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The effect of training status on inter-limb joint stiffness regulation during repeated maximal sprints
Available online at www.sciencedirect.com
Journal of Science and Medicine in Sport 12 (2009) 406–410
Original paper
The effect of training status on inter-limb joint stiffness regulation during repeated maximal sprints Ross A. Clark a,b,∗ a
Department of Health and Human Performance, Faculty of Arts, Health and Science, Central Queensland University, North Rockhampton, QLD 4702, Australia b Centre for Health, Exercise and Sports Medicine, School of Physiotherapy, The University of Melbourne, 202 Berkeley Street, Parkville, VIC 3010, Australia Received 8 March 2007; received in revised form 21 August 2007; accepted 11 December 2007
Abstract The purpose of this study was to examine the effect of anaerobic fatigue and training status on the joint stiffness (JS) regulation of the lower limbs. Twenty-two subjects participated in this study, with a group of athletes (ATH: n = 11, age: 22.1 ± 9.9 yrs, ht: 181.9 ± 6.3 cm, mass: 88.2 ± 12.7 kg) compared to a group of non-athletes (NON: n = 11, age: 20.9 ± 2.3 yrs, ht: 177.8 ± 7.1 cm, mass: 80.9 ± 22.0 kg). A modified phosphate decrement test, which consisted of eight 35 m timed sprints separated by a 30 s active recovery, allowed for inducement of anaerobic fatigue while incorporating measures of sprinting performance and JS. Assessment of JS consisted of a single-legged 2.2 Hz spring-mass hopping protocol, measured for each limb. This test was performed prior to the warm-up and after sprints two, four and six. Data analysis consisted of repeated measures MANOVA comparing groups, limbs and test. Repeated measures ANOVAs were also performed on the sprint times and the magnitude of inter-limb JS difference. For all data analysis the alpha level was set at p < 0.05. Assessment of between limb JS revealed that the ATH group possessed significantly lower inter-limb variation in comparison with the NON group after completion of the first pair of sprints, potentially due to their training status offsetting some of the mechanical and neuromuscular effects of repeated stretch-shortening cycle (SSC) fatigue. This enhanced ability to regulate inter-limb JS, in addition to enhancing performance, may reduce the risk of injury by preserving mechanical efficiency and therefore reducing metabolic cost during SSC contractions. © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. Keywords: Stiffness; Fatigue; Stretch-shortening cycle; Biomechanics
Introduction The relationship between musculotendinous stiffness (MTS) and functional performance, injury prevention and injury management has received a great deal of interest in the current literature.1–4 Part of this interest is due to the apparent adaptability of this structural property, with previous research suggesting that MTS is ‘tuned’ based on prior contraction history.4–6 One of the most effective methods of creating an adaptation in MTS is to perform eccentric contractions, ∗ Centre for Health, Exercise and Sports Medicine, The School of Physiotherapy, University of Melbourne, 202 Berkeley Street, Parkville, VIC 3010, Australia. Tel.: +61 0431 737609. E-mail address: [email protected].
which have been shown to result in an increased stiffness of the system.4 In addition to providing benefits in regards to enhanced joint stability, an increase in MTS also appears to enhance performance during rapid counter movements, such as maximal velocity sprinting.7–10 This is because a stiffer system allows for more efficient elastic energy contribution at high countermovement speeds, thereby enhancing force production during the concentric phase of the movement.8,11,12 The magnitude of this contribution of elastic energy to countermovement performance has resulted in previous research suggesting that stride frequency in running mammals is governed primarily by this property.9,13 In addition to these adaptations in MTS, eccentric training has also been suggested as a method of minimising the risk of soft tissue injury.14–16 This occurs by reducing the
1440-2440/$ – see front matter © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2007.12.003
R.A. Clark / Journal of Science and Medicine in Sport 12 (2009) 406–410
range of motion in which a muscle is undergoing eccentric lengthening during the descending limb of the sarcomeres length-tension relationship.14–16 A vast number of other benefits of eccentric contractions have also been suggested as means of reducing injury risk factors, such as augmented fast-twitch muscle fibre size and strength, and the numerous adaptations deemed responsible for the repeated bouts effect.16 These responses to eccentric exercise appear to optimise the protective strategies of the musculotendinous system, resulting in an improved ability to withstand high intensity eccentric contractions.4,16 In regards to optimising performance, it would appear that the athlete should possess a biomechanical pattern that has the MTS of the involved musculature tuned for the movements performed during their sport. However, this regulation of optimal MTS may become compromised if either the additional structural properties, or the neuromuscular activation signals being received by the involved musculature, are dramatically modified.3,17–19 These two variables are highly susceptible to change during fatigue, especially when unaccustomed eccentric contractions are involved.3,18,19 However, these changes may be offset by the previously mentioned adaptations that arise in response to eccentric contractions, potentially resulting in a more structurally stable musculotendinous system with an improved ability to withstand the trauma that this form of contraction induces.4 Consequently, a portion of the reduction in performance associated with neuromuscular fatigue would be diminished due to an enhanced ability to maintain mechanical efficiency. This would theoretically result in a musculotendinous system that is less prone to fluctuations in MTS as a result of repeated, high intensity eccentric contractions, resulting in enhanced performance and a reduced risk of injury. However, whether athletes who are accustomed to performing repeated, high intensity eccentric contractions display an improved ability to regulate MTS during fatiguing tasks is unknown. Therefore, the purpose of this study was to compare the joint stiffness (JS), of which MTS is a major component, of athletes with extensive eccentric training backgrounds and recreational sporting participants during repeated maximal intensity sprints.
Methodology Testing overview The testing session consisted of a five minute standardised warm-up, followed by performance of a modified phosphate decrement test. This test consisted of eight repetitions of maximal intensity 35 m sprints, with a 30 s active recovery between repetitions. A repeated sprint test was chosen due to both sporting specificity and the high level of eccentric muscular activation, and subsequent fatigue, it induces. An examination of the JS of each lower limb was performed prior to the warm-up and after sprints two, four and six.
407
Subjects Two separate groups of subjects volunteered to participate in this study. The first group possessed extensive athletic training backgrounds (ATH), and included eleven semiprofessional rugby league players (age: 22.1 ± 9.9 yrs, ht: 181.9 ± 6.3 cm, mass: 88.2 ± 12.7 kg). These subjects participated in a minimum of five days of field, resistance and/or plyometric training per week, and had all performed multiple six week microcycles of eccentric specific hamstring training, in the form of the Nordic hamstring stretch exercise. The second group consisted of eleven recreational sporting participants (age: 20.9 ± 2.3 yrs, ht: 177.8 ± 7.1 cm, mass: 80.9 ± 22.0 kg), deemed the non-athlete (NON) subject group. These subjects all performed less than three days of sports related activities per week, but were participating in organised sporting competitions at a recreational level. They were also required to have no prior training history that included a large eccentric component, for example eccentric specific, sprint or plyometric training, in the six months leading up to the study. All subjects completed a pre-activity readiness questionnaire and signed an informed consent document prior to commencement of any data collection. Ethical approval for the methodologies involved in this study was granted by Central Queensland University. Joint stiffness tests The tests of JS consisted of an adaptation of the standard lower limb hopping test performed on a force plate (AMTI, USA), which has been used extensively in previous research.2,4,9 The subjects were instructed to hop on the limb instructed by the examiner, in time to a metronome providing an audible signal at a frequency of 2.2 Hz. During hopping, the subjects were instructed to look forward, maintain hand contact with their hips and attempt to minimise movement of the limb not being tested. Once a steady hopping rhythm at the desired frequency was attained, force and time data were collected via a custom written Labview (National Instruments, USA) data acquisition program sampling at 1000 Hz for three seconds. The value for JS was obtained using an adaptation of the equation previously reported by Farley et al.,9 which is based on the undamped oscillations occurring during simple harmonic motion.20 This equation, along with the extended methodology of the JS tests, is provided in Appendices A of the supplemental material. Due to the intent of the examiner to incorporate a fatiguing intervention which replicated the events occurring during sporting situations, this study was performed on a grass covered rugby oval. To ensure accurate data collection via the force plate, it was thus deemed essential to ensure the stability and orientation of the force plate. This was achieved by mounting the force plate onto a solid aluminium plate, which possessed larger dimensions than the force plate and was approximately 2.5 cm thick. These larger dimensions
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enhanced the stability of the force plate, while the added mass prevented both excess movement and damping due to the grass it was placed on. Resting joint stiffness and warm-up Immediately prior to the warm-up, JS tests of both the dominant and non-dominant limbs were performed according to the previously mentioned protocol, providing a resting JS value. The warm-up consisted of low-intensity agility and plyometric exercises performed for five minutes. These included ladder and mini-hurdle drills designed to prepare the subjects for the subsequent maximal sprints. All subjects performed an identical number of ground contacts during the exercises in the warm-up. The modified phosphate decrement test A diagram of the testing protocol during the modified phosphate decrement test is shown in Fig. 1. An extended methodology for this test is provided in Appendices B of the supplemental material. Data analysis The subjects sprint times for each interval were recorded by the timing lights. This raw data was then modified by calculating the mean sprint time for each group of two sprints performed between the JS tests. This resulted in four values for sprint times for each subject, the mean of sprints one and two, three and four, five and six and finally sprints seven and eight. A repeated measures ANOVA was performed to examine whether any significant changes in sprint times were recorded either within or between groups. The subjects JS results were also examined, with a total of four results recorded for each limb per subject. This resulted in a 2 group × 2 limb × 4 test repeated measures MANOVA. Therefore, an examination was performed to determine whether any significant effects or interactions occurred between limbs, groups or test. In addition, repeated measures ANOVA was performed to examine the magnitude of difference in between limb JS regardless of limb domi-
Figure 1. The layout of the modified phosphate decrement test. The subjects performed eight 35 m sprints with a 30 s active recovery between sprints. JS tests of each limb individually were performed before testing and after sprints two, four and six. Step 1 perform a 35 m sprint. Step 2 decelerate around the marker and walk back to the initial finish line. Step 3 perform a 35 m sprint. Step 4 decelerate to the force plate. Perform dominant limb JS test, step off and walk around marker. Return to the force plate and perform non-dominant limb JS test. Return to starting line for next sprint.
Figure 2. Mean sprint times during each pair of sprints in the modified phosphate decrement test. *ATH recorded significantly lower (p < 0.05) sprint times during each pair of sprints in comparison with the NON group. A significant (p < 0.05) increase in sprint times was also recorded for both groups between each testing interval, signalling fatigue.
nance. This was performed by determining the magnitude of inter-limb difference in JS for each test, with the repeated measures ANOVA examining the relationship between group and testing interval. Fischers LSD post hoc analysis was performed in the event of a significant main effect or interaction in any of the preceding statistical tests, with the alpha level set for all analysis at p < 0.05. Data sphericity (lowerbound epsilon > 0.75) and equal variance (Levene Median Test) were also assessed to ensure the validity of the statistical analysis. Results The results for sprint times are provided in Fig. 2. As expected, the ATH group recorded significantly lower sprint times as a group (F = 16.835, p = 0.002, mean sprint speed 13% lower in the ATH group), with post hoc tests revealing the ATH group recorded significantly (p < 0.05) lower sprint times than the NON group during each interval. In addition, sprint times significantly (p < 0.05) increased for both groups as the testing session progressed. The results for the JS test are supplied in Fig. 3, with the significant main effect for group (F = 11.388, p = 0.003) revealing that the ATH group recorded significantly higher mean JS values than the NON group. No significant differences were observed for dominant versus non-dominant limb JS or JS values during each testing interval. However, when the data was analysed to examine the magnitude of inter-limb difference, shown in Fig. 4, a significant main effect (F = 34.297, p = 0.000) for group was observed. This revealed that the ATH group possessed significantly lower overall inter-limb JS disparity throughout the testing session. Post hoc analysis showed a significant difference (F = 10.351, p = 0.005) between groups in the JS test performed after the first pair of sprints was completed, with the ATH group possessing less than a third of the inter-limb difference observed in the NON group.
R.A. Clark / Journal of Science and Medicine in Sport 12 (2009) 406–410
Figure 3. Hopping JS values during each interval of the modified decrement test. The ATH group recorded significantly (p < 0.05) higher JS values than the NON group both between groups and intervals. No significant differences were observed between either dominant and non-dominant limbs or between groups.
Discussion The results of this study suggest that athletes participating in a training program that includes field, resistance, plyometric and eccentric specific training are more successful than recreational sporting participants at regulating JS during repeated sprints. This was demonstrated by the fact that no significant differences in JS were observed between limbs for the ATH group, while in contrast the NON group showed a significant increase in between limb JS difference after the initial two sprints. Furthermore, although non-significant, the magnitude of between limb JS difference for the NON group was higher during each of the other three tests. This finding suggests that the performance of the first two repeated sprints resulted in a great deal of ‘shock’ to the non-athletes, resulting in a diminished ability to regulate inter-limb JS during the hopping task. The magnitude of JS change between pre-testing and the tests performed after
Figure 4. Inter-limb difference in hopping JS values during each interval of the modified decrement test. A significant (F = 34.297, p = 0.000) main effect for group was observed, with the ATH group displaying a lower overall interlimb JS disparity in comparison with the NON group. *ATH group recorded a significantly (p < 0.05) lower inter-limb JS difference during the first interval after commencement of the sprints.
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the first pair of sprints for the NON group was 10.4% and 4.8% for the dominant and non-dominant limbs respectively, far greater than the 3.9% and 2.0% changes observed in the respective limbs for the ATH group. This finding would suggest that the non-athletes did not possess a neuromuscular coping strategy that allowed for maintenance of neuromuscular control under conditions of unaccustomed, maximal intensity stretch-shortening cycle contractions. This may be due to an ineffective efferent response to the information being received from the afferent somatic sensory receptors, namely the proprioceptors, resulting in an inability to maintain efficient common drive of the musculature. However, as the testing session progressed the difference in between limb JS values diminished, which suggests that the magnitude of inter-limb JS disparity was not directly related to the level of fatigue. This could mean that the nervous system was able to acutely respond to the unaccustomed exercise, and therefore attempt to initiate a neuromuscular control strategy in an effort to enhance inter-limb JS symmetry. Although no previous research has examined the relationship between inter-limb JS and injury risk, theoretically a greater magnitude of between limb asymmetry in the lower body would increase the potential for injury. As previously mentioned in the introduction, MTS is believed to be a vital component of setting stride frequency during sprinting.9 This would suggest that a difference in between limb JS, of which MTS is a major component, in the lower body would result in each limb possessing dissimilar optimal rates for stride frequency. Consequently, unless stride frequency was altered to allow for identical metabolic cost in each limb, the ratio of the contribution of elastic to contractile energy required to successfully perform the countermovement in each limb would be different. Theoretically, this would result in the limb required to contribute a greater proportion of contractile force during each countermovement fatiguing more rapidly than the contralateral limb, potentially increasing the risk of injury. However, even if this hypothesis of an increased risk of injury in relation to a greater magnitude in the difference in between limb JS is unfounded, the aforementioned effect on stride frequency would likely result in a decrease in performance due to reduced mechanical efficiency. Other important findings in this study were that the NON group recorded significantly lower JS levels than the ATH group, which is likely due to both lower levels of muscle mass and the neuromuscular adaptations that resulted in response to the training performed by the athletic subjects. In addition, despite a significant increase in sprint times as the testing session progressed, which was expected due to the fatiguing nature of the testing protocol, no significant differences in the level of JS were observed across tests for either group. This somewhat supports the findings of previous research,3 which showed that repeated maximal intensity countermovement jumps which are not performed to a point of complete exhaustion do not have an effect on the absolute level of JS. In contrast, the same study reported that sub-maximal performance of repeated countermovement jumps performed until
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exhaustion did have a dramatic effect on JS levels. Therefore, further research examining the effect of repeated maximal intensity countermovements performed to failure on JS may provide an insight into potential mechanisms of injury.
Conclusions Athletes who train using maximal intensity stretchshortening cycle and eccentric contractions appear to possess an ability to regulate JS more efficiently than non-athletes when exposed to a maximal intensity repeated sprint intervention. This may subsequently reduce their risk of injury and enhance performance.
Practical implications • High-intensity lower-limb training with an eccentric component may enhance the ability to regulate joint stiffness. • Improved joint stiffness regulation may decrease the risk of injury. • The intensity of repeated maximal sprint training should be gradually increased to enhance mechanical efficiency during running.
Acknowledgements The author wishes to thank Stephanie Hall, Jana Randall, Henry Tass, Elsie Millard, Will Clarke and Greg Capern for their assistance with data collection and analysis.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jsams.2007.12.003.
References 1. Butler RJ, Crowell HP, Davis IM. Lower extremity stiffness: implications for performance and injury. Clin Biomech 2003;18:511–7.
2. Granata KP, Padua DA, Wilson SE. Gender differences in musculoskeletal stiffness. Part II. Quantification of leg stiffness during functional hopping tasks. J Electromyogr Kines 2002;12: 127–35. 3. Kuitunen S, Kyröläinen H, Avela J, Komi PV. Leg stiffness modulation during exhaustive stretch-shortening cycle exercise. Scand J Med Sci Sport 2007;17:67–75. 4. LaStayo PC, Woolf JM, Lewek MD, Snyder-Mackler L, Reich T, Lindstedt SL. Eccentric muscle contractions: their contribution to injury, prevention, rehabilitation, and sport. J Ortho Sport Phys Ther 2003;33(10):557–71. 5. Lindstedt SL, LaStayo PC, Reich TE. When active muscle lengthen: properties and consequences of eccentric contractions. News Phys Sci 2001;16:256–61. 6. Lindstedt SL, Reich TE, Keim P, LaStayo PC. Do muscles function as adaptable motor springs? J Exp Biol 2002;205:2211–6. 7. Arampatzis A, Brugemann G, Klapsing GM. Leg stiffness and mechanical energetic processes during jumping on a sprung surface. Med Sci Sport Exercise 2001;33:923–31. 8. Chelly SM, Denis C. Leg power and hopping stiffness: Relationship with sprint running performance. Med Sci Sport Exerc 2001;33(2):326–33. 9. Farley CT, Blickhan R, Saito J, Taylor CR. Hopping frequency in humans: a test of how springs set stride frequency in bouncing gaits. J Appl Phys 1991;71(6):2127–32. 10. McMahon TA, Cheng GC. The mechanics of running: how does stiffness couple with speed? J Biomech 1990;23:65–78. 11. Anderson FC, Pandy MG. Storage and utilisation of elastic strain energy during jumping. J Biomech 1993;26:1413–27. 12. Latash ML, Zatsiorsky VM. Joint stiffness: myth or reality? Hum Mov Sci 1993;12:653–92. 13. Farley CT, Glasheen J, McMahon TA. Running springs: speed and animal size. J Exp Biol 1993;185:71–86. 14. Clark RA, Bryant A, Culgan J, Hartley B. The effects of eccentric hamstring strength training on dynamic jumping performance and isokinetic strength parameters: a pilot study on the implications for the prevention of hamstring injuries. Phys Ther Sport 2005;6:67–73. 15. Morgan DL, Allen DG. Early events in stretch-induced muscle damage. J Appl Phys 1999;87:2007–15. 16. Proske U, Morgan DL. Muscle damage from eccentric exercise; mechanism, mechanical signs, adaptation and clinical implications. J Appl Phys 2001;537:333–45. 17. Ferris DP, Farley CT. Interaction of leg stiffness and surfaces stiffness during human hopping. J Appl Phys 1997;82:15–22. 18. Horita T, Komi PV, Nicol C, Kyröläinen H. Stretch shortening cycle fatigue: interactions among joint stiffness, reflex, and muscle mechanical performance in the drop jump. Eur J Appl Phys 1996;73: 393–403. 19. Kuitunen S, Avela J, Kyröläinen H, Nicol C, Komi PV. Acute and prolonged reduction in joint stiffness in humans after exhausting stretch-shortening cycle exercise. Eur J Appl Phys 2002;88:107–16. 20. Hibbeler RC. Engineering mechanics: dynamics. 7th ed. Sydney, Australia: Prentice Hall; 1997.
Journal of Science and Medicine in Sport 12 (2009) 406–410
Original paper
The effect of training status on inter-limb joint stiffness regulation during repeated maximal sprints Ross A. Clark a,b,∗ a
Department of Health and Human Performance, Faculty of Arts, Health and Science, Central Queensland University, North Rockhampton, QLD 4702, Australia b Centre for Health, Exercise and Sports Medicine, School of Physiotherapy, The University of Melbourne, 202 Berkeley Street, Parkville, VIC 3010, Australia Received 8 March 2007; received in revised form 21 August 2007; accepted 11 December 2007
Abstract The purpose of this study was to examine the effect of anaerobic fatigue and training status on the joint stiffness (JS) regulation of the lower limbs. Twenty-two subjects participated in this study, with a group of athletes (ATH: n = 11, age: 22.1 ± 9.9 yrs, ht: 181.9 ± 6.3 cm, mass: 88.2 ± 12.7 kg) compared to a group of non-athletes (NON: n = 11, age: 20.9 ± 2.3 yrs, ht: 177.8 ± 7.1 cm, mass: 80.9 ± 22.0 kg). A modified phosphate decrement test, which consisted of eight 35 m timed sprints separated by a 30 s active recovery, allowed for inducement of anaerobic fatigue while incorporating measures of sprinting performance and JS. Assessment of JS consisted of a single-legged 2.2 Hz spring-mass hopping protocol, measured for each limb. This test was performed prior to the warm-up and after sprints two, four and six. Data analysis consisted of repeated measures MANOVA comparing groups, limbs and test. Repeated measures ANOVAs were also performed on the sprint times and the magnitude of inter-limb JS difference. For all data analysis the alpha level was set at p < 0.05. Assessment of between limb JS revealed that the ATH group possessed significantly lower inter-limb variation in comparison with the NON group after completion of the first pair of sprints, potentially due to their training status offsetting some of the mechanical and neuromuscular effects of repeated stretch-shortening cycle (SSC) fatigue. This enhanced ability to regulate inter-limb JS, in addition to enhancing performance, may reduce the risk of injury by preserving mechanical efficiency and therefore reducing metabolic cost during SSC contractions. © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. Keywords: Stiffness; Fatigue; Stretch-shortening cycle; Biomechanics
Introduction The relationship between musculotendinous stiffness (MTS) and functional performance, injury prevention and injury management has received a great deal of interest in the current literature.1–4 Part of this interest is due to the apparent adaptability of this structural property, with previous research suggesting that MTS is ‘tuned’ based on prior contraction history.4–6 One of the most effective methods of creating an adaptation in MTS is to perform eccentric contractions, ∗ Centre for Health, Exercise and Sports Medicine, The School of Physiotherapy, University of Melbourne, 202 Berkeley Street, Parkville, VIC 3010, Australia. Tel.: +61 0431 737609. E-mail address: [email protected].
which have been shown to result in an increased stiffness of the system.4 In addition to providing benefits in regards to enhanced joint stability, an increase in MTS also appears to enhance performance during rapid counter movements, such as maximal velocity sprinting.7–10 This is because a stiffer system allows for more efficient elastic energy contribution at high countermovement speeds, thereby enhancing force production during the concentric phase of the movement.8,11,12 The magnitude of this contribution of elastic energy to countermovement performance has resulted in previous research suggesting that stride frequency in running mammals is governed primarily by this property.9,13 In addition to these adaptations in MTS, eccentric training has also been suggested as a method of minimising the risk of soft tissue injury.14–16 This occurs by reducing the
1440-2440/$ – see front matter © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.jsams.2007.12.003
R.A. Clark / Journal of Science and Medicine in Sport 12 (2009) 406–410
range of motion in which a muscle is undergoing eccentric lengthening during the descending limb of the sarcomeres length-tension relationship.14–16 A vast number of other benefits of eccentric contractions have also been suggested as means of reducing injury risk factors, such as augmented fast-twitch muscle fibre size and strength, and the numerous adaptations deemed responsible for the repeated bouts effect.16 These responses to eccentric exercise appear to optimise the protective strategies of the musculotendinous system, resulting in an improved ability to withstand high intensity eccentric contractions.4,16 In regards to optimising performance, it would appear that the athlete should possess a biomechanical pattern that has the MTS of the involved musculature tuned for the movements performed during their sport. However, this regulation of optimal MTS may become compromised if either the additional structural properties, or the neuromuscular activation signals being received by the involved musculature, are dramatically modified.3,17–19 These two variables are highly susceptible to change during fatigue, especially when unaccustomed eccentric contractions are involved.3,18,19 However, these changes may be offset by the previously mentioned adaptations that arise in response to eccentric contractions, potentially resulting in a more structurally stable musculotendinous system with an improved ability to withstand the trauma that this form of contraction induces.4 Consequently, a portion of the reduction in performance associated with neuromuscular fatigue would be diminished due to an enhanced ability to maintain mechanical efficiency. This would theoretically result in a musculotendinous system that is less prone to fluctuations in MTS as a result of repeated, high intensity eccentric contractions, resulting in enhanced performance and a reduced risk of injury. However, whether athletes who are accustomed to performing repeated, high intensity eccentric contractions display an improved ability to regulate MTS during fatiguing tasks is unknown. Therefore, the purpose of this study was to compare the joint stiffness (JS), of which MTS is a major component, of athletes with extensive eccentric training backgrounds and recreational sporting participants during repeated maximal intensity sprints.
Methodology Testing overview The testing session consisted of a five minute standardised warm-up, followed by performance of a modified phosphate decrement test. This test consisted of eight repetitions of maximal intensity 35 m sprints, with a 30 s active recovery between repetitions. A repeated sprint test was chosen due to both sporting specificity and the high level of eccentric muscular activation, and subsequent fatigue, it induces. An examination of the JS of each lower limb was performed prior to the warm-up and after sprints two, four and six.
407
Subjects Two separate groups of subjects volunteered to participate in this study. The first group possessed extensive athletic training backgrounds (ATH), and included eleven semiprofessional rugby league players (age: 22.1 ± 9.9 yrs, ht: 181.9 ± 6.3 cm, mass: 88.2 ± 12.7 kg). These subjects participated in a minimum of five days of field, resistance and/or plyometric training per week, and had all performed multiple six week microcycles of eccentric specific hamstring training, in the form of the Nordic hamstring stretch exercise. The second group consisted of eleven recreational sporting participants (age: 20.9 ± 2.3 yrs, ht: 177.8 ± 7.1 cm, mass: 80.9 ± 22.0 kg), deemed the non-athlete (NON) subject group. These subjects all performed less than three days of sports related activities per week, but were participating in organised sporting competitions at a recreational level. They were also required to have no prior training history that included a large eccentric component, for example eccentric specific, sprint or plyometric training, in the six months leading up to the study. All subjects completed a pre-activity readiness questionnaire and signed an informed consent document prior to commencement of any data collection. Ethical approval for the methodologies involved in this study was granted by Central Queensland University. Joint stiffness tests The tests of JS consisted of an adaptation of the standard lower limb hopping test performed on a force plate (AMTI, USA), which has been used extensively in previous research.2,4,9 The subjects were instructed to hop on the limb instructed by the examiner, in time to a metronome providing an audible signal at a frequency of 2.2 Hz. During hopping, the subjects were instructed to look forward, maintain hand contact with their hips and attempt to minimise movement of the limb not being tested. Once a steady hopping rhythm at the desired frequency was attained, force and time data were collected via a custom written Labview (National Instruments, USA) data acquisition program sampling at 1000 Hz for three seconds. The value for JS was obtained using an adaptation of the equation previously reported by Farley et al.,9 which is based on the undamped oscillations occurring during simple harmonic motion.20 This equation, along with the extended methodology of the JS tests, is provided in Appendices A of the supplemental material. Due to the intent of the examiner to incorporate a fatiguing intervention which replicated the events occurring during sporting situations, this study was performed on a grass covered rugby oval. To ensure accurate data collection via the force plate, it was thus deemed essential to ensure the stability and orientation of the force plate. This was achieved by mounting the force plate onto a solid aluminium plate, which possessed larger dimensions than the force plate and was approximately 2.5 cm thick. These larger dimensions
408
R.A. Clark / Journal of Science and Medicine in Sport 12 (2009) 406–410
enhanced the stability of the force plate, while the added mass prevented both excess movement and damping due to the grass it was placed on. Resting joint stiffness and warm-up Immediately prior to the warm-up, JS tests of both the dominant and non-dominant limbs were performed according to the previously mentioned protocol, providing a resting JS value. The warm-up consisted of low-intensity agility and plyometric exercises performed for five minutes. These included ladder and mini-hurdle drills designed to prepare the subjects for the subsequent maximal sprints. All subjects performed an identical number of ground contacts during the exercises in the warm-up. The modified phosphate decrement test A diagram of the testing protocol during the modified phosphate decrement test is shown in Fig. 1. An extended methodology for this test is provided in Appendices B of the supplemental material. Data analysis The subjects sprint times for each interval were recorded by the timing lights. This raw data was then modified by calculating the mean sprint time for each group of two sprints performed between the JS tests. This resulted in four values for sprint times for each subject, the mean of sprints one and two, three and four, five and six and finally sprints seven and eight. A repeated measures ANOVA was performed to examine whether any significant changes in sprint times were recorded either within or between groups. The subjects JS results were also examined, with a total of four results recorded for each limb per subject. This resulted in a 2 group × 2 limb × 4 test repeated measures MANOVA. Therefore, an examination was performed to determine whether any significant effects or interactions occurred between limbs, groups or test. In addition, repeated measures ANOVA was performed to examine the magnitude of difference in between limb JS regardless of limb domi-
Figure 1. The layout of the modified phosphate decrement test. The subjects performed eight 35 m sprints with a 30 s active recovery between sprints. JS tests of each limb individually were performed before testing and after sprints two, four and six. Step 1 perform a 35 m sprint. Step 2 decelerate around the marker and walk back to the initial finish line. Step 3 perform a 35 m sprint. Step 4 decelerate to the force plate. Perform dominant limb JS test, step off and walk around marker. Return to the force plate and perform non-dominant limb JS test. Return to starting line for next sprint.
Figure 2. Mean sprint times during each pair of sprints in the modified phosphate decrement test. *ATH recorded significantly lower (p < 0.05) sprint times during each pair of sprints in comparison with the NON group. A significant (p < 0.05) increase in sprint times was also recorded for both groups between each testing interval, signalling fatigue.
nance. This was performed by determining the magnitude of inter-limb difference in JS for each test, with the repeated measures ANOVA examining the relationship between group and testing interval. Fischers LSD post hoc analysis was performed in the event of a significant main effect or interaction in any of the preceding statistical tests, with the alpha level set for all analysis at p < 0.05. Data sphericity (lowerbound epsilon > 0.75) and equal variance (Levene Median Test) were also assessed to ensure the validity of the statistical analysis. Results The results for sprint times are provided in Fig. 2. As expected, the ATH group recorded significantly lower sprint times as a group (F = 16.835, p = 0.002, mean sprint speed 13% lower in the ATH group), with post hoc tests revealing the ATH group recorded significantly (p < 0.05) lower sprint times than the NON group during each interval. In addition, sprint times significantly (p < 0.05) increased for both groups as the testing session progressed. The results for the JS test are supplied in Fig. 3, with the significant main effect for group (F = 11.388, p = 0.003) revealing that the ATH group recorded significantly higher mean JS values than the NON group. No significant differences were observed for dominant versus non-dominant limb JS or JS values during each testing interval. However, when the data was analysed to examine the magnitude of inter-limb difference, shown in Fig. 4, a significant main effect (F = 34.297, p = 0.000) for group was observed. This revealed that the ATH group possessed significantly lower overall inter-limb JS disparity throughout the testing session. Post hoc analysis showed a significant difference (F = 10.351, p = 0.005) between groups in the JS test performed after the first pair of sprints was completed, with the ATH group possessing less than a third of the inter-limb difference observed in the NON group.
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Figure 3. Hopping JS values during each interval of the modified decrement test. The ATH group recorded significantly (p < 0.05) higher JS values than the NON group both between groups and intervals. No significant differences were observed between either dominant and non-dominant limbs or between groups.
Discussion The results of this study suggest that athletes participating in a training program that includes field, resistance, plyometric and eccentric specific training are more successful than recreational sporting participants at regulating JS during repeated sprints. This was demonstrated by the fact that no significant differences in JS were observed between limbs for the ATH group, while in contrast the NON group showed a significant increase in between limb JS difference after the initial two sprints. Furthermore, although non-significant, the magnitude of between limb JS difference for the NON group was higher during each of the other three tests. This finding suggests that the performance of the first two repeated sprints resulted in a great deal of ‘shock’ to the non-athletes, resulting in a diminished ability to regulate inter-limb JS during the hopping task. The magnitude of JS change between pre-testing and the tests performed after
Figure 4. Inter-limb difference in hopping JS values during each interval of the modified decrement test. A significant (F = 34.297, p = 0.000) main effect for group was observed, with the ATH group displaying a lower overall interlimb JS disparity in comparison with the NON group. *ATH group recorded a significantly (p < 0.05) lower inter-limb JS difference during the first interval after commencement of the sprints.
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the first pair of sprints for the NON group was 10.4% and 4.8% for the dominant and non-dominant limbs respectively, far greater than the 3.9% and 2.0% changes observed in the respective limbs for the ATH group. This finding would suggest that the non-athletes did not possess a neuromuscular coping strategy that allowed for maintenance of neuromuscular control under conditions of unaccustomed, maximal intensity stretch-shortening cycle contractions. This may be due to an ineffective efferent response to the information being received from the afferent somatic sensory receptors, namely the proprioceptors, resulting in an inability to maintain efficient common drive of the musculature. However, as the testing session progressed the difference in between limb JS values diminished, which suggests that the magnitude of inter-limb JS disparity was not directly related to the level of fatigue. This could mean that the nervous system was able to acutely respond to the unaccustomed exercise, and therefore attempt to initiate a neuromuscular control strategy in an effort to enhance inter-limb JS symmetry. Although no previous research has examined the relationship between inter-limb JS and injury risk, theoretically a greater magnitude of between limb asymmetry in the lower body would increase the potential for injury. As previously mentioned in the introduction, MTS is believed to be a vital component of setting stride frequency during sprinting.9 This would suggest that a difference in between limb JS, of which MTS is a major component, in the lower body would result in each limb possessing dissimilar optimal rates for stride frequency. Consequently, unless stride frequency was altered to allow for identical metabolic cost in each limb, the ratio of the contribution of elastic to contractile energy required to successfully perform the countermovement in each limb would be different. Theoretically, this would result in the limb required to contribute a greater proportion of contractile force during each countermovement fatiguing more rapidly than the contralateral limb, potentially increasing the risk of injury. However, even if this hypothesis of an increased risk of injury in relation to a greater magnitude in the difference in between limb JS is unfounded, the aforementioned effect on stride frequency would likely result in a decrease in performance due to reduced mechanical efficiency. Other important findings in this study were that the NON group recorded significantly lower JS levels than the ATH group, which is likely due to both lower levels of muscle mass and the neuromuscular adaptations that resulted in response to the training performed by the athletic subjects. In addition, despite a significant increase in sprint times as the testing session progressed, which was expected due to the fatiguing nature of the testing protocol, no significant differences in the level of JS were observed across tests for either group. This somewhat supports the findings of previous research,3 which showed that repeated maximal intensity countermovement jumps which are not performed to a point of complete exhaustion do not have an effect on the absolute level of JS. In contrast, the same study reported that sub-maximal performance of repeated countermovement jumps performed until
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exhaustion did have a dramatic effect on JS levels. Therefore, further research examining the effect of repeated maximal intensity countermovements performed to failure on JS may provide an insight into potential mechanisms of injury.
Conclusions Athletes who train using maximal intensity stretchshortening cycle and eccentric contractions appear to possess an ability to regulate JS more efficiently than non-athletes when exposed to a maximal intensity repeated sprint intervention. This may subsequently reduce their risk of injury and enhance performance.
Practical implications • High-intensity lower-limb training with an eccentric component may enhance the ability to regulate joint stiffness. • Improved joint stiffness regulation may decrease the risk of injury. • The intensity of repeated maximal sprint training should be gradually increased to enhance mechanical efficiency during running.
Acknowledgements The author wishes to thank Stephanie Hall, Jana Randall, Henry Tass, Elsie Millard, Will Clarke and Greg Capern for their assistance with data collection and analysis.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.jsams.2007.12.003.
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