2008), Sanders and Psycharakis (2009), Psycharakis and Sanders (2010), Psycharakis and McCabe (2011), Payton and Sanders (2011) little research has been conducted to investigate yaw in human swimming. Yaw occurs when torques about the vertical axis produced by the limbs on one side of the body are not balanced by those on the other side of the body.
In the 'symmetrical strokes' that is, breaststroke and butterfly, the torques produced by right and left hands and those produced by right and left feet can balance so that there is zero net torque about the vertical axis throughout the stroke. However, if asymmetries in the magnitude of force and its moment arm exist due to bilateral differences misalignment will occur. An early study of breaststroke swimmers (Czabanski and Koszcyc, 1979) revealed that technique asymmetry is very common. Carson (1999) reported a case study of an 11 year old competitive swimmer whose breaststroke technique was so asymmetrical in its movement pattern that he was being repeatedly disqualified. Physical examination revealed strength asymmetries in the shoulders and hips which reflected postural asymmetries and asymmetrical muscle tightness. Carson (1999) reported that therapeutic interventions resulted in improved muscle balance, posture, movement patterns, and race performance.
In our recent analyses of five elite breaststroke swimmers some asymmetries were evident in the techniques of all five swimmers. However, the severity of the asymmetries and likely impact on performance varied greatly among the swimmers. Further, the nature of the technique asymmetries differed among the swimmers.
Work needs to be done to understand the effects of asymmetries on performance to inform decisions on whether interventions to correct asymmetries are warranted. Some asymmetries may not affect performance and so interventions to remove them might be counter-productive and interfere with the swimmer's training program. On the other hand, if asymmetries in strength, flexibility, or technique produce torques that cause poor alignment and increase resistance it may be worthwhile to introduce programs such a strength training, physiotherapy, and technique training to correct the asymmetries.
Bilateral strength differences have been recognised as contributing to asymmetries in swimming actions (Sanders, 2013; Sanders, Thow, and Fairweather, 2011; Sanders et al., 2012). For example, Tourney-Chollet, Seifert, and Chollet (2009) have observed that bilateral differences in isokinetic strength of medial rotator shoulder muscles force symmetry of right and left sides are related to bilateral differences in phase durations in front crawl swimming. Further, it is well known that changes in the three-dimensional kinematics of swimming occur as the swimmer's capacity to produce force declines with fatigue (Figueiredo et al. 2012; 2013). However, there is an extreme paucity of data in the extant literature identifying technique asymmetries and their effect on alignment of the body through the production of torques. This is partly due to the difficulty of calculating torques acting on a swimmer interacting with a fluid environment. In one of the very few quantitative studies of torques acting on a swimmer Yanai (2004) applied the model of Dapena (1978) to determine the role of the buoyancy force in generating body roll in front crawl swimming, concluding that buoyancy is the primary source of generating body roll in front crawl swimming. However, there is a dearth of data relating to the possible effect of technique asymmetries in producing unbalanced torques that cause yawing motion.
In studying contributions to performance, the usual practice is to identify commonalities in kinematics and kinetics associated with performance. Naturally, the ideal approach for this is to obtain data from a sample of swimmers who perform at an elite level or to compare elite and sub elite either by comparing groups or using regression analysis to identify what changes with increasing levels of performance. However, when diagnosis of the limitations and constraints that prevent an individual swimmer from adopting an optimal kinematic and kinetic pattern is required, a case study approach is appropriate. In particular if a swimmer's alignment is disrupted there are myriad possible causes. Thus, the goal of the analysis is to identify the cause of the misalignment for that particular swimmer. Aetiologies include asymmetries in body shape, bilateral strength differences, and bilateral differences in flexibility (Sanders, Thow, and Fairweather, 2011). Further, a swimmer who may have had good swimming technique, may develop faults due to the effect of imbalances arising from training practices or injuries.
In this paper we illustrate by way of an in-depth case study how asymmetrical segmental motions are linked to the production of torques that contribute to yawing motions of a breaststroke swimmer. The paper exemplifies a process of analysis that can be applied in future studies to increase understanding of how technique asymmetries can influence body alignment in swimming.
One elite female breaststroke swimmer (height: 1.78 m; weight: 69.8kg) who competes internationally in 50 m and 100m events was selected for in-depth analysis due to the presence of a consistent pattern of yawing motions. Her personal best time for 100m long course is 1.10.85. She is right side dominant (both arm and leg) and when training front crawl she breathes predominantly to the right. At the time of testing she was free of injuries but had, in the past, injuries to ankles, knees, and left intercostal muscle.
Collection of strength data
Dynamic strength data for shoulder flexion/extension, shoulder internal/external rotation, and knee flexion/extension were collected on a Biodex Dynamometer (Biodex[R], Biodex Corporation, Shirley NY.). These actions are known to be prominent in swimming (Miyashita and Kanehisa, 1979; Payton et al., 1997; Payton et al., 2002). The Biodex was set to 'isokinetic mode' with constant speeds of 60[degrees]/s and 180[degrees]/s. A rotational speed of 60[degrees]/s is one of the most accepted to measure the peak torque of participants (Campenella et al., 2000; Girold et al., 2006; 2007; Lephart et al., 2002; Lund et al., 2005; McCleary and Andersen, 1992; Stickley et al., 2008). A rotational speed of 180[degrees]/s enables assessment of torques that can be applied at rotation speeds similar to those used in actual swimming where, for example, based on our data, the shoulder joints typically flex through approximately 90 degrees in approximately 0.5s in the trials of this breaststroke swimmer. While the knee joint angular velocities commonly exceeded 180[degrees]/s accuracy of the measures obtained on the Biodex dynamometer diminishes at speeds higher than 180[degrees]/s (Mayer et al., 2001) meaning that 180[degrees]/s is the upper limit of the speed range for obtaining accurate and reliable results.
Following familiarisation and warm-up, twelve repetitions of each exercise were performed at maximum effort, first at 60[degrees]/s followed by 180[degrees]/s conducted in accordance with the standard operating guidelines issued by Biodex Inc. The order with respect to right and left was randomised. Asymmetry in dynamic strength across right and left sides was indicated by a difference in average peak torque at each rotation rate and expressed as a percentage of the higher of the two values. Given that the swimmer's results were to guide strength and conditioning and physiotherapy programs the swimmer was highly motivated to comply with the instructions 'push as hard and as fast as you can for every repeat'. The Biodex is known to be reliable among motivated subjects with ICCs of 0.95 at 60[degrees]/s and 0.96 at 180[degrees]/s reported for peak torque during knee flexion and extension (Feiring et al., 1990).
Collection of anthropometric data
The participant, wearing a one piece legless racing competition swimsuit complying with the new rules (i.e. not a body suit) was photographed simultaneously by two digital cameras at a distance of 10m capturing orthogonal front and side views of the subject in accordance with the 'e-Zone' method described by Deffeyes and Sanders (2005). A bespoke MATLAB program, based on Jensen's (1978) 'elliptical zone' method, was used to determine the segment masses, segment centre of mass positions relative to the segment endpoints, and moments of inertia about transverse, antero-posterior, and longitudinal axes of each segment from the digitised landmarks. Briefly, masses segment masses and moments of inertia are based on the volume of the segment modelled as a series of ellipses with depth of 2cm. Using the diameters of the each ellipse obtained from the digitised front and side photographs in conjunction with estimates of density, the mass of each ellipse can be found. The position of the centre of mass of the segment relative to a meaningful landmark or segment endpoint can then be determined by summing moments and the moments of inertia about the three anatomical axes of the segment determined by summing the local and remote terms of the contributions of each...