Comparison and evaluation of MVC measurements from selected reference tasks

Operant conditioning of reflexes can influence motor function; thus, it is being applied to rehabilitation after CNS injury (e.g., spinal cord injury, stroke (Thompson & Wolpaw, 2015)(Prog Brain Res 218: 157-72, 2015)). To analyze and compare EMG data of reflexes, a normalization method is needed. The standard method uses the maximum M-wave to normalize EMG data from electrically evoked muscle reflexes. In laboratory setups, this is a highly reliable method, but it is elaborate and cannot be used with some patients (e.g., those with heart diseases or cardiac pacemaker). In order to simplify the normalization method, this study evaluates the repeatability and relation between different maximum voluntary contraction tasks and the maximum M-wave. Methods: four reference tasks were defined for which maximum EMG was extracted from soleus muscle and compared to Mmax. Between-day and within-day repeatability were assessed. MVC measures were derived from two dynamic tasks (counter movement jumps (CMJ), plyometric drop jumps (DJ) from an elevation of 30 cm) and two isometric tasks in seated position by pressing the toes against a pedal (maximum voluntary contraction of the soleus were measured with an ankle joint angle of 0° and a knee angle of 0°Three repetitions were completed as a standard 5-s isometric contraction (Hold). Additionally, three fast isometric contraction were performed without an extended holding (Fast). To find the Mmax, an input-output curve was generated during upright standing. One stimulation electrode (anode) was positioned above the knee, while the cathode was placed on the back of the knee at the popliteal fossa. Initially, the stimulation intensity was manually increased until the H-reflex disappeared and the M-wave reached its maximum. Then a stimulation protocol applied 20 X 3 stimulation intensities (randomized, 5-7 s inter-stimulus intervals) controlled by the Mr. Kick III Software, enabling a Boltzmann fit to the input-output data. Accordingly, the relationship between Mmax and MVC tasks was evaluated. Calculations on 17 preliminary measurements showed significant correlations between Mmax and CMJ (r = 0.623), Mmax and Fast (r = 0.655) as well as Mmax and Hold (r = 0.606). There was no significant correlation between Mmax and DJ (r = 0.281). Conclusion: Results indicate that combinations of MVC measures from several tasks may be able to replace the time consuming and, in some cases, non-applicable electrical stimulation protocol. Data from more subjects are needed to confirm this multifactorial prediction of Mmax amplitude