Is spinal excitability of the triceps surae mainly affected by muscle

27 The aim of this study was to determine how muscle activity and body orientation contribute to 28 the triceps surae spinal transmission modulation, when moving from a sitting to a standing 29 position. Maximal H-reflex (Hmax) and motor potential (Mmax) were evoked in the soleus 30 (SOL), medial and lateral gastrocnemius in 10 male subjects and in three conditions: passive 31 sitting, upright standing and active sitting, with the same SOL activity in active sitting and 32 upright standing. Moreover Volitional wave (V-wave) was evoked in the two active 33 conditions (i.e. upright standing and active sitting). The results showed that SOL Hmax/Mmax 34 was lower in active sitting than in passive sitting, while for the gastrocnemii it was not 35 significantly altered. For the three plantar flexors, Hmax/Mmax was lower in upright standing 36 than in active sitting whereas V/Mmax was not modulated. SOL H-reflex is therefore affected 37 by the increase in muscle activity and change in body orientation, while in the gastrocnemii, it 38 was only affected by a change in posture. In conclusion, passing from a sitting to a standing 39 position affects the Hmax/Mmax of the whole triceps surae, but the mechanisms responsible for 40 this change differ among the synergist muscles. The V/Mmax does not change when upright 41 stance is assumed. This means that the increased inhibitory activity in orthostatic position is 42 compensated by an increased excitatory inflow to the α-motoneurons of central and/or 43 peripheral origin. 44 45 46 47