Eur J Appl Physiol (2014) 114:365–374 DOI 10.1007/s00421-013-2781-x
Original Article
Maximal and explosive strength training elicit distinct neuromuscular adaptations, specific to the training stimulus Neale A. Tillin · Jonathan P. Folland
Received: 3 July 2013 / Accepted: 20 November 2013 / Published online: 1 December 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose To compare the effects of short-term maximal (MST) vs. explosive (EST) strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes. Methods Male participants completed either MST (n = 9) or EST (n = 10) for 4 weeks. In training participants were instructed to: contract as fast and hard as possible for ~1 s (EST); or contract progressively up to 75 % maximal voluntary force (MVF) and hold for 3 s (MST). Pre- and post-training measurements included recording MVF during maximal voluntary contractions and explosive force at 50-ms intervals from force onset during explosive contractions. Neuromuscular activation was assessed by recording EMG RMS amplitude, normalised to a maximal M-wave and averaged across the three superficial heads of the quadriceps, at MVF and between 0–50, 0–100 and 0–150 ms during the explosive contractions. Results Improvements in MVF were significantly greater (P 100 ms in concentric contractions (Tillin et al. 2012a) and >250 ms in isometric and eccentric contractions (Thorstensson et al. 1976; Tillin et al. 2012a)], and thus explosive strength is considered more important where time available to develop force is limited [e.g. restabilising the body following a loss of balance (Domire et al. 2011; Pijnappels et al. 2008) and sports activities such as sprinting and jumping (de Ruiter et al. 2006; Tillin et al. 2013a)]. Understanding how these components of strength are affected by different strength training modalities has important practical implications for improving health and sports performance. Time under tension and magnitude of the training load are considered important stimuli for developing maximal strength (Crewther et al. 2005), and thus maximal strength training typically involves sustained (>2 s) contractions against loads ≥70 % MVF (Del Balso and Cafarelli 2007; Jones and Rutherford 1987; Kubo et al. 2001). In contrast, training for explosive strength is typically characterised by a series of short (≤1 s) contractions performed as rapidly as possible (Barry et al. 2005; de Ruiter et al. 2012) to practise and improve the rate of force development. Based on the training principle of specificity, it is conceivable that these different training stimuli induce distinct functional adaptations, e.g. with greater gains in the explosive strength after explosive training. On the other hand, maximal and explosive strength are determined by similar physiological mechanisms [e.g. muscle size and neural drive; (Andersen and Aagaard 2006; Duchateau et al. 2006; Hakkinen and Keskinen 1989; Schantz et al. 1983; Tillin et al. 2010)], and might be expected to exhibit similar changes in response to training. Evidence in support of distinct training stimuli for improvements in maximal and explosive strength is equivocal. Individual studies have examined the functional responses to either maximal strength training, explosive strength training, or a combination of both, with some investigations observing distinct effects on maximal and explosive strength (Andersen et al. 2010; Gruber et al. 2007; Rich and Cafarelli 2000; Tillin et al. 2011) and others reporting improvements in both attributes (Barry et al.
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Eur J Appl Physiol (2014) 114:365–374
2005; de Ruiter et al. 2012; Del Balso and Cafarelli 2007; Kubo et al. 2001; Suetta et al. 2004; Tillin et al. 2012b). Methodological differences (e.g. training duration, intensity and volume; instructions to the participants during the training; and the type of training contractions performed) preclude meaningful comparisons of functional responses between separate studies. Therefore a direct comparison of maximal and explosive strength training is required to establish whether there are distinct functional responses to these different training stimuli. Whilst maximal and explosive strength training might be expected to elicit distinct functional responses, the physiological bases of this specificity are unknown. The changes in function with any type of strength training are underpinned by neural and/or morphological adaptations (Folland and Williams 2007), with neural adaptations widely thought to dominate the adaptive response to short-term training (within 4 weeks; Folland and Williams 2007; Sale 2003). Increased maximal strength has been associated with increased neuromuscular activation at MVF (Del Balso and Cafarelli 2007; Tillin et al. 2011), whilst increased explosive strength has been accompanied by greater activation during the explosive phase of contraction (Barry et al. 2005; de Ruiter et al. 2012; Tillin et al. 2012b). However, the degree of specificity and/or transferability of these neural changes between the explosive and maximal phases of contraction, and vice versa, are unclear. Neuromuscular activation can be assessed by EMG amplitude normalised to a maximum evoked compound muscle action potential (M-wave), as a non-volitional reference, to reduce the between-subject variability and increase the sensitivity of the experiment to detect changes in activation (Buckthorpe et al. 2012). In addition, the proportional changes in neuromuscular activation during the explosive and maximum phases of contraction can be examined by assessing EMG during the explosive phase of contraction relative to EMG at MVF. The purpose of this study was to compare the effects of short-term maximal vs. explosive strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes. We hypothesised that the two types of training would elicit distinct functional and neural changes specific to the training stimulus.
Methods Participants Nineteen male participants who were recreationally active (moderate exercise ≤4 times per week), but had not been involved in any form of lower-body strength training for
Eur J Appl Physiol (2014) 114:365–374
367
Table 1 Physical characteristics of the maximal and explosive strength training groups prior to the training Strength training group
Age (years) Height (m) Body mass (kg) MVF (N) MVF/body mass (N.kg−1)
Maximal
Explosive
20.9 ± 1.1 1.82 ± 0.05 81.1 ± 6.8 585 ± 84
20.2 ± 2.4 1.82 ± 0.08 73.6 ± 7.4** 482 ± 58**
7.2 ± 0.8
6.6 ± 0.8
MVF maximal voluntary force ** Denotes a significant difference between the groups (P 6 months prior to the start of the study, were recruited and completed either maximal strength training (MST group; n = 9) or explosive strength training (EST group; n = 10). All participants were healthy, injury free and provided written informed consent prior to their involvement in this study, which was approved by the Loughborough University Ethical Advisory Committee. At baseline the groups were of similar age and height, and whilst they differed in body mass and MVF, there were no differences in MVF relative to body mass (Table 1). Overview Participants completed a familiarisation session and a measurement session (separated by 2–3 days) before, and one measurement session after, 4 weeks of unilateral isometric strength training of the knee extensors. The pre-training measurement session took place within 10 days prior to the start of training, whilst the post-training measurement session took place 2–3 days after the last training session. During the familiarisation session, participants practised the range of measurement tasks without data recording. Preand post-training measurement sessions involved recording external knee extension force and surface EMG of the superficial knee extensors during a series of explosive voluntary and maximal voluntary (MVCs) isometric contractions of the knee extensors. EMG responses (M-waves) to electrical stimulation of the femoral nerve with single, supramaximal impulses were also recorded for normalisation of the EMG signal recorded in the voluntary efforts. Training was of one leg chosen at random and involved isometric contractions of the knee extensors (either EST or MST) performed four times a week for 4 weeks with the same apparatus as the measurement sessions. Some within-group comparisons of the trained vs. untrained leg after maximal (Tillin et al. 2011) and explosive (Tillin et al. 2012b) strength training have previously been reported separately. Participants were instructed to maintain their normal physical activities throughout the period of the study.
Whilst dynamic contractions might be considered more relevant to functional human movement, we chose an isometric contraction model to assess the effects of MST and EST on these two components of strength as it provides a more experimentally controlled situation in which to assess the mechanisms underpinning the changes in function. Training Each training session consisted of a brief warm-up of submaximal isometric knee extensions, followed by four sets (separated by 2 min) of ten isometric contractions of the knee extensors (each set lasting ~60 s). However, the type of contractions performed differed between the training groups. The MST group were instructed to increase force over a 1-s period, up to 75 % of their MVF, hold for 3 s, and then relax for 2 s before completing the next contraction in the set. There was no specific instruction to produce force rapidly during the MST. In contrast, the EST group were instructed to contract as “fast and hard” as possible for ~1 s without producing a prior knee-flexor force, in an attempt to achieve at least 90 % of their MVF, and then relax for 5 s before completing the next contraction in the set. Thus, whilst the duration of loading was greater for the MST group, the peak loads and loading rates were greater for the EST group (Fig. 1). For biofeedback in the MST group, a computer monitor displayed the force–time curve with a horizontal cursor on 75 % MVF, whilst in the EST group the monitor displayed the slope of the force– time curve (established with a 1-ms constant epoch) and the baseline of the force–time curve that was used to confirm that no prior knee-flexor force had occurred. MVF was initially established in familiarisation and the pre-training measurement sessions (see below) and re-established at the start of the first training session each week. Force and EMG recording Measurement and training sessions were completed in an isometric strength testing chair (Bojsen-Moller et al. 2005), with knee and hip angles of 85° and 100°, respectively (180° representing full extension). Participants were secured firmly in the chair with a waist belt and shoulder straps. An ankle strap was consistently placed 3 cm proximal to the medial malleolus and was in series with a calibrated linear response strain gauge (Jones and Parker 1989) positioned perpendicular to the tibia. This low noise strain gauge [range of baseline noise
Original Article
Maximal and explosive strength training elicit distinct neuromuscular adaptations, specific to the training stimulus Neale A. Tillin · Jonathan P. Folland
Received: 3 July 2013 / Accepted: 20 November 2013 / Published online: 1 December 2013 © Springer-Verlag Berlin Heidelberg 2013
Abstract Purpose To compare the effects of short-term maximal (MST) vs. explosive (EST) strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes. Methods Male participants completed either MST (n = 9) or EST (n = 10) for 4 weeks. In training participants were instructed to: contract as fast and hard as possible for ~1 s (EST); or contract progressively up to 75 % maximal voluntary force (MVF) and hold for 3 s (MST). Pre- and post-training measurements included recording MVF during maximal voluntary contractions and explosive force at 50-ms intervals from force onset during explosive contractions. Neuromuscular activation was assessed by recording EMG RMS amplitude, normalised to a maximal M-wave and averaged across the three superficial heads of the quadriceps, at MVF and between 0–50, 0–100 and 0–150 ms during the explosive contractions. Results Improvements in MVF were significantly greater (P 100 ms in concentric contractions (Tillin et al. 2012a) and >250 ms in isometric and eccentric contractions (Thorstensson et al. 1976; Tillin et al. 2012a)], and thus explosive strength is considered more important where time available to develop force is limited [e.g. restabilising the body following a loss of balance (Domire et al. 2011; Pijnappels et al. 2008) and sports activities such as sprinting and jumping (de Ruiter et al. 2006; Tillin et al. 2013a)]. Understanding how these components of strength are affected by different strength training modalities has important practical implications for improving health and sports performance. Time under tension and magnitude of the training load are considered important stimuli for developing maximal strength (Crewther et al. 2005), and thus maximal strength training typically involves sustained (>2 s) contractions against loads ≥70 % MVF (Del Balso and Cafarelli 2007; Jones and Rutherford 1987; Kubo et al. 2001). In contrast, training for explosive strength is typically characterised by a series of short (≤1 s) contractions performed as rapidly as possible (Barry et al. 2005; de Ruiter et al. 2012) to practise and improve the rate of force development. Based on the training principle of specificity, it is conceivable that these different training stimuli induce distinct functional adaptations, e.g. with greater gains in the explosive strength after explosive training. On the other hand, maximal and explosive strength are determined by similar physiological mechanisms [e.g. muscle size and neural drive; (Andersen and Aagaard 2006; Duchateau et al. 2006; Hakkinen and Keskinen 1989; Schantz et al. 1983; Tillin et al. 2010)], and might be expected to exhibit similar changes in response to training. Evidence in support of distinct training stimuli for improvements in maximal and explosive strength is equivocal. Individual studies have examined the functional responses to either maximal strength training, explosive strength training, or a combination of both, with some investigations observing distinct effects on maximal and explosive strength (Andersen et al. 2010; Gruber et al. 2007; Rich and Cafarelli 2000; Tillin et al. 2011) and others reporting improvements in both attributes (Barry et al.
13
Eur J Appl Physiol (2014) 114:365–374
2005; de Ruiter et al. 2012; Del Balso and Cafarelli 2007; Kubo et al. 2001; Suetta et al. 2004; Tillin et al. 2012b). Methodological differences (e.g. training duration, intensity and volume; instructions to the participants during the training; and the type of training contractions performed) preclude meaningful comparisons of functional responses between separate studies. Therefore a direct comparison of maximal and explosive strength training is required to establish whether there are distinct functional responses to these different training stimuli. Whilst maximal and explosive strength training might be expected to elicit distinct functional responses, the physiological bases of this specificity are unknown. The changes in function with any type of strength training are underpinned by neural and/or morphological adaptations (Folland and Williams 2007), with neural adaptations widely thought to dominate the adaptive response to short-term training (within 4 weeks; Folland and Williams 2007; Sale 2003). Increased maximal strength has been associated with increased neuromuscular activation at MVF (Del Balso and Cafarelli 2007; Tillin et al. 2011), whilst increased explosive strength has been accompanied by greater activation during the explosive phase of contraction (Barry et al. 2005; de Ruiter et al. 2012; Tillin et al. 2012b). However, the degree of specificity and/or transferability of these neural changes between the explosive and maximal phases of contraction, and vice versa, are unclear. Neuromuscular activation can be assessed by EMG amplitude normalised to a maximum evoked compound muscle action potential (M-wave), as a non-volitional reference, to reduce the between-subject variability and increase the sensitivity of the experiment to detect changes in activation (Buckthorpe et al. 2012). In addition, the proportional changes in neuromuscular activation during the explosive and maximum phases of contraction can be examined by assessing EMG during the explosive phase of contraction relative to EMG at MVF. The purpose of this study was to compare the effects of short-term maximal vs. explosive strength training on maximal and explosive force production, and assess the neural adaptations underpinning any training-specific functional changes. We hypothesised that the two types of training would elicit distinct functional and neural changes specific to the training stimulus.
Methods Participants Nineteen male participants who were recreationally active (moderate exercise ≤4 times per week), but had not been involved in any form of lower-body strength training for
Eur J Appl Physiol (2014) 114:365–374
367
Table 1 Physical characteristics of the maximal and explosive strength training groups prior to the training Strength training group
Age (years) Height (m) Body mass (kg) MVF (N) MVF/body mass (N.kg−1)
Maximal
Explosive
20.9 ± 1.1 1.82 ± 0.05 81.1 ± 6.8 585 ± 84
20.2 ± 2.4 1.82 ± 0.08 73.6 ± 7.4** 482 ± 58**
7.2 ± 0.8
6.6 ± 0.8
MVF maximal voluntary force ** Denotes a significant difference between the groups (P 6 months prior to the start of the study, were recruited and completed either maximal strength training (MST group; n = 9) or explosive strength training (EST group; n = 10). All participants were healthy, injury free and provided written informed consent prior to their involvement in this study, which was approved by the Loughborough University Ethical Advisory Committee. At baseline the groups were of similar age and height, and whilst they differed in body mass and MVF, there were no differences in MVF relative to body mass (Table 1). Overview Participants completed a familiarisation session and a measurement session (separated by 2–3 days) before, and one measurement session after, 4 weeks of unilateral isometric strength training of the knee extensors. The pre-training measurement session took place within 10 days prior to the start of training, whilst the post-training measurement session took place 2–3 days after the last training session. During the familiarisation session, participants practised the range of measurement tasks without data recording. Preand post-training measurement sessions involved recording external knee extension force and surface EMG of the superficial knee extensors during a series of explosive voluntary and maximal voluntary (MVCs) isometric contractions of the knee extensors. EMG responses (M-waves) to electrical stimulation of the femoral nerve with single, supramaximal impulses were also recorded for normalisation of the EMG signal recorded in the voluntary efforts. Training was of one leg chosen at random and involved isometric contractions of the knee extensors (either EST or MST) performed four times a week for 4 weeks with the same apparatus as the measurement sessions. Some within-group comparisons of the trained vs. untrained leg after maximal (Tillin et al. 2011) and explosive (Tillin et al. 2012b) strength training have previously been reported separately. Participants were instructed to maintain their normal physical activities throughout the period of the study.
Whilst dynamic contractions might be considered more relevant to functional human movement, we chose an isometric contraction model to assess the effects of MST and EST on these two components of strength as it provides a more experimentally controlled situation in which to assess the mechanisms underpinning the changes in function. Training Each training session consisted of a brief warm-up of submaximal isometric knee extensions, followed by four sets (separated by 2 min) of ten isometric contractions of the knee extensors (each set lasting ~60 s). However, the type of contractions performed differed between the training groups. The MST group were instructed to increase force over a 1-s period, up to 75 % of their MVF, hold for 3 s, and then relax for 2 s before completing the next contraction in the set. There was no specific instruction to produce force rapidly during the MST. In contrast, the EST group were instructed to contract as “fast and hard” as possible for ~1 s without producing a prior knee-flexor force, in an attempt to achieve at least 90 % of their MVF, and then relax for 5 s before completing the next contraction in the set. Thus, whilst the duration of loading was greater for the MST group, the peak loads and loading rates were greater for the EST group (Fig. 1). For biofeedback in the MST group, a computer monitor displayed the force–time curve with a horizontal cursor on 75 % MVF, whilst in the EST group the monitor displayed the slope of the force– time curve (established with a 1-ms constant epoch) and the baseline of the force–time curve that was used to confirm that no prior knee-flexor force had occurred. MVF was initially established in familiarisation and the pre-training measurement sessions (see below) and re-established at the start of the first training session each week. Force and EMG recording Measurement and training sessions were completed in an isometric strength testing chair (Bojsen-Moller et al. 2005), with knee and hip angles of 85° and 100°, respectively (180° representing full extension). Participants were secured firmly in the chair with a waist belt and shoulder straps. An ankle strap was consistently placed 3 cm proximal to the medial malleolus and was in series with a calibrated linear response strain gauge (Jones and Parker 1989) positioned perpendicular to the tibia. This low noise strain gauge [range of baseline noise
