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DepartmentDepartmentDepartmentDepartment ofofofof OrthopaedicsOrthopaedicsOrthopaedicsOrthopaedics andandandand Traumatology,Traumatology,Traumatology,Traumatology, PrincePrincePrincePrince ofofofof WalesWalesWalesWales Hospital,Hospital,Hospital,Hospital,FacultyFacultyFacultyFaculty ofofofof Medicine,Medicine,Medicine,Medicine, TheTheTheThe ChineseChineseChineseChinese UniversityUniversityUniversityUniversity ofofofof HongHongHongHong KongKongKongKong,,,, HongHongHongHong,,,, CCCChinahinahinahina1111TheTheTheThe HongHongHongHong KongKongKongKong JockeyJockeyJockeyJockey ClubClubClubClub SportsSportsSportsSports MedicineMedicineMedicineMedicine andandandand HealthHealthHealthHealth SciencesSciencesSciencesSciences Centre,Centre,Centre,Centre,

FacultyFacultyFacultyFaculty ofofofof Medicine,Medicine,Medicine,Medicine, TheTheTheThe ChineseChineseChineseChinese UniversityUniversityUniversityUniversity ofofofof HongHongHongHong KongKongKongKong,,,, HongHongHongHong Kong,Kong,Kong,Kong, ChinaChinaChinaChina 2222

SchoolSchoolSchoolSchool ofofofof MedicalMedicalMedicalMedical SciencesSciencesSciencesSciences,,,, FacultyFacultyFacultyFaculty ofofofof Medicine,Medicine,Medicine,Medicine, TheTheTheThe UniversityUniversityUniversityUniversity ofofofof NewNewNewNew SouthSouthSouthSouthWales,Wales,Wales,Wales, Sydney,Sydney,Sydney,Sydney, AustraliaAustraliaAustraliaAustralia3333

This study tested the performance of a new inexpensive consumer-grade digital camerafor human motion biomechanics research. One subject performed a sprint-running trial athis fastest speed. Three consumer-grade high speed cameras (Casio Exilim EX-F1) wereused to capture the sagittal motion at 300Hz, 600Hz and 1200Hz. External light sourceswere added to ensure the visibility of the subject’s movement. Synchronization was donewith an ON signal from a LED light. The captured video sequences were in general withgood quality when evaluated qualitatively as the limb movements were clearly displayed.Post-study experience sharing from our research group on the picture quality, framingrate setting, capture volume, synchronization technique, and feasibility for telemetric datacollection are discussed.

KEYKEYKEYKEYWORDS:WORDS:WORDS:WORDS: Videography, cinematography, motion analysis, video analysis.

INTRODUCTION:INTRODUCTION:INTRODUCTION:INTRODUCTION:

Videography, or cinematography, has been employed by the sports biomechanists for humanmotion analysis for decades (Andriacchi and Alexander, 2000). The technique originatedfrom Stanford University in 1872, when Leland Stanford, the 8th Governor of California andthe founder of Stanford University, sought out Eadweard Muybridge, a photographer, toprove a debate: “During a horse gallop, all hour hooves were ever off the ground at the sametime”. In 1878, after fine-tuning the shutter speed and exposure of multi-cameras for years,Muybridge successfully photographed three full strides of a fast galloping horse with 24cameras taking pictures at 1/1000 of a second in turns. The series of photo was called “TheHorse in Motion”, and it scientifically proved that all four hooves did leave the ground at thesame time (Leslie, 2001).

The technique was costly – it cost USD 25,000 at that time. Nowadays, even that thetechnique of high speed video capturing is much more mature, it is still costly to purchasehigh speed cameras for precise digital video recording in scientific biomechanics research.For example, a single high speed video camera with a frame rate of 500-1000Hz may costUSD 30,000-40,000. Since this is often not affordable for most research groups, somebiomechanists have been seeking for inexpensive video cameras with high speed videocapturing function for their research work (Castro et al, 2006). An example is the JVCcamera (Model 9600/9800), which provides shooting speeds of 50/60Hz, 100/120Hz or200/240Hz. Each camera costs about USD 1,000 only, however, it has been out of stock forlong already. Recently, there is an inexpensive consumer-grade digital camera which has ahigh speed video capturing function included. The framing rate could be set as high as300Hz, 600Hz and 1200Hz. It is inexpensive – each camera costs about USD 1,000 only.This study tested its performance in capturing a sprint-running motion in a laboratory, andcommented on the feasibility to equip such cameras for human motion biomechanicsresearch.

METHOD:METHOD:METHOD:METHOD:

DataDataDataData Collection:Collection:Collection:Collection: One malesubject performed a barefootedsprint-running trial at his fastestspeed in a laboratory. Fivereflective markers were attachedon the skin surface at the fifthmetatarsal head, lateralmalleolus, calcaneous, tibialtuberosity and lateral femoralepicondyle on his right lowerlimb. Three consumer-gradehigh speed cameras (CasioExilim EX-F1) with tripods werealigned vertically as a column tocapture the sagittal motion at300Hz, 600Hz and 1200Hz(Figure 1). In addition to theroom ceiling lighting in thelaboratory, external light sourceswere added (4 x 60W light bulbs)to ensure the visibility of the subject’s movement as captured in such high speed settings.Synchronization was done with an ON signal from a LED light.

DataDataDataData Analysis:Analysis:Analysis:Analysis: The captured videos in MOV format were transferred to a computer. A totalof 8, 16 and 32 frames were captured from the moment the heel started to raise from theground from the camera with 300Hz, 600Hz and 1200Hz framing rate respectively. The totalduration of this period was about 0.027s (8/300s). Qualitative visual evaluation is done withdiscussion and experience sharing from our research group.

RESULTS:RESULTS:RESULTS:RESULTS:The selected sequence from the cameras with different settings is shown in Figure 2. Thepictures are in general with good quality. In such high speed video capturing, the limbmovements were clearly displayed without much blur, and there was only very tinymovement between each pair of consecutive frames. The actual pixels sizes are 512 x 384for 300Hz, 421 x 192 for 600Hz, and 336 x 96 for 1200Hz framing rate respectively. Therecording time with 1GB memory is about 7 minutes for all settings, and the maximum filesize of a single movie is 4GB.

DISCUSSION:DISCUSSION:DISCUSSION:DISCUSSION:

In 1200Hz setting, the pictures are darker as the sampling frequency increased with a shorterexposure time. If bright and clear pictures are desired, users may have to consider addingextra light source. From Figure 2, the reflective markers are not much brighter than the skincolour. In order to increase the contrast, users may need to consider adding high power ringlight around the camera lens, as this ensures the reflected light from the markers travel backto the lens. At the same time it may be necessary to darken the laboratory room light a bit tomake the skin less visible, or to let the subject wear dark socks and clothing to increase thelight contrast. For capturing less vigorous human motion like gait, users could also sacrificethe sampling frequency for high quality pictures. There are two more settings named HighDefinition (HD) and Full High Definition (FHD) with framing rate of 30Hz and 60Hzrespectively. HD provides un-interlaced pictures of 1280 x 720 pixels, and FHD providesinterlaced pictures of 1920 x 1080 pixels.

Figure 1 – The alignment of the 3 cameras in this study.

Figure 2 – The video sequence captured in this study. 1st column = 300Hz, 2nd column =600Hz, 3rd column = 1200Hz. The displayed views are trimmed to fit the figure. Theexposures are to scale.

From the current study, it was found that the captured pictures are a bit smaller than what weobserved from both the monitor screen and the electronic view finder. We advised users totake videos of a pilot trial, or a calibrating frame which covers the desired capture volume,and play the video back to ensure the motion could be captured without being trimmed off.Users can also consider precisely marking the exact filming area on the monitor screen forreference.

Users should also pay attention to the technique for synchronization. In this study, a LEDlight is used for visual synchronization. We suggested that a fast responsive LED signal isessential, or users will not be able to identify the exact synchronization moment when theyobserve a series of pictures showing the LED progressively lightens up. Anyway, in thecurrent situation, a time mismatch of one frame in these settings is yet considered

acceptable. Another suggestion is to pick a balloon and let it burst for synchronization,however, this is not convenient. Therefore, future video-based motion analysis softwareshould provide control to multi-cameras for synchronized video capturing. In addition, thestored movies are in MOV format, which is often not directly accessible in some computers.Therefore, future video-based motion analysis software should provide a module to convertthe movie format, or to directly access and work on the MOV files.

In this trial we used 8GB memory cards, and they were enough for video shooting for onehour at all 300Hz, 600Hz and 1200Hz framing rate. We conducted the trial with the batteryfully charged and found that they could sustain for 3 hours. Therefore the cameras are goodfor data collection on-field without restriction from power and data cord for collecting videodata of up to one hour.

CONCLUSION:CONCLUSION:CONCLUSION:CONCLUSION:The new consumer-grade digital camera is inexpensive, and is suitable for human motionbiomechanics research. It provides video capturing at 30Hz, 60Hz, 300Hz, 600Hz and1200Hz framing rate. At the highest framing rate (i.e. 1200Hz), the picture quality is still goodfor motion biomechanics analysis. The use of memory cards and battery allows telemetricdata collection of up to one hour, which facilitates on-field data collection. However, thepotential problem is the synchronization. Future video-based motion analysis software shouldtry to provide a function to control multi-cameras for synchronized video capturing.

REFERENCES:REFERENCES:REFERENCES:REFERENCES:Andriacchi, T.P. & Alexander, E.J. (2000). Studies of human locomotion: past, present and future.Journal of Biomechanics, 33(10), 1217-1214.Castro, J.L.G., Medina-Carniced, R. & Galisteo, A.F. (2006). Design and evaluation of a new three-dimensional motion capture system based on video. Gait and Posture, 24(1), 126-129.Leslie, M. (2001). The man who stopped time. Stanford Magazine, 2001 May/June Issue.

AcknowledgementThis research project was made possible by equipment/ resources donated by The Hong Kong JockeyClub Charities Trust.