Robotics in Neurorehabilitation: Locomotor Training

Locomotor training on a treadmill is a rehabilitation procedure for persons suffering spinal cord injury, stroke or traumatic brain injury. Training usually starts four to six weeks after the causal event when patients are incapable of moving their legs themselves. Instead, they are suspended by a harness over the treadmill, with their body partially unloaded by the suspension system.

Historically, leg movements were manually assisted in a physiological way by two physiotherapists helping patients perform stepping movements on the treadmill. These movements are associated with a pattern of leg muscle activation that appears to be generated by locomotor centers within the spinal cord.

Assisted leg movements during the training is important. Optimal input to the spinal cord is achieved only if the legs are moved in a reproducible, rhythmical and physiological manner. This is necessary to stimulate the locomotor centers within the spinal cord to activate leg muscles that cannot be moved voluntarily.

For therapists, moving patients’ legs during the treadmill training is ergonomically unfavorable and tiring work that does not provide optimal gait pattern and is not reproducible.

Hocoma AG, Volketswil, Switzerland has developed a Driven Gait Orthosis (DGO) called Lokomat® (See Figure 1, pg. S17) to improve the treadmill training for patients and reduce the workload of the therapists. Advantages include earlier starts to rehabilitation, longer training sessions and a more physiological and reproducible gait pattern for each patient. Training is less costly because only one therapist is needed.

A user interface allows therapists to operate the Lokomat and adjust training parameters to suit individual patients. Software-controlled gait patterns assist patients’ leg movements, keeping them consistent with normal walking motions, while computer-controlled drives at each hip and knee joint are synchronized with treadmill speed.

Each of the four joints is constantly monitored by software to ensure they are precisely held to the predefined gait pattern. Force transducers at these joints are integrated to accurately measure the interaction between the patient and the Lokomat. Optional optical sensors assist the therapist in supervising the gait therapy by monitoring patients’ feet on the treadmill.

As patients advance in their rehabilitation, the Lokomat guidance force control module encourages them to work harder. A biofeedback system lets patients see their performance (as shown in Figure 1) and is a motivational tool to achieve the most benefit from training sessions.

Adjusting the Lokomat to Different Patients

Using several variable parameters, the Lokomat can be tailored to the training of different patients. The band that is fixed around the chest of the patient is mounted to a back pad, which can be positioned vertically and horizontally. A spindle that moves the two legs apart adjusts the width of the hip orthosis. The length of the thigh and the shank of the orthosis can be changed as well. Both limbs consist of rectangular tubes pushed into one another that can be fixed in different positions by a bolt (See Figure 2, pg. S18).

Finally, the position and size of the leg braces can also be adapted to individual requirements. The braces are connected to a right-angled tube is fixed to the leg orthosis. The tube can be moved in an anterior-posterior direction on the leg orthosis and tightened in the correct position by a small lever. In the same way, the brace becomes fixed at the right position medio-laterally on the other side of the tube. If one brace does not fit with the individual leg, it can easily be replaced by a bigger or smaller brace.

The DGO is available in two sizes (See Figure 3, at left). The adult module and the pediatric module are easily exchangeable. Children at the age of 4 can be trained.

Balance Control

For patients lacking trunk stability, the upper body is stabilized in the vertical direction during training by fixing the DGO to the railing of the treadmill by a rotatable parallelogram shown in Figure 1. This enables the DGO to move only in a vertical direction and prevents tilting to one side. The parallelogram also keeps the DGO in a fixed position over the treadmill and prevents backward movement induced by the moving treadmill belt while permitting the upward and downward movements of the body that occur during walking.

Driving Power

To move the legs of the patient as similarly to normal walking as possible, the drives at the knee and hip joints have to be strong enough to move the limbs even if spastic muscle hypertonia is present. At the same time, the orthosis should be easy to handle and the drives should therefore not be too big or too heavy.

Hip and knee joints are driven by custom-designed drives with a precision ball screw. The nut on the ball screw is driven via a toothed belt by a dc motor (RE40, maxonmotor AG, Switzerland), which delivers a nominal mechanical power of 150W. The motor allows long-time usage at a maximum torque of approximately 180 mNm. For short but repetitive peaks, a torque of up to 1 Nm can be achieved.

Converted to the knee and hip joints of the orthosis (different geometry), an average torque of approximately 30 Nm and 50 Nm, respectively, can be achieved. Peaks of 160 Nm for the knee joint and 280 Nm for the hip joint are possible. The mechanical layout of the orthosis has been designed to optimally profit from the recommended speed characteristics of the motor at walking speeds up to 3 km/h (cadence of 90). The bandwidth (with PD controller) is at least 1 Hz, which is sufficient for normal gait.

Control System

The control setup of the DGO consists of three main hardware parts: the host personal computer (PC), the target PC, and the current controller. Therapists control the DGO via a user interface programmed in LabView and running on the host PC. It consists of a database and an interface to the DGO.

All four driven joints of the DGO are controlled separately via an individual position-controller loop based on a commercially available current controller fed by the output of a position controller. The position controller is implemented in a real-time system running on the target PC. The actual angles of each joint are measured by potentiometers and the corresponding values are transferred via an analog-to-digital converter into the real-time system.

Host PC and target PC communicate with each other using an Ethernet connection. Therapists can change the speed of the DGO at the user interface. The intended speed is transferred to the target PC that adjusts the gait pattern accordingly and also sets the desired speed of the treadmill by a serial port.

This article is an update of an earlier work published in Journal of Rehabilitation Research and Development Vol. 37 No. 6, November/December 2000 — Treadmill training of paraplegic patients using a robotic orthosis — Gery Colombo, MS; Matthias Joerg, MS; Reinhard Schreier, BME; Volker Dietz, MD. Paraplegic Center ParaCare, University Hospital Balgrist, Zurich, Switzerland; Hocoma GmbH, Medical Engineering, Staefa, Switzerland. John H. Williamson is a freelance writer living in Lebanon, NJ.