Sean Clarkson

Calculating Body Segment Inertia Parameters from a Single Rapid Scan Using the Microsoft Kinect

Many biomechanical analyses rely on the availability of reliable body segment inertia parameter (BSIP) estimates. Current processes to obtain these estimates involve many time consuming manual measurements of the human body, used in conjunction with models or equations. While such methods have become the accepted standard they contain many inherent errors arising from manual measurement and significant assumptions made in the underlying data used to form the models and equations. Presented here is an alternative approach to obtaining reliable estimates of body segment inertia parameters through the use of the Microsoft Kinect sensor. A 3D scanning system was developed, comprising four Kinects aligned to a single global coordinate system using rigid body calibration and random sample consensus (RANSAC) optimisation. The system offers the advantage of obtaining BSIP estimates in a single scanning operation of around three seconds, much quicker than the circa thirty minutes of manual measurements required for existing BSIP estimate methods. The results obtained with the system show a mean error of 0.04% and a standard deviation of 2.11% in volumetric measurements of a torso manikin, suggesting comparable and in many cases, greater accuracy volumetric estimates than a commonly used geometric BSIP model. Further work is needed to extend this study to include a full range of BSIP measurements across more of the bodies segments and to include scanning of living human subjects. However, this initial study suggests great potential for a low cost system that can provide quick and accurate subject BSIP estimates.

Assessing the suitability of the Microsoft Kinect for calculating person specific body segment parameters

Many biomechanical and medical analyses rely on the availability of reliable body segment parameter estimates. Current techniques typically take many manual measurements of the human body, in conjunction with geometric models or regression equations. However, such techniques are often criticised. 3D scanning offers many advantages, but current systems are prohibitively complex and costly. The recent interest in natural user interaction (NUI) has led to the development of low cost (~£200) sensors capable of 3D body scanning, however, there has been little consideration of their validity. A scanning system comprising four Microsoft Kinect sensors (a typical NUI sensor) was used to scan twelve living male participants three times. Volume estimates from the system were compared to those from a geometric modelling technique. Results demonstrated high reliability (ICC>0.7, TEM<1 %) and presence of a systematic measurement offset (0.001m\(^{3}\)), suggesting the system would be well received by healthcare and sports communities.

Assessment of a Microsoft Kinect-based 3D scanning system for taking body segment girth measurements: a comparison to ISAK and ISO standards

ABSTRACT Use of anthropometric data to infer sporting performance is increasing in popularity, particularly within elite sport programmes. Measurement typically follows standards set by the International Society for the Advancement of Kinanthropometry (ISAK). However, such techniques are time consuming, which reduces their practicality. Schranz et al. recently suggested 3D body scanners could replace current measurement techniques; however, current systems are costly. Recent interest in natural user interaction has led to a range of low-cost depth cameras capable of producing 3D body scans, from which anthropometrics can be calculated. A scanning system comprising 4 depth cameras was used to scan 4 cylinders, representative of the body segments. Girth measurements were calculated from the 3D scans and compared to gold standard measurements. Requirements of a Level 1 ISAK practitioner were met in all 4 cylinders, and ISO standards for scan-derived girth measurements were met in the 2 larger cylinders only. A fixed measurement bias was identified that could be corrected with a simple offset factor. Further work is required to determine comparable performance across a wider range of measurements performed upon living participants. Nevertheless, findings of the study suggest such a system offers many advantages over current techniques, having a range of potential applications.

Distortion correction of depth data from consumer depth cameras

Since the introduction of the Microsoft Kinect in November 2010, low cost consumer depth cameras have rapidly increased in popularity. Their integral technology provides a means of low cost 3D scanning, extending its accessibility to a far wider audience. Previous work has shown the 3D data from consumer depth cameras to exhibit fundamental measurement errors: likely due to their low cost and original intended application. A number of techniques to correct the errors are presented in the literature, but are typically device specific, or rely on specific open source drivers. Presented here is a simple method of calibrating consumer depth cameras, relying only on 3D scans of a plane filling the field of view: thereby compatible with any device capable of providing 3D point cloud data. Validation of the technique using a Microsoft Kinect sensor has shown non planarity errors to reduce to around ± 3mm: nearing the device’s resolution. Further validation based on circumference measures of a cylindrical object has shown a variable error of up to 45mm to reduce to a systematic overestimation of 10mm, based on a 113mm diameter cylinder. Further work is required to test the proposed method on objects of greater complexity and over greater distances. However, this initial work suggests great potential for a simple method of reducing the error apparent in the 3D data from consumer depth cameras: possibly increasing their suitability for a number of applications.

Breast Volume Calculation Using a Low-Cost Scanning System

Breast volume has been identified as a key metric in assessing patients for reconstructive surgery. Scanning systems have measured breast volume but they have tended to rely on expensive hardware and software. This paper discusses the development and assessment of an algorithm capable of calculating breast volume from 3D point data. A mannequin was scanned (using a custom, Kinect based scanning system) with one of two breast prostheses attached – 400g or 600 g. Each scan was assessed by three independent operators: seven anatomical points were identified representing the boundary of the breast region, which was then isolated. A Coons patch was used to represent the invisible chest surface lying below the breast tissue. A trapezium rule based approach was used to calculate the volume of the enclosed region between the breast and chest surfaces. Breast volume over-estimated by 130 cc with the 400 g prosthesis (30.3%) and 206 cc (33.3%) with the 600 g prosthesis, suggesting positive proportional bias. Average reliability was ± 59.7 cc for the 400 g prosthesis (13.9%) and ± 34.7 cc for the 600 g prosthesis (5.6%) – approaching the levels required to differentiate between implant sizes (25 -50 cc). Future work will focus on refining the hardware and software of this scanning system – minimising proportional basis and maximising reliability of measurement.

3D surface-imaging for volumetric measurement in people with obesity

BACKGROUND Current methods for tracking the progress of people with obesity towards a weight loss goal appear simple and potentially misleading. A technique to quantify change in body shape whilst visualising areas of the body where weight loss occurs would be advantageous, and has the potential to be used as a motivational tool. Three-dimensional (3D) surface-imaging would serve as a good basis for such a technique, however current systems are prohibitively expensive. OBJECTIVE Highlight the use of a cheaper alternative 3D surface-imaging system for volumetric measurement in people with obesity. METHODS A recently developed low-cost 3D surface-imaging system was used, having previously being validated in a healthy population. A total of 61 people with obesity, enrolled on a weight-loss programme, were surface-imaged using the system. RESULTS The findings suggest the low-cost system can obtain 3D surface-images of an obese human body, from which numerical parameters could be calculated and further analysis conducted. CONCLUSIONS Further studies will focus on the validity and reliability of such analyses and the potential of the system to be considered as a long-term instalment in primary healthcare settings as a weight loss aid.

The use of consumer depth cameras for 3D surface imaging of people with obesity: a feasibility study

OBJECTIVE Three dimensional (3D) surface imaging is a viable alternative to traditional body morphology measures, but the feasibility of using this technique with people with obesity has not been fully established. Therefore, the aim of this study was to investigate the validity, repeatability and acceptability of a consumer depth camera 3D surface imaging system in imaging people with obesity. METHODS The concurrent validity of the depth camera based system was investigated by comparing measures of mid-trunk volume to a gold-standard. The repeatability and acceptability of the depth camera system was assessed in people with obesity at a clinic. RESULTS There was evidence of a fixed systematic difference between the depth camera system and the gold standard but excellent correlation between volume estimates (r2=0.997), with little evidence of proportional bias. The depth camera system was highly repeatable - low typical error (0.192L), high intraclass correlation coefficient (>0.999) and low technical error of measurement (0.64%). Depth camera based 3D surface imaging was also acceptable to people with obesity. CONCLUSION It is feasible (valid, repeatable and acceptable) to use a low cost, flexible 3D surface imaging system to monitor the body size and shape of people with obesity in a clinical setting.

The effect of a real-time gait retraining programme on knee angle and ground reaction forces in a group of recreational runners - Abstract only

Gait-retraining using real time visual feedback is an effective intervention for modifying factors associated with overuse injuries in runners (Noehren et al., 2011, Br J Sports Med, 45, 691-696). Decreased knee flexion at initial contact has been associated with increased vertical loading rates, an identified risk factor for tibial stress fracture (Milner et al., 2006, Med Sci Sports Exer, 38, 323-328). Therefore, the aim of this study was to evaluate the effectiveness of a gait retraining program designed to increase knee flexion angle at initial contact . Following institutional ethics approval, eight injury-free recreational runners (5 females and 3 males; mean ± SD: age 24 ± 6.5 years) with an initial knee contact angle lower than 12° at initial screening, were recruited. In a pre-test, participants completed five trials of overground running on a 16m runway at a self-selected speed (mean speed 2.8 m.s-1, SD = 0.5) while force plate data (1000Hz) and sagittal plane video (100Hz) were captured. Participants then completed six 15 minute treadmill-based gait-retraining sessions, over two weeks. Running at a self-selected speed, participants received real-time visual feedback on knee angle via a bespoke system comprising a Microsoft Kinect and custom-written software. The system encouraged participants to maintain an initial knee contact angle of greater than 16°, with feedback gradually removed over the last three sessions. Post-intervention, the overground testing protocol was repeated. The effect of the intervention on knee angle at initial contact, peak knee angle during stance, average and instantaneous vertical loading rates was assessed using paired t-tests and Cohens d effect sizes (d). Knee flexion at initial contact increased from 8.0° ± 2.8° pre to 19.4° ± 2.0° post retraining (P < 0.001, d = 4.7), while maximum knee flexion increased from 39.2° ± 3.1° to 48.6° ± 4.7° following the intervention (P < 0.001, d = 2.3). Average vertical loading rate and instantaneous vertical loading were both reduced following the gait retraining programme with reductions of 30% (P < 0.001, d =1.2), and 25% (P < 0.001, d = 1.1), respectively. Results showed the six session intervention was successful in altering knee kinematics and reducing vertical ground reaction forces, suggesting gait-retraining to be an effective means of reducing injury risk. The real-time feedback system is low cost and portable offering scope for the use of gait-retraining outside of the laboratory in the clinic or home.The relative contribution of lower limb muscle groups changes throughout sustained, high-intensity cycle exercise (Sanderson and Black, 2003, Journal of Sports Sciences, 21, 191–199). This suggests that exercise-induced fatigue is muscle group specific. We asked whether the additional muscular stress associated with hypoxia would alter the muscle specificity of fatigue. With institutional ethics approval, nine male cyclists (mean ± SD VO2max 61.2 ± 3.8 ml · kg · min) pedalled to the limit of tolerance at a fixed work rate (60% of the difference between VO2max and gas-exchange threshold: 306 ± 14 W) and fixed cadence (88 ± 2 rpm) in two conditions: normoxia (FIO2 0.21, SaO2 95 ± 1%) and hypoxia (FIO2 0.15, SaO2 85 ± 2%). Pedal forces, joint kinematics and surface EMG activity were recorded throughout exercise. Joint action powers (hip extension, hip flexion, knee extension, knee flexion, plantar flexion and dorsi flexion) were derived using inverse dynamics. Neuromuscular activation (gluteus maximus, vastus lateralis, biceps femoris, gastrocnemius and soleus) was quantified using EMG root mean square (EMGRMS). For both conditions, data were averaged over the first, middle and final 30 s of the corresponding hypoxia trial to allow for isotime comparisons. Data were also averaged over the final 30 s of the normoxia trial to enable endexercise comparisons. Exercise time was reduced in hypoxia versus normoxia (4.1 ± 0.2 vs. 10.1 ± 1.1 min, P < 0.05). Hip extension power increased throughout exercise in normoxia, whereas knee extension power decreased. The changes in endexercise joint action powers were reduced in hypoxia versus normoxia (113 vs. 127% for hip extension, P < 0.05; 92 vs. 87% for knee extension, P < 0.05), but were relatively well preserved at exercise isotime (P > 0.05 for middle and final 30 s). Gluteus maximus and biceps femoris EMGRMS increased throughout exercise in both conditions. The increases in end-exercise EMGRMS were similar in both conditions, but were elevated in hypoxia versus normoxia at exercise isotime (P < 0.05). In conclusion, time-dependent changes in joint action powers during sustained, high-intensity cycle exercise are relatively well preserved in hypoxia. Rates of rise in electromyographic activity for selected muscles of the lower limb are increased in hypoxia, presumably to ensure the maintenance of joint action power distribution. The results suggest that joint power distribution is a robust property of cycling and is largely independent of arterial hypoxaemia. The increased muscular stress associated with hypoxia appears to require a disproportionate increase in hip-extensor activity to maintain a normal coordinative pattern.

Kinanthropometry Applications of Depth Camera Based 3D Scanning Systems in Cycling: Repeatability and Agreement with Manual Methods

Recent literature suggests that 2D and 3D anthropometric measures are better predictors of sports performance, than traditional 1D measures. The emergence of 3D scanning systems offers a cheap, easy and effective method of estimating these measures. Therefore the aim of this study was to investigate the repeatability of a depth camera based 3D scanning system, and its agreement with manual methods in the extraction of simple thigh measurements. Using 15 healthy, recreationally active male participants, five measurements of the thigh (upper thigh circumference, mid-thigh circumference, knee circumference, knee to mid-thigh length and mid-thigh to upper thigh length) were taken using an anthropometric tape measure and digital callipers, and scanned using a 4-camera Kinect based 3D scanning system (using custom analysis software). Agreement and repeatability was subsequently determined. This study demonstrated a low cost Kinect-based 3D scanning system is capable of extracting length and circumference measures within ~2% and ~3-4%, respectively, with high repeatability, technical error measurements (TEM) of ~1.80% and ~0.7% respectively. The 3D scanning system was able to measure the thigh in good agreement with manual measurement methods, with the presence of systematic bias in circumference. Whilst maintaining a very high degree of repeatability, suggesting it is a suitable method to extract simple thigh measurements.