Leg muscle activity during tandem stance and the control of body balance in the frontal plane Stefania Sozzi a , Jean-Louis Honeine a,b , Manh-Cuong Do b , Marco Schieppati a,⇑ a Centro Studi Attività Motorie (CSAM), Fondazione Salvatore Maugeri (IRCCS) & Dipartimento di Sanità Pubblica e Neuroscienze, University of Pavia, Italy b Laboratoire CIAMS (Complexité, Innovations, Activités Motrices et Sportives), UFR STAPS, Université Paris-Sud, Paris, France article info Article history: Accepted 3 December 2012 Available online xxxx Keywords: Tandem stance Centre of pressure EMG Leg muscles Vision highlights  We studied the effect of tandem stance on muscle activity in tibialis anterior (TA), soleus (SOL) and per- oneus longus (PER) of both legs, and on balance control, with and without vision.  Motion of centre of feet pressure (CoP) was larger in the frontal compared to the sagittal plane and mus- cle activity was larger in the rear than front leg.  Soleus keeps the body upright by tonic activity, while peroneus and tibialis bursts contribute to the con- trol of balance in the frontal plane by reciprocal action. abstract Objectives: We investigated the pattern of activity of the tibialis anterior (TA), soleus (SOL) and peroneus longus (PER) muscles of both legs during tandem stance, in order to highlight their respective role in maintaining balance. Methods: Twelve young healthy subjects were asked to stand with feet in line for successive 15 s-epochs, on a dynamometric platform with (EO) and without (EC) vision. EMG was recorded from the six muscles simultaneously. Collected signals were displacement of the centre of feet pressure (CoP) and EMG. Vari- ables calculated for each recorded epoch were mean level, variability and distribution between legs of EMG, and cross-correlation between EMG and CoP traces and between EMG of homonymous muscles. Results: CoP motion was larger along the medio–lateral (M–L) than antero–posterior (A–P) axis, and larger with EC than EO particularly in the M–L axis. Muscle activity was larger in the rear than in the front leg, as expected, except for PER. Activity increased with the increase in M–L CoP oscillations, except for SOL, which was tonically active, both legs, regardless of the amplitude of the oscillations. Manipulating vision had no effect on the variability of the EMG for equal mean levels of activity, for any muscle. Cross-correlation between EMG of rear leg muscles and M–L CoP sway gave higher coefficients for TA and PER than SOL, and appropriate time-delays between TA or PER and CoP motion, indicating a role of these muscles in the control of M–L sway. Except for the tonically active SOL, the homonymous muscles of the two legs were active out-of-phase, indicating a mutual push–pull action of the pairs. This was confirmed by the reciprocal activation of TA and PER of the same leg. Conclusions: Overall, in spite of a large inter-trial and inter-subject variability, the neural command to the leg muscles during tandem stance implies a task-sharing rule, whereby SOL keeps the body upright while the reci- procal PER and TA activities produce the alternate impulses necessary for body stabilization in the frontal plane. Significance: Knowledge of the normal mode of control of balance in frontal plane can foster new investi- gation in both posture and gait control, in addition to offering tools for understanding balance problems of elderly persons and patients at risk of fall. Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. 1. Introduction Standing with feet in line along the sagittal plane is a demand- ing posture, both because the medio–lateral limits of stability are narrow (Goodworth and Peterka, 2010) and because the effort for balance control elicits a meaningful metabolic energy demand 1388-2457/$36.00 Ó 2012 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved. http://dx.doi.org/10.1016/j.clinph.2012.12.001 ⇑ Corresponding author. Address: CSAM, Fondazione Salvatore Maugeri, Via Salvatore Maugeri 10, I-27100 Pavia, Italy. Tel.: +39 0382 592008; fax: +39 0382 592081. E-mail addresses: marco.schieppati@unipv.it, marco.schieppati@fsm.it (M. Schieppati). Clinical Neurophysiology xxx (2013) xxx–xxx Contents lists available at SciVerse ScienceDirect Clinical Neurophysiology journal homepage: www.elsevier.com/locate/clinph Please cite this article in press as: Sozzi S et al. Leg muscle activity during tandem stance and the control of body balance in the frontal plane. Clin Neu- rophysiol (2013), http://dx.doi.org/10.1016/j.clinph.2012.12.001