# Modeling of Muscle Activation from Electromyography Recordings in Patients with Cerebral Palsy
## 2 Methods  
### Recording of EMG Signal  
EMG signals collected for a previous study [15] were used here to derive muscle activation during cycling exercise on an ergometer. Participants were asked to cycle as regularly as possible on their own (e.g., without external help) with an average speed of 60 revolutions per minute (shown in the ergometer display) and for a period of 5 min. During cycling, the foot was fixed to the ergometer's foot support (the angle leg-ankle-foot was not fixed, though). A set of four reflectors fixed to the wheel of the ergometer together with a distance sensor attached to the pedal were used to determine the crank angle during cycling. This allowed also synchronization between the cycling movement and the EMG recordings. During cycling, the circular path traveled by the foot was divided into four quadrants as illustrated in Figure 2. The crank angle secured by the foot at any instant is defined by the angle formed by the following two radial lines: one stretching from the center of the wheel to the highest point possible--the reference position--and the other one stretching from the center of the circle to the current position. When the foot is at the point of maximum height, this corresponds to a crank angle of 0\({}^{\circ}\) (Figure 2). Before starting a recording, the distance of the participant to the ergometer was adjusted such that, when the crank angle was 90\({}^{\circ}\), the leg stretched to make an \"upper leg-to-lower leg\" angle of 170\({}^{\circ}\).  
EMG signals were recorded from the following four muscles in each leg: Musculus rectus femoris (M. rectus femoris), Musculus biceps femoris (M. biceps femoris), Musculus tibialis anterior (M. tibialis anterior), and Musculus gastrocnemius (M. gastrocnemius). These muscles were chosen because spasticity in any of these four muscles, either individually or in combination/simultaneously with another of these muscles, can lead to contractures, limit the range of motion and influence movement/mobility. Regular training for better coordination of these muscles may increase muscle strength and improve functionality of movement.Placement of electrodes followed the recommendations of the SENIAM (Surface ElectroMyography for the Non-Invasive Assessment of Muscles) european project [22]. The skin was cleaned with an alcohol containing cleansing liquid before placing the electrodes in bipolar configuration. A contact gel was used between the electrode  
Figure 1: Schematic representation of steps followed in the present work and to be followed in future work. EMG: electromyography.  
and skin to improve electrical conductance. The EMG analog signal was amplified, half-wave rectified and smoothed (low-pass filter with a 2-Hz cut-off frequency). The resulting envelope signal was then amplified again and recorded for offline analysis with a 77 Hz sampling rate. Even though this is a much lower sampling rate than recommended for EMG signals, it is enough to reproduce the envelope of the raw EMG signal, with maximum frequency at 2 Hz, here used to derive muscle activation. The reader is referred to the previous studies [15; 16] for further details on the experimental procedure.  
EMG signals analyzed here were collected for a group of fourteen adult patients with CP (nine with diplegia and five with hemiplegia) with ages between 23 to 60 years and a group of ten healthy adults aged between 25 and 55 years. A patient's degree of mobility according to the Gross Motor Function Classification Scale (GMFCS) [23] varied from Level I to Level IV. The degree of muscle spasticity classified according to the Modified Ashworth Spasticity (MAS) scale [24], ranged from 0 to 3, with a higher MAS level indicating a higher level of spasticity.  
Experimental procedures were approved by the ethics committee of the Faculty of Medicine of the Technical University of Munich. Participation in the study was voluntary, and all subjects, or in some cases their legal protectors, gave their written informed consent before performing the tests.