###### Abstract  
The aim of this study was to investigate the influence of transcranial direct current stimulation (tDCS) on the biceps brachii and flexor carpi radialis stiffness in children with cerebral palsy (CP). The authors also aimed to verify the relationship between spasticity and muscle stiffness. _Methods_: Twelve children with CP (mean \(\pm\) SD; age, \(8\pm 1.3\) years; height, \(118.7\pm 4.1\) cm; weight, \(23.0\pm 2.2\) kg) were involved in the study. Muscle stiffness was estimated using a MyotonPRO device in a MultiScan pattern of five measurements. Simultaneously, the tDCS stimulation was performed. Spasticity was assessed by a neurologist using the Ashworth Scale. _Results_: Stiffness of the flexor carpi radialis muscle decreased significantly after tDCS therapy (\(p=0.04\)). There was no significant change in stiffness of the biceps brachii. For all participants, the Spearman rank correlation showed statistically significant and positive relationships between muscle stiffness and the Ashworth Scale (\(p=0.04\)). _Conclusions_: Transcranial direct current stimulation has a decreasing effect on stiffness and spasticity of the flexor carpi radialis in children with CP. The MyotonPRO device provides objective data and correlates with spasticity measurements.  
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active investigation of tDCS, with over 100 articles written in the past 10 years in peer-reviewed journals [5].  
A systematic review of interventions for children with CP showed several methods of managing spasticity, i.e., selective dorsal rhizotomy and administration of diazepam or botulinum toxin. However, tDCS was not mentioned in that review [6]. As a low-cost, easy-to-administer, and well-tolerated technique, tDCS in combination with conventional physical rehabilitation may potentiate neuroplasticity [7]. Therefore, it seems reasonable to investigate tDCS as an optional (additional) method to manage spasticity in CP children.  
The main contributors to increased joint resistance, which is a typical symptom in CP, are increased stiffness of muscle and hyperactivity of the stretch reflex (clinically labeled as \"spasticity\") [8]. It is generally assumed that increased resistance, explained by an acceleration in involuntary muscle activation during passive movements, is related to impaired voluntary motor function [9]. As stated by [PERSON] (2005), improvements in the upper limb and hand function can be achieved by spasticity reduction. Precise evaluation of spasticity is crucial to establish the efficacy of medical and physical therapeutic interventions. The Ashworth Scale grades the manual sensation of mechanical resistance experienced by the examiner during a 1-s joint rotation over the full range of motion [10]. Rating depends on examiner experience and testing technique, which may have a significant effect on subjectivity and may not provide a valid measure of spasticity [11].  
To support our study with more objective data, we obtained muscle stiffness measurements using a MyotonPRO (Myoton Muscle Diagnostics, Tallinn, Estonia) device for clinical purposes. Myotonometry is becoming increasingly popular in assessing the viscoelastic properties of muscles, both before and after applying physiotherapy treatment [12]. It is non-invasive, portable, and easy to administer [13].  
To the best of our knowledge, no study addressed the effect of tDCS on muscle stiffness, assessed by myotonometry, in CP children. Our study aimed to investigate the influence of tDCS on the biceps brachii and flexor carpi radialis stiffness in children with CP and the relationship between spasticity and muscle stiffness.