Effects of Motor Task Difficulty on Postural Control Complexity during Dual Tasks in Young Adults: A Nonlinear Approach.

Few studies have evaluated the effect of a secondary motor task on the standing posture based on nonlinear analysis. However, it is helpful to extract information related to the complexity, stability, and adaptability to the environment of the human postural system. This study aimed to analyze the effect of two motor tasks with different difficulty levels in motor performance complexity on the static standing posture in healthy young adults. Thirty-five healthy participants (23.08 ± 3.92 years) performed a postural single task (ST: keep a quiet standing posture) and two motor dual tasks (DT). i.e., mot-DT(A)­perform the ST while performing simultaneously an easy motor task (taking a smartphone out of a bag, bringing it to the ear, and putting it back in the bag)­and mot-DT(T)­perform the ST while performing a concurrent difficult motor task (typing on the smartphone keyboard). The approximate entropy (ApEn), Lyapunov exponent (LyE), correlation dimension (CoDim), and fractal dimension (detrending fluctuation analysis, DFA) for the mediolateral (ML) and anterior-posterior (AP) center-of-pressure (CoP) displacement were measured with a force plate while performing the tasks. A significant difference was found between the two motor dual tasks in ApEn, DFA, and CoDim-AP (p < 0.05). For the ML CoP direction, all nonlinear variables in the study were significantly different (p < 0.05) between ST and mot-DT(T), showing impairment in postural control during mot-DT(T) compared to ST. Differences were found across ST and mot-DT(A) in ApEn-AP and DFA (p < 0.05). The mot-DT(T) was associated with less effectiveness in postural control, a lower number of degrees of freedom, less complexity and adaptability of the dynamic system than the postural single task and the mot-DT(A).

Standing Posture in Motor and Cognitive Dual-Tasks during Smartphone Use: Linear and Nonlinear Analysis of Postural Control.

Analysis of the center of pressure (CoP) during cognitive or motor dual-tasking is widely used to characterize postural control. Most studies use traditional measures of CoP to quantify postural control, but given its complexity, nonlinear analysis of CoP is of growing interest in the area. This study aims to analyze CoP behavior in healthy young adults during standing posture performance while simultaneously performing motor or cognitive tasks on a smartphone, using linear and nonlinear analysis of CoP. Thirty-six healthy participants (23.08 ± 3.92 years) were found eligible for this study. They performed a single task (ST), cognitive dual-task (cog-DT), and motor dual-task (mot-DT). The total excursion of CoP, displacement of CoP in the anterior-posterior and medial-lateral directions, mean total velocity of CoP, and mean anterior-posterior and medial-lateral velocities of CoP were measured with a force plate. Approximate entropy (ApEn) of the anterior-posterior (ApEn-AP) and medial-lateral (ApEn-ML) displacement of CoP were also calculated. The results showed that dual-task costs for the total excursion, displacement in the anterior-posterior direction, mean total velocity, and mean anterior-posterior velocity of CoP were greater during the cog-DT than the mot-DT (p < 0.05). In the nonlinear analysis of the CoP, there was no difference (p > 0.05) between the cog-DT and mot-DT for ApEn values of the anterior-posterior and medial-lateral time series of the CoP. Both linear and nonlinear analyses showed differences between the cog-DT and ST (p < 0.05), revealing a decline in postural control during the cog-DT compared with the ST. In conclusion, performing a cog-DT causes sway impairments and lower postural control efficacy compared with motor single and dual-tasks. Furthermore, both linear and nonlinear analyses were able to distinguish between conditions.

Comparison of shoulder and trunk muscle activation between different pullover exercises / Comparación de la activación de los músculos del hombro y del tronco entre diferentes ejercicios de pullover

OBJECTIVE: To quantify and compare the electromyographic activity of trunk and upper limb muscles in three different pullover exercises. METHODS: 15 healthy men, with at least two years of experience in resistance training, executed in random order six repetitions with 60% of 1 Maximum Repetition for three different pullover exercises: lying on a step with a barbell, grip 100% biacromial (E1); lying on a step with a barbell, grip 150% (E2); lying on a Swiss ball with a barbell, grip 100% (E3). Surface electromyography was recorded from the Deltoideus (Clavicular and Spinalis Pars), Pectoralis Major (Clavicular and Sternocostalis Pars), Serratus Anterior, Triceps Brachii (Long Head), Latissimus Dorsi, Infraspinatus, Rectus Abdominis, Obliquus Internus Abdominis and Transversus Abdominis. The normalized electromyogram of maximal voluntary isometric contraction of each muscle was calculated for each exercise. RESULTS: The most engaged muscles were Infraspinatus (51-53% Electromyogram maximal voluntary isometric contraction) and Posterior Deltoid (49-51% Electromyogram maximal voluntary isometric contraction). Surface electromyography activity was similar between the E1, E2 and E3 exercises. CONCLUSIONS: This study quantified muscular solicitation during pullover exercises performed with 60% Maximum Repetition. The muscles with higher level of activation were the Posterior Deltoid and the Infraspinatus, suggesting that pullover may be a valid option for strengthening the dynamic stabilizing muscles of shoulder joint in trained individuals. No significant differences in muscle electromyography intensity were observed when grip distance and trunk stabilization were altered, showing that these conditions do not influence muscle activation levels. However, the 1 Maximum Repetition was lower when the pullover was performed on a Swiss ball, suggesting that it is possible to obtain higher level of muscle recruitment with lower weights in unstable exercises

OBJETIVO: cuantificar y comparar la actividad electromiográfica de diez músculos en tres diferentes ejercicios de pullover. MÉTODO: 15 hombres sanos, con al menos dos años de experiencia en entrenamiento de resistencia, realizaron seis repeticiones al 60% de 1 Repetición Máxima en orden aleatorio para tres ejercicios de pullover diferentes: acostados en una tabla con mancuernas y agarre 100% biacromial (E1), acostados en una tabla con mancuernas, agarre 150% biacromial (E2) y acostado en una pelota suiza con mancuernas, agarre 100% biacromial (E3). Se registró la señal electromiográfica de superfície de Deltoides (anterior y posterior), Pectoral Mayor (clavicular y esternocostal), Serrato Anterior, Tríceps Braquial (porción externa), Dorsal Grande, Infraespinoso, Recto Abdominal, Oblicuo Interno y Transverso del Abdominal. Se calculó la Repetición Máxima para normalizar la señal electromiográfica de cada músculo y para cada ejercicio. RESULTADOS: los músculos más involucrados fueron el Infraespinoso (51-53% señal electromiográfica de superfície 1 Repetición Máxima) y el Deltoides Posterior (49-51% señal electromiográfica de superfície 1 Repetición Máxima). La actividad electromiográfica de superficie fue similar entre los ejercicios E1, E2 y E3. CONCLUSIONES: este estudio cuantificó las demandas musculares durante los ejercicios pullover realizados con un 60% de la Repetición Máxima. Los músculos con mayor nivel de activación fueron el Deltoides Posterior e Infraespinoso, lo que sugiere que el pullover puede ser una opción válida para fortalecer los músculos estabilizadores dinámicos de la articulación del hombro en individuos entrenados. No se observaron diferencias significativas en el nivel de la activación muscular cuando se modificó la distancia del agarre y la estabilización del tronco, lo que demuestra que estas condiciones no influyen en los niveles de activación muscular. Sin embargo, 1 Repetición Máxima fue menor cuando el pullover se realizó en una pelota suiza, lo que sugiere que es posible obtener un mayor nivel de reclutamiento muscular con pesos menores en ejercicios inestables