Abstract
The aim of this study was to determine how muscle activity and body orientation contribute to the triceps surae spinal transmission modulation, when moving from a sitting to a standing position. Maximal Hoffmann-reflex (Hmax) and motor potential (Mmax) were evoked in the soleus (SOL), medial and lateral gastrocnemius in 10 male subjects and in three conditions, passive sitting, active sitting and upright standing, with the same SOL activity in active sitting and upright standing. Moreover volitional wave (V) was evoked in the two active conditions (i.e., active sitting and upright standing). The results showed that SOL Hmax/Mmax was lower in active sitting than in passive sitting, while for the gastrocnemii it was not significantly altered. For the three plantar flexors, Hmax/Mmax was lower in upright standing than in active sitting, whereas V/Mmax was not modulated. SOL H-reflex is therefore affected by the increase in muscle activity and change in body orientation, while, in the gastrocnemii, it was only affected by a change in posture. In conclusion, passing from a sitting to a standing position affects the Hmax/Mmax of the whole triceps surae, but the mechanisms responsible for this change differ among the synergist muscles. The V/Mmax does not change when upright stance is assumed. This means that the increased inhibitory activity in orthostatic position is compensated by an increased excitatory inflow to the α-motoneurons of central and/or peripheral origin.
Highlights
THE MONOSYNAPTIC HOFFMANN REFLEX (H-reflex) has been extensively used to investigate the transmission efficiency of group Ia projection onto the ␣-motoneuron pool
If we consider 1) that the density of muscle spindles is more than two times higher in SOL than in gastrocnemii (i.e., SOL receives greater spindle feedback) (Tucker and Turker 2004; Voss 1971); 2) that the primary afferents principally depolarize slow-twitch ␣-motoneurons (Koerber and Mendell 1991; LevTov 1987), the proportion of which is higher in SOL than in gastrocnemii, and 3) that presynaptic inhibition could be differently organized in gastrocnemii than in SOL (Nielsen and Kagamihara 1993), it can be expected that SOL and gastrocnemii H-reflexes are differently modulated by changes in posture and muscle activity
The target torque developed in the sitting position, when subjects reproduced the SOL EMG-root mean square (RMS) activity recorded in upright standing (US), was 28.2 Ϯ 10.5 N·m, (i.e., 11.9 Ϯ 4.7% of the plantar flexors (PFs) maximum voluntary contraction (MVC))
Summary
THE MONOSYNAPTIC HOFFMANN REFLEX (H-reflex) has been extensively used to investigate the transmission efficiency of group Ia projection onto the ␣-motoneuron pool. Several investigations have shown that the H-reflex in the plantar flexors (PFs) is downregulated when subjects are in a quiet standing condition compared with sitting (Katz et al 1988; Kawashima et al 2003) or to supine (Chalmers and Knutzen 2002) and prone positions (Angulo-Kinzler et al 1998; Bove et al 2006; Koceja et al 1993, 1995) This modulation could be the consequence of a change in body position and/or a change in the background activity of the muscles involved in the postural task. If we consider 1) that the density of muscle spindles is more than two times higher in SOL than in gastrocnemii (i.e., SOL receives greater spindle feedback) (Tucker and Turker 2004; Voss 1971); 2) that the primary afferents principally depolarize slow-twitch ␣-motoneurons (Koerber and Mendell 1991; LevTov 1987), the proportion of which is higher in SOL than in gastrocnemii, and 3) that presynaptic inhibition could be differently organized in gastrocnemii than in SOL (Nielsen and Kagamihara 1993), it can be expected that SOL and gastrocnemii H-reflexes are differently modulated by changes in posture and muscle activity
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