Background:
A well-learned motor skill is characterized by the efficient activation of muscles that are involved in movement execution. However, it is unclear if practice-related changes in motor performance correlate with those in quantitative markers of muscle activity and if so, whether the association is different with respect to the investigated muscle (i.e., agonist and antagonist) and quantitative myoelectric parameter. Thus, we conducted a systematic review and characterized associations between practice-related changes in motor performance and muscle activity in healthy individuals.

Methods:
A computerized systematic literature search was performed in the electronic databases PubMed, Web of Science, and SPORTDiscus up to September 2017 to capture all relevant articles. A systematic approach was applied to evaluate the 1670 articles identified for initial review. Studies were included only if they investigated healthy subjects aged 6 years and older and tested at least one measure of motor performance (e.g., error score, movement time) and quantitative muscle activity (i.e., amplitude domain: iEMG [integrated electromyography], RMS [root mean square]; time domain: duration of muscle activity, time to peak muscle activation). In total, 24 studies met the inclusionary criteria for review. The included studies were coded for the following criteria: age, learning task, practice modality, and investigated muscles (i.e., agonist and antagonist). Correlation coefficients for the relationship of motor performance changes with changes in electromyography (EMG) amplitude, and duration were extracted, transformed (i.e., Fisher’s z-transformed rz value), aggregated (i.e., weighted mean rz value), and back-transformed to r values. To increase sample size, we additionally extracted pre and post practice data for motor performance and myoelectric variables and calculated percent change values as well as associations between both. Correlations were classified according to their magnitude (i.e., small r ≤ 0.69, medium r ≤ 0.89, large r ≥ 0.90).

Results:
Five studies reported correlation coefficients for the association between practice-related alterations in motor performance and EMG activity. We found small associations (range r = 0.015–0.50) of practice-related changes in motor performance with measures of agonist and antagonist EMG amplitude and duration. A secondary analysis (17 studies) that was based on the calculation of percent change values also revealed small correlations for changes in motor performance with agonist (r = − 0.25, 11 studies) and antagonist (r = − 0.24, 7 studies) EMG amplitude as well as agonist (r = 0.46, 8 studies) and antagonist (r = 0.29, 5 studies) EMG duration.

Conclusions:
Our systematic review showed small-sized correlations between practice-related changes in motor performance and agonist and antagonist EMG amplitude and duration in healthy individuals. These findings indicate that practice-related changes can only partly be explained by quantitative myoelectric measures. Thus, future studies investigating biomechanical mechanisms of practice-related changes in motor performance should additionally include qualitative measures of muscle activity (e.g., timing of muscle activity, level of coactivation) and other biomechanical variables (i.e., kinetics, kinematics).