Ultrasound reveals negligible cocontraction during isometric plantar flexion and dorsiflexion despite the presence of antagonist electromyographic activity

Abstract

Because of the approximate linear relationship between muscle force and muscle activity, muscle forces are often estimated during maximal voluntary isometric contractions (MVICs) from torque and surface electromyography (sEMG) measurements. However, sEMG recordings from a target muscle may contain cross-talk originating from nearby muscles, which could lead to erroneous force estimates. Here we used ultrasound imaging to measure in vivo muscle fascicle length (Lf) changes and sEMG to measure muscle activity of the tibialis anterior, medial gastrocnemius, lateral gastrocnemius, and soleus muscles during ramp MVICs in plantar and dorsiflexion directions (n = 8). After correcting longitudinal Lf changes for ankle rotation, the antagonist Lf at peak antagonist root-mean-square (RMS) amplitude were significantly longer than the agonist Lf at this sEMG-matched level. On average, Lf shortened from resting length by 1.29 to 2.90 mm when muscles acted as agonists and lengthened from resting length by 0.43 to 1.16 mm when muscles acted as antagonists (depending on the muscle of interest). The lack of fascicle shortening when muscles acted as antagonists indicates that cocontraction was likely to be negligible, despite cocontraction as determined by sEMG of between 7 and 23% MVIC across all muscles. Different interelectrode distances (IEDs) over the plantar flexors revealed significantly higher antagonist RMS amplitudes for the 4-cm IEDs compared with the 2-cm IEDs, which further indicates that cross-talk was present. Consequently, investigators should be wary about performing agonist torque corrections for isometric plantar flexion and dorsiflexion based on the antagonist sEMG trace and predicted antagonist moment.

Quantification of muscle co-contraction using supersonic shear wave imaging

“…This result indicates that there was negligible active force generated by the LG muscle during isometric dorsiflexion, despite a measured increase in sEMG activity. This supports our previous assertions that the antagonist LG sEMG signal is primarily due to cross-talk contamination (Raiteri et al, 2015). However, our results also demonstrate that antagonist muscle force did increase during isometric dorsiflexion due to passive lengthening of the antagonist.…”

“…Three-dimensional position data of the foot and shank was collected at 120 Hz using a four-camera optoelectronic system (Natural Point Inc., OptiTrack, Tracking Tools) to quantify movement of the foot and shank during the contractions and passive rotations (Raiteri et al, 2015). Muscle activity of the LG was recorded with sEMG using two electrodes (8 mm recording diameter, Ag/AgCl, Covidien, Mansfield, MA) placed just distal to the most prominent muscle bulge in a bipolar configuration with an interelectrode spacing of 2 cm (centre to centre).…”

“…Therefore, it may be a useful technique for quantifying co-contraction during isometric contractions. In a recent report (Raiteri et al, 2015), we used conventional B-mode ultrasound imaging to show that there was negligible cocontraction of the each of the triceps surae muscles during maximal isometric dorsiflexion, despite recorded antagonist surface electromyography (sEMG) root-meansquare (RMS) amplitudes of 12-23% MVC. We concluded that the antagonist sEMG signal was primarily due to cross-talk, based largely on the observed lack of fascicle shortening.…”

Specialized properties of the triceps surae muscle‐tendon unit in professional ballet dancers

“…Plantar flexion torque during the ramped contractions was calculated by multiplying the load cell force by the perpendicular distance to the axis of joint rotation. As ankle rotation was minimal (4.0 ± 2.6° across groups), the recorded torques were not corrected for TA coactivation . Tendon force was calculated by dividing plantar flexion torque by the instantaneous tendon moment arm as derived from ankle joint angle and leg length using equations by Spoor et al …”

“…As ankle rotation was minimal (4.0 ± 2.6° across groups), the recorded torques were not corrected for TA coactivation. 33 Tendon force was calculated by dividing plantar flexion torque by the instantaneous tendon moment arm as derived from ankle joint angle and leg length using equations by Spoor et al 34 Displacement of the MTJs was measured relative to the skin marker by semiautomated tracking of the ultrasound videos in video-tracking software (Tracker Video Analysis and Modeling Tool V.4.62, Open Source Physics, Aptos, CA, USA). At each scan location, displacement was interpolated at 50 N intervals using a spline function and the valid trials were averaged.…”