Evaluating Knee Joint Stresses During Kneeling Work

Goal of the Project:

Evaluate the effectiveness of knee pads and other protective equipment designed to reduce loading of the knees while performing kneeling work. Identify optimum materials and designs to develop an educational campaign that disseminates information to reduce exposure to hazards that have been associated with the development of knee osteoarthritis.

Summary of progress:

Literature Review:

A literature review was conducted with some of the key words like “Knee”, “Osteoarthritis”, “Kneeling”, “Injury”, “Workplace”. The papers related to the workplace injury due to kneeling, squatting etc were collected. From the numerous papers collected only the papers related to the current topic were studied and used. Most of the papers collected have significant importance in background for the project. From the literature review only couple of papers were found which had a significant amount of relation with the project. These papers are

a) Exposure Assessment of Kneeling Work Activities among Floor Layers by Jensen LK
External knee forces were measured in five different kneeling work positions in 10 floor layers using computer dynography. The study showed that floor layer spent significant amount of time in knee straining positions.
b) Pressures Applied to Anatomical Landmarks of the Knee while in Kneeling Postures by Moore SM
The objective of this study was to determine the pressure applied to the knee during static postures used in low seam mining, with and without knee pads.  Three postures were simulated kneeling in full flexion, kneeling at 90o of knee flexion and kneeling on one knee.
These papers did an analysis of the pressure being experience by the knee during different kneeling postures. These initial literature studies helped with knowing the effects of kneeling on the knee joint, also in preparing the questionnaires for the subjects performing the task.

Sensors Design and Testing:

In order to measure the pressure applied on the knee joint a pressure sensors have to be developed. These pressure sensors will be placed on the knee of the subjects. These sensors will measure the pressure produced in the knee of the subjects performing kneeling work. Hence the designing and testing of these sensors become very important in conducting of the project.

In the process of developing the sensors the first step is to search for the appropriate sensor which is thin and also can measure high pressures. There are many pressure sensors available but depending on our requirement and cost we selected the Force Sensing Resistors (FSRs) which were produced by Interlink Electronics. The FSRs are a polymer thick film device which exhibits a decrease in resistance with an increase in the force applied to the active surface. Once the first task of selecting the sensors was completed the nest task was to design a circuit which would read the output from the sensors. Some of the circuits were suggested by the Interlink Electronics and one of the very basic circuit was considered. The required material for building the circuit was bought and arranged as given in the FSR manual.

This initial circuit was tested and calibrated to calculate the voltage output for an applied load. A dynamometer was used in measuring the applied load on the FSRs. Once the calibration was completed it showed that though the initial difference in voltage for change in force was larger it reduced as the force increase. Hence amplification of the output voltage was considered and the values were recalibrated. The circuit was tested with two times amplification firstly and then with an amplification of three times. It was noticed that sensors with larger area yielded desired results with two times amplification and sensors with smaller area yielded desired results with three times amplification. This output was to be fed into a vicon system so that the output can be easily transferred in the computer and its input voltage was limited to 10V (a voltage above 10V as input would show nothing but a straight line in the output display i.e, it clips off the higher voltage). Since the amplification of three times for sensors with smaller area never reached 10V, they were used in the circuit with three times amplification but the sensors with larger area were used with two times amplification.

enter table here

After selection of the amplification and calibration for the circuit then the next step is to decide on the number of sensors and the placement of the sensors. In order to decide on the placement of the sensors on the knee a pressure plate was used. A trail run was conducted with kneeling on the pressure plate and conducting different kneeling tasks so that the shift in center of mass of the person can be seen and from this the placement of the sensors can be decided. Eight sensors are distributed on the knee depending on the data obtained from the pressure plate. In order to handle eight sensors on each knee the LM324 Op-amp is used.
A permanent circuit board has been prepared based on testing circuit on a solderable circuit board with sensors connected to it as shown in the figure on the right.

Epoxy has been used to seal the connection of sensors with the wires so that they are prevented from any kind of breakage. Epoxy is thermosetting polymer formed from the reaction of an epoxide “resin” with polyamine “hardner.”

Upcoming Tasks:

• Testing the sensors on subjects using different knee pads and collecting the data.

Different kinds of knee pads made by different manufacturers are considered in this study.

• Preparation of questionnaire
• Analyzing the collected data