Altered lower leg muscle activation patterns in patients with cerebral palsy during cycling on an ergometer

Abstract

Objective: Cycling on a recumbent ergometer constitutes one of the most popular rehabilitation exercises in cerebral palsy (CP). However, no control is performed on how muscles are being used during training. Given that patients with CP present altered muscular activity patterns during cycling or walking, it is possible that an incorrect pattern of muscle activation is being promoted during rehabilitation cycling. This study investigated patterns of muscular activation during cycling on a recumbent ergometer in patients with CP and whether those patterns are determined by the degree of spasticity and of mobility.

Methods: Electromyographic (EMG) recordings of lower leg muscle activation during cycling on a recumbent ergometer were performed in 14 adult patients diagnosed with CP and five adult healthy participants. EMG recordings were done with an eight-channel EMG system built in the laboratory. The activity of the following muscles was recorded: Musculus rectus femoris, Musculus biceps femoris, Musculus tibialis anterior, and Musculus gastrocnemius. The degree of muscle spasticity and mobility was assessed using the Modified Ashworth Scale and the Gross Motor Function Classification System, respectively. Muscle activation patterns were described in terms of onset and duration of activation as well as duration of cocontractions.

Results: Muscle activation in CP was characterized by earlier onsets, longer periods of activation, a higher occurrence of agonist-antagonist cocontractions, and a more variable cycling tempo in comparison to healthy participants. The degree of altered muscle activation pattern correlated significantly with the degree of spasticity.

Conclusion: This study confirmed the occurrence of altered lower leg muscle activation patterns in patients with CP during cycling on a recumbent ergometer. There is a need to develop feedback systems that can inform patients and therapists of an incorrect muscle activation during cycling and support the training of a more physiological activation pattern.

Keywords: cocontraction; electromyography; mobility; rehabilitation; spasticity.

Figure 1

Schematic representation of the participant’s position during recording and the definition of foot position. Notes: The participant sat in front of the ergometer. During cycling, the path traveled by the foot is circular, such that the travel path can be divided into four different quadrants as illustrated in the inset in the right panel. The position of the foot in 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 and the point where the foot is located. When the foot is at the point of maximum height, this corresponds to an angle of 0° as illustrated in the left panel. Before starting a recording, the distance of the participant to the ergometer was adjusted such that, when the foot was at a 90° position, the leg was not completely stretched but made a “upper leg-to-lower leg” angle of 170° (right panel).

Figure 2

EMG signal amplitude recorded in each turning cycle from the left Musculus rectus (blue lines) and the average muscle activity (red line) computed over all valid full revolutions recorded. Note: Mean activity was calculated and included in further analysis when a minimum of 15 valid cycles were available. Abbreviations: EMG, electromyography; V, volt.

Figure 3

Definition of onset and offset points derived from the normalized mean muscle activity (red line) during a cycle revolution. Note: Onset and offset points were defined as the angle points at which the signal amplitude was half of the maximum amplitude (full-width at half maximum criterion). Abbreviation: EMG, electromyography.

Figure 4

Polar diagrams of muscle activation as a function of the cycle angle. Notes: Movement is performed in clockwise direction, and 0° angle corresponds to the position when the right leg is at the highest position. Each circumference represents the data for a different participant. Periods of muscular activation are represented in colors, blue for healthy participants and orange and red for patients, interleaved to facilitate distinction between individuals. The absence of a line indicates that no valid mean EMG could be obtained from the muscle. Each panel contains data collected for a different muscle, as indicated in the upper left corner of each panel. Gray circle segments illustrate the range of reference muscular activation periods. This reference range was obtained by averaging, across healthy participants, the individual mean EMG signals. Abbreviation: EMG, electromyography.

Figure 5

Difference in angle of onset of activation of the left Musculus rectus femoris relative to the reference onset angle, as a function of the duration of muscle activation. Notes: The reference angle of onset was derived as the onset angle averaged across healthy participants and is indicated in the figure by the blue dot at 0°. Each red/green dot represents the result for a single healthy participant/patient, respectively. The black line segment illustrates the parameter B for a single patient.

Figure 6

B factor as a function of MAS level (for healthy participants MAS =0). Notes: Different rows correspond to different muscles, and left and right columns correspond to muscles of the left and right legs, respectively. Red dots illustrate results for healthy participants; and green, blue, light green, and purple dots illustrate results for patients with increasing MAS spasticity level. Blue crosses indicate the mean B value within each MAS group. The blue line is a linear regression along all B values at all levels. Abbreviation: MAS, Modified Ashworth Spasticity.

Figure 7

Polar diagrams indicating periods of cocontractions during a full cycling revolution (360°). Notes: Each gray line illustrates results for a different participant. No line indicates that no clear EMG recording could be made. Red and blue circles indicate periods of cocontraction for patients and healthy participants, respectively. The upper/lower left panels show results obtained for the muscles of the upper/lower part of the left leg, respectively, and the left/right panels show results obtained for the muscles of the left/right leg, respectively. Abbreviation: EMG, electromyography.

Figure 8

Cocontraction factor K as a function of the spasticity level MAS for different pairs of agonist–antagonist muscles as indicated on the side of each panel. Note: Red dots illustrate results for healthy participants, green, blue light green and purple dots illustrate results for patients with increasing MAS level of spasticity. Blue crosses indicate the mean K factor within each group and the blue line illustrates a linear regression on mean K factors across MAS level groups. Abbreviation: MAS, Modified Ashworth Spasticity.

Figure 9

Mean proportion of time spent on a given quadrant in relation to total cycle duration, expressed in percentage, for each individual participant. Notes: Error bars illustrate the standard deviation of the mean. Darker areas indicate the range of times observed for the test and reference groups.