Reliability of the elliptical zone method of estimating body segment parameters of swimmers.

Author:Sanders, Ross H.
Position::Research article


Obtaining accurate body segment parameter (BSP) data such as segment masses, segment centre of mass positions and moments of inertia is fundamental to calculating the kinematics and kinetics of human motion from anatomical reference points digitised from video. While personalised BSP data may be useful in the analysis of any human motion, it is particularly important in analysis of mid-pool swimming where direct measurement of kinetics is extremely difficult. Net force, the product of the second derivative of whole body centre of mass (COM) position and body mass, can indicate the instantaneous balance between propulsive and resistive forces, thereby enabling an assessment of the effectiveness of technique and the relative contributions by the right and left limbs. Direct measurement of swimming speed has been used to gather this information by attaching a light line to a fixed point on the swimmer's body, commonly the hips (e.g. Payton and Wilcox, 2006). However, the speeds and derived accelerations based on the motion of the hips have been shown to differ considerably from the motion of the COM (Figueiredo et al., 2009; Psycharakis and Sanders, 2009). With respect to angular motion, the inverse dynamics approach of Dapena (1978) has been applied to swimming to estimate net torques about the longitudinal axis (body roll) (Yanai, 2004), the transverse axis (pitch) (Yanai, 2001) and about the vertical axis (yaw) (Sanders and McCabe, 2014). The patterns of roll (Yanai, 2004) and yaw (Sanders and McCabe, 2014) have been found to be linked to technique asymmetries and to swimming performance.

In addition to understanding and assessing technique of able bodied swimmers, quantification of linear and angular kinetics are necessary to explore the effects of disabilities on performance and to shed light on the issue of classification in Paralympic swimming. In many cases, the disabilities create morphological asymmetries which affect the balance of torques acting during swimming, for example, the effect of missing limbs on the roll and pitch of the body in response to bilateral asymmetries in the torques due to gravity and buoyancy. Thus, input of personalised segment BSP data is essential to obtain realistic results. Additionally, change in the anthropometric data themselves is of interest when assessing the effect of body mass, and its distribution, on swimming performance. For this reason it is important to establish the sensitivity of the BSP measurements and the confidence with which changes over time can be detected.

Several methods have been developed for estimating BSP data. These include using data based on cadavers (Depster, 1955; Dempster and Gaughran, 1967; Clauser et al., 1969), dual energy X-ray absorptiometry (Durkin et al., 2002), magnetic resonance imaging (Martin et. al., 1989; Pearsall et al., 1994), computer tomography (Pearsall et al.,), gamma ray scanning (Casper et al., 1971), surface 3D scanning combined with computer aid design (Ma et al., 2011).

One method of obtaining personalised estimates of these parameters inexpensively and non-invasively is by modelling segments as a series of ellipses of known depth and diameters (Jensen, 1978). The volume of each ellipse is then determined and, 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 about the three anatomical axes of the segment. The moments of inertia about each of the anatomical axes of each segment can be determined by summing the local and remote terms of the contributions of each ellipse by applying the parallel axis theorem (Hay, 1993).

The diameters of the ellipses are obtained by tracing the outline of the segments from two photographs taken from orthogonal perspectives. The reliability of the measurements depends on the consistency of tracing those outlines as well as digitising the anatomical landmarks that define each body segment. Reliability of the method is particularly important when the body segment parameter data are used in longitudinal studies in which changes in body morphology and mass distribution are likely.

Sanders (1991) showed that forces derived from COM data based on BSP data obtained using the elliptical zone method matched the actual forces measured by a force plate in a drop jumping task with the exception of the high frequency contributions associated with impact. Therefore, it could be expected that the method could be applied well to mid-pool swimming where forces are comprised of low frequency contributions.

Reliability of the elliptical zone method has been maximised in the past by projecting slides onto large digitising tablets (Jensen and Fletcher, 1994; Tupling et al., 1984). Tupling et al. (1984) indicated that the adapted elliptical zone method is consistent in day-to-day measuring or by different assessors. Therefore, the past elliptical zone methods have been applied in longitudinal studies to determine the variation of humans' body shape, volume, mass, radius, and moments of inertia of segments (Jensen and Nassas, 1985; Jensen 1986a; 1986b; 1987; 1989; Yokoi et al., 1986).

With the advancement in software, enabling user interaction with digital photographs, digitising and tracing of body segments can be achieved readily on personal computers. A MATLAB program (E-Zone) enabling the acquisition of the ellipse diameters and subsequent calculation of body segment parameter data using the elliptical zone method has been developed (Deffeyes and Sanders, 2005). Figure 1 shows some graphical output of the MATLAB program with the body modelled as ellipses.

Table 1 indicates that E-Zone is the only BSP data collection method that meets all the criteria relating to accuracy, cost, portability, accessibility, speed, and being free of health risks. More body dimensions (segment breadths and depths) are used by E-Zone than other mathematical models so more accurate BSP data can be obtained for biomechanical analysis.

E-zone requires only two digital cameras allowing easy availability, low cost, and portability compared with photonic or medical scanners. Digitising and calculations of a subject can be completed within 20 minutes.

To date, there is a paucity of data regarding the reliability of the measurements obtained both in terms of within assessor variability and between assessor variability. One of the very few papers to report reliability of these methods was a study of front crawl swimmers by Psycharakis et al. (2010) in which within-operator standard deviation of total body mass was reported as 0.4kg and the coefficient of variation as 0.3%.

The purpose of the current study was to establish the reliability of body segment parameter data obtained using the elliptical zone method with segment endpoints and outlines being digitised and traced manually on a personal computer screen using E-Zone. The contribution to variability of differences between assessors trained in digitising and tracing the body segments, and the contribution to variability of differences within assessors between repeated digitisations and tracings, were evaluated. For application to analysis of both able bodied and Paralympic swimming, establishing the sensitivity of measurement was of particular interest. In future studies of swimming this will inform the confidence with which the effect of bilateral differences on torsional balance, and the effect on performance of longitudinal changes in morphology with exercise and diet, can be assessed.



Participants in this study comprised 11 single arm amputee swimmers (9 females, 2 males) whose body segment parameter data were required for subsequent video based three-dimensional (3D) analysis of swimming technique. The mass of the nine female single arm amputee swimmers ranged from 44.8 kg to 67.4 kg while the masses of the two male single arm amputee swimmers were 99.1 kg and 71.6 kg (Table 2). Heights ranged from 1.57m to 1.67m (females) and 1.84m to 1.86m for the two males (Table 2). Swimmers wore nylon competition swimming suits (not body suits) so that all segment endpoint landmarks could be palpated and marked as described by Deffeyes and Sanders (2005).


Five assessors digitised and traced the segment outlines from digital photographs of the swimmers. The assessors underwent training to gain familiarity with the MATLAB program 'E-Zone' (Deffeyes and Sanders, 2005) used for digitising/tracing of the body segments and subsequent output of body segment parameter data applying the modelling methods of Jensen (1978). The training, conducted by the first author, comprised a demonstration and explanation of the elliptical zone method and the digitising/tracing techniques required to obtain accurate and reliable results. The most experienced assessor supervised three practice trials of the other assessors ensuring that the digitising and tracing protocols were applied consistently according to the original developers of this method (Jensen, 1978) and the developers of the E-Zone MATLAB program (Deffeyes and Sanders, 2005). Moreover training was utilised to assess the accuracy of each trial by comparing the participant's actual total body mass (weighing scales) to the calculated total body mass (E-Zone output).


Swimmers were weighed on a set of Seca 712 column scales (Germany) and their height measured (without shoes) on a Seca 225 stadiometer (Germany) ensuring that the spine at lumbar, thoracic and cervical regions...

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