Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C13
Neuromechanical behaviour of synergistic muscles during prolonged human walking
N. Cronin1, J. Peltonen2, T. Sinkjaer3, J. Avela2
1. School of Physiotherapy & Exercise Science, Griffith University, Gold Coast, Queensland, Australia. 2. Department of Biology of Physical Activity, University of Jyv?skyl?, Jyv?skyl?, Finland. 3. Department of Health Science and Technology, Aalborg University, Aalborg, Denmark.
During unconstrained human walking, the muscle activation strategy is assumed to remain approximately constant across consecutive steps when walking for a short time, but it is unknown whether this strategy is maintained over a longer walking duration. In a previous study, we reported evidence of an increase in tendinous tissue compliance after a prolonged walking protocol (1). This could influence activation patterns of individual muscles, thus altering the activation strategy adopted by the central nervous system (see also references 2 and 3). In this study, we sought to investigate the hypothesis that muscle activity may be up- or down-regulated in individual triceps surae muscles in response to a relatively low intensity, prolonged period of walking. Thirteen healthy subjects walked on a treadmill for 60 minutes at 4.5 km/h. Soleus (Sol) and medial gastrocnemius (MG) muscle activity, ankle and knee joint kinematics and maximal muscle activation (M-max) were recorded at 5 minute intervals, resulting in the collection of 13 data sets per subject. Muscle activity data were expressed relative to M-max measured within the same 5 minute interval. During the first and last measurement intervals (0 and 60 minutes, respectively), ultrasound data were collected to enable fascicle length to be estimated. Throughout the walking protocol, M-max amplitude varied considerably in both muscles, with a general decrease being apparent in MG and an increase in Sol. After normalising the muscle activity data to M-max, Sol activation increased by 9.3±0.2% (RM ANOVA: F1,13=2.941; P<0.05) and MG activation decreased by 9.3±0.3% (F1,13=2.349; P<0.01) after 60 minutes of walking. No consistent changes were observed in fascicle length in MG (Paired t-test: decreased by 5.6±4.9%; P = 0.424) or Sol (increased by 2.6±9.5%; P = 0.502). When failing to take into account non-physiological changes in the electromyographic signal (e.g. due to geometric artefact), the data suggest that muscle activity decreased in Sol and MG. However, when normalising the data to M-max measured during walking, MG activity decreased, and this may have been compensated by a corresponding increase in Sol activation. These data show that muscle activity may be up- or down-regulated in individual triceps surae muscles after prolonged walking, thus highlighting the ability of the central nervous system to maintain a relatively constant movement pattern in spite of neural changes in individual muscles.
Where applicable, experiments conform with Society ethical requirements