Standardized voluntary force measurement in a lower extremity rehabilitation robot

Journal of NeuroEngineering and Rehabilitation BioMed Central Research Open Access Standardized voluntary force measurement in a lower extremity rehabilitation robot Marc Bolliger*1,2, Raphael Banz1, Volker Dietz1 and Lars Lünenburger1 Address: 1Spinal Cord Injury Center, Balgrist University Hospital, Zurich, Switzerland and 2Sensory-Motor Systems Laboratory, ETH Zurich, Switzerland Email: Marc Bolliger* - ; Raphael Banz - ; Volker Dietz - ; Lars Lünenburger - * Corresponding author Published: 28 October 2008 Journal of NeuroEngineering and Rehabilitation 2008, 5:23 doi:10.1186/1743-0003-5-23 Received: 12 December 2007 Accepted: 28 October 2008 This article is available from: http://www.jneuroengrehab.com/content/5/1/23 © 2008 Bolliger et al; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract Background: Isometric force measurements in the lower extremity are widely used in rehabilitation of subjects with neurological movement disorders (NMD) because walking ability has been shown to be related to muscle strength. Therefore muscle strength measurements can be used to monitor and control the effects of training programs. A new method to assess isometric muscle force was implemented in the driven gait orthosis (DGO) Lokomat. To evaluate the capabilities of this new measurement method, inter- and intra-rater reliability were assessed. Methods: Reliability was assessed in subjects with and without NMD. Subjects were tested twice on the same day by two different therapists to test inter-rater reliability and on two separate days by the same therapist to test intra-rater reliability. Results: Results showed fair to good reliability for the new measurement method to assess isometric muscle force of lower extremities. In subjects without NMD, intraclass correlation coefficients (ICC) for inter-rater reliability ranged from 0.72 to 0.97 and intra-rater reliability from 0.71 to 0.90. In subjects with NMD, ICC ranged from 0.66 to 0.97 for inter-rater and from 0.50 to 0.96 for intra-rater reliability. Conclusion: Inter- and intra- rater reliability of an assessment method for measuring maximal voluntary isometric muscle force of lower extremities was demonstrated. We suggest that this method is a valuable tool for documentation and controlling of the rehabilitation process in patients using a DGO. Background Muscle force testing is a well established method of assessing muscle function in subjects with neurological movement disorder (NMD) [1,2], despite the fact that these tests are in generally not sensitive enough to assess the force of a single muscle. Isometric force measurements are widely used because walking ability has been shown to be related to muscle strength [3-6]. Therefore, monitoring of muscle force can be used to control the effects of rehabilitation treatments. Furthermore, in rehabilitation hospitals, manual muscle tests (e.g. Manual Muscle Test, ASIA Motor score, Medical Research Council, Lower Extremity Motor Score) are the most commonly used methods of documenting impaired muscle strength. However, these tests are based on subjective assessment, produce ordinal (not scalar) data, require comprehensive training of therPage 1 of 8 (page number not for citation purposes) Journal of NeuroEngineering and Rehabilitation 2008, 5:23 http://www.jneuroengrehab.com/content/5/1/23 apists, and have poor inter- and intra-rater reliability [7,8]. In addition, these tests are usually not sensitive to small or moderate changes in muscle strength [1,9]. Robotic gait training devices have gradually become established to treat individuals with a locomotor dysfunction, such as spinal cord injury (SCI), stroke and traumatic brain injury [10-13]. A widely used device is the driven gait orthosis (DGO) Lokomat (Hocoma AG, Volketswil, Switzerland). This DGO is equipped with force transducers to assess the activity of patients while walking with the DGO. A detailed description of the Lokomat is published elsewhere [14,15]. Recently a novel measurement method for assessing muscle force using this DGO was developed. The method can be applied during a standard Lokomat training session and requires minimal additional time. The mechanical properties of the device allow hip and knee flexion and extension measurements. The aim of this study was to analyze the reliability of a measurement method that assesses voluntary isometric force of leg muscles with a driven gait orthosis. We determined inter- and intra-rater reliability of force measurements in subjects with and without NMD. If reliability can be demonstrated, the new assessment method can be established as a tool to investigate and control the rehabilitation process of patients. Methods Isometric force measurement with the DGO The DGO Lokomat is used in combination with a treadmill and a dynamic body weight support system. The DGO controls the patient's leg trajectories in the sagittal plane during walking [14,15]. The hip and knee joints of the DGO are actuated by linear back-drivable actuators integrated into an exoskeleton structure. In every actuator, a force transducer measures the linear forces, whereas potentiometers measure the actual joint angles. The torques acting on each joint are calculated online from these position and linear force values based on the known geometry. For the isometric force assessment, subjects wear a harness and are fixed to the DGO by straps around the trunk and the pelvis. The legs of the device are attached to the subject's legs with cuffs around the thighs and calves. Proximal and distal leg structures of the DGO are adjusted to align hip and knee joints of the subjects with the joint axes of the DGO. Subjects are lifted above the treadmill (unloading from 100% body weight) and the software sets the device to position control mode with preset fixed joint angles (hip 30° flexion, knee 45° flexion; see Figure 1). In this position subjects are asked to perform either a flexion or extension movement in hip or knee joint in left or right leg and push against the orthosis legs according to a defined sequence of tests. The system Figure 1 position of subject in DGO Measuring Measuring position of subject in DGO. Subject in the position used for the force measurement in the DGO. The device is set to position control mode with preset fixed joint angles (hip 30° flexion, knee 45° flexion). Page 2 of 8 (page number not for citation purposes) Journal of NeuroEngineering and Rehabilitation 2008, 5:23 controls the drives to keep this position and measures forces acting on the force transducers. Visual feedback of the forces applied to the DGO is displayed for the subjects (Figure 2). Forces applied in the desired movement direction for t (...truncated)