Postural challenge affects motor cortical activity in young and old adults

Abstract

When humans voluntarily activate a muscle, intracortical inhibition decreases. Such a decrease also occurs in the presence of a postural challenge and more so with increasing age. Here, we examined age-related changes in motor cortical activity during postural and non-postural contractions with varying levels of postural challenge. Fourteen young (age 22) and twelve old adults (age 70) performed three conditions: (1) voluntary contraction of the soleus muscle in sitting and (2) leaning forward while standing with and (3) without being supported. Subthreshold transcranial magnetic stimulation was applied to the soleus motor area suppressing ongoing EMG, as an index of motor cortical activity. The area of EMG suppression was~60% smaller (p b 0.05) in unsupported vs. supported leaning and sitting, with no difference between these latter two conditions (p N 0.05). Even though in absolute terms young compared with old adults leaned farther (p = 0.018), there was no age effect or an age by condition interaction in EMG suppression. Leaning closer to the maximum without support correlated with less EMG suppression (rho = −0.44, p = 0.034). We conclude that the critical factor in modulating motor cortical activity was postural challenge and not contraction aim or posture. Age did not affect the motor control strategy as quantified by the modulation of motor cortical activity, but the modulation appeared at a lower task difficulty with increasing age.

The use of transcranial magnetic stimulation to evaluate cortical excitability of lower limb musculature: Challenges and opportunities

“…The double-cone coil has been used most frequently (Jayaram & Stinear, 2009; Madhavan, Rogers, & Stinear, 2010; Papegaaij, Baudry, et al, 2016; Papegaaij, Taube, Hogenhout, et al, 2014; Papegaaij, Taube, et al, 2016). Other studies have used the batwing coil (Barthelemy et al, 2011; Needle, Palmer, Kesar, Binder-Macleod, & Swanik, 2013; Palmer, Hsiao, Awad, & Binder-Macleod, 2016; Palmer, Hsiao, Wright, & Binder-Macleod, 2017; Palmer, Needle, Pohlig, & Binder-Macleod, 2016; Palmer, Zarzycki, Morton, Kesar, & Binder-Macleod, 2017) and flat figure of eight coil (Smith et al, 2017).…”

“…Studying the effects of rehabilitation programs on neurophysiologic and neuro-imaging metrics post-stroke may provide innovative insights about LE rehabilitation mechanisms. Interesting perspectives about LE TMS may also be gained by evaluating how the suppression of ongoing EMG in response to sub-threshold TMS (Papegaaij, Taube, et al, 2016) or activation-related modulation of intra-cortical inhibition (Papegaaij, Baudry, et al, 2016) vary across different LE muscles of the contra- and ipsilateral leg.…”

“…For example, short-interval intra-cortical inhibition of the soleus muscle was greater during supported standing (Papegaaij, Baudry, et al, 2016) and during standing on a foam rather than a rigid surface (Papegaaij, Taube, Hogenhout, et al, 2014). Subthreshold TMS-induced EMG suppression of ongoing soleus EMG activity, purported to be mediated through modulation of intra-cortical circuitry, was greater during an unsupported forward leaning postural task (Papegaaij, Taube, et al, 2016). When compared to TMS recordings in the seated posture, LE TMS data recorded during standing, challenging postural tasks, and walking may provide more sensitive indices of neuroplasticity.…”

“…Cortical centers help modify the motor plan to avoid obstacles, change direction and speed, respond to perturbations, and enable cognitive or motor dual-tasks during locomotion (Barthelemy et al, 2011; Charalambous, Bowden, & Adkins, 2016a; Rothwell, 1997). In addition to the increasing evidence for the role of M1 in the control of posture and locomotion (Papegaaij, Baudry, Negyesi, Taube, & Hortobagyi, 2016; Papegaaij, Taube, Baudry, Otten, & Hortobagyi, 2014; Papegaaij, Taube, Hogenhout, Baudry, & Hortobagyi, 2014; Papegaaij, Taube, et al, 2016), other descending pathways such as reticulo-spinal and vestibulo-spinal tracts are relevant to lower limb motor control, and merit more investigation (Prentice & Drew, 2001; Ting et al, 2015). Additionally, fast and slow conducting pyramidal neurons may play a differential task-specific role during LE movements (Stout & Beloozerova, 2013).…”

Effects of posture and coactivation on corticomotor excitability of ankle muscles

“…TMS-evoked MEP amplitudes could be influenced by modulation of excitability within the cortex (volitional drive, intra-cortical inhibition/facilitation), descending pathways (reticulospinal pathway activation, pre-synaptic inhibition), as well as excitability of spinal interneurons and lower motoneurons. In an attempt to identify the specific site and mechanism underlying the effects of posture and coactivation, future studies may evaluate the effects of age (Papegaaij, Taube, Baudry, Otten, & Hortobagyi, 2014), postural challenge (Papegaaij et al, 2016), and pathology, as well as additional measures such as cortical silent period and MEP duration (Brum, Cabib, & Valls-Sole, 2015). …”