Eur J Appl Physiol DOI 10.1007/s00421-014-3063-y
ORIGINAL ARTICLE
Explosive hamstrings‑to‑quadriceps force ratio of males versus females Ricci Hannah · Jonathan P. Folland · Stephanie L. Smith · Claire Minshull
Received: 30 September 2014 / Accepted: 24 November 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Females are known to exhibit a greater risk of ACL injury compared to males. Lower explosive hamstrings-to-quadriceps (H/Q) force ratio in the first 150 ms from activation onset could reflect an impaired capacity for knee joint stabilisation and increased risk of ACL injury. However, the explosive H/Q force ratio has not been compared between the sexes. Methods The neuromuscular performance of untrained males and females (20 of each) was assessed during a series of isometric knee flexor and extensor contractions, specifically explosive and maximum voluntary contractions of each muscle group. Force, in absolute terms and normalised to body mass, and surface EMG of the hamstrings and quadriceps were recorded. Hamstrings force was expressed Communicated by Olivier Seynnes. R. Hannah (*) Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, 146, Queen Square, London WC1 3BG, UK e-mail: [email protected] J. P. Folland School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK S. L. Smith School of Health and Life Sciences, Glasgow Caledonian University, Glasgow, UK C. Minshull School of Clinical Sciences, University of Edinburgh, Edinburgh, UK C. Minshull School of Health Sciences, Queen Margaret University, Edinburgh, UK
relative to quadriceps force to produce ratios of explosive H/Q force and H/Q maximum voluntary force (MVF). For the explosive contractions, agonist electromechanical delay (EMD) and agonist neural activation were also assessed. Results The H/Q MVF ratio was greater in males (56 %) than females (50 %; P 300 ms (Thorstensson et al. 1976). Therefore, we previously suggested that because MVF is unlikely to be attained within 50 ms, the capacity for knee joint stabilisation might depend more on the relative ability of the knee muscles to rapidly or “explosively” exert force during the initial rising phase of contraction, i.e. the first 150 ms following neuromuscular activation (Hannah et al. 2014). In a male cohort we previously showed that from first activation [electromyography (EMG) onset] the ability to produce explosive force with the hamstrings relative to the quadriceps (explosive H/Q force ratio) in the early phase of explosive contraction was very low (0–17 %; 25–50 ms after activation onset) and substantially less than the commonly reported H/Q MVF ratio (56 %). This low early ratio of explosive H/Q force could render the knee particularly unstable in the early phase of contraction when ACL injuries are thought to occur (Krosshaug et al. 2007). The early
13
Eur J Appl Physiol
phase explosive H/Q force ratio has not been investigated previously in females. Given that females exhibit a lower H/Q MVF ratio than males (Andrade et al. 2012; Calmels et al. 1997; Griffin et al. 1993; Hewett et al. 1996; Holm and Vollestad 2008), studying the time course of explosive H/Q force production in females compared to males may further inform our understanding of why females are at relatively higher risk of ACL injury (Griffin et al. 2006; Renstrom et al. 2008). Understanding the differences in knee extensor and flexor function (maximal and explosive) between males and females may help to reveal the basis of any differences in these ratios. Given the greater body size and muscle mass of males [e.g. (O’Brien et al. 2010)], strength measurements normalised to body mass may provide a more appropriate and meaningful comparison between males and females than absolute values. Furthermore, force production capabilities in relation to body mass may reflect the ability to move the body and/or stabilise the knee during locomotor activities and thus have greater functional relevance. Finally, the explosive force produced by a muscle group may depend on the ability to explosively express the available force generating capacity, which can be assessed by examining explosive force production relative to MVF (de Ruiter et al. 2004; Folland et al. 2013; Hannah et al. 2014; Tillin et al. 2010). Electromechanical delay (EMD), which is defined as the time difference between the onset of EMG activity and the onset of force (Cavanagh and Komi 1979), is an important contributor to explosive neuromuscular performance. In men, we previously found hamstrings EMD to be almost twice as long as quadriceps EMD, thus delaying the production of hamstrings force relative to quadriceps force. Hamstrings EMD also explained a large proportion (72 %) of the inter-individual variance in the explosive H/Q force ratio at 50 ms from activation onset in men (Hannah et al. 2014). Therefore, any sex differences in hamstrings or quadriceps EMD, or the H/Q EMD ratio, could have implications for the explosive H/Q force ratio. Furthermore, agonist neuromuscular activation has also been found to influence explosive force production (Aagaard 2002; Folland et al. 2013; Tillin et al. 2010) and could explain any differences in explosive force production between males and females. The purpose of this study was to compare the absolute and relative (to MVF and body mass) explosive neuromuscular performance of males and females, with particular attention to the explosive H/Q force ratio. These comparisons were made with the knee close to full extension (150°), which reflects a position of the knee close to where the greatest strain on the ACL occurs (Shimokochi and Shultz 2008), where the majority of ACL injuries occur (Krosshaug et al. 2007) and when the foot contacts
Eur J Appl Physiol
the ground during athletic locomotor movements (Gehring et al. 2009; Malinzak et al. 2001).
Methods Participants Twenty untrained males and twenty untrained females with similar low-to-moderate habitual physical activity levels were recruited. Participants were categorised using the International Physical Activity Questionnaire Short Format (http://www.ipaq.ki.se/ipaq.htm; Craig et al. 2003). To minimise the potentially confounding effects of training background on neuromuscular performance and particularly H/Q ratios, exclusion criteria included any history of systematic strength, power or aerobic training. To control for the possibility of menstrual cycle phase influencing the neuromuscular performance [e.g. (Phillips et al. 1996)], female participants were required to have been taking the combined monophasic oral contraceptive pill for >6 months and were only tested between days 7 and 21 of pill consumption to minimise any fluctuations in endogenous gonadal hormones. All participants were healthy and free from musculoskeletal injury to the lower limbs. Volunteers provided written informed consent prior to their participation in this study that was approved by the University Human Ethical Review Committee. Study design Participants visited the laboratory for 60 min on two separate occasions to complete one familiarisation and one test session. The familiarisation and test sessions were separated by 1 week, and both sessions consisted of the same protocol. Participants were positioned in custombuilt dynamometers to obtain measures of isometric hamstrings and quadriceps performance of the dominant limb. The order of hamstrings and quadriceps testing was randomised. A series of maximum voluntary contractions and explosive voluntary contractions were completed, in that order, in each dynamometer. Measurements Knee extension force Participants were seated supine in a custom-built dynamometer (Hannah et al. 2013; Minshull et al. 2011) with hip and knee joint angles of 140° and 150° (180° = full extension; Hannah et al. 2014), respectively, and adjustable strapping placed across the pelvis and shoulders prevented extraneous movement. An ankle cuff was attached to the dominant
leg of the participant ~3 cm proximal to the lateral malleolus and was in series with a linear response S-beam load cell (615, Tedea-Huntleigh, Herzliya, Israel) oriented perpendicular to the tibia. The force signal was amplified (1,000×), and sampled at 2,000 Hz using an external A/D converter (1401; CED, Cambridge, UK), interfaced with a personal computer (PC) using Spike 2 software (CED, Cambridge, UK). Knee flexion force Participants were secured in a prone position on a custombuilt dynamometer (Minshull et al. 2007, 2009), with hip and knee joint angles of 180° and 150° (Hannah et al. 2014), and adjustable strapping placed across the pelvis prevented extraneous movement. The same ankle cuff and load cell used for knee extension force measurements were attached to the dominant leg of the participant ~3 cm proximal to the lateral malleolus and oriented perpendicular to the tibia. The force signal was sampled in the same manner as knee extension force. The hardware and software for assessments of knee flexion and extension force were the same and the use of two dynamometers ensured that agonist muscle EMG electrodes were not subject to compression or movement artefacts. For instance, performing knee flexion contractions whilst in a seated position could affect hamstring EMG by virtue of the electrodes being between the hamstrings and seat. Individual positioning for the prescribed joint configurations was established during the familiarisation session whilst the participant performed sub-maximal voluntary contractions to ~50 % MVF. The characteristics of the dynamometers ensured minimal changes in knee angles during contraction. In pilot experiments with five participants, an electrogoniometer (SG150, Biometrics Ltd., Gwent, UK; pre-amplified lead, Noraxon, Scottsdale, USA) was attached to either side of the lateral aspect of the knee and calibrated on the individual before being used to record changes in knee joint angle during flexion and extension contractions. Signals were sampled at 2,000 Hz and acquired into Spike 2 simultaneously with the force signals via the same A/D converter. Changes in knee joint angle of 4° ± 1° from rest to MVF were found with both dynamometers, which seems small in light of the >15° joint movement measured via X-ray video fluoroscopy during knee extension on a commercially available dynamometer (Tsaopoulos et al. 2007). Electromyography (EMG) Electromyographic signals were recorded from the superficial quadriceps and hamstrings: rectus femoris, vastus
13
medialis, vastus lateralis, biceps femoris long head and semitendinosus. After preparation of the skin by shaving, light abrasion and cleaning with alcohol, bipolar surface electrodes (3 cm inter-electrode distance; silver/ silver chloride, 95 mm2 area, Ambu Blue Sensor, Ambu, Ballerup, Denmark) were attached over each muscle at standardised percentages of thigh length measured from the knee joint space to greater trochanter: rectus femoris, 60 %; vastus lateralis, biceps femoris and semitendinosus 40 %; vastus medialis, 20 %. Electrodes were positioned parallel to the presumed orientation of the muscle fibres. EMG signals were pre-amplified by active EMG leads (input impedance 100 MΩ, CMMR > 100 dB, base gain 500, first-order high-pass filter set to 10 Hz; Noraxon, Scottsdale, USA) connected in series to a custom-built junction box and subsequently to the same A/D converter and PC software that enabled synchronisation with the force data. The signals were sampled at 2,000 Hz. EMG data were band-pass filtered in both directions between 20 and 450 Hz using a fourth-order zero-lag Butterworth filter prior to analysis. Protocol Maximum voluntary knee extension and flexion contractions A brief warm-up of three sub-maximal knee extension contractions at 50, 75 and 90 % of the participants’ perceived maximal force were performed, followed by a series of three maximum voluntary contractions. All contractions lasted ~3 s and were preceded by ~30 s rest. For the maximum voluntary contractions participants were instructed to contract as hard as possible for 3 s. They received strong verbal encouragement during the contractions, together with online feedback of the force signal and a marker of their maximum force during that session displayed on-screen. MVF of the quadriceps (QMVF) and hamstrings (HMVF) was defined as the greatest instantaneous force produced during the relevant series of contractions and expressed in absolute terms (N) and relative to body mass (N•kg−1). The H/Q MVF ratio was calculated as HMVF divided by QMVF and expressed as a percentage of QMVF (%). Maximal EMG amplitude of each of the agonist muscle recording sites during knee extension and flexion maximum voluntary contractions was calculated as the root mean square of a 500 ms epoch surrounding MVF (Buckthorpe et al. 2012), and was averaged across the respective sites to calculate mean quadriceps (QEMGmax) and hamstrings (HEMGmax) values. Antagonist muscle coactivation during extension and flexion maximum voluntary contractions was recorded over the same 500 ms epoch
13
Eur J Appl Physiol
and expressed as a percentage of the respective EMGmax of each muscle, prior to calculating mean quadriceps and hamstrings values, respectively. In the absence of an appropriate reference value for normalisation of QEMGmax and HEMGmax, absolute HEMGmax was expressed as a percentage of absolute QEMGmax (H/Q EMGmax ratio). Whilst crude, this ratio may remove many of the confounders of absolute EMG comparisons between males and females (e.g. differences in subcutaneous adipose tissue), and provide some indication of the relative ability to activate the knee muscles during maximal efforts. Explosive voluntary knee extension and flexion contractions Participants completed a series of ≥10 explosive isometric contractions (for each muscle group), each separated by 20 s rest. Starting from a completely relaxed state, they were instructed to respond to an auditory signal by extending their knee “as fast and hard as possible” for ~1 s, with an emphasis on “fast”. An on-screen cursor was used to provide online feedback on their explosive performance, displaying the maximum rate of force development (2 ms time constant) of their best attempt. Strong verbal encouragement was provided to participants to exceed this target during each subsequent contraction. A visual marker on the screen depicted 80 % of MVF, which participants were expected to achieve or exceed during each explosive contraction. The resting force was also displayed on a sensitive scale during all explosive contractions to aid the detection of pre-tension or countermovement. The explosive contractions were performed until ten contractions with no prior countermovement or pre-tension been had been recorded. The three contractions with the greatest maximum rate of force development and no prior countermovement or pre-tension were analysed. Analyses involved measurement of the force–time and EMG-time traces in short time periods after their onsets. Force and EMG onsets were identified visually (Hodges and Bui 1996; Staude 2001) by the same investigator and in accordance with a previously published method (Tillin et al. 2010, 2013). EMD, defined as the time difference between the onset of EMG and force, was determined for each of the agonist muscles, and the longest EMD of the agonist muscles for each contraction was described as EMDmax (i.e. HEMDmax during knee flexion, and QEMDmax for knee extension). The rationale for using the EMDmax, rather than the average EMD of the agonist muscles, is that the first muscle head to fire (i.e. the one with the longest EMD) is the likely the first to contribute to the force generated by the muscle. Thus, this likely reflects the true EMD of the muscle group. EMDmax was averaged across the three contractions and expressed in absolute terms for
Eur J Appl Physiol
from EMG onset (0–50, 50–100 and 100–150 ms), i.e. first agonist muscle to be activated. EMG values for each muscle recording site were then expressed as a percentage of their maximal EMG [recorded at MVF (Buckthorpe et al. 2012)], prior to calculating mean quadriceps (QEMG) and hamstrings (HEMG) values for each time window. All force and EMG measurements were averaged across the three contractions selected for analysis. Statistical analyses Descriptive and outcome statistics are presented as mean ± standard deviation (SD). For indices measured at two or more time points such as during the explosive contractions, e.g. explosive force (absolute, relative to BM, relative to MVF) and EMG (agonist, antagonist), the effects of sex and time were analysed using a two-way repeated measures ANOVA (sex × time) for each muscle group (hamstrings and quadriceps). A two-way repeated measures ANOVA (sex × time) was also used to detect differences in explosive H/Q force over time, including H/Q MVF as the final time point since this is the ultimate end point for explosive H/Q force production. When a significant sex × time interaction was observed, pairwise comparisons with a Bonferroni correction were performed to locate the differences between groups at specific time points, i.e. P values obtained from statistical analyses were multiplied by n comparisons and compared to the critical P = 0.05. All other comparisons between sexes or muscle groups (age, height, body mass, IPAQ scores, MVF, EMDmax, EMGmax) were assessed using a paired samples t test. Statistical tests were all performed using SPSS version 21 (SPSS inc., Chicago, IL, USA) and statistical significance was accepted at P
ORIGINAL ARTICLE
Explosive hamstrings‑to‑quadriceps force ratio of males versus females Ricci Hannah · Jonathan P. Folland · Stephanie L. Smith · Claire Minshull
Received: 30 September 2014 / Accepted: 24 November 2014 © Springer-Verlag Berlin Heidelberg 2014
Abstract Purpose Females are known to exhibit a greater risk of ACL injury compared to males. Lower explosive hamstrings-to-quadriceps (H/Q) force ratio in the first 150 ms from activation onset could reflect an impaired capacity for knee joint stabilisation and increased risk of ACL injury. However, the explosive H/Q force ratio has not been compared between the sexes. Methods The neuromuscular performance of untrained males and females (20 of each) was assessed during a series of isometric knee flexor and extensor contractions, specifically explosive and maximum voluntary contractions of each muscle group. Force, in absolute terms and normalised to body mass, and surface EMG of the hamstrings and quadriceps were recorded. Hamstrings force was expressed Communicated by Olivier Seynnes. R. Hannah (*) Sobell Department of Motor Neuroscience and Movement Disorders, Institute of Neurology, University College London, 146, Queen Square, London WC1 3BG, UK e-mail: [email protected] J. P. Folland School of Sport, Exercise and Health Sciences, Loughborough University, Loughborough, UK S. L. Smith School of Health and Life Sciences, Glasgow Caledonian University, Glasgow, UK C. Minshull School of Clinical Sciences, University of Edinburgh, Edinburgh, UK C. Minshull School of Health Sciences, Queen Margaret University, Edinburgh, UK
relative to quadriceps force to produce ratios of explosive H/Q force and H/Q maximum voluntary force (MVF). For the explosive contractions, agonist electromechanical delay (EMD) and agonist neural activation were also assessed. Results The H/Q MVF ratio was greater in males (56 %) than females (50 %; P 300 ms (Thorstensson et al. 1976). Therefore, we previously suggested that because MVF is unlikely to be attained within 50 ms, the capacity for knee joint stabilisation might depend more on the relative ability of the knee muscles to rapidly or “explosively” exert force during the initial rising phase of contraction, i.e. the first 150 ms following neuromuscular activation (Hannah et al. 2014). In a male cohort we previously showed that from first activation [electromyography (EMG) onset] the ability to produce explosive force with the hamstrings relative to the quadriceps (explosive H/Q force ratio) in the early phase of explosive contraction was very low (0–17 %; 25–50 ms after activation onset) and substantially less than the commonly reported H/Q MVF ratio (56 %). This low early ratio of explosive H/Q force could render the knee particularly unstable in the early phase of contraction when ACL injuries are thought to occur (Krosshaug et al. 2007). The early
13
Eur J Appl Physiol
phase explosive H/Q force ratio has not been investigated previously in females. Given that females exhibit a lower H/Q MVF ratio than males (Andrade et al. 2012; Calmels et al. 1997; Griffin et al. 1993; Hewett et al. 1996; Holm and Vollestad 2008), studying the time course of explosive H/Q force production in females compared to males may further inform our understanding of why females are at relatively higher risk of ACL injury (Griffin et al. 2006; Renstrom et al. 2008). Understanding the differences in knee extensor and flexor function (maximal and explosive) between males and females may help to reveal the basis of any differences in these ratios. Given the greater body size and muscle mass of males [e.g. (O’Brien et al. 2010)], strength measurements normalised to body mass may provide a more appropriate and meaningful comparison between males and females than absolute values. Furthermore, force production capabilities in relation to body mass may reflect the ability to move the body and/or stabilise the knee during locomotor activities and thus have greater functional relevance. Finally, the explosive force produced by a muscle group may depend on the ability to explosively express the available force generating capacity, which can be assessed by examining explosive force production relative to MVF (de Ruiter et al. 2004; Folland et al. 2013; Hannah et al. 2014; Tillin et al. 2010). Electromechanical delay (EMD), which is defined as the time difference between the onset of EMG activity and the onset of force (Cavanagh and Komi 1979), is an important contributor to explosive neuromuscular performance. In men, we previously found hamstrings EMD to be almost twice as long as quadriceps EMD, thus delaying the production of hamstrings force relative to quadriceps force. Hamstrings EMD also explained a large proportion (72 %) of the inter-individual variance in the explosive H/Q force ratio at 50 ms from activation onset in men (Hannah et al. 2014). Therefore, any sex differences in hamstrings or quadriceps EMD, or the H/Q EMD ratio, could have implications for the explosive H/Q force ratio. Furthermore, agonist neuromuscular activation has also been found to influence explosive force production (Aagaard 2002; Folland et al. 2013; Tillin et al. 2010) and could explain any differences in explosive force production between males and females. The purpose of this study was to compare the absolute and relative (to MVF and body mass) explosive neuromuscular performance of males and females, with particular attention to the explosive H/Q force ratio. These comparisons were made with the knee close to full extension (150°), which reflects a position of the knee close to where the greatest strain on the ACL occurs (Shimokochi and Shultz 2008), where the majority of ACL injuries occur (Krosshaug et al. 2007) and when the foot contacts
Eur J Appl Physiol
the ground during athletic locomotor movements (Gehring et al. 2009; Malinzak et al. 2001).
Methods Participants Twenty untrained males and twenty untrained females with similar low-to-moderate habitual physical activity levels were recruited. Participants were categorised using the International Physical Activity Questionnaire Short Format (http://www.ipaq.ki.se/ipaq.htm; Craig et al. 2003). To minimise the potentially confounding effects of training background on neuromuscular performance and particularly H/Q ratios, exclusion criteria included any history of systematic strength, power or aerobic training. To control for the possibility of menstrual cycle phase influencing the neuromuscular performance [e.g. (Phillips et al. 1996)], female participants were required to have been taking the combined monophasic oral contraceptive pill for >6 months and were only tested between days 7 and 21 of pill consumption to minimise any fluctuations in endogenous gonadal hormones. All participants were healthy and free from musculoskeletal injury to the lower limbs. Volunteers provided written informed consent prior to their participation in this study that was approved by the University Human Ethical Review Committee. Study design Participants visited the laboratory for 60 min on two separate occasions to complete one familiarisation and one test session. The familiarisation and test sessions were separated by 1 week, and both sessions consisted of the same protocol. Participants were positioned in custombuilt dynamometers to obtain measures of isometric hamstrings and quadriceps performance of the dominant limb. The order of hamstrings and quadriceps testing was randomised. A series of maximum voluntary contractions and explosive voluntary contractions were completed, in that order, in each dynamometer. Measurements Knee extension force Participants were seated supine in a custom-built dynamometer (Hannah et al. 2013; Minshull et al. 2011) with hip and knee joint angles of 140° and 150° (180° = full extension; Hannah et al. 2014), respectively, and adjustable strapping placed across the pelvis and shoulders prevented extraneous movement. An ankle cuff was attached to the dominant
leg of the participant ~3 cm proximal to the lateral malleolus and was in series with a linear response S-beam load cell (615, Tedea-Huntleigh, Herzliya, Israel) oriented perpendicular to the tibia. The force signal was amplified (1,000×), and sampled at 2,000 Hz using an external A/D converter (1401; CED, Cambridge, UK), interfaced with a personal computer (PC) using Spike 2 software (CED, Cambridge, UK). Knee flexion force Participants were secured in a prone position on a custombuilt dynamometer (Minshull et al. 2007, 2009), with hip and knee joint angles of 180° and 150° (Hannah et al. 2014), and adjustable strapping placed across the pelvis prevented extraneous movement. The same ankle cuff and load cell used for knee extension force measurements were attached to the dominant leg of the participant ~3 cm proximal to the lateral malleolus and oriented perpendicular to the tibia. The force signal was sampled in the same manner as knee extension force. The hardware and software for assessments of knee flexion and extension force were the same and the use of two dynamometers ensured that agonist muscle EMG electrodes were not subject to compression or movement artefacts. For instance, performing knee flexion contractions whilst in a seated position could affect hamstring EMG by virtue of the electrodes being between the hamstrings and seat. Individual positioning for the prescribed joint configurations was established during the familiarisation session whilst the participant performed sub-maximal voluntary contractions to ~50 % MVF. The characteristics of the dynamometers ensured minimal changes in knee angles during contraction. In pilot experiments with five participants, an electrogoniometer (SG150, Biometrics Ltd., Gwent, UK; pre-amplified lead, Noraxon, Scottsdale, USA) was attached to either side of the lateral aspect of the knee and calibrated on the individual before being used to record changes in knee joint angle during flexion and extension contractions. Signals were sampled at 2,000 Hz and acquired into Spike 2 simultaneously with the force signals via the same A/D converter. Changes in knee joint angle of 4° ± 1° from rest to MVF were found with both dynamometers, which seems small in light of the >15° joint movement measured via X-ray video fluoroscopy during knee extension on a commercially available dynamometer (Tsaopoulos et al. 2007). Electromyography (EMG) Electromyographic signals were recorded from the superficial quadriceps and hamstrings: rectus femoris, vastus
13
medialis, vastus lateralis, biceps femoris long head and semitendinosus. After preparation of the skin by shaving, light abrasion and cleaning with alcohol, bipolar surface electrodes (3 cm inter-electrode distance; silver/ silver chloride, 95 mm2 area, Ambu Blue Sensor, Ambu, Ballerup, Denmark) were attached over each muscle at standardised percentages of thigh length measured from the knee joint space to greater trochanter: rectus femoris, 60 %; vastus lateralis, biceps femoris and semitendinosus 40 %; vastus medialis, 20 %. Electrodes were positioned parallel to the presumed orientation of the muscle fibres. EMG signals were pre-amplified by active EMG leads (input impedance 100 MΩ, CMMR > 100 dB, base gain 500, first-order high-pass filter set to 10 Hz; Noraxon, Scottsdale, USA) connected in series to a custom-built junction box and subsequently to the same A/D converter and PC software that enabled synchronisation with the force data. The signals were sampled at 2,000 Hz. EMG data were band-pass filtered in both directions between 20 and 450 Hz using a fourth-order zero-lag Butterworth filter prior to analysis. Protocol Maximum voluntary knee extension and flexion contractions A brief warm-up of three sub-maximal knee extension contractions at 50, 75 and 90 % of the participants’ perceived maximal force were performed, followed by a series of three maximum voluntary contractions. All contractions lasted ~3 s and were preceded by ~30 s rest. For the maximum voluntary contractions participants were instructed to contract as hard as possible for 3 s. They received strong verbal encouragement during the contractions, together with online feedback of the force signal and a marker of their maximum force during that session displayed on-screen. MVF of the quadriceps (QMVF) and hamstrings (HMVF) was defined as the greatest instantaneous force produced during the relevant series of contractions and expressed in absolute terms (N) and relative to body mass (N•kg−1). The H/Q MVF ratio was calculated as HMVF divided by QMVF and expressed as a percentage of QMVF (%). Maximal EMG amplitude of each of the agonist muscle recording sites during knee extension and flexion maximum voluntary contractions was calculated as the root mean square of a 500 ms epoch surrounding MVF (Buckthorpe et al. 2012), and was averaged across the respective sites to calculate mean quadriceps (QEMGmax) and hamstrings (HEMGmax) values. Antagonist muscle coactivation during extension and flexion maximum voluntary contractions was recorded over the same 500 ms epoch
13
Eur J Appl Physiol
and expressed as a percentage of the respective EMGmax of each muscle, prior to calculating mean quadriceps and hamstrings values, respectively. In the absence of an appropriate reference value for normalisation of QEMGmax and HEMGmax, absolute HEMGmax was expressed as a percentage of absolute QEMGmax (H/Q EMGmax ratio). Whilst crude, this ratio may remove many of the confounders of absolute EMG comparisons between males and females (e.g. differences in subcutaneous adipose tissue), and provide some indication of the relative ability to activate the knee muscles during maximal efforts. Explosive voluntary knee extension and flexion contractions Participants completed a series of ≥10 explosive isometric contractions (for each muscle group), each separated by 20 s rest. Starting from a completely relaxed state, they were instructed to respond to an auditory signal by extending their knee “as fast and hard as possible” for ~1 s, with an emphasis on “fast”. An on-screen cursor was used to provide online feedback on their explosive performance, displaying the maximum rate of force development (2 ms time constant) of their best attempt. Strong verbal encouragement was provided to participants to exceed this target during each subsequent contraction. A visual marker on the screen depicted 80 % of MVF, which participants were expected to achieve or exceed during each explosive contraction. The resting force was also displayed on a sensitive scale during all explosive contractions to aid the detection of pre-tension or countermovement. The explosive contractions were performed until ten contractions with no prior countermovement or pre-tension been had been recorded. The three contractions with the greatest maximum rate of force development and no prior countermovement or pre-tension were analysed. Analyses involved measurement of the force–time and EMG-time traces in short time periods after their onsets. Force and EMG onsets were identified visually (Hodges and Bui 1996; Staude 2001) by the same investigator and in accordance with a previously published method (Tillin et al. 2010, 2013). EMD, defined as the time difference between the onset of EMG and force, was determined for each of the agonist muscles, and the longest EMD of the agonist muscles for each contraction was described as EMDmax (i.e. HEMDmax during knee flexion, and QEMDmax for knee extension). The rationale for using the EMDmax, rather than the average EMD of the agonist muscles, is that the first muscle head to fire (i.e. the one with the longest EMD) is the likely the first to contribute to the force generated by the muscle. Thus, this likely reflects the true EMD of the muscle group. EMDmax was averaged across the three contractions and expressed in absolute terms for
Eur J Appl Physiol
from EMG onset (0–50, 50–100 and 100–150 ms), i.e. first agonist muscle to be activated. EMG values for each muscle recording site were then expressed as a percentage of their maximal EMG [recorded at MVF (Buckthorpe et al. 2012)], prior to calculating mean quadriceps (QEMG) and hamstrings (HEMG) values for each time window. All force and EMG measurements were averaged across the three contractions selected for analysis. Statistical analyses Descriptive and outcome statistics are presented as mean ± standard deviation (SD). For indices measured at two or more time points such as during the explosive contractions, e.g. explosive force (absolute, relative to BM, relative to MVF) and EMG (agonist, antagonist), the effects of sex and time were analysed using a two-way repeated measures ANOVA (sex × time) for each muscle group (hamstrings and quadriceps). A two-way repeated measures ANOVA (sex × time) was also used to detect differences in explosive H/Q force over time, including H/Q MVF as the final time point since this is the ultimate end point for explosive H/Q force production. When a significant sex × time interaction was observed, pairwise comparisons with a Bonferroni correction were performed to locate the differences between groups at specific time points, i.e. P values obtained from statistical analyses were multiplied by n comparisons and compared to the critical P = 0.05. All other comparisons between sexes or muscle groups (age, height, body mass, IPAQ scores, MVF, EMDmax, EMGmax) were assessed using a paired samples t test. Statistical tests were all performed using SPSS version 21 (SPSS inc., Chicago, IL, USA) and statistical significance was accepted at P
