Assessment of Lower Limb Muscle Strength: Feasibility and Implementation on Exxentric’s SingleExx Machines

Sammanfattning: Measuring lower limb strength and symmetry is a common practice in elite sports to determine the return to sports point in time during rehabilitation, and this information could be useful for amateur athletes too. However, the devices used for this are highly sophisticated and hardly accessible. The aim of this project is to devise an affordable system for lower limb strength measurement which can be integrated with a common training system, to make this type of information widely available. The system is built up on a flywheel-based leg extension machine (LegExx by Exxentric AB) that allows quadriceps training at maximal force in concentric and eccentric contraction. Inexpensive standard components and software were used for prototyping. The parameters of interest were angular velocity of extension/flexion in the knee and force applied by the athlete. In the first step, evaluation was limited to the concentric phase of movement. Angular velocity and leg position was measured with a gyroscope sensor (Movesense by Suunto). For force measurement, two approaches were tested: In prototype A, the tension of the drive belt, which ultimately puts the flywheel into rotation, was measured with a sensor for tensile forces (Tindeq Progressor 300). In prototype B, a more direct measurement was used, with force sensors mounted under the contact point of the athlete’s shank with the swing beam of the LegExx. Sensor data were transmitted via Bluetooth to a mobile device or a laptop and displayed graphically after synchronization of the data streams. Force values were converted to torque using inverse kinematics to make the two prototypes comparable. The first prototypes were shown to be workable and yielded similar readings for concentric peak torque, with realistic wave forms in the graphical display. Comparison with results of a gold standard isokinetic dynamometer revealed, however, major discrepancies mainly regarding the absolute torque values. Oscillation in the belt system were identified as an issue in prototype A, while cross talk between sensors and vulnerability to leg placement occurred in prototype B. However, these issues are not insurmountable, and it is suggested to proceed with the development of prototype B, as it has the advantage of simultaneous measurement and direct comparison of both legs.