doi:10.1017/S1478061508017027
Comparative Exercise Physiology 5(3-4); 107–109
Comparison between a sensor (3D accelerometer) and ProReflex motion capture systems to measure stride frequency of horses on a treadmill Kathryn Nankervis1, Diana Hodgins2 and David Marlin1,* 1
Hartpury Equine Veterinary and Therapy Centre, Hartpury College, Hartpury, Gloucestershire GL19 3BE, UK 2 European Technology for Business Ltd, Codicote Innovation Centre, Codicote, Hertfordshire SG4 8WH, UK * Corresponding author:
[email protected] Short Communication
Submitted 6 February 2008: Accepted 26 May 2008
Abstract The most popular techniques used for studying equine kinematics are videographic recording combined with the analysis using a commercial software package or optoelectronic systems based on emission and detection of infrared or visible light. Such systems can be expensive, complicated to use and their use may be restricted to indoor use (in the case of infrared systems) or a limited number of strides when used outdoors. Recently, there has been considerable interest in monitoring motion using inertial measuring systems. The purpose of the present study was to determine the accuracy of the Pegasus system (European Technology for Business Ltd., Codicote, UK), which determines stride frequency directly when mounted on the withers. Eight horses of different breeds and sizes (447–588 kg) were studied at walk, trot and two speeds at canter on a treadmill. Simultaneous measurements of stride were made using the Pegasus and ProReflex motion capture systems. Mean stride frequencies (n ¼ 8 horses) for the ProReflex and Pegasus systems at walk (0.86 ^ 0.05 and 0.87 ^ 0.05 strides per second, respectively), trot (1.36 ^ 0.07 and 1.36 ^ 0.07 strides per second, respectively) and canter (7 m s21: 1.92 ^ 0.05 and 1.92 ^ 0.05 strides per second, respectively; 8 m s21: 1.93 ^ 0.05 and 1.94 ^ 0.05 strides per second, respectively) were not significantly different (P . 0.05). The mean difference between the two systems for all four speeds was 2 0.002 strides per second (lower 95% CI: 2 0.016; upper 95% CI: 0.011; P ¼ 0.309). In conclusion, the differences between stride frequency measurements made with the Pegasus and ProReflex systems in horses exercising at walk, trot and slow canter on a treadmill are , 1% and not likely to be of physiological significance. Keywords: exercise; gait analysis; field; locomotion; inertial; treadmill
Introduction The most popular techniques used for studying equine kinematics are videographic recording combined with the analysis using a commercial software package or optoelectronic systems based on emission and detection of infrared or visible light1. The ProReflex motion capture system (Qualisys, Sweden) is an established system used in many equine biomechanics laboratories and involves one (for 2D) or more (for 3D) cameras tracking reflective markers placed on anatomical landmarks on the skin surface. Calibration of the field of view is necessary prior to data collection.
Sampling frequency varies between 120 and 240 Hz. Using this system, stride duration can be determined by placing a marker on a hoof wall in order to determine consecutive stride cycles. Stride duration is then given by the period of time elapsed between defined points of consecutive strides, e.g. beginning of stance phase of a hind hoof. Stride frequency is then given by 1/stride duration. Such systems also have the capacity to provide extremely detailed information concerning movement. However, video-based systems are relatively expensive, analysis can be complicated and time consuming, and both are limited in application. For example, qCambridge University Press 2009
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infrared systems cannot be used outdoors and the amount of strides that can be analysed is usually limited to those within a narrow and fixed field of view. Over the last 10 years, there has been a considerable amount of research and development in the monitoring of motion using inertial measuring systems2–4. A new system known as Pegasus (European Technology for Business Ltd., Codicote, UK) determines the stride frequency directly from sensors in a unit mounted on the withers, and has been developed for use by riders and trainers. The aim of the present study was to compare the stride frequency measured using the Pegasus motion sensor and ProReflex motion capture systems. Materials and methods This study was carried out at Hartpury Equine Veterinary and Therapy Centre. Eight horses (Table 1), considered sound, free of clinical signs of disease and with a mean (^ SD) height of 161 ^ 6 cm and a mean weight of 521 ^ 46 kg, completed a treadmill exercise test in walk, trot and canter, while stride frequency was measured using both the Pegasus motion sensor and ProReflex motion capture systems simultaneously. All horses were fully accustomed to treadmill exercise. A two-camera ProReflex system (Qualisys, Sweden) with a sampling frequency of 120 Hz was used to track the movement of a hemispherical marker (30 mm in diameter) placed on the lateral wall of the left hind hoof for the determination of stride frequency. All horses carried out a treadmill exercise test according to the following protocol: walk at 1.4 m s21 on a 0% incline for 5 min; trot at 3.0 m s21 on a 0% incline for 3 min; canter at 7.0 m s21 on a 3% incline for 1 min and canter at 8.0 m s21 on a 3% incline for 1 min. A numnah containing the Pegasus unit was placed on the horse and secured with a surcingle. The unit, measuring 78 £ 36 £ 18 mm and weighing 52 g, was nominally placed with its long axis in line with the spine of the horse. Accurate location or alignment of
the unit was not required, according to the manufacturer’s specification. Collection of accelerometric data began immediately prior to the start of the exercise. Data were collected continuously throughout walk, trot and canter using the Pegasus system. For both the Pegasus and ProReflex systems, the arithmetic mean and the standard deviation of stride frequency were taken over the same 20-s period at the end of each of the gait/speed steps. For the Pegasus system, the value generated by the software every second represents a 5-s rolling average, which is ^ 2 s around the 1-s interval. Therefore, in order to obtain data from the same period, the average value was taken over a 16-s period within the 20-s average used for the ProReflex. The times of the two systems were synchronized to within 1 s. Results Stride frequency during walk The results from the ProReflex system gave a mean stride frequency for the eight horses ranging from 0.81 to 0.93 strides per second. From the Pegasus system, the stride frequency for the eight horses ranged from 0.81 to 0.94 strides per second. The mean difference between the two systems was 2 0.002 strides per second (n ¼ 8; lower 95% CI: 2 0.010; upper 95% CI: 0.005; P ¼ 0.351). Stride frequency during trot For the ProReflex system, the mean stride frequency varied between 1.28 and 1.50 strides per second (n ¼ 8). For the Pegasus system, the stride frequency for the eight horses ranged from 1.27 to 1.47 strides per second. The mean difference between the two systems was 0.001 strides per second (n ¼ 8; lower 95% CI: 2 0.013; upper 95% CI: 0.016; P ¼ 0.815). Stride frequency during canter For the ProReflex system, the mean stride frequency varied between 1.87 and 2.00 strides per second for the eight horses. For the Pegasus system, the stride frequency for the eight horses ranged from 1.86 to 1.99
Table 1 Horse details Breed
Discipline
1. Arab 2. Warmblood 3. Thoroughbred 4. Thoroughbred 5. Thoroughbred £ Irish Draught 6. Thoroughbred 7. Thoroughbred 8. Irish Hunter Mean SD
Endurance Showjumper Dressage Racehorse Dressage Event Racehorse Event
Height (cm)
Age (years)
Weight (kg)
149 165 157 165 165 162 165 163 161 6
13 9 16 5 10 5 5 10 9 4
447 580 500 492 518 534 510 588 521 46
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did not change significantly (walk 0.82%, trot 1.07% and canter 0.79% (7 m s21), 0.85% (8 m s21)). Discussion
FIG . 1 Bland–Altman plot showing the difference between measurements of stride frequency (strides per second) obtained with the ProReflex and Pegasus systems as a function of the values obtained from the ProReflex system in eight horses during treadmill exercise at walk (1.4 m s21, 0% incline), trot (3.0 m s21, 0% incline) and two speeds of canter (7 m s21 and 8 m s21, both on a 3% incline). The mean value for each horse for each speed/gait is represented by a set of four symbols
strides per second. The mean difference between the two systems at 7 m s21 was 0.000 strides per second (n ¼ 8; lower 95% CI: 2 0.015; upper 95% CI: 0.015; P ¼ 1.000). At 8 m s21, the mean difference between the two systems was 2 0.009 strides per second (n ¼ 8; lower 95% CI: 2 0.025; upper 95% CI: 0.008; P ¼ 0.175).
Stride frequency over all speeds/gaits The differences between stride frequency measured with the ProReflex and Pegasus systems are shown in Fig. 1. For the ProReflex system, the mean stride frequency varied between 1.85 and 1.99 strides per second for the eight horses. For the Pegasus system, the stride frequency for the eight horses ranged from 1.86 to 2.01 strides per second. Mean difference between the systems over all speeds was 2 0.002 strides per second (lower 95% CI: 2 0.016; upper 95% CI: 0.011; P ¼ 0.309). There was a trend for the absolute difference between the two systems to increase with increasing stride frequency, but the CV
The Pegasus system produced results for stride frequency that were very close to those produced by the ProReflex system, and the differences are unlikely to be considered of physiological or practical significance. The results were obtained from the eight horses of different sizes and weights and from different equine disciplines when tested at four speeds and three gaits – walk, trot and canter – on the treadmill. The Pegasus system can therefore be considered accurate under these circumstances. The Pegasus system was quick and easy to use and produced data that were comparable with that obtained using the ProReflex motion capture system. As optoelectronic systems require controlled lighting environments, they are not always suitable for work outdoors or indoors with significant natural light. The Pegasus system can, however, be used in any environment. Further work is required to validate the Pegasus system at faster speeds and galloping gait and during overground locomotion. In conclusion, the differences between stride frequency measurements made with the Pegasus and ProReflex systems in horses exercising at walk, trot and slow canter on a treadmill are minimal. References 1 2 3 4
Clayton HM and Schamhardt HC (2001). Measurement techniques for gait analysis. In: Back W and Clayton HM (eds) Equine Locomotion. London: W.B. Saunders. Barrey E and Galloux P (1997). Analysis of the equine jumping technique by accelerometry. Equine Veterinary Journal Supplement 23: 45–49. Leleu C, Gloria E, Renault G and Barrey E (2002). Analysis of trotter gait on the track by accelerometry and image analysis. Equine Veterinary Journal Supplement 34: 344–348. Pfau T, Witte TH and Wilson AM (2004). A method for deriving displacement data during cyclical movement using an inertial sensor. Journal of Experimental Biology 207: 3639–3648.
