Postgraduate R.O. Solodilov
Dr.Biol., Professor S.I. Loginov
Surgut State University KhMAR-Yugra, Surgut

Keywords: biomechanics, osteoarthritis, knee joint, markerless motion capture system.

Introduction. Over the past few years, the markerless motion capture technology has made a big push, having gone a long way from the expensive professional equipment to the product available to almost every researcher. In the last few years, along with the launch of Microsoft Kinect aimed at unlocking the potential of the use of this technology, the researchers from all over the world have become increasingly excited about the development of applications for this device. Its scope ranges from the evaluation of athletes' biomechanical indices to the implementation of rehabilitation measures based on neurofeedback. Microsoft Kinect is an inexpensive (only $110 as of this writing) and a long-range substitute for the comparatively expensive marker motion capture systems.

A person's capability to smoothly perform simple, everyday actions (for example, rising from a chair) is an important indicator of his/her level of functional independence and living standards in general. The biomechanical analysis of the "sit-to-stand" test was conducted in different conditions with the involvement of different contingents of testees; however, as of today, there are not enough research papers dedicated to the study of biomechanics of the lower extremity of people with osteoarthritis.

The purpose of the study was to develop the three-axis biomechanical model of knee joint motion when standing up for individuals aged 40-65 who did not have any symptoms of osteoarthritis of the knee.

Materials and methods. 42 healthy people with no visible signs of osteoarthritis of the knee (OAK) (control group - CG) and 31 people with OAK (experimental group - EG) aged 40-65 were involved in the study (Table1).

Table 1. Study group indices

Upon matching against the inclusion and exclusion criteria (Table 2) there remained 30 people in CG and 26 in EG (Table 3).

Тable 2. I/E criteria

* Seat height – 44 cm

The biomechanical analysis was carried out using a contactless sensor controller "Kinect", equipped with a motion capture system and specially developed software. The participants performed the chair-rise task, the seat height was adjusted at the level of 110% of the knee height, the arms were crossed at chest height. The feet and ankles were in a natural position.

Table 3. Study group indices upon selection procedure

To record the time of rising more accurately a contact switch was attached to the seat of the chair. The test execution speed was at the discretion of the participants. The non-contact sensor was located on a tripod of 80 cm height, the distance from the sensor to the chair equaled 210 cm. The whole cycle of motion was divided into 3 phases, these phases - into four events (t1-t4) (Fig. 1). The motion onset was defined as the moment when the trunk in its sagittal plane was inclined by 1 degree or more. The motion completion was defined as the time when the angular velocity of the hip reached 0 C°/sec.

The cycle of motion was standardized as follows: 0% - the beginning of the test, 100% - the completion of the test (Fig. 1). The lower extremity angles were measured using the software «Brekel Pro Body». This software is used to capture the human skeleton motions using the Microsoft SDK calculations. The data were stored in the CSV format from the standpoint of the three degrees of freedom XYZ of each joint against the sensor.

The first degree of freedom is associated with the transverse axis X, towards which the flexion and extension motions in the sagittal plane are made. The second degree of freedom of the knee joint is related to the rotation around the tibial longitudinal axis Y with the knee bent. The axis Z is front-to-rear and square with the other two axes. This axis is not the third degree of freedom, but owing to a certain mechanical vibration in the joint resulting from the relaxation of lateral ligaments, lateral motions with the bent knee occur towards this axis (within the range of 1-2 cm, when measured at the level of the ankle). At full extension of the knee, these motions disappear due to the tension of lateral ligaments, and their preservation, usually, indicates the incidence of ligament pathology [2].

Fig. 1. Rising phases

Phase I – support phase; Phase II – momentum transfer phase; Phase III – stabilization phase. t1 – onset (the trunk is inclined by 1 degree or more), t2 – rising (the contact switches on the chair turned off), t3 – maximum ankle flexion, t4 – completion (hip joint rotation rate is 0°/sec)

The statistical analysis was conducted using the software package Statistica 10 (StatSoft, Inc., USA). We calculated the average <X> and standard deviation <SD>. The indices of the control group were averaged and standardized.

Results and discussion. Temporal characteristics. The control group needs on the average 1.83±0.12 sec to complete the test. This value is significantly lower than that demonstrated by the experimental group while taking the test, 2.03±0.14 sec. The experimental group needs on average more time to complete the first and second phases, 0.79±0.04 sec (37% of normalized time) (NT) and 0.41±0.03 sec (19% of NT), respectively. At the same time, it took the control group 0.66±0.04 sec (30% of NT) and 0.33±0.03 sec (16% of NT) to complete the same phases.

Kinematics. We detected significant differences between the biomechanical model and parameters of a person with OAK.

Fig. 2. Comparison of biomechanical model (A) and biomechanical values of knee extensor mechanism (B) in 58-year-old man with OAK.

Along the axis X significant differences were noted in Phases I and III. While extending the knee joint, a person with OAK was high angled by default (Fig. 2). Along the axis Y the differences were detected at the end of Phase III (Fig. 3).

Fig. 3. Comparison of biomechanical model (A) and biomechanical values of knee joint rotation rate in 58-year-old man with OAK (B).

Along the axis Z the differences were observed in all the three phases. Throughout the test, the knee joint was turned outward. Besides, greater abduction was registered in Phases II and III (Fig. 4).

Fig. 4. Comparison of biomechanical model (A) and biomechanical values of "lateral motions" of knee joint in 58-year-old man with OAK (B).

As of today, osteoarthritis (OA) is one of the most common diseases both in Russia and abroad. According to the broad-scale research carried out in seven cities of the former USSR, OA was detected in 6.43% of patients [1]. In general, the OA prevalence rate varies enormously in different countries. In the USA, for instance, more than 26.9 million people over 25 years old suffer from this disease, which is approximately 8.4% of the population [7].

In everyday life, as well as in sports, it is often the knee joint that falls within the scope of OA, which significantly disables a person in his/her everyday life [1, 7, 8]. The study conducted by H.H. Huberti and W.C. Hayes, revealed that people with enhanced valgus or varus knee deformity were observed to have retrogressive changes in the hip joint, which contributed to an increase in the knee joint tension [6]. High joint tension results from the development and progression of OAK [4]. The knee joint tension increases with forced knee flexion, since counter-force of the knee joint reaches its peak when performing actions that include forced contraction of the quadriceps muscle of thigh as the knee joint is flexed the most [13]. Therefore, the load on the knee joint while ascending the sloping surface is equal to 0.5 of the body mass, whereas while rising from a chair the load on the knee joint equals 6.7 of the body mass [5].

We detected significant differences in the test execution time between EG and CG. Thus, it takes EG more time to complete the I and II phases of motion. The average execution time, demonstrated by EG during our studies, was less than that of EG in the research paper of Pai et al. [11]. These differences could be caused by different seat height. The stool used by Pai et al. [11] had a settled height of 45 cm, while the height of the stool used in our study was adjusted to the height equal to 110% of participants' knee height, and amounted to 49±2 cm (mean±SD). This was done to control the difference in body mass and length of the lower extremities. Higher test execution speed could be due to the fact that the testees from EG of Pai et al had the 2nd and 3rd stages of OAK, while our participants had only the 1st stage. In addition, in our study the participants from both groups were younger than those in the study conducted by Pai et al [11]: the mean age of our testees (in EG and CG) was 56.1 (5.8) and 51.6 (7.1) years respectively (the range of 41-65 years old), whereas in the study by Pai et al. - 69.6 (4.6) and 70.7 (5.4) years (mean, SD) (the range of 64-78 years old).

The differences between the groups were also identified during the biochemical analysis and registered in all three axes of the knee. During the test, the knee joint of the participants from EG was at the greater angle compared to CG, this difference was recorded at the beginning and at the end of the test. This position may cause the center-of-gravity to shift slightly forward, which would potentially reduce the power generated by the quadriceps (QUADS) to maintain the knee extension, thus reducing the counter-force and pain in the knee joint. We also found that the testees from EG had a more pronounced knee abduction. Maintenance of large knee abduction by the end of the test testifies to the incidence of pathology of the knee ligaments. A large degree of deviation of the knee joint from the norm increases the vector of QUADS valgus, which leads to an increase in the lateral-oriented efforts of QUADS. Re-orientation of QUADS increases the knee joint loading as well as load on the cartilage of the lateral edge of patella [6]. Increased load on joints can be considered as one of the main factors of development and progression of OAK [3, 10, 12]. Increment of loading on the lateral articular surface of patella may adversely affect the patellar cartilage during rising, as this position requires an increase in the force of QUADS contraction. We also registered slight external rotation at the end of the test. Increased external rotation might also be indicative of OAK [9]. In our further studies we plan to determine whether keeping the knee joint abducted will enable to reduce the load when rising from a chair.

Conclusion. The presented biomechanical model of the knee joint locomotions by the markerless motion capture system enables us to accurately register the motion speed in different phases of rising and record the biomechanical values of behavior of the knee joint with respect to the norm and abnormalities, and thereupon implement scientifically-based rehabilitation measures. Using this method one can develop the models aimed at the study of biomechanical values not only in the field of rehabilitation, but also in the sphere of physical culture and sports for the purpose of evaluation and registration of the accuracy of performance of various physical exercises.

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Corresponding author: apokin_vv@mail.ru