Timing of muscle activation of the lower limbs can be modulated to maintain a constant pedaling cadence
Introduction
The primary goal of motor control research is to study motor skill variability to obtain insight into the central organization of the system to solve the problems of motor redundancy (Bernstein, 1967). One common way to characterize pedaling skill is to study muscle activation by measuring the surface EMG activity of the lower limb muscles (Hug and Dorel, 2007, Li, 2004, Raasch et al., 1997, Rouffet and Hautier, 2008).
Variations in the muscle activity have been widely analyzed during pedaling as researchers can easily modify the pedaling conditions by manipulating the control parameters of the movement and/or the environmental constraints. In general, manipulations of the pedaling conditions result into significant changes in the activation level of the lower limb muscles. For example, the activation level of the lower limb muscles varies when subjects are asked to optimize the energy expenditure/power rate ratio, to maximize pedaling smoothness, or to maximize the power they produce (Raasch and Zajac, 1999). Other works demonstrate that activation level of the lower limb muscles is influenced by changes in pedaling position (Brown et al., 1996, Chapman et al., 2007a, Chapman et al., 2007b, Hakansson and Hull, 2005), movement kinematics (Neptune and Herzog, 2000, Neptune et al., 2000, Ting et al., 1999), pedaling cadence (Neptune and Herzog, 1999, Neptune et al., 1997, Suzuki et al., 1982) or external load (Ericson et al., 1985, Ericson et al., 1986, MacIntosh et al., 2000). But, only small modifications of the timing of activation of the muscles are reported when pedaling conditions are manipulated (Chapman et al., 2007a, Chapman et al., 2007b, Takaishi et al., 1998). These results suggest that the different lower limb muscles are activated in fixed phases of the pedaling cycle, depending on their functional role (Neptune et al., 1997). However, the biomechanical function of bi-articular muscles may vary depending on environmental constraints of the movement (van Ingen Schenau, 1989) and such changes could be potentially evidenced by recording the EMG activity.
A simple way to modify the environmental constraints during cycling consists in modifying the relative contribution of muscular forces and non muscular forces. Some authors have previously manipulated surface incline or pedaling position to modify the relative influence of the non muscular forces (Baum and Li, 2003, Brown et al., 1996, Li and Caldwell, 1998). In this study, the relative influence of gravitational and inertial components of the non muscular forces (Kautz and Hull, 1993) was modified by suppressing the external resistance applied to the pedaling movement. It was hypothesized that maintaining a constant cadence while pedaling during an unloaded condition would require a modification of the biomechanical function of the bi-articular muscles that could be illustrated by changes in the timing of activation. EMG, biomechanical and physiological measurements have been combined to determine what are the changes in the functional role of the lower limb muscles induced by modifications of the relative contribution of muscular and non muscular forces during pedaling.
Section snippets
Subjects
Eleven male subjects (age: 24 ± 2 years; height: 176 ± 4 cm; body mass: 71.6 ± 9.9 kg) volunteered to participate in this experiment after reading and signing an informed consent form. All participants were students at the Sport Science Faculty and were physically well-trained (3–5 h per week of non-cycling specific aerobic activity). However, highly trained cyclists were not selected to avoid adaptations due to their specific training.
Experimental protocol
The subjects first performed a 10 min warm-up consisting of
Results
Our results confirm that the subjects modify the timing of activation of the lower limb muscles when pedaling during unloaded condition in order to adjust the forces applied to the pedals and to maintain a constant cadence. Typical EMG and forces data obtained from one subject at the different pedaling conditions are illustrated in Fig. 2.
Discussion
The main result of this study is that the subjects dramatically modified the timing of activation of RF and BF bi-articular muscles to maintain a constant cadence when pedaling during unloaded condition. Our results also showed that the level of activation of all bi-articular (RF, BF and GAS) and one mono-articular (TA) muscles were higher than those extrapolated to 0 W from the loaded conditions. In the following discussion, the pedaling cycle was decomposed in four quadrants (i.e. first
Conclusion
The analysis of the movement performed in this study shows that the timings of activation and the biomechanical function of RF and BF bi-articular muscles can be modified to counterbalance changes in the environmental constraints of the pedaling movement. However, mechanical effectiveness and metabolic efficiency criteria may not be satisfied simultaneously by modulating the activation of bi-articular muscles. Finally, our results demonstrate that approaches combining EMG and biomechanical
Acknowledgments
The authors would like to acknowledge Dr. Stéphane Champely and Dr. Antoine Couturier for their advice in this study and Dr. Muriel Bourdin for their material assistance that made this experiment possible. The authors also gratefully acknowledge the reviewers of this article for their constructive criticism and valuable suggestions.
David Rouffet received his Ph.D. degree in Physiology and Biomechanics from Lyon University, France, in 2007. He is a post-doctoral researcher at the Centre for Aging Rehabilitation and Exercise, Victoria University of Melbourne, Australia. His research deals with the influence of environmental constraints and control parameters of the movement on neuromuscular, biomechanical and metabolic responses during sport exercises and locomotion.
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David Rouffet received his Ph.D. degree in Physiology and Biomechanics from Lyon University, France, in 2007. He is a post-doctoral researcher at the Centre for Aging Rehabilitation and Exercise, Victoria University of Melbourne, Australia. His research deals with the influence of environmental constraints and control parameters of the movement on neuromuscular, biomechanical and metabolic responses during sport exercises and locomotion.
Guillaume Mornieux received his Ph.D. degree in Biomechanics from Saint-Etienne University, France, in 2005. He is a research assistant at the Institute for Sports and Sport Sciences at the University of Freiburg, Germany. His research deals with the evaluation of the biomechanical and neuromuscular responses associated with different environmental constraints during sport exercises.
Karim Zameziati received his Ph.D. degree in Biomechanics from Saint-Etienne University, France, in 2006. He is a research assistant at the Saint-Etienne University, France. His research deals with the evaluation of the biomechanical and metabolic responses constraints during sport exercises.
Alain Belli received his Biomedical engineering diploma in 1986 from the Compiègne Technology University, France. He received Ph.D. degree in Exercise Biology from Saint-Etienne University, France, in 1992. He is a Professor in Biomechanics at University of Saint Etienne since September 1999. He is also the president of the French Economical Cluster for Sport and Leisure Industry. His research mainly deals with the evaluation of the biomechanical and physiologic responses associated with different material and equipment during sport exercises.
Christophe A. Hautier received his Ph.D. degree in exercise physiology from Saint-Etienne University, France, in 1996. He is an assistant professor (MCU) at the University Claude Bernard (Lyon 1) and is working in the Center of Research and Innovation on Sport (Faculty of Sport Sciences). His research topics concern determinants of maximal power in human movements and cycling performance.