Cerebral palsy (CP) is the most common cause of physical disability in children and is associated with muscle weakness, spasticity, and impaired motor control. These impairments often lead to compensatory movement strategies, in which other muscles are recruited or movements are adapted to offload weaker target muscles. Although lower limb strengthening is widely used to improve motor function in children with CP, outcomes remain inconsistent. One potentially important but rarely examined factor is whether the intended muscles are actually activated during training. This study aims to develop and apply an activation-driven assessment protocol that combines individualized exercise selection, real-time electromyography (EMG) biofeedback, and compensation monitoring to identify conditions that promote target-muscle activation while limiting compensatory mechanisms. The focus is on three key muscle groups: hip extensors (HE), knee extensors (KE), and plantar flexors (PF). In this prospective\[JV2.1\], within-subject repeated-measures study, 30 ambulant children aged 5-12 years with spastic CP and Gross Motor Function Classification System levels I-III will complete three assessment visits in which exercise conditions and feedback strategies are compared using surface EMG and three-dimensional movement analysis.
CP causes primary neurological impairments such as spasticity, muscle weakness, and reduced selective motor control. Over time, these impairments contribute to secondary musculoskeletal changes, including muscle contractures and skeletal deformities, which further limit mobility and affect gait. Muscle weakness in CP is multifactorial and results not only from reduced force-generating capacity but also from impaired muscle activation, altered muscle morphology, and biomechanical disadvantages. Consequently, children may compensate through alternative muscle recruitment and altered movement strategies that offload weaker muscles. Lower limb strength training is a common intervention and has shown benefits for muscle strength, gait, and gross motor function. However, findings across studies are highly variable. One potentially important but rarely examined factor is whether strengthening exercises activate the intended target muscles. Due to impaired selective motor control, children with CP may unintentionally train compensatory muscles rather than the weaker muscles that require strengthening. To address this limitation, the present study proposes an activation-driven approach to lower limb strengthening. The protocol combines individualized exercise selection, EMG-biofeedback, and explicit management of compensatory mechanisms to promote more selective muscle activation. The ultimate goal is to improve training specificity and support the development of more effective strengthening interventions for children with spastic CP. The study specifically focuses on the gluteus maximus, vastus lateralis, and soleus, as these muscles play a crucial role in gait, are commonly weakened in CP, and may be offloaded through compensatory muscle recruitment. In this prospective, \[JV3.1\]within-subject repeated-measures study, 30 ambulant children aged 5-12 years with spastic CP and Gross Motor Function Classification System levels I-III will complete three assessment visits. Visit 1 will evaluate 3-5 bodyweight exercise conditions per muscle group and select up to three individualized conditions based on target-muscle activation and compensatory mechanisms. Visit 2 will compare standardized instruction, patient-tailored verbal feedback, and visual single-target EMG feedback. Visit 3 will compare single-target EMG feedback with dual-target EMG feedback and combined EMG and biomechanical feedback. Surface EMG, three-dimensional motion capture, and force platforms will be used to assess the immediate effects of these conditions on target-muscle activation, compensatory muscle activity, and compensatory movement patterns.
Study Type
OBSERVATIONAL
Enrollment
30
UZ Leuven
Leuven, Vlaams-Brabant, Belgium
Peak EMG amplitude of the target muscles
Peak normalized EMG amplitude during the exercise, used as a measure of maximal targeted activation of the hip extensors (HE), knee extensors (KE), and plantar flexors (PF).
Time frame: Assessed across three study visits over approximately 4 weeks.
Integrated EMG of the target muscles
Integrated normalized EMG over the duration of the complete task or phase of interest, used as a measure of total targeted muscle activation.
Time frame: Assessed across three study visits over approximately 4 weeks.
Co-activation index
Quantification of simultaneous activation of the target muscle and relevant antagonist or compensatory muscle, used to assess co-activation during strengthening exercises. It is calculated as: Co-activation index (%) = 2 × (Common Area of A and B / (Area of A + Area of B)) × 100 where A and B represent the two muscles being compared. Values range from 0% to 100%, with higher values indicating greater simultaneous muscle activation (co-activation).
Time frame: Assessed across three study visits over approximately 4 weeks.
Selectivity ratio
Integrated EMG of the point-by-point ratio between the time-normalized activation waveform of the target muscle and the corresponding activation waveform of a relevant synergistic or compensatory muscle, used as a measure of target muscle selectivity during the exercise.
Time frame: Assessed across three study visits over approximately 4 weeks.
Joint angle compensation parameter
A predefined joint angle (expressed in °) selected according to the exercise task, used to quantify compensatory movement strategies.
Time frame: Assessed across three study visits over approximately 4 weeks.
Joint moment compensation parameter
A predefined joint moment (expressed in Nm/kg) selected according to the exercise task, used to quantify compensatory movement strategies.
Time frame: Assessed across three study visits over approximately 4 weeks.
Joint power compensation parameter
A predefined joint power (expressed in W/kg) selected according to the exercise task, used to quantify compensatory movement strategies.
Time frame: Assessed across three study visits over approximately 4 weeks.
Continuous EMG waveforms
Time-normalized EMG waveforms of target, antagonist, and compensatory muscles across the full exercise cycle and relevant task phases.
Time frame: Assessed across three study visits over approximately 4 weeks.
Continuous joint angle waveforms
Time-normalized joint angle waveforms across the full exercise cycle and task-specific phases. Values are expressed in degrees (°) and reported over 0-100% of the exercise cycle.
Time frame: Assessed across three study visits over approximately 4 weeks.
Continuous joint moment waveforms
Time-normalized joint moment waveforms across the full exercise cycle and task-specific phases. Values are expressed in Nm/kg and reported over 0-100% of the exercise cycle.
Time frame: Assessed across three study visits over approximately 4 weeks.
Continuous joint power waveforms
Time-normalized joint power waveforms across the full exercise cycle and task-specific phases. Values are expressed in W/kg and reported over 0-100% of the exercise cycle.
Time frame: Assessed across three study visits over approximately 4 weeks.
Mean EMG amplitude
Mean normalized EMG amplitude of the target muscle during the exercise, expressed as a percentage (%) relative to the reference signal used for EMG normalization.
Time frame: Assessed across three study visits over approximately 4 weeks.
Muscle activation timing
Activation onset and offset timing of the target muscle, determined from the normalized EMG signal and expressed in milliseconds (ms).
Time frame: Assessed across three study visits over approximately 4 weeks.
Additional co-activation measures
Co-activation indices (expressed in %) for other relevant muscle pairs, including agonist-antagonist and proximal-distal muscle combinations. Co-activation is calculated as: Co-activation Index (%) = 2 × (Common Area of A and B / (Area of A + Area of B)) × 100 where A and B represent the two muscles being compared. Values range from 0% to 100%, with higher values indicating greater simultaneous muscle activation.
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Time frame: Assessed across three study visits over approximately 4 weeks.
Additional selectivity measures
Selectivity ratios involving other target, synergistic, antagonist, or compensatory muscles to further describe the specificity of muscle recruitment. The selectivity ratio is calculated as the integrated EMG of the point-by-point ratio between the time-normalized activation waveform of the target muscle and the corresponding activation waveform of the comparison muscle. Values are expressed as a %, with higher values indicating greater selective activation of the target muscle relative to the comparison muscle.
Time frame: Assessed across three study visits over approximately 4 weeks.
Additional kinematic parameters
Exercise-specific kinematic variables, including joint range of motion, peak joint angles, trunk inclination, pelvic movement, knee position, ankle strategy. Values are expressed in degrees (°).
Time frame: Assessed across three study visits over approximately 4 weeks.
Peak joint moments
Exercise-specific peak joint moments. Values are expressed in Nm/kg.
Time frame: Assessed across three study visits over approximately 4 weeks.
Peak joint powers
Exercise-specific peak joint powers. Values are expressed in W/kg.
Time frame: Assessed across three study visits over approximately 4 weeks.
Movement variability
Variability of biomechanical parameters across repeated exercise trials, calculated as the standard deviation of the parameter of interest.
Time frame: Assessed across three study visits over approximately 4 weeks.
Number of repetitions completed
Number of exercise repetitions successfully completed during the exercise task.
Time frame: Assessed across three study visits over approximately 4 weeks.
Exercise duration parameters
Task execution measures including movement duration and task phase duration during strengthening exercises. Values are expressed in seconds.
Time frame: Assessed across three study visits over approximately 4 weeks.
Task execution success
Success of task execution during strengthening exercises, expressed as the proportion of successfully completed repetitions to the total number of attempted repetitions.
Time frame: Assessed across three study visits over approximately 4 weeks.
Ability to perform the exercise under feedback conditions
Ability to perform the exercise across the different feedback conditions, expressed as the proportion of feedback conditions in which the participant is able to complete the exercise.
Time frame: Assessed across three study visits over approximately 4 weeks.
Maximal isometric strength
Maximal voluntary isometric strength of the hip extensors (HE), knee extensors (KE), and plantar flexors (PF), measured with a hand-held dynamometer during the maximal voluntary isometric contraction (MVIC) procedures to characterize participant-specific strength capacity.
Time frame: Assessed across three study visits over approximately 4 weeks.
Age
Age of the participant at study inclusion (expressed in years).
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Sex
Sex of the participant (male, female, other).
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Gross Motor Function Classification System (GMFCS) level
GMFCS Levels I-III (higher levels indicate greater functional limitation).
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
General treatment history
General treatment history relevant to the management of cerebral palsy.
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Previous strength training experience
Previous participation in strength training activities.
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Botulinum neurotoxin history
Previous treatment with botulinum neurotoxin.
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Orthopaedic surgery history
Previous lower-limb orthopaedic surgery.
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.
Use of orthoses or assistive devices
Current use of orthoses or assistive devices.
Time frame: Collected during screening and/or Visit 1, before the experimental exercise assessments.