Force fluctuations during steady plantar flexion and dorsiflexion in young adults.

The aim of the study was to evaluate the difference on force fluctuations during isometric plantar flexion (PF) and dorsiflexion (DF) and the potential influence of changes in ankle joint angle. Twelve young adults performed 2 maximal voluntary contractions (MVC) and 3 steady submaximal isometric contractions at three ankle angles (70o, 90o and 110o) and five target forces (5, 10, 20, 40 and 60% MVC) with both the plantar flexors and dorsiflexors. The electromyographic (EMG) activity of Medial Gastrocnemius (MGAS), Soleus (SOL) and Tibialis Anterior (TA) was recorded. MVC force was greater during PF compared to DF for all ankle angles (p > 0.05). The normalized amplitude (coefficient of variation, CoV) of the force fluctuations was similar between PF and DF. However, the absolute amplitude (standard deviation) was higher during PF compared to DF, regardless of ankle angle and target force (p < 0.05). The CoV for force exhibited a gradual decrease from 5% to 20% MVC, and then it plateaued at 40% and 60% MVC force. During plantar flexion, the normalized EMG amplitude (root mean square) of the agonists MGAS and SOL increased progressively from 5% to 60% MVC force (p > 0.05), as the same happened with the TA during DF. These findings indicate that muscle groups with large differences in maximal force production are likely to have the same force steadiness during submaximal contractions, apparently because the neural activation of agonist and antagonist muscles has common features. [ABSTRACT FROM AUTHOR]