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ARTICLE Muscle-tendon unit stiffness does not independently affect voluntary explosive force production or muscle intrinsic contractile properties Ricci Hannah and Jonathan P. Folland
Abstract: This study examined the relationship of muscle-tendon unit (MTU) stiffness and explosive force production during voluntary and evoked contractions of the knee extensors. Thirty-four untrained participants performed a series of explosive voluntary and electrically evoked (octets (8 pulses, 300 Hz) via femoral nerve stimulation) isometric contractions. Maximum voluntary force (MVF) was assessed during maximum voluntary contractions. Explosive force production was assessed as the time taken, from force onset (0 N), to achieve specific levels of absolute (25–300 N) and relative force (5%–75% MVF) during the explosive contractions. Ultrasonic images of the vastus lateralis were recorded during 10-s ramp contractions to assess MTU stiffness, which was expressed in absolute (N·mm−1) and relative (to MVF and resting tendon-aponeurosis length) terms. Bivariate correlations suggested that absolute MTU stiffness was associated with voluntary explosive force (time to achieve 150–300 N: r = –0.35 to –0.54, P < 0.05). However, no relationships between stiffness and voluntary explosive force were observed when the influence of MVF was removed, either via partial correlations of absolute values (P ≥ 0.49) or considering relative values (P ≥ 0.14). Similarly, absolute MTU stiffness was related to explosive force during evoked octet contractions (r = –0.41 to –0.64, P < 0.05), but these correlations were no longer present when accounting for the influence of MVF (P ≥ 0.15). Therefore, once maximum strength was considered, MTU stiffness had no independent relationship with voluntary explosive force production or the evoked capacity for explosive force. Key words: muscle strength, rate of force development, tendon stiffness, ultrasonography, electrical stimulation, skeletal muscle contraction. Résumé : Cette étude examine la relation entre la raideur de l’ensemble muscle-tendon (« MTU ») et la production de force explosive au cours d’une extension volontaire du genou et de contractions élicitées électriquement. Trente-quatre participants non entraînés effectuent des séries de contractions isométriques volontaires et sont soumis a` une stimulation électrique (octets (8 impulsions, 300 Hz) du nerf fémoral). On détermine la force maximale volontaire (« MVF ») au cours de contractions maximales volontaires. On évalue la production de force explosive par le temps écoulé entre le début de l’expression de la force (0 N) et le moment correspondant a` l’atteinte d’une force absolue donnée (25–300 N) et relative (5–75 % MVF) au cours de contractions explosives. On enregistre des images ultrasoniques du vaste externe au cours de contractions d’intensité graduelle de 10 s pour évaluer la raideur de MTU de façon absolue (N·mm–1) et relative (a` la MVF et a` la longueur de repos du tendon-aponévrose). D’après l’analyse de corrélation bivariée, la valeur absolue de la raideur de MTU est associée a` la force explosive volontaire (temps pour atteindre 150300 N: r = –0,35 a` –0,54, P < 0,05). Toutefois, on n’observe aucune relation entre la raideur et la force explosive volontaire quand l’influence de la MVF est retirée, que ce soit par l’analyse des corrélations partielles des valeurs absolues (P ≥ 0,49) ou relatives (P ≥ 0,14). Aussi, la valeur absolue de la raideur de MTU est associée a` la force explosive durant les huit contractions élicitées (r = entre –0,41 et –0,64, P < 0,05), mais ces corrélations disparaissent après la prise en compte de l’influence de la MVF (P ≥ 0,15). En conséquence, une fois la force maximale prise en compte, la raideur de MTU ne présente pas de relation indépendante avec la production de force explosive volontaire ou la capacité de force explosive élicitée. [Traduit par la Rédaction] Mots-clés : force musculaire, taux de production de la force, raideur tendineuse, ultrasonographie, électrostimulation, contraction musculaire squelettique.
Introduction Explosive force production by human skeletal muscle is considered to be functionally important during explosive movements such as jumping and sprinting (de Ruiter et al. 2006; Korhonen et al. 2006; Tillin et al. 2013a) and injury-related situations where a joint or the whole body must be rapidly stabilised following a loss of balance (Gruber and Gollhofer 2004; Izquierdo et al. 1999).
As such, a greater understanding of the mechanisms underpinning the ability for explosive force production could have important implications for sports performance, the prevention of joint, and fall-related injuries. Explosive force production may be defined as the capability to increase force from a low or resting level as quickly as possible. It has been quantified in a variety of ways, including the force pro-
Received 18 August 2014. Accepted 30 September 2014. R. Hannah.* Sport, Health and Performance Enhancement (SHAPE) Research Group, School of Science and Technology, Nottingham Trent University, Nottingham, UK; School of Sport, Exercise, and Health Sciences, Loughborough University, Loughborough, UK. J.P. Folland. School of Sport, Exercise, and Health Sciences, Loughborough University, Loughborough, UK. Corresponding author: Ricci Hannah (e-mail: [email protected]). *Present address: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Box 146, Queen Square, London WC1 3BG, UK. Appl. Physiol. Nutr. Metab. 40: 87–95 (2015) dx.doi.org/10.1139/apnm-2014-0343
Published at www.nrcresearchpress.com/apnm on 3 October 2014.
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duced at specific time points from contraction onset (Hannah et al. 2012; Tillin et al. 2010), the rate of force development (RFD) over a particular time period (Aagaard et al. 2002; Hannah et al. 2012; Tillin et al. 2010) or the time taken to achieve a given level of force (Korhonen et al. 2006; Viitasalo and Komi 1978). Physiological factors thought to influence explosive force production include maximum strength (maximum voluntary force, MVF; Andersen and Aagaard 2006), agonist neural drive (Aagaard et al. 2002; de Ruiter et al. 2004, 2006; Folland et al. 2014; Tillin et al. 2010), and muscle fibre type composition (Harridge et al. 1996; Viitasalo and Komi 1978). Muscle-tendon unit (MTU) stiffness is also thought to influence the time course of force production by affecting the efficacy of force transmission from muscle to bone (Kubo et al. 2001; Reeves et al. 2003). In theory, a stiffer MTU would more effectively transmit force to bone, and this would be reflected by a shorter time taken to achieve a given level of force. However, the influence of MTU stiffness on explosive contractile performance remains poorly understood. Ultrasonic imaging of the tendon or tendon aponeurosis can be combined with force measurements during muscle contractions in vivo to measure MTU stiffness; defined as the slope of the relationship between muscle force and elongation of the distal MTU segment (Kubo et al. 2001; Reeves et al. 2003). Concomitant changes in MTU stiffness and explosive force production in response to interventions, for example concurrent increases in both variables following resistance training (Kubo et al. 2001; Reeves et al. 2003) and decreases following unloading (de Boer et al. 2007), provide some support for an influence of MTU stiffness on explosive force production. The relationship between MTU stiffness and explosive force production for a range of individuals has been considered by 2 studies that reported significant positive relationships (r = 0.57– 0.69, n = 16–17; Bojsen-Moller et al. 2005; Wang et al. 2012). However, the participants within these studies included different populations and potentially confounding variables; for example, either healthy and tendinopathic limbs (Wang et al. 2012) or discrete athletic groups (Bojsen-Moller et al. 2005). It is likely that these different limbs/athletic populations had several distinctive characteristics (e.g., muscle composition, maximum strength, neuromuscular activation, and pain); therefore, these studies may not have isolated their respective experimental variable very effectively. Considering maximum strength, MTU stiffness and explosive force production are known to be related to MVF (Andersen and Aagaard 2006; Folland et al. 2014; Stenroth et al. 2012). However, whether there is an independent relationship between MTU stiffness and explosive force production or simply a coincidental one because of the influence of MVF on both variables remains unknown. Finally, previous studies also evaluated stiffness at a range of forces (50%–90% or 50%–100% of MVF; Bojsen-Moller et al. 2005; Wang et al. 2012) that did not equate to the range of forces used to measure explosive force production, typically assessed from zero (e.g., 0%–50%MVF). This presents a clear discrepancy between the range of forces over which explosive force production and MTU stiffness were measured. Quantifying stiffness and explosive force production over the same range of forces may circumvent this potential confounder. The intrinsic capacity of the muscle-tendon unit (MTU) for explosive force production can be assessed with evoked, involuntary contractions that are independent of voluntary neural control. This intrinsic contractile property of the muscle is thought to reflect muscle morphology and tissue mechanics (Harridge et al. 1996; Oda et al. 2007). Evoked octet contractions (8 pulses at 300 Hz) produce the maximum possible RFD and thus facilitate the measurement of the muscle’s maximum capacity for explosive force production (de Ruiter et al. 2004). MTU stiffness might be expected to show a more pronounced effect on evoked octet explosive force than on voluntary explosive force, which is also influenced by neural activation (Aagaard et al. 2002; de Ruiter
Appl. Physiol. Nutr. Metab. Vol. 40, 2015
et al. 2004; Folland et al. 2014; Tillin et al. 2010). However, the relationship between MTU stiffness and the evoked capacity for explosive force production has not been examined. The aim of this study was to assess the relationship of MTU stiffness with voluntary and evoked explosive force in a large, healthy and relatively homogeneous population. Explosive force was assessed at the time taken to achieve given absolute and relative (to MVF) force levels, whilst MTU stiffness was evaluated over an identical range of absolute and relative forces. We hypothesised that absolute explosive force during voluntary and evoked contractions would be related to absolute MTU stiffness and this association would remain independent of maximum strength. To elucidate whether MTU stiffness influenced explosive force production independently from maximum strength, the influence of maximum strength was factored out by (i) performing partial correlations that control for the influence of MVF, and (ii) by examining the relationship between normalised stiffness and the time taken to achieve relative (%MVF) force levels.
Materials and methods Ethical approval Volunteers provided written informed consent prior to their participation in this study, which was approved by the University Human Ethical Review Committee at Nottingham Trent University to the standard set by the Declaration of Helsinki. Participants Thirty-four participants (17 males and 17 females; age, 22 ± 2 years; height, 174 ± 5 cm; body mass, 68 ± 9 kg) were recruited. To control for the potentially confounding effects of training background, we studied untrained individuals with similar low to moderate levels of habitual physical activity. Physical activity was assessed using the International Physical Activity Questionnaire (Short Format, www.ipaq.ki.se/ipaq.htm; Craig et al. 2003). Exclusion criteria included any previous participation in systematic strength or power training. To control for the possibility of menstrual cycle phase influencing the neuromechanical function of the knee extensors (Phillips et al. 1996), female participants were required to have been taking the combined monophasic oral contraceptive pill for ≥6 months. Furthermore, females were only tested between days 7–21 of pill consumption to minimise any fluctuations in endogenous gonadal hormones (Onambele et al. 2007). All participants were healthy and free from musculoskeletal injury to the lower limb. Volunteers provided written informed consent prior to their participation in this study, which was approved by the Ethical Advisory Committee at Loughborough University. Study design Participants visited the laboratory for 60–90 min on 2 separate occasions, separated by 1 week, to complete a familiarization and 1 test session. All sessions commenced between 1200 and 1630 h, and measurements were made on the preferred leg only. Participants were seated in a custom-built dynamometer and completed a series of isometric voluntary and electrically evoked contractions of the knee extensors. Specifically, maximum voluntary contractions (MVCs), explosive voluntary contractions, electrically evoked contractions (octet stimulation), and ramped voluntary contractions were performed in this order. In addition, a series of knee flexion MVCs were performed at the end of each session. We have previously reported the influence of sex on the explosive neuromuscular performance of this cohort (Hannah et al. 2012), showing that explosive voluntary force, the intrinsic contractile properties and MTU stiffness were statistically similar for both the sexes once the greater maximum strength of males was accounted for. To provide a wide range of values in the dependent variables, particularly absolute explosive force and MTU stiffness, male and female data were pooled for the present analysis. Published by NRC Research Press
Hannah and Folland
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Measurements Knee extension and flexion force Participants were secured in the dynamometer (Parker et al. 1990), with hip and knee joint angles of 100° and 85° (where 180° = full extension), respectively. Adjustable strapping across the pelvis and shoulders prevented extraneous movement. A strap, 40-mm width reinforced canvas webbing, was placed proximal to the ankle (15% of tibial length above the medial malleolus), positioned perpendicular to the tibia and in series with a calibrated strain gauge. This low-noise strain gauge (range of baseline noise
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ARTICLE Muscle-tendon unit stiffness does not independently affect voluntary explosive force production or muscle intrinsic contractile properties Ricci Hannah and Jonathan P. Folland
Abstract: This study examined the relationship of muscle-tendon unit (MTU) stiffness and explosive force production during voluntary and evoked contractions of the knee extensors. Thirty-four untrained participants performed a series of explosive voluntary and electrically evoked (octets (8 pulses, 300 Hz) via femoral nerve stimulation) isometric contractions. Maximum voluntary force (MVF) was assessed during maximum voluntary contractions. Explosive force production was assessed as the time taken, from force onset (0 N), to achieve specific levels of absolute (25–300 N) and relative force (5%–75% MVF) during the explosive contractions. Ultrasonic images of the vastus lateralis were recorded during 10-s ramp contractions to assess MTU stiffness, which was expressed in absolute (N·mm−1) and relative (to MVF and resting tendon-aponeurosis length) terms. Bivariate correlations suggested that absolute MTU stiffness was associated with voluntary explosive force (time to achieve 150–300 N: r = –0.35 to –0.54, P < 0.05). However, no relationships between stiffness and voluntary explosive force were observed when the influence of MVF was removed, either via partial correlations of absolute values (P ≥ 0.49) or considering relative values (P ≥ 0.14). Similarly, absolute MTU stiffness was related to explosive force during evoked octet contractions (r = –0.41 to –0.64, P < 0.05), but these correlations were no longer present when accounting for the influence of MVF (P ≥ 0.15). Therefore, once maximum strength was considered, MTU stiffness had no independent relationship with voluntary explosive force production or the evoked capacity for explosive force. Key words: muscle strength, rate of force development, tendon stiffness, ultrasonography, electrical stimulation, skeletal muscle contraction. Résumé : Cette étude examine la relation entre la raideur de l’ensemble muscle-tendon (« MTU ») et la production de force explosive au cours d’une extension volontaire du genou et de contractions élicitées électriquement. Trente-quatre participants non entraînés effectuent des séries de contractions isométriques volontaires et sont soumis a` une stimulation électrique (octets (8 impulsions, 300 Hz) du nerf fémoral). On détermine la force maximale volontaire (« MVF ») au cours de contractions maximales volontaires. On évalue la production de force explosive par le temps écoulé entre le début de l’expression de la force (0 N) et le moment correspondant a` l’atteinte d’une force absolue donnée (25–300 N) et relative (5–75 % MVF) au cours de contractions explosives. On enregistre des images ultrasoniques du vaste externe au cours de contractions d’intensité graduelle de 10 s pour évaluer la raideur de MTU de façon absolue (N·mm–1) et relative (a` la MVF et a` la longueur de repos du tendon-aponévrose). D’après l’analyse de corrélation bivariée, la valeur absolue de la raideur de MTU est associée a` la force explosive volontaire (temps pour atteindre 150300 N: r = –0,35 a` –0,54, P < 0,05). Toutefois, on n’observe aucune relation entre la raideur et la force explosive volontaire quand l’influence de la MVF est retirée, que ce soit par l’analyse des corrélations partielles des valeurs absolues (P ≥ 0,49) ou relatives (P ≥ 0,14). Aussi, la valeur absolue de la raideur de MTU est associée a` la force explosive durant les huit contractions élicitées (r = entre –0,41 et –0,64, P < 0,05), mais ces corrélations disparaissent après la prise en compte de l’influence de la MVF (P ≥ 0,15). En conséquence, une fois la force maximale prise en compte, la raideur de MTU ne présente pas de relation indépendante avec la production de force explosive volontaire ou la capacité de force explosive élicitée. [Traduit par la Rédaction] Mots-clés : force musculaire, taux de production de la force, raideur tendineuse, ultrasonographie, électrostimulation, contraction musculaire squelettique.
Introduction Explosive force production by human skeletal muscle is considered to be functionally important during explosive movements such as jumping and sprinting (de Ruiter et al. 2006; Korhonen et al. 2006; Tillin et al. 2013a) and injury-related situations where a joint or the whole body must be rapidly stabilised following a loss of balance (Gruber and Gollhofer 2004; Izquierdo et al. 1999).
As such, a greater understanding of the mechanisms underpinning the ability for explosive force production could have important implications for sports performance, the prevention of joint, and fall-related injuries. Explosive force production may be defined as the capability to increase force from a low or resting level as quickly as possible. It has been quantified in a variety of ways, including the force pro-
Received 18 August 2014. Accepted 30 September 2014. R. Hannah.* Sport, Health and Performance Enhancement (SHAPE) Research Group, School of Science and Technology, Nottingham Trent University, Nottingham, UK; School of Sport, Exercise, and Health Sciences, Loughborough University, Loughborough, UK. J.P. Folland. School of Sport, Exercise, and Health Sciences, Loughborough University, Loughborough, UK. Corresponding author: Ricci Hannah (e-mail: [email protected]). *Present address: Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, Box 146, Queen Square, London WC1 3BG, UK. Appl. Physiol. Nutr. Metab. 40: 87–95 (2015) dx.doi.org/10.1139/apnm-2014-0343
Published at www.nrcresearchpress.com/apnm on 3 October 2014.
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duced at specific time points from contraction onset (Hannah et al. 2012; Tillin et al. 2010), the rate of force development (RFD) over a particular time period (Aagaard et al. 2002; Hannah et al. 2012; Tillin et al. 2010) or the time taken to achieve a given level of force (Korhonen et al. 2006; Viitasalo and Komi 1978). Physiological factors thought to influence explosive force production include maximum strength (maximum voluntary force, MVF; Andersen and Aagaard 2006), agonist neural drive (Aagaard et al. 2002; de Ruiter et al. 2004, 2006; Folland et al. 2014; Tillin et al. 2010), and muscle fibre type composition (Harridge et al. 1996; Viitasalo and Komi 1978). Muscle-tendon unit (MTU) stiffness is also thought to influence the time course of force production by affecting the efficacy of force transmission from muscle to bone (Kubo et al. 2001; Reeves et al. 2003). In theory, a stiffer MTU would more effectively transmit force to bone, and this would be reflected by a shorter time taken to achieve a given level of force. However, the influence of MTU stiffness on explosive contractile performance remains poorly understood. Ultrasonic imaging of the tendon or tendon aponeurosis can be combined with force measurements during muscle contractions in vivo to measure MTU stiffness; defined as the slope of the relationship between muscle force and elongation of the distal MTU segment (Kubo et al. 2001; Reeves et al. 2003). Concomitant changes in MTU stiffness and explosive force production in response to interventions, for example concurrent increases in both variables following resistance training (Kubo et al. 2001; Reeves et al. 2003) and decreases following unloading (de Boer et al. 2007), provide some support for an influence of MTU stiffness on explosive force production. The relationship between MTU stiffness and explosive force production for a range of individuals has been considered by 2 studies that reported significant positive relationships (r = 0.57– 0.69, n = 16–17; Bojsen-Moller et al. 2005; Wang et al. 2012). However, the participants within these studies included different populations and potentially confounding variables; for example, either healthy and tendinopathic limbs (Wang et al. 2012) or discrete athletic groups (Bojsen-Moller et al. 2005). It is likely that these different limbs/athletic populations had several distinctive characteristics (e.g., muscle composition, maximum strength, neuromuscular activation, and pain); therefore, these studies may not have isolated their respective experimental variable very effectively. Considering maximum strength, MTU stiffness and explosive force production are known to be related to MVF (Andersen and Aagaard 2006; Folland et al. 2014; Stenroth et al. 2012). However, whether there is an independent relationship between MTU stiffness and explosive force production or simply a coincidental one because of the influence of MVF on both variables remains unknown. Finally, previous studies also evaluated stiffness at a range of forces (50%–90% or 50%–100% of MVF; Bojsen-Moller et al. 2005; Wang et al. 2012) that did not equate to the range of forces used to measure explosive force production, typically assessed from zero (e.g., 0%–50%MVF). This presents a clear discrepancy between the range of forces over which explosive force production and MTU stiffness were measured. Quantifying stiffness and explosive force production over the same range of forces may circumvent this potential confounder. The intrinsic capacity of the muscle-tendon unit (MTU) for explosive force production can be assessed with evoked, involuntary contractions that are independent of voluntary neural control. This intrinsic contractile property of the muscle is thought to reflect muscle morphology and tissue mechanics (Harridge et al. 1996; Oda et al. 2007). Evoked octet contractions (8 pulses at 300 Hz) produce the maximum possible RFD and thus facilitate the measurement of the muscle’s maximum capacity for explosive force production (de Ruiter et al. 2004). MTU stiffness might be expected to show a more pronounced effect on evoked octet explosive force than on voluntary explosive force, which is also influenced by neural activation (Aagaard et al. 2002; de Ruiter
Appl. Physiol. Nutr. Metab. Vol. 40, 2015
et al. 2004; Folland et al. 2014; Tillin et al. 2010). However, the relationship between MTU stiffness and the evoked capacity for explosive force production has not been examined. The aim of this study was to assess the relationship of MTU stiffness with voluntary and evoked explosive force in a large, healthy and relatively homogeneous population. Explosive force was assessed at the time taken to achieve given absolute and relative (to MVF) force levels, whilst MTU stiffness was evaluated over an identical range of absolute and relative forces. We hypothesised that absolute explosive force during voluntary and evoked contractions would be related to absolute MTU stiffness and this association would remain independent of maximum strength. To elucidate whether MTU stiffness influenced explosive force production independently from maximum strength, the influence of maximum strength was factored out by (i) performing partial correlations that control for the influence of MVF, and (ii) by examining the relationship between normalised stiffness and the time taken to achieve relative (%MVF) force levels.
Materials and methods Ethical approval Volunteers provided written informed consent prior to their participation in this study, which was approved by the University Human Ethical Review Committee at Nottingham Trent University to the standard set by the Declaration of Helsinki. Participants Thirty-four participants (17 males and 17 females; age, 22 ± 2 years; height, 174 ± 5 cm; body mass, 68 ± 9 kg) were recruited. To control for the potentially confounding effects of training background, we studied untrained individuals with similar low to moderate levels of habitual physical activity. Physical activity was assessed using the International Physical Activity Questionnaire (Short Format, www.ipaq.ki.se/ipaq.htm; Craig et al. 2003). Exclusion criteria included any previous participation in systematic strength or power training. To control for the possibility of menstrual cycle phase influencing the neuromechanical function of the knee extensors (Phillips et al. 1996), female participants were required to have been taking the combined monophasic oral contraceptive pill for ≥6 months. Furthermore, females were only tested between days 7–21 of pill consumption to minimise any fluctuations in endogenous gonadal hormones (Onambele et al. 2007). All participants were healthy and free from musculoskeletal injury to the lower limb. Volunteers provided written informed consent prior to their participation in this study, which was approved by the Ethical Advisory Committee at Loughborough University. Study design Participants visited the laboratory for 60–90 min on 2 separate occasions, separated by 1 week, to complete a familiarization and 1 test session. All sessions commenced between 1200 and 1630 h, and measurements were made on the preferred leg only. Participants were seated in a custom-built dynamometer and completed a series of isometric voluntary and electrically evoked contractions of the knee extensors. Specifically, maximum voluntary contractions (MVCs), explosive voluntary contractions, electrically evoked contractions (octet stimulation), and ramped voluntary contractions were performed in this order. In addition, a series of knee flexion MVCs were performed at the end of each session. We have previously reported the influence of sex on the explosive neuromuscular performance of this cohort (Hannah et al. 2012), showing that explosive voluntary force, the intrinsic contractile properties and MTU stiffness were statistically similar for both the sexes once the greater maximum strength of males was accounted for. To provide a wide range of values in the dependent variables, particularly absolute explosive force and MTU stiffness, male and female data were pooled for the present analysis. Published by NRC Research Press
Hannah and Folland
Appl. Physiol. Nutr. Metab. Downloaded from www.nrcresearchpress.com by NORTHEASTERN UNIVERSITY on 02/22/15 For personal use only.
Measurements Knee extension and flexion force Participants were secured in the dynamometer (Parker et al. 1990), with hip and knee joint angles of 100° and 85° (where 180° = full extension), respectively. Adjustable strapping across the pelvis and shoulders prevented extraneous movement. A strap, 40-mm width reinforced canvas webbing, was placed proximal to the ankle (15% of tibial length above the medial malleolus), positioned perpendicular to the tibia and in series with a calibrated strain gauge. This low-noise strain gauge (range of baseline noise
