Knee angle-specific MVIC for triceps surae EMG signal normalization in weight and non weight-bearing conditions
Introduction
It is standard in research to express electromyography (EMG) signals as a percentage of a reference value to quantify the relative level of muscle activity. One of the most common methods used in science is to normalize EMG signals to those associated with a maximal voluntary isometric contraction (MVIC). Such normalization reduces the between- and within-subject variability of EMG recordings, improves the accuracy and reproducibility of results, and allows valid and reliable comparison of data from different individuals and experimental conditions (Burden, 2010).
At the same time, several factors must be considered to optimize the amplitude, validity and specificity of EMG signals collected during an MVIC for normalization. These factors include the time of day (Castaingts et al., 2004, Guette et al., 2005), rate of force development (Ricard et al., 2005), lateralization (Ball and Scurr, 2011) and body position (Carlsson et al., 2001). One key factor is joint angles because these can significantly influence the amplitudes of EMG signals from an MVIC, as often reported in the case of the triceps surae muscles (Arampatzis et al., 2006, Arndt et al., 1998, Carlsson et al., 2001, Signorile et al., 2002).
Although these ankle plantar-flexors all insert distally into the posterior aspect of the calcaneus through the Achilles tendon, the muscles comprising the triceps surae group are considered to be anatomically and functionally distinct. For instance, the soleus is mono-articular and plays a role predominantly in postural stability and horizontal propulsion during gait; whereas the gastrocnemius is bi-articular, possesses medial and lateral heads, and is important in the generation and transmission of forces and power (Jacobs et al., 1996, Neptune et al., 2001). Therefore, selective MVIC procedures based primarily on different knee-joint angles, body positions and loading patterns have been proposed for the acquisition of normalization signals from the soleus (SOL), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL) muscles.
All methods proposed for collecting MVIC signals from the triceps surae for normalization have specific advantages and drawbacks with respect to the level of muscle selectivity and signal amplitude. Hébert-Losier et al. (2011) reported that the amplitudes of SOL, GM and GL surface EMG signals during unilateral plantar-flexion MVICs were greater in weight-bearing and 0° or 45° of knee flexion (KF) than in non weight-bearing and 90° of KF, for all three triceps surae muscles. Overall, their findings support the use of both 0° and 45° of KF during collection of plantar-flexion MVIC for normalization of EMG signals from the triceps surae, while emphasizing that no single knee position is specific or selective to the respective maximal activation of the SOL or gastrocnemius muscles.
Since the late 1960s, isokinetic dynamometers demonstrated their practical value, utility and effectiveness in connection with muscle rehabilitation, training and testing in both research and clinical settings (Baltzopoulos and Brodie, 1989, Kraemer et al., 2006). Dynamometric investigations have improved our understanding of neuromuscular function (Duclay et al., 2009), adaptations to interventions (Alfredson et al., 1998b, Horstmann et al., 2012) and fluctuations in muscle performance as a function of time of day (Castaingts et al., 2004, Guette et al., 2005). In isometric, isokinetic or isotonic-based research studies, dynamometers are commonly employed to capture EMG signals during plantar-flexion MVIC trials for signal normalization of the triceps surae muscle group (Cresswell et al., 1995, Gerdle and Fugl-Meyer, 1992, Hubley-Kozey and Earl, 2000, Kay and Blazevich, 2009, Pinniger, 2003). This approach minimizes participant displacement and is convenient and time-efficient in laboratory settings, but does not necessarily provide the maximal activation of each individual triceps surae muscle (Ball and Scurr, 2010, Carlsson et al., 2001).
In fact, more pronounced activation of the triceps surae during plantar-flexion contractions has been observed in connection with a weight-bearing (WB) standing position than a non weight-bearing (NWB) sitting or supine position (Carlsson et al., 2001, Perry et al., 1981). Consequently, unless configuration of the isokinetic dynamometer permits positioning in an upright stance, the traditional approach use to collect plantar-flexion MVIC values for EMG signal normalization using isokinetic devices may not elicit the same level of triceps surae muscle activity as the unipedal standing heel-raise MVIC method. Clearly, the greatest EMG signal recorded during an MVIC from any given muscle should be utilized in subsequent EMG signal normalization (Winter, 1991).
The present investigation compares the surface EMG recordings from the three triceps surae muscles during WB and NWB plantar-flexion MVICs at both 0° and 45° of KF. The primary purpose was to determine the condition that elicited the greatest signal during plantar-flexion MVICs from SOL, GM and GL, respectively, for EMG signal normalization. Our hypotheses were that WB would elicit the greatest EMG signal more frequently than NWB from all muscles; and that no single knee position would specifically or consistently capture peak SOL, GM or GL activity. However, on the basis of a previous investigation (Hébert-Losier et al., 2011), we speculated that 0° of KF would elicit the greatest EMG signal amplitudes from the GM and GL in approximately 70% of the study group, and the greatest amplitude from the SOL in about 40%. A secondary aim was to document the plantar-flexion torque and force values associated with the various MVIC trials. Because of the positive relationship between generation of force and muscle length (Arndt et al., 1998), it was assumed that 0° of KF would elicit higher plantar-flexion output than 45° both with and without weight-bearing.
Section snippets
Experimental design
This study used a repeated-measures design (Fig. 1) that required each participant to attend a single experimental session at the muscle performance laboratory of the Swedish Winter Sports Research Centre. The project was pre-approved by the Regional Ethical Review Board (2011-385-31M, Umeå, Sweden) and adhered to the latest amendment of the Declaration of Helsinki.
Participants
After providing verbal and written informed consent, 54 volunteers purposefully recruited to balance the cohort with respect to sex
Results
The highest rms amplitude elicited from each triceps surae muscle under each experimental condition is summarized in Table 1. The number of times that each experimental condition evoked the highest EMG signal from the GM, GL and SOL muscles is also documented in Table 1. The findings specific to WB and NWB trials, and to 0° and 45° of KF, are presented in Table 2.
The highest EMG signals were elicited from the GM and GL muscles ∼40% of the time during WB0°, which was significantly more often
Discussion
Our hypothesis that weight-bearing would elicit peak triceps surae muscle activity more often than non weight-bearing was confirmed for the soleus muscle (65% versus 35%), but not for the gastrocnemius medialis (56% versus 44%) and lateralis (50% versus 50%) muscles. As expected, neither 0° nor 45° of KF consistently elicited the peak EMG signals from the GM, GL or SOL muscles during an MVIC. The observation of peak GM and GL activities at 0° of KF in approximately 80% of our study population
Conclusions
In light of the fact that the EMG signal from the SOL muscle during plantar-flexion MVIC trials was greater here with than without weight-bearing, we recommend that the former condition be used more routinely to collect EMG signals from the SOL muscle for normalization. However, in connection with electromyographic studies on the gastrocnemius, plantar-flexion MVIC trials may not elicit the greatest signal amplitude if performed only with or without weight-bearing, rather than both. In any
Acknowledgements
The authors acknowledge the research assistance from Sarah Willis, MSc, and Maria Hansson, PT, during the data collection. The authors would also like to thank all participants who volunteered to take part in this study. There are no relevant conflicts of interest to declare and no source of funding was necessary for the preparation of this paper.
Kim Hébert-Losier received a BSc degree in Physiotherapy (Hons) in Canada from the Université de Montréal in 2006 and worked as a physiotherapist in musculoskeletal and sports until 2008. In 2011, she obtained her PhD from the University of Otago working under the Sports and Exercise Research Group at the Centre for Physiotherapy Research in New Zealand. She is currently a post doctoral researcher for the Swedish Winter Sports Research Centre at the Mid Sweden University. Her fields of research
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Kim Hébert-Losier received a BSc degree in Physiotherapy (Hons) in Canada from the Université de Montréal in 2006 and worked as a physiotherapist in musculoskeletal and sports until 2008. In 2011, she obtained her PhD from the University of Otago working under the Sports and Exercise Research Group at the Centre for Physiotherapy Research in New Zealand. She is currently a post doctoral researcher for the Swedish Winter Sports Research Centre at the Mid Sweden University. Her fields of research interest include muscle function, 3D motion analysis, sports performance, injury prevention and rehabilitation of musculoskeletal injuries, with a particular focus on the lower-extremity.
Hans-Christer Holmberg completed his Naprapathy education in Stockholm, Sweden, and worked for several years with elite level athletes. In 2005, he obtained his PhD from Karolinska Institutet, Sweden, studying the physiology of cross-country skiing. He is currently Professor at the Department of Health Sciences within Mid Sweden University and Director of the Swedish Winter Sports Research Centre. His research interests include human movement science and the physiology of exercise, with a special interest in elite level sports.