Bilateral activation of the abdominal muscles induces longer reaction time
Introduction
For muscles of the limbs, reaction time is longer during bilateral activation when compared with unilateral activation (Kelso et al., 1979; Kerr et al., 1963; Ohtsuki, 1981). Similarly, maximal voluntary strength is reduced during bilateral contractions compared to unilateral contractions (Oda and Moritani, 1994; Ohtsuki, 1983). This is known as bilateral deficit. Two mechanisms have been proposed (Ohtsuki, 1994). The first argues that there is greater demand for attention during concurrent performance of two movements when compared with the demand during performance of individual movements. The second argues that the effect is due to crossed-inhibition between the motor cortices. In addition, recent studies have demonstrated that reductions in strength during bilateral movements may also be associated with differences in muscle fibre shortening velocities rather than differences in neural drive (Bobbert et al., 2006).
Increase in demand for attention during bilateral movements has been suggested to increase processing time leading to slower/reduced activation. However, simultaneous contractions of non-homologous muscles at anatomically disparate sites do not lead to reductions in strength during maximum voluntary efforts (Howard and Enoka, 1991). Furthermore, investigations of markers of attention demand using electroencephalography failed to identify differences between unilateral and bilateral limb movements (Taniguchi et al., 2001). These results suggest that at least for limb muscles, increased attention demand can not explain for bilateral deficit, and that the alternative mechanism of interhemispheric inhibition between cortical areas that control the limbs is the most likely explanation.
It is argued that during bilateral movements, when both the left and right motor cortices are activated, there is inhibition between hemispheres, leading to increased reaction time (Ohtsuki, 1994). Studies using paired transcranial magnetic stimulation have demonstrated that stimulation to one cortex leads to inhibition of the response in the distal limb muscles from stimulation of the opposite motor cortex (Di Lazzaro et al., 1999; Ferbert et al., 1992). Individuals with agenesis or surgical resection of the corpus callosum do not demonstrate this inhibition (Rothwell et al., 1991) and have similar reaction times between unilateral and bilateral activation (Aglioti et al., 1993; Gazzaniga and Sperry, 1966). Thus it is argued that transcallosal pathways are likely to be responsible for bilateral deficit. As muscles of the limbs are controlled almost solely by contralateral corticospinal pathways, inhibition of the opposite motor cortex through transcallosal pathways is thought to restrict movement to the target limb (Di Stefano et al., 1980).
Studies of bilateral deficit and its mechanisms have focused predominantly on muscles of the limbs. However, the organisation of control of more proximal muscles in the central nervous system (CNS) differs from that of the distal limb muscles. Apart from voluntary efforts, the axial muscles are also involved in postural control when internal and external forces perturb the body, such as those associated with voluntary movements (Belen’kii et al., 1967; Bouisset and Zattara, 1981; Hodges and Richardson, 1997). Notably, studies have shown that activation of the axial muscles during postural tasks can vary depending on specific demands of individual tasks (Horak et al., 1984; Lee et al., 1987; Zattara and Bouisset, 1986). During voluntary contractions, the axial muscles receive a greater bilateral drive compared to the distal muscles of the limbs (Carr et al., 1994; Marsden et al., 1999; Muellbacher et al., 2001). This drive involves corticospinal projections from the motor cortex (Berardelli et al., 1991; Pearce et al., 2003; Plassman and Gandevia, 1989) and includes a greater proportion of ipsilateral corticospinal projections than the distal limb muscles (Berardelli et al., 1991; Gooden et al., 1999; Quartarone et al., 1999; Strutton et al., 2004). The bilateral distribution of projections to axial muscles is consistent with the greater propensity for bilateral contractions. This raises the question whether bilateral deficit is present for these muscles. Notably, it has been suggested that bilateral deficit may be absent in proximal limb muscles (Anson and Bird, 1993). This study examined the reaction time of the axial muscles (abdominal muscles) during unilateral and bilateral voluntary contractions.
Section snippets
Participants
Eight healthy right-handed individuals (27 ± 6 (mean ± SD) years, 5 females, 3 males) were recruited for this study. Hand dominance was determined using the Edinburgh Handedness Inventory (Oldfield, 1971). Exclusion criteria included any circulatory, orthopaedic, neurological or respiratory conditions, recent or current pregnancies, previous surgery to abdomen or back, hearing deficits, or performance of regular abdominal or back exercises in the past 12 months. The Institutional Medical Research
Results
Fig. 2 illustrates the reaction time of all abdominal muscles during bilateral or unilateral movements for both simple and choice reaction time tasks. There was a main effect for Task (F = 328.9, p < 0.001) and Activation (F = 141.5, p < 0.001), and a significant interaction between these two factors (F = 4.9, p = 0.035). Post hoc analysis revealed that the reaction time of the abdominal muscles during unilateral contractions was significantly faster than during bilateral efforts during both simple and
Discussion
The abdominal muscles are commonly activated bilaterally during functional tasks and involve descending corticospinal pathways that include a greater proportion of ipsilateral projections than those of the limb muscles (Strutton et al., 2004). Thus it was surprising to find that bilateral contraction increases the reaction time of abdominal muscle. These findings demonstrate that bilateral activation of muscles which are organised and controlled through a descending motor system that is
Acknowledgement
HT and PH were funded by the National Health and Medical Research Council of Australia.
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