Authors: Markus Due Jakobsen, MSc Emil Sundstrup, MSc Christoffer H. Andersen, PhD Roger Persson, PhD Mette K. Zebis, PhD Lars L. Andersen, PhD
Electromyography
ORIGINAL RESEARCH ARTICLE
Affiliations: From the National Research Centre for the Working Environment, Copenhagen, Denmark (MDJ, ES, CHA, LLA); Department of Psychology, Lund University, Lund, Sweden (RP); Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark (MDJ, ES); and Arthroscopic Centre Amager and Gait Analysis Laboratory, Copenhagen University Hospital, Hvidovre, Denmark (MKZ).
Effectiveness of Hamstring Knee Rehabilitation Exercise Performed in Training Machine vs. Elastic Resistance Electromyography Evaluation Study
Correspondence: All correspondence and requests for reprints should be addressed to: Markus Due Jakobsen, MSc, National Research Centre for the Working Environment, LersL Parkalle 105, DK-2100 Copenhagen, Denmark.
ABSTRACT Jakobsen MD, Sundstrup E, Andersen CH, Persson R, Zebis MK, Andersen LL: Effectiveness of hamstring knee rehabilitation exercise performed in training machine vs. elastic resistance: electromyography evaluation study. Am J Phys Med Rehabil 2014;93:320Y327.
Objective: The aim of this study was to evaluate muscle activity during ham-
Disclosures: Supported by a grant from the Danish Working Environment Research Fund (grant no. 48-2010-03) (to L.L.A.). Financial disclosure statements have been obtained, and no conflicts of interest have been reported by the authors or by any individuals in control of the content of this article.
0894-9115/14/9304-0320 American Journal of Physical Medicine & Rehabilitation Copyright * 2013 by Lippincott Williams & Wilkins DOI: 10.1097/PHM.0000000000000043
string rehabilitation exercises performed in training machine compared with elastic resistance.
Design: Six women and 13 men aged 28Y67 yrs participated in a crossover study. Electromyographic (EMG) activity was recorded in the biceps femoris and the semitendinosus during the concentric and the eccentric phase of hamstring curls performed with TheraBand elastic tubing and Technogym training machines and normalized to maximal voluntary isometric contractionYEMG (normalized EMG). Knee joint angle was measured using electronic inclinometers. Results: Training machines and elastic resistance showed similar high levels of muscle activity (biceps femoris and semitendinosus peak normalized EMG 980%). EMG during the concentric phase was higher than during the eccentric phase regardless of exercise and muscle. However, compared with machine exercise, slightly lower (P G 0.05) normalized EMG values were observed using elastic resistance at 30- to 50-degree knee joint angle for the semitendinosus and the biceps femoris during the concentric and the eccentric phase, respectively. Perceived loading (Borg CR10) was significantly higher (P G 0.001) during hamstring curl performed with elastic resistance (7.58 T 0.08) compared with hamstring curl performed in a machine (5.92 T 0.03).
Conclusions: Hamstring rehabilitation exercise performed with elastic resistance induces similar peak hamstring muscle activity but slightly lower EMG values at more extended knee angles and with higher perceived loading as hamstring curls using training machines. Key Words:
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Strength Training, Hamstring, Perceived Exertion, Borg
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K
nee joint injuries may occur in many situations, for example, during sports, at work, or during other daily activities involving movement. Some of the individual consequences that have frequently been reported to follow knee joint trauma are severe pain, muscular atrophy, osteoarthritis, central activation failure, and reduced mobility leading to functional disability.1Y4 However, there are also social and economic consequences in that knee pain may be disabling in working life. Among a representative sample of 5600 blue and white collar workers, the prevalence of severe knee pain was 16% and 12%, respectively.5 Because knee pain increases the risk for long-term sickness absence, knee pain also has societal costs.5 Accordingly, it seems important to find and evaluate methods for rehabilitation. Rehabilitation after knee injury or surgery is a very challenging task. The availability of strength training equipment such as machines and free weights is often a limiting factor during rehabilitation and training interventions.6 As a consequence, the use of more accessible equipment such as elastic resistance exercises is increasing during knee rehabilitation after injury and/or surgery. In terms of knee injury rehabilitation, the main aims are to regain full pain-free mobility of the knee followed by increased strength of the surrounding muscles (quadriceps, hamstrings, and calf muscles). Because the early stages of knee injury rehabilitation are mainly focused on mobility rather than high-intensity strength training, the use of elastic resistance exercises is highly advised during this stage.7Y9 Nevertheless, as the strength of the surrounding muscles increases through the stages of rehabilitation, the use of more traditional, but less accessible, exercises with weights or machines is recommended to ensure sufficient range of motion and intensity for gains in strength. Until now, only a few studies have investigated muscular activity (electromyography [EMG]) during hamstring knee rehabilitation exercises using elastic resistance.7,8 These studies demonstrated activation values lower than the recommended 60% threshold for muscular hypertrophy and gains in strength.10 Accordingly, although recent studies have demonstrated elastic resistance to be equally effective in strengthening smaller muscles in the neck, the shoulder, and the arm compared with free weight training,11,12 it remains questionable whether elastic resistance activities with larger muscle groups such as during hamstring knee rehabilitation exercises can induce sufficiently high intensity of muscle activation, www.ajpmr.com
without compromising the range of motion, as using traditional training machines. In addition, the EMGYjoint angle relationship that is well described for conventional strength training exercises13 remains largely unexplored for high-intensity elastic resistance exercise. The purpose of the present study was to evaluate muscle activity pattern during hamstring curl exercises performed with a training machine or with elastic resistance. The authors hypothesized that hamstring curl exercise performed with elastic tubing would induce similar high muscle activity as hamstring curl exercise performed with a training machine.
MATERIALS AND METHODS Subjects This study was performed in Copenhagen, Denmark. A group of 19 untrained adults (6 women and 13 men) were recruited from a large workplace with various job tasks. Exclusion criteria were blood pressure higher than 160/100, disk prolapse, or serious chronic disease. Complete testing was performed on all participants, that is, with both elastic resistance and in a machine. All subjects were informed about the purpose and the content of the project and gave written informed consent to participate in this study, which conformed to the Declaration of Helsinki and was approved by the Local Ethical Committee (H-3-2010-062).
Study Design The participants visited the researchers on two occasions. On the first occasion, 1 wk before testing, the participants received training and were tested for maximum performance to determine the load used for testing. On the second occasion, the participants were tested with EMG and inclinometry with the aim to compare two different exercises in a crossover design.
Exercise Equipment Two different types of training equipment were used: (1) elastic tubing with resistances ranging from light to very heavy (red, green, blue, black, and gray colors; TheraBand, Akron, United States) and (2) machine (vertical seated hamstring curl; Technogym, Gambettola, Italy) (cf. Fig. 1).
Exercise Description During the training session, the participants performed a 10 repetition maximum test (10 RM) for the two exercises. During the elastic resistance Hamstring Knee Rehabilitation Exercise
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FIGURE 1 Illustration of the hamstring curl exercise with elastic tubing (A) and in the training machine (B).
exercise, the 10 RM loading was found using one or a combination of several color-coded elastic tubes with resistances ranging from light to very heavy (red, green, blue, black, and gray), whereas the 10 RM loading of the machine exercise was determined in kilograms. All exercises were performed unilaterally. On the day of EMG measurements, the participants warmed up with submaximal loads and then performed three consecutive repetitions for each exercise using a 10 RM load to avoid the influence of fatigue on the subsequent exercises. Exercises were performed in a controlled manner at a slow constant speed (each repetition lasting È3 secs). The order of exercises was randomized for each subject, and the rest period between different exercises was approximately 5 mins. The exercises are described in detail below (Fig. 1).
Hamstring Curl with Elastic Resistance The participant was sitting on a high chair placed 2.5 m from a wooden rib with one leg fully extended and a 90-degree hip flexion, whereas the other leg was hanging loose. The elastic tubing was fixated at one end to the participant’s ankle and attached in the other end to the wooden rib at 1.7-m height. The elastic tubing was stretched to È200% of the initial length (initial length, 1.2 m). The participant started flexing the knee (concentric phase) until È90-degree knee joint angle and then extended the knee (eccentric phase) until full extension (È0-degree knee joint angle) (Fig. 1A).
Hamstring Curl in Machine The participant was seated in a Technogym vertical leg curl machine with the leg fully extended and a 100-degree hip flexion. The participant started flexing the knee (concentric phase) until È90-degree knee joint angle and then extended the knee (eccentric phase) until full extension (È0-degree knee joint angle) (Fig. 1B).
Inclinometer Sampling and Analysis Knee joint angle was continuously measured using two electronic inclinometers (2-dimensional
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Direct Transmission System (DTS) inclination sensor; Noraxon, Arizona, United States) placed at the lateral side of the tibia and the femur. The inclinometer data were synchronously sampled with the EMG data, using the 16-channel 16-bit personal computer-interface receiver (TeleMyo DTS Telemetry; Noraxon, Arizona, United States). The dimension of the probes was 3.4 cm 2.4 cm 3.5 cm. During subsequent analysis, the inclinometer signals were digitally low-pass filtered using a fourth-order zerolag Butterworth filter (3-Hz cutoff frequency). The momentary knee joint angle was calculated as the difference in angular position, with respect to the gravitational line, between the tibia and femur inclinometers. Knee joint angles ranged from a 90-degree flexed position to a 0-degree full knee extension. The concentric and eccentric phases were defined as periods with negative and positive angular velocity, respectively (going from 90 to 0 degrees and 0 to 90 degrees, respectively). Angle at peak EMG was calculated within the concentric and eccentric phases.
Maximal Voluntary Isometric Contraction Before the dynamic exercises, maximal voluntary isometric contractions (MVCs) were performed according to standardized procedures during (1) static knee extension and flexion maneuvers (positioned in a Biodex dynamometer: knee angle, 70 degrees, and hip angle, 110 degrees), (2) hip adduction (lying flat on the back and pressing the knees against a solid ball), (3) hip abduction (lying flat on the back and pressing the knees outward against a rigid band) and (4) hip extension (lying flat on the stomach with the knee flexed [90 degrees] and pressing the foot upward against the instructor’s hands), and (5) trunk extension (in standing posture and the pelvis fixated with the trunk extended against a rigid band) to induce a maximal EMG response in the tested muscles.14 Two MVCs were performed for each muscle, and the trial with the highest root mean square (RMS) EMG value was subsequently used for normalization of the RMS EMG signals obtained in the resistance exercises. During the MVCs, the subjects were instructed to gradually increase muscle contraction
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force toward maximum for a period of 2 secs, sustain the MVC for 3 secs, and then slowly release the force again. Strong and standardized verbal encouragement was given during all trials by the test leader, who repeatedly commanded BPush, Push, Push.[
EMG Signal Sampling and Analysis EMG signals were recorded from 10 leg, abdominal, and lower back muscles: vastus medialis, vastus lateralis, rectus femoris, biceps femoris (BF), semitendinosus (ST), gluteus medius, adductors (adductor longus), right erector spinae, right obliques externus, and right rectus abdominis. A bipolar surface EMG configuration (Blue Sensor N-00-S; Ambu A/S, Ballerup, Denmark) and an interelectrode distance of 2 cm were used.13,15 Before affixing the electrodes, the skin of the respective area was prepared with scrubbing gel (Acqua gel; Meditec, Parma, Italy) to effectively lower the impedance to less than 10 k6.11 Electrode placements followed Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) recommendations (www.seniam.org). The EMG electrodes were connected directly to wireless probes that preamplified the signal (gain 400) and transmitted data in real time to a 16-channel 16-bit PC-interface receiver (TeleMyo DTS Telemetry; Noraxon, Arizona, United States). The dimension of the probes was 3.4 cm 2.4 cm 3.5 cm. Data sampling rate was 1500 Hz with a bandwidth of 10Y500 Hz to avoid aliasing. Common mode rejection ratio was higher than 100 dB. During later offline analysis, all raw EMG signals obtained during MVCs and during the exercises were digitally filtered by a Butterworth fourth-order highpass filter (10-Hz cutoff frequency). For each individual muscle, maximal moving RMS (500 milliseconds constant) EMG was used to identify peak EMG within the concentric and the eccentric phase, whereas the RMS of the high-pass filtered EMG signal was calculated within each 10-degree angle interval (0Y10 degrees, 10Y20 degrees, I, 80Y90 degrees) of the concentric and the eccentric phase and normalized to the maximal moving RMS (500-milliseconds time constant) EMG obtained during MVC.11,15,16 Contraction time was calculated according to procedures previously described.15
Perceived Loading Immediately after each set of exercise, the Borg CR10 scale17 was used to rate perceived loading during the resistance exercise. The authors have previously validated this scale in the evaluation www.ajpmr.com
of neck/shoulder resistance exercises with elastic resistance.11
Statistical Analysis A two-way repeated-measures analysis of variance (Proc Mixed, SAS version 9; SAS Institute, Cary, NC) was used to locate differences between exercises and range of knee joint motion for each muscle and contraction mode (concentric or eccentric), vastus medialis to vastus lateralis activation ratio, perceived loading (BORG), and contraction time. Factors included in the model were exercise (elastic resistance and machine) and knee joint angle (0Y90 degrees) as well as exercise by knee joint angle interaction. The analysis was controlled for sex and age. Normalized EMG (nEMG) was the dependent variable. Values are reported as least square means (standard error) unless otherwise stated. P e 0.05 was considered statistically significant. A priori power analysis showed that 19 participants in this paired design were sufficient to obtain a statistical power of 80% at a minimal relevant difference of 10% and a type I error probability of 1%, assuming an SD of 10% based on previous research in the authors’ laboratory.18
RESULTS Demographics Table 1 shows the demographics.
Normalized EMG Figure 2 shows the nEMG-angle relationship for the selected muscles during hamstring curl exercises performed in a machine or with elastic resistance during the 0- to 90-degree knee joint range of motion. There was no significant difference in maximal hamstring EMG between elastic resistance and machine exercise (Table 2). However, in the ST, the concentric EMG-angle pattern (0Y10 degrees, 10Y20 degrees, I, 80Y90 degrees) was significantly TABLE 1 Demographics of the participants
n Age, yrs Height, cm Weight, kg BMI
Men
Women
13 39 (13) 178 (7) 80 (7) 25 (2)
6 42 (10) 167 (8) 68 (17) 24 (6)
Data presented as mean (standard deviation). BMI, body mass index.
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FIGURE 2 nEMG and angular position during the concentric and the eccentric contraction phase of hamstring curl exercises with elastic tubing and in the training machine in the BF (top) and ST (bottom) muscles.
higher (P G 0.05) at 30Y50 degrees during the machine exercise compared with the elastic resistance. Similarly, the ST and the BF were higher (P G 0.05) at 30Y50 degrees during the eccentric contraction phase performed with the machine (Fig. 1). Hamstring EMG was highest during the middle part (46Y57 degrees) of concentric phase range of motion (Table 2). However, EMG activity during the machine exercise seemed to decline more during the final part of concentric contraction phase (i.e., 60- to 90-degree knee angle) compared with hamstring curl exercises performed with elastic resistance, probably as a result of elastic force generation (i.e., external loading) being greatest at the more flexed knee angles. The eccentric contraction phase showed a somewhat different EMG-angle pattern than the concentric contraction phase. For both training modalities and the prime movers (BF and ST), the EMG-angle pattern
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showed a rather constant increase toward the more flexed knee joint positions, resulting in an angle at peak EMG ranging from 56 to 77 degrees during the eccentric phase. Nevertheless, a more flexed knee joint angle at peak EMG was seen during the elastic resistance compared with the machine exercise (P G 0.001) (Table 2). Maximal adductor and vastus lateralis EMG activity were higher (P G 0.05) during the eccentric phase when performing hamstring curls with elastic resistance compared with machine (Table 2). For the remainder of the muscles, there was no significant difference between the two types of exercise.
External Load and Contraction Time The mean T SE load of the machine exercise was 24.81 T 2.13 kg, ranging from 12.5 to 40 kg. The 10 RM elastic resistance ranged from a combination
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TABLE 2 Maximal nEMG (percentage of maximum) and angle at maximal nEMG obtained during the concentric and the eccentric phase of leg curls performed with machine and elastic resistance nEMG (% of Max)
Muscle BF ST Adductors Gluteus medius Erector spinae Obliques externus Rectus abdominis Rectus femoris Vastus lateralis Vastus medialis
Elastic
Angle at Peak EMG (degree)
Machine
Contraction Type
Mean
SE
Mean
SE
CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC
80.8 58.8 78.9 53.1 82.3 60.3 29.5 24.7 26.9 21.4 13.0 12.3 5.0 5.9 6.0 13.8 8.6 20.5 10.1 18.0
5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4
83.0 57.4 92.4 55.1 82.2 49.9 17.8 12.5 28.1 19.2 11.5 9.1 4.3 3.7 5.3 5.0 4.3 4.2 9.9 7.6
5.8 4.5 5.8 4.5 5.8 4.5 5.8 4.5 5.8 4.5 6.0 4.6 6.0 4.6 5.8 4.5 5.8 4.5 5.8 4.5
Elastic
NS NS NS NS NS a
NS NS NS NS NS NS NS NS NS NS NS b
NS NS
Machine
Mean
SE
Mean
SE
53.1 75.5 57.0 76.9 65.9 77.0 70.8 75.1 57.1 70.1 69.3 68.4 64.0 52.8 56.7 15.5 19.9 12.3 45.8 31.5
5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9
46.0 56.3 48.4 62.0 46.5 63.3 51.7 54.0 46.9 49.9 52.7 59.5 51.7 61.0 53.4 57.7 42.5 49.9 53.6 56.9
5.8 6.0 5.8 6.0 5.8 6.0 5.8 6.0 5.8 6.0 5.9 6.1 5.9 6.1 5.8 6.0 5.8 6.0 5.8 6.0
NS b
NS b b a b b a b b
NS a
NS NS b b b
NS b
Data presented as mean and SE (of the least square means). a Denotes a significant (P G 0.05) difference between elastic and machine exercise. b Denotes a significant (P G 0.01) difference between elastic and machine exercise. CON, concentric; ECC, eccentric; Max, maximum; NS, not significant.
of 1 blue to 3 silver and 1 blue (TheraBand elastic tubes). Irrespective of training modality (machine vs. elastic resistance) and contraction mode (concentric vs. eccentric), there was no significant difference in contraction time (i.e., time under tension) (1753 T 94 milliseconds and 1853 T 129 milliseconds, respectively, and 1736 T 94 milliseconds and 1791 T 102 milliseconds, respectively) during the hamstring curl exercise.
Perceived Loading and Influence of Age and Sex Perceived loading as determined by the Borg CR10 scale was significantly higher (P G 0.001) during elastic hamstring curls (7.58 T 0.08) compared with hamstring curls in training machine (5.92 T 0.03). There were no significant effects of age and sex on muscle activity (P = 0.39 and P = 0.27, respectively).
DISCUSSION The main finding of this study was that hamstring curl rehabilitation exercises performed with elastic resistance induce similar high peak hamstring muscle activity as hamstring curls using training machines. www.ajpmr.com
Although the use of elastic resistance rehabilitation exercises is increasing, the recommendations are primarily minded on the early stages of knee injury rehabilitation. The later stages of rehabilitation, however, require high-intensity activation throughout full range of motion, and although it is questionable whether elastic resistance can ensure this, physicians tend to recommend training machines over elastic resistance. Nevertheless, the present data demonstrate that hamstring curl exercises using elastic resistance induce equally high muscle activity (980%) as training machines without compromising the range of motion. Despite similar peak EMG values, the EMG amplitude was slightly lower at 20Y50 degrees for the ST during the concentric phase and for the ST and the BF during the eccentric phase of hamstring curls performed with elastic resistance. The lower EMG activity during the extended knee angles is probably a result of the linear increase in elastic force generation (i.e., external loading) increasing toward the more flexed knee angles. Nevertheless, the high compliance of the modern elastic tubes makes it possible to induce high hamstring muscle activity throughout large deformations, that is, increasing from È30% at full knee extension to È80% at more flexed knee angles. Consequently, in Hamstring Knee Rehabilitation Exercise
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the ST and the BF, angle at peak EMG was higher during the eccentric contraction phase when performing hamstring curls using elastic resistance. Furthermore, in contrast to the machine exercise, the elastic hamstring EMG-angle pattern seemed to decline less during the final part of the concentric phase (i.e., from angle at peak at È55 degrees vs. È47 degrees, elastic vs. machine exercise, respectively, to 90-degree knee flexion). Hence, elastic resistance hamstring exercises can induce high muscle activity without compromising the range of motion. Another minor visual difference (cf. Fig. 2) was an abrupt increase in EMG activity during the extended knee positions (20Y30 degrees) of the eccentric contraction phase of the machine exercise. This phenomenon may indicate the need for sudden active deceleration of the lever arm’s inertia compared with the more constant increase in EMG activity observed during the eccentric contraction with elastic resistance. As previously indicated, isolated hamstring curls using training machines are widely used and recommended in clinical rehabilitation of knee injury or hamstring musculoskeletal disorders. The seated hamstring curl has been reported to specifically target the medial hamstring (ST), whereas hamstring curl in a prone position has been reported to equally target the medial and the lateral hamstring (BF) muscle.19 Although both training modalities were performed as seated hamstring curls, the authors did not observe a difference, between muscles and/or exercise modalities, in medial or lateral hamstring muscle activity pattern in the present study. The relatively high levels of EMG amplitude during the more flexed knee joint position using elastic resistance may have important clinical relevance. Strength training performed with high force during flexed hip and knee positions has been suggested as a more functional approach to anterior cruciate ligament (ACL) injury rehabilitation and prophylactic training because the ACL injury often occurs during landing or side cutting with flexed hip and knees.19,20 Consequently, it may be speculated that the similar high peak EMG and the higher angle at peak EMG make the hamstring curl exercise using elastic resistance better or at least equally effective in ACL injury rehabilitation compared with the machine exercise. Perceived exertion was higher during the hamstring curls performed with elastic resistance compared with hamstring curls performed in training machines. A possible explanation may be
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the multiple degrees of freedom of the lower leg when performing the hamstring curl with elastic resistance compared with the more rigid lever arm movement of the training machine. Performing training exercises with multiple degrees of freedom may induce increased co-contraction from the stabilizing muscles. Accordingly, significantly higher adductor and vastus lateralis muscle activity were observed during the eccentric phase of hamstring curls performed with elastic resistance compared with the machine, thus potentially contributing to the higher degree of perceived exertion. Another contributing factor for these differences in perceived exertion could be the different seating arrangements. It seems plausible that the training machine, in comparison with the stool used during the elastic resistance training, was more comfortable because it had a softer and wider seat. Regardless of exercise modality, the hamstring curl exercise induced high adductor muscle activity (concentric and eccentric phase peak EMG of 980% and 950%, respectively). The adductors are uniarticular muscles working at the hip joint. Their primary function is to adduct the hip; however, the adductor longus also contributes in flexing the hip. Accordingly, the high adductor activity observed during both training modalities may have been induced as dynamic hip flexion or as a stabilizing static moment around the hip joint.8 It should be noted that the high adductor activity may be influenced by cross-talk from the medial hamstring. However, the EMG pattern and angle at peak EMG of the adductors and nearby ST were quite different. Thus, cross-talk induced by the ST seems highly unlikely. The hamstring curl exercise performed with elastic resistance seems to be a feasible and simple method for inducing relevant high levels of muscle activity to induce potential strength improvements of both the medial and lateral hamstring muscles. From a practical perspective, its portability makes it ideal for early and later stages of rehabilitation and for resistance training at work site, at home, in the hospital, or at training fields with limited resources for large and expensive training equipment. For instance, in hospital settings, a setup could be to fixate the elastic resistance tubing on the bed next to the patient and perform the exercises without having to move the patient to training facilities located elsewhere. Therapists and strength coaches working in the field of prevention and rehabilitation of injuries can use this knowledge to train their clients in settings where training machines are not accessible.
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Strengths and Limitations The fact that the EMG measurements were performed on untrained adults obviously limits the generalization of this study’s findings to novice trainees. On the other hand, because a large part of the population is fairly physically inactive during both work and leisure time, they might still be valid for a substantial part of the population. The fact that the participants learned the exercise technique and performed the repetition maximum tests to determine the appropriate load a week before testing is a strength because it minimizes fatigue on the actual day of testing. It should also be noted that although some training and control for correct performance were important to evaluate differences between exercises, the exercises require little instruction.
CONCLUSIONS In novice trainees, hamstring curls performed with elastic resistance induce similar high hamstring muscle activity as hamstring curls using training machines. Notably, elastic resistance seems to be a simple and feasible method for training mobility, with high muscle activity during all stages of rehabilitation. However, the perceived loading was higher when exercising with the elastic tubing compared with the training machines. This may in part be explained by the multiple degrees of freedom of the lower leg and the associated higher adductor muscle and vastus lateralis EMG activity when performing the hamstring curl exercise with elastic resistance. ACKNOWLEDGMENTS
workers in eldercare. Int Arch Occup Environ Health. 2012;82:615Y22 6. Zebis MK, Andersen LL, Pedersen MT, et al: Implementation of neck/shoulder exercises for pain relief among industrial workers: A randomized controlled trial. BMC Musculoskelet Disord 2011;12:205 7. Hopkins JT, Ingersoll CD, Sandrey MA, et al: An electromyographic comparison of 4 closed chain exercises. J Athl Train 1999;34:353Y7 8. Hintermeister RA, Bey MJ, Lange GW, et al: Quantification of elastic resistance knee rehabilitation exercises. J Orthop Sports Phys Ther 1998;28:40Y50 9. Bynum EB, Barrack RL, Alexander AH: Open versus closed chain kinetic exercises after anterior cruciate ligament reconstruction. A prospective randomized study. Am J Sports Med 1995;23:401Y6 10. Rhea MR, Alvar BA, Burkett LN, et al: A meta-analysis to determine the dose response for strength development. Med Sci Sports Exerc 2003;35:456Y64 11. Andersen LL, Andersen CH, Mortensen OS, et al: Muscle activation and perceived loading during rehabilitation exercises: Comparison of dumbbells and elastic resistance. Phys Ther 2010;90:538Y49 12. Andersen LL, Saervoll CA, Mortensen OS, et al: Effectiveness of small daily amounts of progressive resistance training for frequent neck/shoulder pain: Randomised controlled trial. Pain 2011;152:440Y6 13. Andersen LL, Magnusson SP, Nielsen M, et al: Neuromuscular activation in conventional therapeutic exercises and heavy resistance exercises: Implications for rehabilitation. Phys Ther 2006;86:683Y97 14. Zebis MK, Bencke J, Andersen LL, et al: The effects of neuromuscular training on knee joint motor control during sidecutting in female elite soccer and handball players. Clin J Sport Med 2008;18:329Y37
The authors thank the physical therapy students from Metropolitan University College for practical help during the project.
15. Jakobsen MD, Sundstrup E, Andersen CH, et al: Evaluation of muscle activity during a standardized shoulder resistance training bout in novice individuals. J Strength Cond Res 2011;26:2515Y22
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Affiliations: From the National Research Centre for the Working Environment, Copenhagen, Denmark (MDJ, ES, CHA, LLA); Department of Psychology, Lund University, Lund, Sweden (RP); Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark (MDJ, ES); and Arthroscopic Centre Amager and Gait Analysis Laboratory, Copenhagen University Hospital, Hvidovre, Denmark (MKZ).
Effectiveness of Hamstring Knee Rehabilitation Exercise Performed in Training Machine vs. Elastic Resistance Electromyography Evaluation Study
Correspondence: All correspondence and requests for reprints should be addressed to: Markus Due Jakobsen, MSc, National Research Centre for the Working Environment, LersL Parkalle 105, DK-2100 Copenhagen, Denmark.
ABSTRACT Jakobsen MD, Sundstrup E, Andersen CH, Persson R, Zebis MK, Andersen LL: Effectiveness of hamstring knee rehabilitation exercise performed in training machine vs. elastic resistance: electromyography evaluation study. Am J Phys Med Rehabil 2014;93:320Y327.
Objective: The aim of this study was to evaluate muscle activity during ham-
Disclosures: Supported by a grant from the Danish Working Environment Research Fund (grant no. 48-2010-03) (to L.L.A.). Financial disclosure statements have been obtained, and no conflicts of interest have been reported by the authors or by any individuals in control of the content of this article.
0894-9115/14/9304-0320 American Journal of Physical Medicine & Rehabilitation Copyright * 2013 by Lippincott Williams & Wilkins DOI: 10.1097/PHM.0000000000000043
string rehabilitation exercises performed in training machine compared with elastic resistance.
Design: Six women and 13 men aged 28Y67 yrs participated in a crossover study. Electromyographic (EMG) activity was recorded in the biceps femoris and the semitendinosus during the concentric and the eccentric phase of hamstring curls performed with TheraBand elastic tubing and Technogym training machines and normalized to maximal voluntary isometric contractionYEMG (normalized EMG). Knee joint angle was measured using electronic inclinometers. Results: Training machines and elastic resistance showed similar high levels of muscle activity (biceps femoris and semitendinosus peak normalized EMG 980%). EMG during the concentric phase was higher than during the eccentric phase regardless of exercise and muscle. However, compared with machine exercise, slightly lower (P G 0.05) normalized EMG values were observed using elastic resistance at 30- to 50-degree knee joint angle for the semitendinosus and the biceps femoris during the concentric and the eccentric phase, respectively. Perceived loading (Borg CR10) was significantly higher (P G 0.001) during hamstring curl performed with elastic resistance (7.58 T 0.08) compared with hamstring curl performed in a machine (5.92 T 0.03).
Conclusions: Hamstring rehabilitation exercise performed with elastic resistance induces similar peak hamstring muscle activity but slightly lower EMG values at more extended knee angles and with higher perceived loading as hamstring curls using training machines. Key Words:
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Strength Training, Hamstring, Perceived Exertion, Borg
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K
nee joint injuries may occur in many situations, for example, during sports, at work, or during other daily activities involving movement. Some of the individual consequences that have frequently been reported to follow knee joint trauma are severe pain, muscular atrophy, osteoarthritis, central activation failure, and reduced mobility leading to functional disability.1Y4 However, there are also social and economic consequences in that knee pain may be disabling in working life. Among a representative sample of 5600 blue and white collar workers, the prevalence of severe knee pain was 16% and 12%, respectively.5 Because knee pain increases the risk for long-term sickness absence, knee pain also has societal costs.5 Accordingly, it seems important to find and evaluate methods for rehabilitation. Rehabilitation after knee injury or surgery is a very challenging task. The availability of strength training equipment such as machines and free weights is often a limiting factor during rehabilitation and training interventions.6 As a consequence, the use of more accessible equipment such as elastic resistance exercises is increasing during knee rehabilitation after injury and/or surgery. In terms of knee injury rehabilitation, the main aims are to regain full pain-free mobility of the knee followed by increased strength of the surrounding muscles (quadriceps, hamstrings, and calf muscles). Because the early stages of knee injury rehabilitation are mainly focused on mobility rather than high-intensity strength training, the use of elastic resistance exercises is highly advised during this stage.7Y9 Nevertheless, as the strength of the surrounding muscles increases through the stages of rehabilitation, the use of more traditional, but less accessible, exercises with weights or machines is recommended to ensure sufficient range of motion and intensity for gains in strength. Until now, only a few studies have investigated muscular activity (electromyography [EMG]) during hamstring knee rehabilitation exercises using elastic resistance.7,8 These studies demonstrated activation values lower than the recommended 60% threshold for muscular hypertrophy and gains in strength.10 Accordingly, although recent studies have demonstrated elastic resistance to be equally effective in strengthening smaller muscles in the neck, the shoulder, and the arm compared with free weight training,11,12 it remains questionable whether elastic resistance activities with larger muscle groups such as during hamstring knee rehabilitation exercises can induce sufficiently high intensity of muscle activation, www.ajpmr.com
without compromising the range of motion, as using traditional training machines. In addition, the EMGYjoint angle relationship that is well described for conventional strength training exercises13 remains largely unexplored for high-intensity elastic resistance exercise. The purpose of the present study was to evaluate muscle activity pattern during hamstring curl exercises performed with a training machine or with elastic resistance. The authors hypothesized that hamstring curl exercise performed with elastic tubing would induce similar high muscle activity as hamstring curl exercise performed with a training machine.
MATERIALS AND METHODS Subjects This study was performed in Copenhagen, Denmark. A group of 19 untrained adults (6 women and 13 men) were recruited from a large workplace with various job tasks. Exclusion criteria were blood pressure higher than 160/100, disk prolapse, or serious chronic disease. Complete testing was performed on all participants, that is, with both elastic resistance and in a machine. All subjects were informed about the purpose and the content of the project and gave written informed consent to participate in this study, which conformed to the Declaration of Helsinki and was approved by the Local Ethical Committee (H-3-2010-062).
Study Design The participants visited the researchers on two occasions. On the first occasion, 1 wk before testing, the participants received training and were tested for maximum performance to determine the load used for testing. On the second occasion, the participants were tested with EMG and inclinometry with the aim to compare two different exercises in a crossover design.
Exercise Equipment Two different types of training equipment were used: (1) elastic tubing with resistances ranging from light to very heavy (red, green, blue, black, and gray colors; TheraBand, Akron, United States) and (2) machine (vertical seated hamstring curl; Technogym, Gambettola, Italy) (cf. Fig. 1).
Exercise Description During the training session, the participants performed a 10 repetition maximum test (10 RM) for the two exercises. During the elastic resistance Hamstring Knee Rehabilitation Exercise
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FIGURE 1 Illustration of the hamstring curl exercise with elastic tubing (A) and in the training machine (B).
exercise, the 10 RM loading was found using one or a combination of several color-coded elastic tubes with resistances ranging from light to very heavy (red, green, blue, black, and gray), whereas the 10 RM loading of the machine exercise was determined in kilograms. All exercises were performed unilaterally. On the day of EMG measurements, the participants warmed up with submaximal loads and then performed three consecutive repetitions for each exercise using a 10 RM load to avoid the influence of fatigue on the subsequent exercises. Exercises were performed in a controlled manner at a slow constant speed (each repetition lasting È3 secs). The order of exercises was randomized for each subject, and the rest period between different exercises was approximately 5 mins. The exercises are described in detail below (Fig. 1).
Hamstring Curl with Elastic Resistance The participant was sitting on a high chair placed 2.5 m from a wooden rib with one leg fully extended and a 90-degree hip flexion, whereas the other leg was hanging loose. The elastic tubing was fixated at one end to the participant’s ankle and attached in the other end to the wooden rib at 1.7-m height. The elastic tubing was stretched to È200% of the initial length (initial length, 1.2 m). The participant started flexing the knee (concentric phase) until È90-degree knee joint angle and then extended the knee (eccentric phase) until full extension (È0-degree knee joint angle) (Fig. 1A).
Hamstring Curl in Machine The participant was seated in a Technogym vertical leg curl machine with the leg fully extended and a 100-degree hip flexion. The participant started flexing the knee (concentric phase) until È90-degree knee joint angle and then extended the knee (eccentric phase) until full extension (È0-degree knee joint angle) (Fig. 1B).
Inclinometer Sampling and Analysis Knee joint angle was continuously measured using two electronic inclinometers (2-dimensional
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Direct Transmission System (DTS) inclination sensor; Noraxon, Arizona, United States) placed at the lateral side of the tibia and the femur. The inclinometer data were synchronously sampled with the EMG data, using the 16-channel 16-bit personal computer-interface receiver (TeleMyo DTS Telemetry; Noraxon, Arizona, United States). The dimension of the probes was 3.4 cm 2.4 cm 3.5 cm. During subsequent analysis, the inclinometer signals were digitally low-pass filtered using a fourth-order zerolag Butterworth filter (3-Hz cutoff frequency). The momentary knee joint angle was calculated as the difference in angular position, with respect to the gravitational line, between the tibia and femur inclinometers. Knee joint angles ranged from a 90-degree flexed position to a 0-degree full knee extension. The concentric and eccentric phases were defined as periods with negative and positive angular velocity, respectively (going from 90 to 0 degrees and 0 to 90 degrees, respectively). Angle at peak EMG was calculated within the concentric and eccentric phases.
Maximal Voluntary Isometric Contraction Before the dynamic exercises, maximal voluntary isometric contractions (MVCs) were performed according to standardized procedures during (1) static knee extension and flexion maneuvers (positioned in a Biodex dynamometer: knee angle, 70 degrees, and hip angle, 110 degrees), (2) hip adduction (lying flat on the back and pressing the knees against a solid ball), (3) hip abduction (lying flat on the back and pressing the knees outward against a rigid band) and (4) hip extension (lying flat on the stomach with the knee flexed [90 degrees] and pressing the foot upward against the instructor’s hands), and (5) trunk extension (in standing posture and the pelvis fixated with the trunk extended against a rigid band) to induce a maximal EMG response in the tested muscles.14 Two MVCs were performed for each muscle, and the trial with the highest root mean square (RMS) EMG value was subsequently used for normalization of the RMS EMG signals obtained in the resistance exercises. During the MVCs, the subjects were instructed to gradually increase muscle contraction
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force toward maximum for a period of 2 secs, sustain the MVC for 3 secs, and then slowly release the force again. Strong and standardized verbal encouragement was given during all trials by the test leader, who repeatedly commanded BPush, Push, Push.[
EMG Signal Sampling and Analysis EMG signals were recorded from 10 leg, abdominal, and lower back muscles: vastus medialis, vastus lateralis, rectus femoris, biceps femoris (BF), semitendinosus (ST), gluteus medius, adductors (adductor longus), right erector spinae, right obliques externus, and right rectus abdominis. A bipolar surface EMG configuration (Blue Sensor N-00-S; Ambu A/S, Ballerup, Denmark) and an interelectrode distance of 2 cm were used.13,15 Before affixing the electrodes, the skin of the respective area was prepared with scrubbing gel (Acqua gel; Meditec, Parma, Italy) to effectively lower the impedance to less than 10 k6.11 Electrode placements followed Surface ElectroMyoGraphy for the Non-Invasive Assessment of Muscles (SENIAM) recommendations (www.seniam.org). The EMG electrodes were connected directly to wireless probes that preamplified the signal (gain 400) and transmitted data in real time to a 16-channel 16-bit PC-interface receiver (TeleMyo DTS Telemetry; Noraxon, Arizona, United States). The dimension of the probes was 3.4 cm 2.4 cm 3.5 cm. Data sampling rate was 1500 Hz with a bandwidth of 10Y500 Hz to avoid aliasing. Common mode rejection ratio was higher than 100 dB. During later offline analysis, all raw EMG signals obtained during MVCs and during the exercises were digitally filtered by a Butterworth fourth-order highpass filter (10-Hz cutoff frequency). For each individual muscle, maximal moving RMS (500 milliseconds constant) EMG was used to identify peak EMG within the concentric and the eccentric phase, whereas the RMS of the high-pass filtered EMG signal was calculated within each 10-degree angle interval (0Y10 degrees, 10Y20 degrees, I, 80Y90 degrees) of the concentric and the eccentric phase and normalized to the maximal moving RMS (500-milliseconds time constant) EMG obtained during MVC.11,15,16 Contraction time was calculated according to procedures previously described.15
Perceived Loading Immediately after each set of exercise, the Borg CR10 scale17 was used to rate perceived loading during the resistance exercise. The authors have previously validated this scale in the evaluation www.ajpmr.com
of neck/shoulder resistance exercises with elastic resistance.11
Statistical Analysis A two-way repeated-measures analysis of variance (Proc Mixed, SAS version 9; SAS Institute, Cary, NC) was used to locate differences between exercises and range of knee joint motion for each muscle and contraction mode (concentric or eccentric), vastus medialis to vastus lateralis activation ratio, perceived loading (BORG), and contraction time. Factors included in the model were exercise (elastic resistance and machine) and knee joint angle (0Y90 degrees) as well as exercise by knee joint angle interaction. The analysis was controlled for sex and age. Normalized EMG (nEMG) was the dependent variable. Values are reported as least square means (standard error) unless otherwise stated. P e 0.05 was considered statistically significant. A priori power analysis showed that 19 participants in this paired design were sufficient to obtain a statistical power of 80% at a minimal relevant difference of 10% and a type I error probability of 1%, assuming an SD of 10% based on previous research in the authors’ laboratory.18
RESULTS Demographics Table 1 shows the demographics.
Normalized EMG Figure 2 shows the nEMG-angle relationship for the selected muscles during hamstring curl exercises performed in a machine or with elastic resistance during the 0- to 90-degree knee joint range of motion. There was no significant difference in maximal hamstring EMG between elastic resistance and machine exercise (Table 2). However, in the ST, the concentric EMG-angle pattern (0Y10 degrees, 10Y20 degrees, I, 80Y90 degrees) was significantly TABLE 1 Demographics of the participants
n Age, yrs Height, cm Weight, kg BMI
Men
Women
13 39 (13) 178 (7) 80 (7) 25 (2)
6 42 (10) 167 (8) 68 (17) 24 (6)
Data presented as mean (standard deviation). BMI, body mass index.
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FIGURE 2 nEMG and angular position during the concentric and the eccentric contraction phase of hamstring curl exercises with elastic tubing and in the training machine in the BF (top) and ST (bottom) muscles.
higher (P G 0.05) at 30Y50 degrees during the machine exercise compared with the elastic resistance. Similarly, the ST and the BF were higher (P G 0.05) at 30Y50 degrees during the eccentric contraction phase performed with the machine (Fig. 1). Hamstring EMG was highest during the middle part (46Y57 degrees) of concentric phase range of motion (Table 2). However, EMG activity during the machine exercise seemed to decline more during the final part of concentric contraction phase (i.e., 60- to 90-degree knee angle) compared with hamstring curl exercises performed with elastic resistance, probably as a result of elastic force generation (i.e., external loading) being greatest at the more flexed knee angles. The eccentric contraction phase showed a somewhat different EMG-angle pattern than the concentric contraction phase. For both training modalities and the prime movers (BF and ST), the EMG-angle pattern
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showed a rather constant increase toward the more flexed knee joint positions, resulting in an angle at peak EMG ranging from 56 to 77 degrees during the eccentric phase. Nevertheless, a more flexed knee joint angle at peak EMG was seen during the elastic resistance compared with the machine exercise (P G 0.001) (Table 2). Maximal adductor and vastus lateralis EMG activity were higher (P G 0.05) during the eccentric phase when performing hamstring curls with elastic resistance compared with machine (Table 2). For the remainder of the muscles, there was no significant difference between the two types of exercise.
External Load and Contraction Time The mean T SE load of the machine exercise was 24.81 T 2.13 kg, ranging from 12.5 to 40 kg. The 10 RM elastic resistance ranged from a combination
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TABLE 2 Maximal nEMG (percentage of maximum) and angle at maximal nEMG obtained during the concentric and the eccentric phase of leg curls performed with machine and elastic resistance nEMG (% of Max)
Muscle BF ST Adductors Gluteus medius Erector spinae Obliques externus Rectus abdominis Rectus femoris Vastus lateralis Vastus medialis
Elastic
Angle at Peak EMG (degree)
Machine
Contraction Type
Mean
SE
Mean
SE
CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC CON ECC
80.8 58.8 78.9 53.1 82.3 60.3 29.5 24.7 26.9 21.4 13.0 12.3 5.0 5.9 6.0 13.8 8.6 20.5 10.1 18.0
5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4 5.7 4.4
83.0 57.4 92.4 55.1 82.2 49.9 17.8 12.5 28.1 19.2 11.5 9.1 4.3 3.7 5.3 5.0 4.3 4.2 9.9 7.6
5.8 4.5 5.8 4.5 5.8 4.5 5.8 4.5 5.8 4.5 6.0 4.6 6.0 4.6 5.8 4.5 5.8 4.5 5.8 4.5
Elastic
NS NS NS NS NS a
NS NS NS NS NS NS NS NS NS NS NS b
NS NS
Machine
Mean
SE
Mean
SE
53.1 75.5 57.0 76.9 65.9 77.0 70.8 75.1 57.1 70.1 69.3 68.4 64.0 52.8 56.7 15.5 19.9 12.3 45.8 31.5
5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9 5.7 5.9
46.0 56.3 48.4 62.0 46.5 63.3 51.7 54.0 46.9 49.9 52.7 59.5 51.7 61.0 53.4 57.7 42.5 49.9 53.6 56.9
5.8 6.0 5.8 6.0 5.8 6.0 5.8 6.0 5.8 6.0 5.9 6.1 5.9 6.1 5.8 6.0 5.8 6.0 5.8 6.0
NS b
NS b b a b b a b b
NS a
NS NS b b b
NS b
Data presented as mean and SE (of the least square means). a Denotes a significant (P G 0.05) difference between elastic and machine exercise. b Denotes a significant (P G 0.01) difference between elastic and machine exercise. CON, concentric; ECC, eccentric; Max, maximum; NS, not significant.
of 1 blue to 3 silver and 1 blue (TheraBand elastic tubes). Irrespective of training modality (machine vs. elastic resistance) and contraction mode (concentric vs. eccentric), there was no significant difference in contraction time (i.e., time under tension) (1753 T 94 milliseconds and 1853 T 129 milliseconds, respectively, and 1736 T 94 milliseconds and 1791 T 102 milliseconds, respectively) during the hamstring curl exercise.
Perceived Loading and Influence of Age and Sex Perceived loading as determined by the Borg CR10 scale was significantly higher (P G 0.001) during elastic hamstring curls (7.58 T 0.08) compared with hamstring curls in training machine (5.92 T 0.03). There were no significant effects of age and sex on muscle activity (P = 0.39 and P = 0.27, respectively).
DISCUSSION The main finding of this study was that hamstring curl rehabilitation exercises performed with elastic resistance induce similar high peak hamstring muscle activity as hamstring curls using training machines. www.ajpmr.com
Although the use of elastic resistance rehabilitation exercises is increasing, the recommendations are primarily minded on the early stages of knee injury rehabilitation. The later stages of rehabilitation, however, require high-intensity activation throughout full range of motion, and although it is questionable whether elastic resistance can ensure this, physicians tend to recommend training machines over elastic resistance. Nevertheless, the present data demonstrate that hamstring curl exercises using elastic resistance induce equally high muscle activity (980%) as training machines without compromising the range of motion. Despite similar peak EMG values, the EMG amplitude was slightly lower at 20Y50 degrees for the ST during the concentric phase and for the ST and the BF during the eccentric phase of hamstring curls performed with elastic resistance. The lower EMG activity during the extended knee angles is probably a result of the linear increase in elastic force generation (i.e., external loading) increasing toward the more flexed knee angles. Nevertheless, the high compliance of the modern elastic tubes makes it possible to induce high hamstring muscle activity throughout large deformations, that is, increasing from È30% at full knee extension to È80% at more flexed knee angles. Consequently, in Hamstring Knee Rehabilitation Exercise
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the ST and the BF, angle at peak EMG was higher during the eccentric contraction phase when performing hamstring curls using elastic resistance. Furthermore, in contrast to the machine exercise, the elastic hamstring EMG-angle pattern seemed to decline less during the final part of the concentric phase (i.e., from angle at peak at È55 degrees vs. È47 degrees, elastic vs. machine exercise, respectively, to 90-degree knee flexion). Hence, elastic resistance hamstring exercises can induce high muscle activity without compromising the range of motion. Another minor visual difference (cf. Fig. 2) was an abrupt increase in EMG activity during the extended knee positions (20Y30 degrees) of the eccentric contraction phase of the machine exercise. This phenomenon may indicate the need for sudden active deceleration of the lever arm’s inertia compared with the more constant increase in EMG activity observed during the eccentric contraction with elastic resistance. As previously indicated, isolated hamstring curls using training machines are widely used and recommended in clinical rehabilitation of knee injury or hamstring musculoskeletal disorders. The seated hamstring curl has been reported to specifically target the medial hamstring (ST), whereas hamstring curl in a prone position has been reported to equally target the medial and the lateral hamstring (BF) muscle.19 Although both training modalities were performed as seated hamstring curls, the authors did not observe a difference, between muscles and/or exercise modalities, in medial or lateral hamstring muscle activity pattern in the present study. The relatively high levels of EMG amplitude during the more flexed knee joint position using elastic resistance may have important clinical relevance. Strength training performed with high force during flexed hip and knee positions has been suggested as a more functional approach to anterior cruciate ligament (ACL) injury rehabilitation and prophylactic training because the ACL injury often occurs during landing or side cutting with flexed hip and knees.19,20 Consequently, it may be speculated that the similar high peak EMG and the higher angle at peak EMG make the hamstring curl exercise using elastic resistance better or at least equally effective in ACL injury rehabilitation compared with the machine exercise. Perceived exertion was higher during the hamstring curls performed with elastic resistance compared with hamstring curls performed in training machines. A possible explanation may be
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the multiple degrees of freedom of the lower leg when performing the hamstring curl with elastic resistance compared with the more rigid lever arm movement of the training machine. Performing training exercises with multiple degrees of freedom may induce increased co-contraction from the stabilizing muscles. Accordingly, significantly higher adductor and vastus lateralis muscle activity were observed during the eccentric phase of hamstring curls performed with elastic resistance compared with the machine, thus potentially contributing to the higher degree of perceived exertion. Another contributing factor for these differences in perceived exertion could be the different seating arrangements. It seems plausible that the training machine, in comparison with the stool used during the elastic resistance training, was more comfortable because it had a softer and wider seat. Regardless of exercise modality, the hamstring curl exercise induced high adductor muscle activity (concentric and eccentric phase peak EMG of 980% and 950%, respectively). The adductors are uniarticular muscles working at the hip joint. Their primary function is to adduct the hip; however, the adductor longus also contributes in flexing the hip. Accordingly, the high adductor activity observed during both training modalities may have been induced as dynamic hip flexion or as a stabilizing static moment around the hip joint.8 It should be noted that the high adductor activity may be influenced by cross-talk from the medial hamstring. However, the EMG pattern and angle at peak EMG of the adductors and nearby ST were quite different. Thus, cross-talk induced by the ST seems highly unlikely. The hamstring curl exercise performed with elastic resistance seems to be a feasible and simple method for inducing relevant high levels of muscle activity to induce potential strength improvements of both the medial and lateral hamstring muscles. From a practical perspective, its portability makes it ideal for early and later stages of rehabilitation and for resistance training at work site, at home, in the hospital, or at training fields with limited resources for large and expensive training equipment. For instance, in hospital settings, a setup could be to fixate the elastic resistance tubing on the bed next to the patient and perform the exercises without having to move the patient to training facilities located elsewhere. Therapists and strength coaches working in the field of prevention and rehabilitation of injuries can use this knowledge to train their clients in settings where training machines are not accessible.
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Strengths and Limitations The fact that the EMG measurements were performed on untrained adults obviously limits the generalization of this study’s findings to novice trainees. On the other hand, because a large part of the population is fairly physically inactive during both work and leisure time, they might still be valid for a substantial part of the population. The fact that the participants learned the exercise technique and performed the repetition maximum tests to determine the appropriate load a week before testing is a strength because it minimizes fatigue on the actual day of testing. It should also be noted that although some training and control for correct performance were important to evaluate differences between exercises, the exercises require little instruction.
CONCLUSIONS In novice trainees, hamstring curls performed with elastic resistance induce similar high hamstring muscle activity as hamstring curls using training machines. Notably, elastic resistance seems to be a simple and feasible method for training mobility, with high muscle activity during all stages of rehabilitation. However, the perceived loading was higher when exercising with the elastic tubing compared with the training machines. This may in part be explained by the multiple degrees of freedom of the lower leg and the associated higher adductor muscle and vastus lateralis EMG activity when performing the hamstring curl exercise with elastic resistance. ACKNOWLEDGMENTS
workers in eldercare. Int Arch Occup Environ Health. 2012;82:615Y22 6. Zebis MK, Andersen LL, Pedersen MT, et al: Implementation of neck/shoulder exercises for pain relief among industrial workers: A randomized controlled trial. BMC Musculoskelet Disord 2011;12:205 7. Hopkins JT, Ingersoll CD, Sandrey MA, et al: An electromyographic comparison of 4 closed chain exercises. J Athl Train 1999;34:353Y7 8. Hintermeister RA, Bey MJ, Lange GW, et al: Quantification of elastic resistance knee rehabilitation exercises. J Orthop Sports Phys Ther 1998;28:40Y50 9. Bynum EB, Barrack RL, Alexander AH: Open versus closed chain kinetic exercises after anterior cruciate ligament reconstruction. A prospective randomized study. Am J Sports Med 1995;23:401Y6 10. Rhea MR, Alvar BA, Burkett LN, et al: A meta-analysis to determine the dose response for strength development. Med Sci Sports Exerc 2003;35:456Y64 11. Andersen LL, Andersen CH, Mortensen OS, et al: Muscle activation and perceived loading during rehabilitation exercises: Comparison of dumbbells and elastic resistance. Phys Ther 2010;90:538Y49 12. Andersen LL, Saervoll CA, Mortensen OS, et al: Effectiveness of small daily amounts of progressive resistance training for frequent neck/shoulder pain: Randomised controlled trial. Pain 2011;152:440Y6 13. Andersen LL, Magnusson SP, Nielsen M, et al: Neuromuscular activation in conventional therapeutic exercises and heavy resistance exercises: Implications for rehabilitation. Phys Ther 2006;86:683Y97 14. Zebis MK, Bencke J, Andersen LL, et al: The effects of neuromuscular training on knee joint motor control during sidecutting in female elite soccer and handball players. Clin J Sport Med 2008;18:329Y37
The authors thank the physical therapy students from Metropolitan University College for practical help during the project.
15. Jakobsen MD, Sundstrup E, Andersen CH, et al: Evaluation of muscle activity during a standardized shoulder resistance training bout in novice individuals. J Strength Cond Res 2011;26:2515Y22
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