ELECTROMYOGRAPHIC RESPONSES OF THE HUMAN TRICEPS SURAE AND FORCE TREMOR DURING SUSTAINED SUBMAXIMAL ISOMETRIC PLANTAR FLEXION

The objective was to investigate electromyographic activity (EMG) and isometric force tremor (IFT) changes during a sustained sub-maximal is ometric contraction in two muscles acting upon the same joint but diff ering in muscle fibre composition. Surface and intra-muscular EMG acti vity from the gastrocnemius and soleus muscles and IFT were recorded d uring an exhausting isometric plantar flexion (30% of maximal voluntar y contraction). Surface EMG amplitude (RMS) of both gastrocnemius and soleus muscles increased significantly over time. Gastrocnemius EMG RM S increased in a non-linear fashion while soleus EMG RMS increased lin early. A significant linear decrease of surface EMG mean power frequen cy (MPF) was observed over time for both muscles. The decrease in gast rocnemius MPF was significantly greater than that for soleus. Intra-mu scular EMG results showed similar trends. Correlations of intramuscula r EMG RMS and MPF with time were, however, characterized by lower corr elation coefficients than those from the surface EMG. Isometric force tremor RMS significantly increased non-linearly with duration of contr action, while IFT MPF showed a significant linear decrease with time. Changes in surface EMG RMS were correlated to changes seen in IFT RMS, in particular, for the predominantly fast twitch gastrocnemius muscle . Correlation coefficients of surface EMG MPF and IFT MPF were lower t han RMS correlations. The associated changes in IFT and EMG with fatig ue indicate alterations in motor unit firing rate, recruitment and syn chronization. The muscle specificity of the EMG and IFT changes sugges ts a coupling to muscle fibre type composition, although differences i n the relative force contribution of each muscle could also affect the results.