Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus Débora Cantergi a,⇑, Jefferson Fagundes Loss a, Azim Jinha b, Guilherme Auler Brodt a,c, Walter Herzog b a
Universidade Federal do Rio Grande do Sul, Exercise Research Laboratory, School of Physical Education, Brazil University of Calgary, Human Performance Laboratory, School of Kinesiology, Canada c Universidade de Caxias do Sul, Health Science Center, Brazil b
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 26 August 2014 Accepted 31 August 2014 Available online xxxx Keywords: Pilates Optimization
a b s t r a c t Considering the kinematics of leg extensions performed on a Reformer apparatus, one would expect high activation of hip and knee extensor muscle groups. However, because of the bi-articular nature of some lower limb muscles, and the possibility to vary the direction of force application on the Reformer bar, muscles can be coordinated theoretically in a variety of ways and still achieve the desired outcome. Hence, the aim of this study was to determine the knee and hip moments during leg extensions performed on the Reformer apparatus and to estimate the forces in individual muscles crossing these joints using static optimization. Fifteen subjects performed leg extensions exercises on the Reformer apparatus using an individually chosen resistance. To our big surprise, we found that subjects performed the exercise using two conceptually different strategies (i) the first group used simultaneous hip and knee extension moments, (ii) while the second group used simultaneous hip flexion and knee extension moments to perform the exercise. These different strategies were achieved by changing the direction of the resultant force applied by the subject’s feet on the Reformer bar. While leg extensions on the Reformer apparatus have been thought to strengthen the hip and knee extensors muscles, our results demonstrate that patients can perform the exercise in a different and unexpected way. In order to control the hip and knee moments and achieve the desired outcome of the exercise, the direction of force application on the Reformer bar must be controlled carefully. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Pilates exercising has been used for a long time in the dance community because of its perceived effect on accelerating rehabilitation from injury and return to activity (Anderson and Spector, 2005; La Touche et al., 2008; Self et al., 1996). Recently, Pilates has also become an accepted exercise method for the general public, with a rapidly growing number of participants (Blum, 2002; Rydeard et al., 2006). Pilates as a rehabilitation tool has been treated cautiously with calls for research evaluating its effects on the body (Brodt et al., 2014; Gagnon et al., 2001; La Touche et al., 2008; Von Sperling de Souza and Vieira, 2006). Although there are previous studies on the biomechanics of Pilates, the majority of them were conducted in the past 20 years (Self et al., 1996; Silva et al., 2009; Queiroz et al., 2010; Melo et al., 2011). Self et al. (1996) compared the ballet movement, Demi Plié, performed in the standing position and on the Reformer ⇑ Corresponding author at: Federal University of Rio Grande do Sul, Physical Education School, Rua Felizardo, 750 – Porto Alegre, Rio Grande do Sul 90690-200, Brazil. Tel.: +55 51 3308 5822. E-mail address: [email protected] (D. Cantergi).
apparatus, and Silva et al. (2009) studied the external forces for hip extension exercises on the Cadillac apparatus. Muscle activities for Pilates exercises have also been quantified using electromyography (Queiroz et al., 2010) and inverse dynamics approaches aimed at quantifying the resultant hip joint moments and muscles forces (Melo et al., 2011). Leg extension exercises are commonly performed in strengthening activities, aimed at improving functionality in activities of daily living, such as rising from a chair or lifting a load from the floor. Strength training exercises, such as squatting, present a similar kinematics to the hip and knee as the extension exercise performed on a Reformer apparatus. Squats are performed by flexing and extending the hip and knee simultaneously, producing extensor moments at both joints throughout the exercise, with the extensor moments increasing during joint flexion and decreasing during joint extension phases (Wretenberg et al., 1993; Escamilla et al., 1998; Robertson et al., 2008). Considering the kinematic nature of the task (leg extension), it would be normal to expect a high activation of hip and knee extensor muscle groups. However, because of the bi-articular nature of some of the muscles involved (knee extensors that also function as hip flexors and hip extensors that also function as knee flexors),
http://dx.doi.org/10.1016/j.jelekin.2014.08.016 1050-6411/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
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D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx
and the possibility to change the direction of force application on the Reformer apparatus, the contribution of muscles to the leg extension exercise is not necessarily obvious. Previous studies on the Pilates method showed that small changes in body position or force direction can dramatically affect the resultant joint moments, and thus the muscle groups involved in the exercise (Sacco et al., 2005; Melo et al., 2011). Hence, the aim of this study was to characterize knee and hip moments during leg extensions performed on the Reformer apparatus and to estimate the forces in individual muscles crossing these joints. We measured the forces and movements of leg extensions on the Reformer apparatus, calculated the resultant hip, knee and ankle moments using an inverse dynamics approach, and estimated the individual muscle forces using non-linear constrained optimization (Crowninshield and Brand, 1981; Erdemir et al., 2007; Herzog, 1996; Tsirakos et al., 1997). We hypothesized, that depending on the technique used, leg extensions on the Reformer machine might primarily be performed using a hip extension or a knee extension strategy. 2. Methods Fifteen subjects (3 males; 12 females, mean weight of 62.3 ± 11.6 kg and mean height of 1.65 ± 0.07 m), aged between 20 and 50 years, gave written informed consent to participate in this study. Subjects were physically active and had no history of injury to the lower limbs or trunk. Besides being free of injuries, it was also an inclusion criterion that subjects were currently practicing Pilates, for at least six months, with a frequency of twice a week or higher. Kinematics and kinetics were obtained for leg extension exercises performed on a Reformer apparatus using the Pilates Method (Fig. 1). The movement was performed with the heels pressing against the foot bar and the feet positioned parallel to each other, in order to avoid contributions from the ankle extensor muscles to the leg extension exercise. Knee and hip motion during the exercise ranged from about 90° of flexion in both joint to zero degrees of flexion at the knee and 15° of flexion at the hip. The position of the bar relative to the subject is the distance between the mobile cart and the bar, which was adjusted so as to match the initial hip and knee angles for all subjects. Each subject performed ten leg extensions against the spring resistance of the Reformer apparatus. The sagittal plane leg extension forces were measured using an instrumented and calibrated foot bar. The foot bar consisted of four load cells, which were positioned in the extremities of the bar, measuring forces in the horizontal and vertical directions. From these measurements, the resultant force direction and magnitude could be calculated. More information on the instrumentation of the force bar can be found elsewhere (Brodt et al., 2014). Sagittal plane movements were measured using a video system (JVC GR-DVL 9800; 25 Hz) and reflective markers (15 mm of diameter) placed at the base of the fifth metatarsus, lateral malleolus, lateral condyle, greater trochanter, acromion process, and the mobile cart. The kinematics system used in this paper has been
tested and described elsewhere (Ribeiro e Loss, 2010), and the error propagation of the proximal reaction force (PRF) and proximal net moments (PNM), has been described. They found that the uncertainty for the proximal reaction force was, on average, 0.27 N and uncertainty for the proximal net moments, was on average 0.97 Nm. Also, the accuracy, bias and precision of the kinematics system were calculated, resulting in mean values of 1.7 mm, 0.5 mm, and 1.6 mm, respectively (Ribeiro and Loss, 2010). In the Reformer apparatus used for the study, up to five springs may be used for resistance. Prior to the data collection, subjects were asked to select their resistance setting for the exercise. The subjects chose the resistance individually in such a manner that it provided sufficient challenge but so that they still could perform the exercise and all required repetitions comfortably. After selecting a combination of springs, they performed two repetitions of the exercise in order to feel the load, while an experiment Pilates instructor checked the kinematics characteristics of the movement. A complete exercise cycle consisted of an extension and flexion of the leg. The force data were processed using a 4th order, low-pass, recursive Butterworth filter (MATLAB Signal Processing Toolbox). The cut-off frequency (5 Hz) was calculated using a residual analysis criterion (Winter, 2005). Leg angles and positions were obtained by digitizing the joint markers in the recorded video of each trial using software Dvideow (Figueroa et al., 2003). Joint velocities and accelerations were then obtained by differentiating the displacement–time data with respect to time once or twice, respectively. Resultant net joint moments at the knee and hip were calculated using an inverse dynamics approach (Andrews, 1974) and a link segment method, with inertial data obtained from regression equations given by Clauser (1969). Muscle forces were estimated using a nonlinear, constrained, static optimization approach and a cost function minimizing the cubed muscle stresses, as proposed by Crowninshield and Brand (1981) using MatlabÒ 7.0. Thirty-six muscle tendon units were used to represent the primary lower limb flexor/extensor muscles. Moment arms and physiological cross sectional areas were obtained from Pierrynowski (1982). 3. Results Joint moments were defined as positive for extension and negative for flexion. Resultant net moments at the hip and knee presented two distinct patterns between groups of subjects (Fig. 2): those whose knee and hip moments were essentially positive (extensor) for all (most) of the movement (subjects e–p) and those who had a substantial phase of negative (flexor) hip moments (subjects a–d). Independent of the strategy used for performing the leg extension exercise on the Reformer apparatus, the knee extensor muscles contributed the most to the exercise (Fig. 3). However, subjects using the hip and knee extension strategy used the hip extensor muscles secondarily with the hip flexors essentially silent (Fig. 3a), while subjects using the hip flexion and knee extension strategy used the hip flexors secondarily while the hip extensors remained essentially silent (Fig. 3b). 4. Discussion
Fig. 1. Starting and finishing position for the leg extension exercise.
This study was aimed at characterizing leg extension exercises performed on the Reformer apparatus. While performing the exercise, subjects pushed against the Reformer apparatus bar, generating a reaction force on the subject’s foot. This reaction force creates the primary net moments at the hip and knee joints. Depending on
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx
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Fig. 2. Mean hip ( ) and knee (- -) resultant moments normalized to the extension phase of the movement. We defined joint moments as positive for extension and negative for flexion. Each graph represents the mean value of ten repetitions of leg extension from one subject.
(a) mean muscle force prediction for 11 subjects
(b) mean muscle force prediction for 4 subjects
Fig. 3. Mean normalized force predictions for the muscles contributing to the leg extensor force for (a) subjects using the knee and hip extension strategy and for (b) subjects using the knee extension and hip flexion strategy.
the direction of the line of action of the reaction force, which can be controlled by the subjects, there are three possible ways for performing this exercise, depending on how the reaction force may be directed: (i) above the knee, creating knee flexor and hip extensor moments; (ii) between knee and hip, creating knee and hip extensor moments; or (iii) below the hip, creating knee extensor and hip flexor moments (Fig. 4). Although possible, the strategy with the line of action above the knee joint was not used by any of the subjects in this study. The line of action was kept mostly below the hip by four subjects (Fig. 2, a–d), and between the knee and hip for the majority (Fig. 2, e–p) of the subjects tested. A first attempt at possibly explaining the different strategies observed in this study was directed toward the subject’s physical characteristics, specifically, their thigh and lower limb segment lengths. Once the initial position of the bar relative to the subject had been chosen, lower limb segmental lengths might affect the direction of the reaction force and, consequently, the direction of the hip and knee moments. It may be speculated that someone taller or with longer limbs may keep the knees higher relative to the force bar, and this positioning might affect the direction of the force of the feet on the Reformer bar. However, we found no apparent relationship between the strategy chosen by subjects and their segmental leg lengths. Apparently, the strategy for performing leg
extensions was less related to physical characteristics but was more influenced by learning or choice on how to perform the exercise. Similar to the leg extensions performed on the Reformer apparatus, leg extension exercises are part of many other exercise regimes, for example squats. However, in contrast to squats, where the line of action of the resultant ground reaction force cannot vary much because of the stability required to remain standing during the exercise, leg extensions with the Reformer apparatus allow for great variation in the line of action of the reaction forces. Therefore, leg extensions in a Reformer apparatus can be performed using distinctly different strategies as outlined above, whereas such vastly differing strategies cannot be utilized in a squat exercise. Because of the tight restrictions on the direction of the ground reaction force in a squat exercise, squatting always involves a simultaneous hip and knee extensor moment (Escamilla et al., 2001), while, as discussed above, this is not true for leg extensions using the Reformer apparatus. In most subjects, it was observed that the vastus lateralis and vastus intermedius were predicted to contribute more than 25% of the total muscle force to the leg extension exercise on the Reformer apparatus. However, for many subjects (d, e, f, l, m, n, and o, Fig. 2), the resultant knee moment became flexor toward the very
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
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Fig. 4. The three possibilities that theoretically exist for performing leg extension exercises on the Reformer apparatus: (a) reaction force line of action is directed above the knee joint, and generates knee flexor and hip extensor resultant moments, (b) reaction force line of action is directed between the knee and hip joints, and generates knee and hip extensor resultant moments, and (c) reaction force line of action is directed below the hip joint, and generates knee extensor and hip flexor resultant moments.
end of leg extension, and the knee flexor muscles were predicted to contribute to that final leg extension. This behavior can readily be understood using Fig. 4, and imagining that toward the end of leg extension, the knee is at level or even below the foot rest of the Reformer apparatus and the line of action may easily be directed above the knee, thereby causing the knee flexor moment. For the four subjects using the knee extension/hip flexion strategy (Fig. 3b), it was observed that the rectus femoris was predicted to be the primary contributor to the leg extension exercise. This result makes perfect sense as the rectus femoris is the sole muscle that functions as a knee extensor and hip flexor. The role of the hamstring muscles is greatly decreased in this group compared to the knee extension/hip extension group (Fig. 3a), as the two joint hamstring muscles function as hip extensors and knee flexors, the exact opposite strategy than that used for leg extensions on the Reformer apparatus. Amarantini et al. (2010) studied squat exercises using an EMGand-optimization method for muscle force estimation. Not surprisingly, they found that the knee extensor muscles were the primary contributors to the squat exercise. This is in agreement with our findings of leg extension, which also involved the knee extensors to a great degree for both techniques chosen by the subjects in this study. In addition, there would have been the theoretical possibility to perform the leg extensions with a knee flexion/hip extension strategy (Fig. 4), but none of the subjects opted for this solution. Leg extensions on the Reformer apparatus are supposed to strengthen the same muscle groups as in the squat exercise. The major difference is the load. In squat exercises, the lowest possible resistance is the subject’s weight, while during leg extension, the subject’s weight is not an important factor and subjects can adjust the resistance by increasing the stiffness of springs, thus allowing weak individuals or individuals in rehabilitation, to perform leg extensor strengthening in a gentle manner that would not be possible with squat exercises. Furthermore, joint moments in the squat are extensor in both knee and hip throughout the exercise (Escamilla et al., 1998). While leg extensions on the Reformer may be performed using continuous hip and knee extensor moments, the exercise may also
be performed with extension moments at one joint and flexion moments at the other, as we showed here for the first time. There are two important outcomes from these results. First, Pilates instructors may now be aware that patients can perform the exercise in unexpected ways, and to control for that, they need to be careful about the direction of force application of the feet on the Reformer bar. And, based on this knowledge, Pilates instructors can now instruct patients to alter the Reformer bar reaction forces so as to achieve the desired strengthening outcomes. The use of a self-selected external load may be considered a limitation of this study. However, we believe that allowing subjects to use their own exercise resistance did not affect the conceptual results of this study. Another limitation of our work is the use of static optimization in the prediction of the individual muscle force contributions. However, static optimization is still the most frequently used approach to determine individual muscle forces during human movement (Erdemir et al., 2007). Also, static optimization has been compared to dynamic (Anderson and Pandy, 2001) and computed muscle control (Mokhtarzadeh et al., 2014) methods and was considered satisfactory, particularly when movements were performed slowly and at sub-maximal levels of activation when dynamic muscle effects are not limiting. Prescribing the direction in which a Pilates practitioner may push during an exercise performed on the Reformer device is not a usual instruction. However, our results suggest that this should be taken into consideration. A slight difference in the direction of the line of force was enough to completely change the characteristics of the exercise and the muscle groups involved. This finding could be exploited in exercise and rehabilitation strategies to work specifically on a muscle group that might need attention. Thus, performing leg extensions on the Reformer apparatus allows for flexibility in execution that would not be possible in a regular squat exercise, and thus might be useful in addressing specific weaknesses of the leg musculature in a controlled manner. 5. Conclusion Leg extension exercises on the Reformer apparatus are primarily performed with a knee extension/hip extension strategy, although four of 15 subjects opted for a knee extension/hip flexion strategy. At the beginning and end of the exercise, knee extensor moments dominated, while mid-way through the leg extension, hip extensor moments dominated for most, but not all subjects. Independent of the strategy used, the knee extensor muscles contributed the most to the exercise. However, subjects using the hip and knee extension strategy used the hip extensor muscles secondarily, while subjects using the hip flexion and knee extension strategy used the hip flexors secondarily. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements We would like to thank CAPES, CNPq, ELAP and FP Pilates Equipamentos for the resources and equipment used in this research. References Amarantini D, Rao G, Berton E. A two-step EMG-and-optimization process to estimate muscle force during dynamic movement. J Biomech 2010;43(9):1827–30. Anderson FC, Pandy MG. Static and dynamic optimization solutions for gait are practically equivalent. J Biomech 2001;34(2):153–61.
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx Anderson BD, Spector A. Introduction to Pilates-based rehabilitation. Orthop Phys Ther Clin North Am 2005;9:395–410. Andrews JG. Biomechanical analysis of human motion. Kinesiology 1974;4:32–42. Blum CL. Chiropractic and Pilates therapy for the treatment of adult scoliosis. J Manip Physiol Ther 2002;25(4):e3. Brodt GA, Cantergi D, Gertz LC, Loss JF. An instrumented footbar for evaluating external forces in Pilates. J Appl Biomech 2014;30(3):483–90. Clauser CE, McConville JT, Young JW. Weight, volume, and center of mass of segments of the human body. In: AMRL technical report 69–70. OH: Wright Patterson Air Force Base; 1969. Crowninshield RD, Brand RA. A physiologically based criterion of muscle force prediction in locomotion. J Biomech 1981;14(11):793–801. Erdemir A, McLean S, Herzog W, van den Bogert AJ. Model-based estimation of muscle forces exerted during movements. Clin Biomech 2007;22(2):131–54. Escamilla RF, Fleising GS, Zheng N, Barrentine SW, Wilk KE, Andrews JR. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sports Exerc 1998;30(4):556. Escamilla RF, Fleisig GS, Lowry TM, Barrentine SW, Andrews JR. A three-dimensional biomechanical analysis of the squat during varying stance widths. Med Sci Sports Exerc 2001;33(6):984–98. Figueroa PJ, Leite NJ, Barros RML. A flexible software for tracking of markers used in human motion analysis. Comput Methods Programs Biomed 2003;72(2):155–65. Gagnon D, Larivière C, Loisel P. Comparative ability of EMG, optimization, and hybrid modelling approaches to predict trunk muscle forces and lumbar spine loading during dynamic sagittal plane lifting. Clin Biomech 2001;16(5):359–72. Herzog W. Force-sharing among synergistic muscles: theoretical considerations and experimental approaches. Exerc Sport Sci Rev 1996;24:173–202. La Touche R, Escalante K, Linares MT. Treating non-specific chronic low back pain through the Pilates method. J Bodywork Movement Ther 2008;12(4):364–70. Melo MO, Gomes LE, Silva YO, Bonezi A, Loss JF. Assessment of resistance torque and resultant muscular force during Pilates hip extension exercise and its implications to prescription and progression. Revista Brasileira de Fisioterapia 2011;15(1):23–30. Mokhtarzadeh H, Perraton L, Fok L, Muñoz MA, Clark R, Pivonka P, et al. A comparison of optimisation methods and knee joint degrees of freedom on muscle force predictions during single-leg hop landings. J Biomech 2014 [in press]. Pierrynowski M. A physiological model for the solution of individual muscle forces during normal human walking. Burnaby: Simon Fraser University; 1982. Queiroz B, Cagliari M, Amorim C, Sacco I. Muscle activation during four Pilates core stability exercises in quadruped position. Arch Phys Med Rehabil 2010;91(1):86–92. Ribeiro DC, Loss JF. Assessment of the propagation of uncertainty on link segment model results. Mot Control 2010;14(4):411–23. Robertson D, Wilson J, St. Pierre T. Lower extremity muscle functions during squats. J Appl Biomech 2008;24(4):333. Rydeard R, Leger A, Smith D. Pilates-based therapeutic exercise: effect on subjects with nonspecific chronic low back pain and functional disability: a randomized controlled trial. J Orthop Sports Phys Ther 2006;36(7):472–84. Sacco ICN, Andrade MS, Souza PS, Nisiyama M, Cantuária Al, Maeda FYI, et al. Pilates method in review: biomechanical aspects of specific movements for postural reorganization – cases report. Revista Brasileira de Ciência e Movimento 2005;13(4):65–78. Self BP, Bagley AM, Triplett TL, Paulos LE. Functional biomechanical analysis of the Pilates-based Reformer during Demi-Plie movements. J Appl Biomech 1996;12(3):326–37. Silva Y, Melo M, Gomes L, Bonezi A, Loss J. Analysis of the external resistance and electromyographic activity of hip extension performed according to the Pilates method. Revista Brasileira de Fisioterapia 2009;13(1):82–8. Tsirakos D, Baltzopoulos V, Bartlett R. Inverse optimization: functional and physiological considerations related to the force-sharing problem. Crit Rev Biomed Eng 1997;25(4–5):371–407. Von Sperling de Souza M, Vieira CB. Who are the people looking for the Pilates method? J Bodywork Movement Ther 2006;10(4):328–34. Winter DA. Biomechanics and motor control of human movement. 3rd ed. Hoboken, New Jersey: John Wiley & Sons; 2005. Wretenberg O, Fend Y, Lindberg F. Joint moments of force and quadriceps muscle activity during squatting exercise. Scandinavian J Med Sci Sports 1993;3(4):244–50.
Debora Cantergi is a PHD student in Human Movement Science at the Federal University of Rio Grande do Sul, in Porto Alegre, Brazil. She has a bachelor’s degree in Physical Education (2007) and a master’s degree (2011) at the same University in Brazil. Her research interests are on biomechanics of human movement, with emphasis on sports performance, training and health.
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Jefferson Fagundes Loss is professor in the Department of Physical Education at the Federal University of Rio Grande do Sul – Brazil. He earned an MSc in Mechanical Engineering and a Ph.D. in biomechanics from the same university. Dr. Loss served as Researcher and Professor of the Biomechanical Laboratory at the Federal University of Rio Grande do Sul where he is an Associate Professor of Biomechanics. His research focuses on kinetic and kinematic analysis of human movement with specific interests in musculoskeletal effects.
Azim Jinha did his undergraduate training in Mechanical Engineering at the University of Ottawa in Ottawa, Ontario (1995), completed his master’s research in biomechanics at the University of Calgary in 2002. Currently, Azim Jinha is a research support technician and software developer in Kinesiology at the University of Calgary. His research interests include modeling of skeletal muscle, numerical and computational algorithms for data analysis and modeling.
Guilherme Auler Brodt did his undergraduate studies in Physical Education at the Federal University of Rio Grande do Sul (2011). He has a master’s degree in Human Movement Science at the Federal University of Rio Grande do Sul (2013). He is a professor of the Physical Education school at the University of Caxias do Sul. His interests are in the subjects of physical education, especially in biomechanics, training and health, focusing on study and analysis og physical capacities and its effects in several populations.
Walter Herzog did his undergraduate training in Physical Education at the Federal Technical Institute in Zurich, Switzerland (1979), completed his doctoral research in biomechanics at the University of Iowa (USA) in 1985, and completed postdoctoral fellowships in Neuroscience and Biomechanics in Calgary, Canada in 1987. Currently, Dr. Herzog is a Professor of Biomechanics with appointments in Kinesiology, Medicine, Engineering, and Veterinary Medicine, holds the Canada Research Chair for Cellular and Molecular Biomechanics, and is appointed the Killam Memorial Chair for Inter-Disciplinary Research at the University of Calgary. His research interests are in musculoskeletal biomechanics with emphasis on mechanisms of muscle contraction and the biomechanics of joints with focus on mechanisms of onset and progression of osteoarthritis. Dr. Herzog is the recipient of the Borelli Award from the American Society of Biomechanics, the Career Award from the Canadian Society for Biomechanics and is the past president of the International, American and Canadian Societies for Biomechanics.
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
Contents lists available at ScienceDirect
Journal of Electromyography and Kinesiology journal homepage: www.elsevier.com/locate/jelekin
Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus Débora Cantergi a,⇑, Jefferson Fagundes Loss a, Azim Jinha b, Guilherme Auler Brodt a,c, Walter Herzog b a
Universidade Federal do Rio Grande do Sul, Exercise Research Laboratory, School of Physical Education, Brazil University of Calgary, Human Performance Laboratory, School of Kinesiology, Canada c Universidade de Caxias do Sul, Health Science Center, Brazil b
a r t i c l e
i n f o
Article history: Received 21 August 2013 Received in revised form 26 August 2014 Accepted 31 August 2014 Available online xxxx Keywords: Pilates Optimization
a b s t r a c t Considering the kinematics of leg extensions performed on a Reformer apparatus, one would expect high activation of hip and knee extensor muscle groups. However, because of the bi-articular nature of some lower limb muscles, and the possibility to vary the direction of force application on the Reformer bar, muscles can be coordinated theoretically in a variety of ways and still achieve the desired outcome. Hence, the aim of this study was to determine the knee and hip moments during leg extensions performed on the Reformer apparatus and to estimate the forces in individual muscles crossing these joints using static optimization. Fifteen subjects performed leg extensions exercises on the Reformer apparatus using an individually chosen resistance. To our big surprise, we found that subjects performed the exercise using two conceptually different strategies (i) the first group used simultaneous hip and knee extension moments, (ii) while the second group used simultaneous hip flexion and knee extension moments to perform the exercise. These different strategies were achieved by changing the direction of the resultant force applied by the subject’s feet on the Reformer bar. While leg extensions on the Reformer apparatus have been thought to strengthen the hip and knee extensors muscles, our results demonstrate that patients can perform the exercise in a different and unexpected way. In order to control the hip and knee moments and achieve the desired outcome of the exercise, the direction of force application on the Reformer bar must be controlled carefully. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Pilates exercising has been used for a long time in the dance community because of its perceived effect on accelerating rehabilitation from injury and return to activity (Anderson and Spector, 2005; La Touche et al., 2008; Self et al., 1996). Recently, Pilates has also become an accepted exercise method for the general public, with a rapidly growing number of participants (Blum, 2002; Rydeard et al., 2006). Pilates as a rehabilitation tool has been treated cautiously with calls for research evaluating its effects on the body (Brodt et al., 2014; Gagnon et al., 2001; La Touche et al., 2008; Von Sperling de Souza and Vieira, 2006). Although there are previous studies on the biomechanics of Pilates, the majority of them were conducted in the past 20 years (Self et al., 1996; Silva et al., 2009; Queiroz et al., 2010; Melo et al., 2011). Self et al. (1996) compared the ballet movement, Demi Plié, performed in the standing position and on the Reformer ⇑ Corresponding author at: Federal University of Rio Grande do Sul, Physical Education School, Rua Felizardo, 750 – Porto Alegre, Rio Grande do Sul 90690-200, Brazil. Tel.: +55 51 3308 5822. E-mail address: [email protected] (D. Cantergi).
apparatus, and Silva et al. (2009) studied the external forces for hip extension exercises on the Cadillac apparatus. Muscle activities for Pilates exercises have also been quantified using electromyography (Queiroz et al., 2010) and inverse dynamics approaches aimed at quantifying the resultant hip joint moments and muscles forces (Melo et al., 2011). Leg extension exercises are commonly performed in strengthening activities, aimed at improving functionality in activities of daily living, such as rising from a chair or lifting a load from the floor. Strength training exercises, such as squatting, present a similar kinematics to the hip and knee as the extension exercise performed on a Reformer apparatus. Squats are performed by flexing and extending the hip and knee simultaneously, producing extensor moments at both joints throughout the exercise, with the extensor moments increasing during joint flexion and decreasing during joint extension phases (Wretenberg et al., 1993; Escamilla et al., 1998; Robertson et al., 2008). Considering the kinematic nature of the task (leg extension), it would be normal to expect a high activation of hip and knee extensor muscle groups. However, because of the bi-articular nature of some of the muscles involved (knee extensors that also function as hip flexors and hip extensors that also function as knee flexors),
http://dx.doi.org/10.1016/j.jelekin.2014.08.016 1050-6411/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
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D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx
and the possibility to change the direction of force application on the Reformer apparatus, the contribution of muscles to the leg extension exercise is not necessarily obvious. Previous studies on the Pilates method showed that small changes in body position or force direction can dramatically affect the resultant joint moments, and thus the muscle groups involved in the exercise (Sacco et al., 2005; Melo et al., 2011). Hence, the aim of this study was to characterize knee and hip moments during leg extensions performed on the Reformer apparatus and to estimate the forces in individual muscles crossing these joints. We measured the forces and movements of leg extensions on the Reformer apparatus, calculated the resultant hip, knee and ankle moments using an inverse dynamics approach, and estimated the individual muscle forces using non-linear constrained optimization (Crowninshield and Brand, 1981; Erdemir et al., 2007; Herzog, 1996; Tsirakos et al., 1997). We hypothesized, that depending on the technique used, leg extensions on the Reformer machine might primarily be performed using a hip extension or a knee extension strategy. 2. Methods Fifteen subjects (3 males; 12 females, mean weight of 62.3 ± 11.6 kg and mean height of 1.65 ± 0.07 m), aged between 20 and 50 years, gave written informed consent to participate in this study. Subjects were physically active and had no history of injury to the lower limbs or trunk. Besides being free of injuries, it was also an inclusion criterion that subjects were currently practicing Pilates, for at least six months, with a frequency of twice a week or higher. Kinematics and kinetics were obtained for leg extension exercises performed on a Reformer apparatus using the Pilates Method (Fig. 1). The movement was performed with the heels pressing against the foot bar and the feet positioned parallel to each other, in order to avoid contributions from the ankle extensor muscles to the leg extension exercise. Knee and hip motion during the exercise ranged from about 90° of flexion in both joint to zero degrees of flexion at the knee and 15° of flexion at the hip. The position of the bar relative to the subject is the distance between the mobile cart and the bar, which was adjusted so as to match the initial hip and knee angles for all subjects. Each subject performed ten leg extensions against the spring resistance of the Reformer apparatus. The sagittal plane leg extension forces were measured using an instrumented and calibrated foot bar. The foot bar consisted of four load cells, which were positioned in the extremities of the bar, measuring forces in the horizontal and vertical directions. From these measurements, the resultant force direction and magnitude could be calculated. More information on the instrumentation of the force bar can be found elsewhere (Brodt et al., 2014). Sagittal plane movements were measured using a video system (JVC GR-DVL 9800; 25 Hz) and reflective markers (15 mm of diameter) placed at the base of the fifth metatarsus, lateral malleolus, lateral condyle, greater trochanter, acromion process, and the mobile cart. The kinematics system used in this paper has been
tested and described elsewhere (Ribeiro e Loss, 2010), and the error propagation of the proximal reaction force (PRF) and proximal net moments (PNM), has been described. They found that the uncertainty for the proximal reaction force was, on average, 0.27 N and uncertainty for the proximal net moments, was on average 0.97 Nm. Also, the accuracy, bias and precision of the kinematics system were calculated, resulting in mean values of 1.7 mm, 0.5 mm, and 1.6 mm, respectively (Ribeiro and Loss, 2010). In the Reformer apparatus used for the study, up to five springs may be used for resistance. Prior to the data collection, subjects were asked to select their resistance setting for the exercise. The subjects chose the resistance individually in such a manner that it provided sufficient challenge but so that they still could perform the exercise and all required repetitions comfortably. After selecting a combination of springs, they performed two repetitions of the exercise in order to feel the load, while an experiment Pilates instructor checked the kinematics characteristics of the movement. A complete exercise cycle consisted of an extension and flexion of the leg. The force data were processed using a 4th order, low-pass, recursive Butterworth filter (MATLAB Signal Processing Toolbox). The cut-off frequency (5 Hz) was calculated using a residual analysis criterion (Winter, 2005). Leg angles and positions were obtained by digitizing the joint markers in the recorded video of each trial using software Dvideow (Figueroa et al., 2003). Joint velocities and accelerations were then obtained by differentiating the displacement–time data with respect to time once or twice, respectively. Resultant net joint moments at the knee and hip were calculated using an inverse dynamics approach (Andrews, 1974) and a link segment method, with inertial data obtained from regression equations given by Clauser (1969). Muscle forces were estimated using a nonlinear, constrained, static optimization approach and a cost function minimizing the cubed muscle stresses, as proposed by Crowninshield and Brand (1981) using MatlabÒ 7.0. Thirty-six muscle tendon units were used to represent the primary lower limb flexor/extensor muscles. Moment arms and physiological cross sectional areas were obtained from Pierrynowski (1982). 3. Results Joint moments were defined as positive for extension and negative for flexion. Resultant net moments at the hip and knee presented two distinct patterns between groups of subjects (Fig. 2): those whose knee and hip moments were essentially positive (extensor) for all (most) of the movement (subjects e–p) and those who had a substantial phase of negative (flexor) hip moments (subjects a–d). Independent of the strategy used for performing the leg extension exercise on the Reformer apparatus, the knee extensor muscles contributed the most to the exercise (Fig. 3). However, subjects using the hip and knee extension strategy used the hip extensor muscles secondarily with the hip flexors essentially silent (Fig. 3a), while subjects using the hip flexion and knee extension strategy used the hip flexors secondarily while the hip extensors remained essentially silent (Fig. 3b). 4. Discussion
Fig. 1. Starting and finishing position for the leg extension exercise.
This study was aimed at characterizing leg extension exercises performed on the Reformer apparatus. While performing the exercise, subjects pushed against the Reformer apparatus bar, generating a reaction force on the subject’s foot. This reaction force creates the primary net moments at the hip and knee joints. Depending on
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx
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Fig. 2. Mean hip ( ) and knee (- -) resultant moments normalized to the extension phase of the movement. We defined joint moments as positive for extension and negative for flexion. Each graph represents the mean value of ten repetitions of leg extension from one subject.
(a) mean muscle force prediction for 11 subjects
(b) mean muscle force prediction for 4 subjects
Fig. 3. Mean normalized force predictions for the muscles contributing to the leg extensor force for (a) subjects using the knee and hip extension strategy and for (b) subjects using the knee extension and hip flexion strategy.
the direction of the line of action of the reaction force, which can be controlled by the subjects, there are three possible ways for performing this exercise, depending on how the reaction force may be directed: (i) above the knee, creating knee flexor and hip extensor moments; (ii) between knee and hip, creating knee and hip extensor moments; or (iii) below the hip, creating knee extensor and hip flexor moments (Fig. 4). Although possible, the strategy with the line of action above the knee joint was not used by any of the subjects in this study. The line of action was kept mostly below the hip by four subjects (Fig. 2, a–d), and between the knee and hip for the majority (Fig. 2, e–p) of the subjects tested. A first attempt at possibly explaining the different strategies observed in this study was directed toward the subject’s physical characteristics, specifically, their thigh and lower limb segment lengths. Once the initial position of the bar relative to the subject had been chosen, lower limb segmental lengths might affect the direction of the reaction force and, consequently, the direction of the hip and knee moments. It may be speculated that someone taller or with longer limbs may keep the knees higher relative to the force bar, and this positioning might affect the direction of the force of the feet on the Reformer bar. However, we found no apparent relationship between the strategy chosen by subjects and their segmental leg lengths. Apparently, the strategy for performing leg
extensions was less related to physical characteristics but was more influenced by learning or choice on how to perform the exercise. Similar to the leg extensions performed on the Reformer apparatus, leg extension exercises are part of many other exercise regimes, for example squats. However, in contrast to squats, where the line of action of the resultant ground reaction force cannot vary much because of the stability required to remain standing during the exercise, leg extensions with the Reformer apparatus allow for great variation in the line of action of the reaction forces. Therefore, leg extensions in a Reformer apparatus can be performed using distinctly different strategies as outlined above, whereas such vastly differing strategies cannot be utilized in a squat exercise. Because of the tight restrictions on the direction of the ground reaction force in a squat exercise, squatting always involves a simultaneous hip and knee extensor moment (Escamilla et al., 2001), while, as discussed above, this is not true for leg extensions using the Reformer apparatus. In most subjects, it was observed that the vastus lateralis and vastus intermedius were predicted to contribute more than 25% of the total muscle force to the leg extension exercise on the Reformer apparatus. However, for many subjects (d, e, f, l, m, n, and o, Fig. 2), the resultant knee moment became flexor toward the very
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
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Fig. 4. The three possibilities that theoretically exist for performing leg extension exercises on the Reformer apparatus: (a) reaction force line of action is directed above the knee joint, and generates knee flexor and hip extensor resultant moments, (b) reaction force line of action is directed between the knee and hip joints, and generates knee and hip extensor resultant moments, and (c) reaction force line of action is directed below the hip joint, and generates knee extensor and hip flexor resultant moments.
end of leg extension, and the knee flexor muscles were predicted to contribute to that final leg extension. This behavior can readily be understood using Fig. 4, and imagining that toward the end of leg extension, the knee is at level or even below the foot rest of the Reformer apparatus and the line of action may easily be directed above the knee, thereby causing the knee flexor moment. For the four subjects using the knee extension/hip flexion strategy (Fig. 3b), it was observed that the rectus femoris was predicted to be the primary contributor to the leg extension exercise. This result makes perfect sense as the rectus femoris is the sole muscle that functions as a knee extensor and hip flexor. The role of the hamstring muscles is greatly decreased in this group compared to the knee extension/hip extension group (Fig. 3a), as the two joint hamstring muscles function as hip extensors and knee flexors, the exact opposite strategy than that used for leg extensions on the Reformer apparatus. Amarantini et al. (2010) studied squat exercises using an EMGand-optimization method for muscle force estimation. Not surprisingly, they found that the knee extensor muscles were the primary contributors to the squat exercise. This is in agreement with our findings of leg extension, which also involved the knee extensors to a great degree for both techniques chosen by the subjects in this study. In addition, there would have been the theoretical possibility to perform the leg extensions with a knee flexion/hip extension strategy (Fig. 4), but none of the subjects opted for this solution. Leg extensions on the Reformer apparatus are supposed to strengthen the same muscle groups as in the squat exercise. The major difference is the load. In squat exercises, the lowest possible resistance is the subject’s weight, while during leg extension, the subject’s weight is not an important factor and subjects can adjust the resistance by increasing the stiffness of springs, thus allowing weak individuals or individuals in rehabilitation, to perform leg extensor strengthening in a gentle manner that would not be possible with squat exercises. Furthermore, joint moments in the squat are extensor in both knee and hip throughout the exercise (Escamilla et al., 1998). While leg extensions on the Reformer may be performed using continuous hip and knee extensor moments, the exercise may also
be performed with extension moments at one joint and flexion moments at the other, as we showed here for the first time. There are two important outcomes from these results. First, Pilates instructors may now be aware that patients can perform the exercise in unexpected ways, and to control for that, they need to be careful about the direction of force application of the feet on the Reformer bar. And, based on this knowledge, Pilates instructors can now instruct patients to alter the Reformer bar reaction forces so as to achieve the desired strengthening outcomes. The use of a self-selected external load may be considered a limitation of this study. However, we believe that allowing subjects to use their own exercise resistance did not affect the conceptual results of this study. Another limitation of our work is the use of static optimization in the prediction of the individual muscle force contributions. However, static optimization is still the most frequently used approach to determine individual muscle forces during human movement (Erdemir et al., 2007). Also, static optimization has been compared to dynamic (Anderson and Pandy, 2001) and computed muscle control (Mokhtarzadeh et al., 2014) methods and was considered satisfactory, particularly when movements were performed slowly and at sub-maximal levels of activation when dynamic muscle effects are not limiting. Prescribing the direction in which a Pilates practitioner may push during an exercise performed on the Reformer device is not a usual instruction. However, our results suggest that this should be taken into consideration. A slight difference in the direction of the line of force was enough to completely change the characteristics of the exercise and the muscle groups involved. This finding could be exploited in exercise and rehabilitation strategies to work specifically on a muscle group that might need attention. Thus, performing leg extensions on the Reformer apparatus allows for flexibility in execution that would not be possible in a regular squat exercise, and thus might be useful in addressing specific weaknesses of the leg musculature in a controlled manner. 5. Conclusion Leg extension exercises on the Reformer apparatus are primarily performed with a knee extension/hip extension strategy, although four of 15 subjects opted for a knee extension/hip flexion strategy. At the beginning and end of the exercise, knee extensor moments dominated, while mid-way through the leg extension, hip extensor moments dominated for most, but not all subjects. Independent of the strategy used, the knee extensor muscles contributed the most to the exercise. However, subjects using the hip and knee extension strategy used the hip extensor muscles secondarily, while subjects using the hip flexion and knee extension strategy used the hip flexors secondarily. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements We would like to thank CAPES, CNPq, ELAP and FP Pilates Equipamentos for the resources and equipment used in this research. References Amarantini D, Rao G, Berton E. A two-step EMG-and-optimization process to estimate muscle force during dynamic movement. J Biomech 2010;43(9):1827–30. Anderson FC, Pandy MG. Static and dynamic optimization solutions for gait are practically equivalent. J Biomech 2001;34(2):153–61.
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D. Cantergi et al. / Journal of Electromyography and Kinesiology xxx (2014) xxx–xxx Anderson BD, Spector A. Introduction to Pilates-based rehabilitation. Orthop Phys Ther Clin North Am 2005;9:395–410. Andrews JG. Biomechanical analysis of human motion. Kinesiology 1974;4:32–42. Blum CL. Chiropractic and Pilates therapy for the treatment of adult scoliosis. J Manip Physiol Ther 2002;25(4):e3. Brodt GA, Cantergi D, Gertz LC, Loss JF. An instrumented footbar for evaluating external forces in Pilates. J Appl Biomech 2014;30(3):483–90. Clauser CE, McConville JT, Young JW. Weight, volume, and center of mass of segments of the human body. In: AMRL technical report 69–70. OH: Wright Patterson Air Force Base; 1969. Crowninshield RD, Brand RA. A physiologically based criterion of muscle force prediction in locomotion. J Biomech 1981;14(11):793–801. Erdemir A, McLean S, Herzog W, van den Bogert AJ. Model-based estimation of muscle forces exerted during movements. Clin Biomech 2007;22(2):131–54. Escamilla RF, Fleising GS, Zheng N, Barrentine SW, Wilk KE, Andrews JR. Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Med Sci Sports Exerc 1998;30(4):556. Escamilla RF, Fleisig GS, Lowry TM, Barrentine SW, Andrews JR. A three-dimensional biomechanical analysis of the squat during varying stance widths. Med Sci Sports Exerc 2001;33(6):984–98. Figueroa PJ, Leite NJ, Barros RML. A flexible software for tracking of markers used in human motion analysis. Comput Methods Programs Biomed 2003;72(2):155–65. Gagnon D, Larivière C, Loisel P. Comparative ability of EMG, optimization, and hybrid modelling approaches to predict trunk muscle forces and lumbar spine loading during dynamic sagittal plane lifting. Clin Biomech 2001;16(5):359–72. Herzog W. Force-sharing among synergistic muscles: theoretical considerations and experimental approaches. Exerc Sport Sci Rev 1996;24:173–202. La Touche R, Escalante K, Linares MT. Treating non-specific chronic low back pain through the Pilates method. J Bodywork Movement Ther 2008;12(4):364–70. Melo MO, Gomes LE, Silva YO, Bonezi A, Loss JF. Assessment of resistance torque and resultant muscular force during Pilates hip extension exercise and its implications to prescription and progression. Revista Brasileira de Fisioterapia 2011;15(1):23–30. Mokhtarzadeh H, Perraton L, Fok L, Muñoz MA, Clark R, Pivonka P, et al. A comparison of optimisation methods and knee joint degrees of freedom on muscle force predictions during single-leg hop landings. J Biomech 2014 [in press]. Pierrynowski M. A physiological model for the solution of individual muscle forces during normal human walking. Burnaby: Simon Fraser University; 1982. Queiroz B, Cagliari M, Amorim C, Sacco I. Muscle activation during four Pilates core stability exercises in quadruped position. Arch Phys Med Rehabil 2010;91(1):86–92. Ribeiro DC, Loss JF. Assessment of the propagation of uncertainty on link segment model results. Mot Control 2010;14(4):411–23. Robertson D, Wilson J, St. Pierre T. Lower extremity muscle functions during squats. J Appl Biomech 2008;24(4):333. Rydeard R, Leger A, Smith D. Pilates-based therapeutic exercise: effect on subjects with nonspecific chronic low back pain and functional disability: a randomized controlled trial. J Orthop Sports Phys Ther 2006;36(7):472–84. Sacco ICN, Andrade MS, Souza PS, Nisiyama M, Cantuária Al, Maeda FYI, et al. Pilates method in review: biomechanical aspects of specific movements for postural reorganization – cases report. Revista Brasileira de Ciência e Movimento 2005;13(4):65–78. Self BP, Bagley AM, Triplett TL, Paulos LE. Functional biomechanical analysis of the Pilates-based Reformer during Demi-Plie movements. J Appl Biomech 1996;12(3):326–37. Silva Y, Melo M, Gomes L, Bonezi A, Loss J. Analysis of the external resistance and electromyographic activity of hip extension performed according to the Pilates method. Revista Brasileira de Fisioterapia 2009;13(1):82–8. Tsirakos D, Baltzopoulos V, Bartlett R. Inverse optimization: functional and physiological considerations related to the force-sharing problem. Crit Rev Biomed Eng 1997;25(4–5):371–407. Von Sperling de Souza M, Vieira CB. Who are the people looking for the Pilates method? J Bodywork Movement Ther 2006;10(4):328–34. Winter DA. Biomechanics and motor control of human movement. 3rd ed. Hoboken, New Jersey: John Wiley & Sons; 2005. Wretenberg O, Fend Y, Lindberg F. Joint moments of force and quadriceps muscle activity during squatting exercise. Scandinavian J Med Sci Sports 1993;3(4):244–50.
Debora Cantergi is a PHD student in Human Movement Science at the Federal University of Rio Grande do Sul, in Porto Alegre, Brazil. She has a bachelor’s degree in Physical Education (2007) and a master’s degree (2011) at the same University in Brazil. Her research interests are on biomechanics of human movement, with emphasis on sports performance, training and health.
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Jefferson Fagundes Loss is professor in the Department of Physical Education at the Federal University of Rio Grande do Sul – Brazil. He earned an MSc in Mechanical Engineering and a Ph.D. in biomechanics from the same university. Dr. Loss served as Researcher and Professor of the Biomechanical Laboratory at the Federal University of Rio Grande do Sul where he is an Associate Professor of Biomechanics. His research focuses on kinetic and kinematic analysis of human movement with specific interests in musculoskeletal effects.
Azim Jinha did his undergraduate training in Mechanical Engineering at the University of Ottawa in Ottawa, Ontario (1995), completed his master’s research in biomechanics at the University of Calgary in 2002. Currently, Azim Jinha is a research support technician and software developer in Kinesiology at the University of Calgary. His research interests include modeling of skeletal muscle, numerical and computational algorithms for data analysis and modeling.
Guilherme Auler Brodt did his undergraduate studies in Physical Education at the Federal University of Rio Grande do Sul (2011). He has a master’s degree in Human Movement Science at the Federal University of Rio Grande do Sul (2013). He is a professor of the Physical Education school at the University of Caxias do Sul. His interests are in the subjects of physical education, especially in biomechanics, training and health, focusing on study and analysis og physical capacities and its effects in several populations.
Walter Herzog did his undergraduate training in Physical Education at the Federal Technical Institute in Zurich, Switzerland (1979), completed his doctoral research in biomechanics at the University of Iowa (USA) in 1985, and completed postdoctoral fellowships in Neuroscience and Biomechanics in Calgary, Canada in 1987. Currently, Dr. Herzog is a Professor of Biomechanics with appointments in Kinesiology, Medicine, Engineering, and Veterinary Medicine, holds the Canada Research Chair for Cellular and Molecular Biomechanics, and is appointed the Killam Memorial Chair for Inter-Disciplinary Research at the University of Calgary. His research interests are in musculoskeletal biomechanics with emphasis on mechanisms of muscle contraction and the biomechanics of joints with focus on mechanisms of onset and progression of osteoarthritis. Dr. Herzog is the recipient of the Borelli Award from the American Society of Biomechanics, the Career Award from the Canadian Society for Biomechanics and is the past president of the International, American and Canadian Societies for Biomechanics.
Please cite this article in press as: Cantergi D et al. Muscle strategies for leg extensions on a ‘‘Reformer’’ apparatus. J Electromyogr Kinesiol (2014), http:// dx.doi.org/10.1016/j.jelekin.2014.08.016
