Journal of Strength and Conditioning Research Publish Ahead of Print DOI: 10.1519/JSC.0000000000002246
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ORIGINAL RESEARCH
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Shod versus barefoot effects on force and power
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development during a conventional deadlift
Mark E. Hammer, 1 Rudi A. Meir, 1 John W. Whitting, 1 Zachary J. Crowley-McHattan
School of Health and Human Sciences, Southern Cross University, Lismore, New South Wales, Australia
Running title: Shod and un-shod deadlift
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Correspondence:
R. Meir School of Health and Human Sciences Southern Cross University Lismore Australia Email: [email protected] Tel: +2-66203911 Fax: +2-66203022
Word count: 3,361 Submitted to: The Journal of Strength and Conditioning Research Date: July, 2017 Copyright ª 2017 National Strength and Conditioning Association
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ABSTRACT
The kinetics of a conventional deadlift in shod (S) versus unshod (US) footwear conditions in 10 male participants (mean ± SD, age = 27.0 ± 5.8 years; weight = 78.7 ± 11.5 kg; height = 175.8 ± 8.2 cm; 1RM deadlift = 155.8 ± 25.8 kg) was assessed in two testing sessions. A counterbalanced, crossover
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experimental design was used with different loads (60% and 80% 1RM). Four sets of four repetitions
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were prescribed per session with two sets per shoe and with each shoe condition involving one set per load. Peak vertical force (PF), rate of force development (RFD), time to peak force (TPF), anteriorposterior (COP-AP) and medio-lateral (COP-ML) center of pressure excursion, and barbell peak power (PP) data were recorded during all repetitions. Except for RFD (F = 6.389; p = 0.045; ƞp2 = 0.516) and
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ML-COP (F = 6.696; p = 0.041; ƞp2 = 0.527), there were no other significant main effects of shoe. There were significant main effects of load for PF (p < 0.05), COP-AP (p = 0.011), TPF (p = 0.018) and COPAP (p = 0.011). There were no significant interactions found between session, shoe and load (p range from 0.944 to 0.086). While the unshod condition may have produced changes in RFD and ML-COP compared with the shod condition, there is only limited evidence in the current study to support this
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lifting technique for the conventional deadlift. Further investigation is required to clarify any possible
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implications of this result and its benefit to lifters.
Word count: 250/250
Keywords: lifting technique, force production, footwear
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INTRODUCTION The deadlift is a closed chain, multi-joint resistance exercise that involves moving a stationary barbell from the floor until extension is established at the hips and knees. Within the strength and conditioning fields, the deadlift is seen as an important part of an athletic training program for the development of strength and power in the posterior chain i.e. back, hips and hamstrings (10, 34, 35, 37, 40). In
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Powerlifting competitions, the deadlift is the final of three contested lifts and is essential in developing a
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competitive total weight lifted (16). In the rehabilitation community, the deadlift and its variations are a common protocol in the post-operative and non-operative rehabilitation of the lower back, anterior cruciate ligament (ACL) and hamstrings (25).
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As with the squat, there is debate among weightlifting equipment forums about the best shoes to wear while deadlifting. Strength and conditioning researchers, and a range of industry practitioners (e.g. 7, 9, 14, 19, 20), have stated that weightlifting shoes, non-compressive soled shoes, or unshod (socks only or barefoot) conditions are essential in providing a stable platform and effective force transfer from the ground to the bar. In contrast, it could be argued that soft soled shoes, such as traditional running shoes,
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delay force transmission and create instability, altering the direction of the resultant ground reaction force
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(GRF), thus compromising lifting performance.
There appears to be no research published on how different footwear affect the kinetics or kinematics of the deadlift. However, with respect to the barbell back squat Shorter et al. (30) found that when using loads at 80% of 1RM shod lifters wearing lower profile, soft sole training shoe produced higher peak power and peak GRF than lifters wearing minimalist shoes or no shoes. Furthermore, Shorter et al. (30) found that the shod condition displayed a larger excursion in the center of pressure (COP) than the minimalist or no shoe conditions. Interestingly however, research conducted by Whitting et al. (38) reported a significantly larger anterior-posterior COP excursion during the barbell back squat for Copyright ª 2017 National Strength and Conditioning Association
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participants wearing a solid, rear-foot elevated weightlifting shoe when compared to a standard sports shoe. Although one might reasonably surmise that the stable weightlifting shoe could elicit similar effects in COP excursion to minimalist or barefoot conditions, it seems that the findings of these two studies are at odds with one another. Further research comparing a shod condition with an unshod condition in the barbell back squat has not provided kinetic data to help elucidate this small body of research (14, 28). As
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a result, it appears that shoe effects on kinetic parameters in common weightlifting modes remains poorly
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understood.
The deadlift and its variations have been studied extensively, with a number of biomechanical analyses of the deadlift reported in the literature (4, 34-37). Several studies have focused on joint and segment angles
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in deadlift variations (29, 39), with two studies specifically focused on the comparison of kinematics and kinetics of the sumo deadlift and the conventional deadlift (10, 12). Further research has been completed on muscle activation patterns via electromyography in both stable and unstable conditions (6, 11, 18, 24). However, none of the aforementioned studies made reference to the types of footwear worn during testing conditions, and nor was COP data provided.
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Research (16) has also found that there are significant kinematic differences between the back squat and deadlift, indicating that the data compiled for the squat may not be entirely relevant for the deadlift.
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Therefore, given current and yet to be supported beliefs that deadlifting performance may be enhanced by wearing minimalist shoes or by being barefoot, it was the purpose of this study to determine possible kinetic effects of different footwear conditions (shod and unshod) while performing a conventional deadlift. Specifically, this study aimed to determine whether the peak vertical ground reaction force (PF), peak vertical power (PP), rate of force development of the peak force (RFD), time to peak force (TPF), and center of pressure excursions in the anterior-posterior axis (COP-AP) and medio-lateral axis (COPML) were affected by these shoe conditions. It was hypothesized that a deadlift in an unshod condition
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would provide a higher magnitude of PF, PP, RFD and a reduced COP excursion on the ML and AP axis when compared to a shod condition.
METHODS Experimental Approach to the Problem
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A within-subject, counter-balanced, crossover experimental design was used to quantify the impact of the two lifting conditions (S = shod in regular training shoes, US = Unshod) on the kinetics of the
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conventional deadlift at different %1RM loads. Data was collected for each participant over 3 sessions (initial 1RM testing and 2 trial sessions), each separated by a minimum of 72 hours to avoid any possible influence of fatigue. All sessions were conducted at the institutional biomechanics laboratory. During the
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trial sessions, participants completed 4 repetitions of the conventional deadlift with pronated grip using 60 and 80% of their predetermined 1RM. A total of 4 sets were completed on each of the 2 trial sessions. All lifts were completed in both shod and unshod conditions with the order of footwear condition reversed for the second testing session. Participants were required to wear those shoes that they normally wore (all soft soled “trainers”) during training in the shod condition, and to be barefoot in the unshod
Subjects
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condition.
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A total of 10 male participants (group mean ±SD: age = 27.0 ± 5.8 years, weight = 78.7 ± 11.5 kg, height = 175.8 ± 8.2 cm and 1RM = 155.8 ± 25.8 kg) volunteered for this study. To be eligible to participate, participants were required to have a minimum of 2 years training age using the conventional deadlift and to have used maximal and near maximal loads in this lift as part of their normal training program. Participants had to be free from injury and illness and complete a pre-screening exercise and health questionnaire to identify any contraindications to their participation. Prior to agreeing to their involvement participants were provided with an information sheet that described all aspects of the
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research, including any potential risks and benefits. Having agreed to participate each participant attended a familiarization session where the test process was explained verbally and written informed consent was obtained. At this session, each participant had their 1RM deadlift score established under the supervision of an accredited Australian Weightlifting Federation coach. Participants were instructed to avoid both caffeine and lifting maximal loads in the 24 hours immediately prior to each data collection
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session. Participants were randomly assigned to Group 1 (G1) or Group 2 (G2) to determine the ordering of their first shoe test condition (S or US respectively) in their first testing session. All participants
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reversed the order of shoe condition during their second session to control the potential effect of shoe order on the kinetic outcomes. All procedures used in this research were approved by the institutional
Procedures Establishing deadlift 1RM
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ethics committee (ECN-16-072) and complied with the Declaration of Helsinki.
Upon agreeing to participate, baseline measurements (age, height and weight) were recorded for each participant. Participants then performed a standardized warm-up of approximately 8 minutes that comprised of 2 minutes on a Monarch cycle ergometer (utilizing a low load while maintaining a cadence
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of ~70-80 rpm), and a series of prescribed dynamic stretches. Participants were then tested for their deadlift 1RM wearing their normal training shoes. Deadlift technique was visually assessed by an
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accredited Australian Weightlifting Federation coach prior to establishing their 1RM. Participants performed the following conventional deadlift sequence to attain their 1RM: i) 5-10 repetitions on a 20kg bar; followed by time to load the bar; ii) 4-5 repetitions at 70kg (approximately 40-60% of predicted 1RM) followed by 3 minutes rest; iii) 1 repetition at 70-80% of predicted 1RM followed by 3 minutes rest; iv) 1 repetition at approximately 90% of their predicted 1RM followed by a minimum of 3 minutes rest. Participants were then given six attempts to reach their 1RM with a minimum of 3 minutes rest between attempts as established in previous protocols (5, 14, 18, 21). The final 1RM was considered as
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the load successfully lifted immediately prior to the failed attempted load. A successful lift was considered as observation of full extension of the hip and knee. Chalk was the only supportive aid used to prevent hands from slipping on the bar due to sweat.
Deadlift trial conditions
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On Arrival for session 2, participants performed the standardized warm-up and then performed the following deadlift sequence: i) 5-10 repetitions with a 20kg bar; followed by time to load bar; ii) 5-6
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repetitions at 50% of their first experimental load (i.e. approx. 30% of 1RM) followed by 1 minute rest; iii) 4 repetitions at 60% of 1RM followed by 3 minutes rest; iv) 4 repetitions at 80% of 1RM followed by 5 minutes rest. Following this rest period, participants completed the lifting sequence as identified in
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points iii-iv above, in the second experimental condition. All deadlifts were performed as a conventional deadlift with both hands pronated. No feedback was given during data capture; however, prior to lifting, all participants were instructed to perform the concentric phase of the lift as fast as possible.
GRF data were sampled (1000Hz) for each trial as participants stood with both feet on a single Kistler force plate (Type 9287; Winterthur, Switzerland) using Nexus software (version 1.8.4, 2013). GRF were
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filtered (ƒc = 100 Hz) with a Butterworth fourth-order low-pass filter. Filtered data were used to calculate the PF, RFD and COP excursions in the anterior-posterior (AP) and mediolateral (ML) directions. Data
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for PP were collected using a Gym Aware Power Tool 5 (Kinetic Performance Technology, Canberra, Australia) as seen in Figure. 1. All variables of interest were determined from data extracted from the concentric phase of the lift, corresponding to the period of time where force and power is developed during the lift. Insert Figure 1 about here
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Statistical Analysis A three-factor (session x shoe x load) repeated-measures ANOVA was used to determine whether there were main effects of session, shoe condition (S and US) or load (60 and 80% 1RM), and whether there were any interactions (session x shoe, session x load, session x shoe x load, shoe x load). Upon identification of significant effects, post-hoc analyses with a Bonferroni adjustment were used to
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minimize the chance of type I errors. Processed data were identified and entered into an Excel spreadsheet
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for the development of all descriptive values. A priori sample size calculation using G*Power with an α level of 0.05, and a power of 0.8, indicated that a sample size of 8 was sufficient for statistical analysis. All statistical analyses were conducted using SPSS version 20 for Windows with an α level set at 0.05. Effect size interpretation was in accordance with Cohen (1988), which states that for partial eta squared a
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small effect is 0.01 – 0.059, medium effect is 0.06 – 0.139 and large is 0.14 and above.
RESULTS
Means and standard deviations (±SD) for all variables are displayed in Table. 1. Shoe condition main
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effects were found to be significant for RFD (F = 6.389; p = 0.045; ƞp2 = 0.516) and ML-COP (F = 6.696; p = 0.041; ƞp2 = 0.527), with post-hoc comparisons indicating the unshod condition producing a
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greater rate of force development (Figure. 2) and lower ML-COP displacements (Figure. 3) than the shod condition respectively. Non-significant shoe effects were found for PF, TPF, PP, or AP-COP (F range from 0.270 to 4.567; p range from 0.618 to 0.076). No significant main effects of session (F range from 0.103 to 2.861; p range from 0.758 to 0.142) were found for any of the independent variables. Main effects of load were significant for PF (F = 83.090; p < 0.05; ƞp2 = 0.912), TPF (F = 9.411; p = 0.018; ƞp2 = 0.573), and COP-AP (F = 10.914; p = 0.011; ƞp2 = 0.577), with pair-wise post-hoc comparisons indicating a significant increase in these variables at 80% compared to 60% of 1RM. No significant main effects of load were found for PP (F = 0.005; p = 0.944; ƞp2 < 0.05), RFD (F = 3.632; p = 0.105; ƞp2 = Copyright ª 2017 National Strength and Conditioning Association
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0.377), or COP-ML (F = 4.055; p = 0.091; ƞp2 = 0.403). There were no significant interactions found between session, shoe and load (F range from < 0.05 – 4.222; p range from 0.944 to 0.086).
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Insert Table 1 and Figures 2 & 3 about here
DISCUSSION
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The purpose of this study was to determine possible kinetic effects of different footwear conditions (shod versus unshod) while performing a conventional deadlift; specifically during the concentric phase of the lift using submaximal training loads. Biomechanical effects of the deadlift exercise have been studied
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extensively (2, 3, 5, 10-12, 16, 17, 29, 34, 35). However, to the best of our knowledge there has been no research published with reference to the kinetic effects of shod or unshod conditions during this exercise.
Data analysis showed significant main effects of shoe on RFD and ML-COP; with unshod lifters able to apply more force to the floor at a faster rate. RFD was assessed as one of the main variables to investigate
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anecdotal beliefs in the strength and conditioning community that there is a perceived ‘disconnect’ between the ground and the feet in a soft soled shoe (9, 13, 19, 20). This lifting strategy is based on an
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assumption that the soft sole in these shoes is likely to produce instability, since the shoe sole must be compressed before any effective force transfer can occur between the ground and the feet. The extension of this belief is that an impeded force transmission will result in a delayed and reduced vertical GRF, thereby compromising lifting performance. Upon initial analysis of the data in the present study, a significantly higher RFD in the unshod compared to shod condition lends some weight to these anecdotal comments. However, this finding needs to be viewed with caution for a number of reasons. Firstly, the statistical strength appears weak to moderate (p = 0.045; ƞp2 = 0.516; observed power = 0.563). Secondly, given Figure 2, it is questionable as to whether the means (±SE) of each data set are functionally different. Copyright ª 2017 National Strength and Conditioning Association
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Lastly, and perhaps most importantly, there were no shoe x load interactions, and no other significant main effects of shoe for the other primary indicators of power and force output (PP and PF). As such, this study does not provide substantive evidence to support the hypotheses developed to prove current anecdote and practice i.e. that there is an advantage for producing force and power when deadlifting
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unshod.
It should also be noted, that kinematic data were not collected during the present study. As a result, it
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could not be determined whether compression of the shoe sole might be a factor correlated with a possibly slower RFD in the shod condition. Furthermore, with TPF only significantly different in main effects of load, it would appear that the duration of the concentric phase of the deadlift may be unaffected
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by shoe condition, and that any possible delay in GRF at the start of the lift may have been overcome during the concentric phase. Future biomechanical investigations of the deadlift, concerning different footwear conditions, should collect kinematic data to determine any possible links between performer and shoe movements, and kinetic outputs such as RFD.
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As expected, peak vertical ground reaction forces (PF) were found to significantly differ between the load
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conditions. Post-hoc analyses indicates a greater magnitude of peak force at 80% of 1RM than at 60% of 1RM, thereby confirming the results of previous studies (3, 5, 34, 39). Peak power was similar between loads of 60% and 80% of 1RM, respectively. Previous studies that focused on PF, velocity and PP (3, 5, 34) for the deadlift and upper body (33), have reported a larger magnitude of power at 60% than at 80% of 1RM. Power, being a product of force and velocity (P = F.V) (15, 26, 33) may be generated more easily with submaximal resistance from lighter loads, predominantly due to substantially greater velocities, despite slightly lower peak forces. While not reported, velocity data taken from the Gym Aware showed a 20% increase in bar velocity at 60% of 1RM compared to loads lifted at 80% of 1RM,
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while GRF data showed a 9% increase in PF when lifting at 80% of 1RM. These values show a somewhat proportional increase in force to the decrease in velocity, thereby explaining the similarity in our absolute PP values. While we are unable to confirm the cause of the discrepancy in our PP data when compared to the previously mentioned studies, the majority of participants noted several grip issues with the pronated grip, which was standardized in this analysis. A majority of the lifters in this study performed the deadlift
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with an alternating grip during training sessions, and so it is speculated that this may have had an effect
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on lift velocity.
The control of bipedal stance involves the complex integration of sensory inputs, perceptual processing of those inputs, and the production of appropriate motor commands in response (23). The measurement of
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the COP trajectory changes during the deadlift in the current study showed that COP-AP displayed significant main effects of load, and COP-ML showed an effect of shoe. Post-hoc analyses of COP-AP indicated larger COP displacement at 80% versus 60% of 1RM while the COP-ML exhibited larger COP displacements when shod versus unshod. This suggests that the motor control systems appears to alter independently the motor behavior of the postural control system when experiencing changes in either load
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or surface conditions. To our knowledge however, this is the first study to investigate COP excursions in the deadlift and the potential effect of differing loads and shod conditions on COP behaviour, and
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therefore direct comparisons of findings is difficult.
One possible way to interpret these findings could be to explain the results through the Equilibrium Point (EP) hypothesis. This hypothesis states that the central nervous system (CNS) initiates and modifies movement by using sensory input to shift the equilibrium states of the motor system (22). Since the motor control system controls the COP movement and directs it towards steady state behaviors, under heavier
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loads or more compliant plantar surface conditions, the motor system may be achieving its final planned behaviour through flexibility in its control strategies and hence leading to larger COP movements.
Tahayori et al. (36) investigated the effects of differential loading on the COP behaviour in quiet stance and found that a load equal to 15% of body mass to the back of the torso was effective in increasing COP
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movement, especially in the ML direction. Interestingly, the application of the same load to the front of the torso led to no change in COP movement behaviour, which is contrary to the COP changes found in
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the current study during dynamic frontal loading (i.e. the deadlift). These differences may be the result of the considerable methodological differences between the studies, including static v dynamic actions and relatively low loads v relatively high loads. Therefore it is suggested that further research be undertaken
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to elucidate the potential mechanism of the changes in COP behavior with differing loads.
The deadlift is performed with a variety of techniques (e.g. sumo, clean deadlift, snatch deadlift, alternating grip) and with a variety of assistive equipment, such as straps, belts and deadlifting suits. As such, limiting preferred technique and assistance equipment may influence an individual’s deadlifting
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performance, which may explain large variances between subjects in this and other studies.
PRACTICAL APPLICATION This study was developed from an observed practice within the strength and conditioning field regarding possible or perceived effects of different footwear on performance during the conventional deadlift. Motives for the participation in the deadlift vary and it is performed using a variety of techniques. Thus it is essential that rehabilitation and strength and conditioning practitioners employ evidence-based practices to reduce the chance of injury and to provide optimal outcomes. The findings of this current
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Table 1: Mean (±SD) of recorded variables displayed for each shoe (S, US) and load (60 and 80% 1RM) condition (N = 10). 80% 1RM US
Average PF (N)†
2060.1 (236.4)
Average PP (W) Average RFD (N/s)‡ Average TPF (s)†
S
US
2072.4 (240.5)
2259.3 (269.3)
2257.4 (289.0)
1129.0 (142.5)
1135.5 (148.4)
1093.6 (74.6)
1117.7 (75.2)
1840.4 (794.2)
2099.7 (915.9)
1593.7 (524.0)
2008.1 (772.3)
0.5 (0.2)
0.7 (0.3)
0.6 (0.3)
45.6 (16.4)
55.0 (16.7)
56.4 (22.0)
15.8 (7.2)
23.3 (7.9)
18.6 (5.4)
0.5 (0.3)
Average COP-AP (mm)†
41.7 (13.6)
Average COP-ML (mm)§
18.8 (7.0)
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S
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60% 1RM
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1RM = 1 repetition maximum; S = Shod; US = Unshod; COP = center of pressure; AP = anterior-posterior; ML = medio-lateral.
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† Indicates a main effect of load – post-hoc comparison showing larger magnitude of PF, TPF and COP-AP. ‡ Indicates main effect of shoe condition – post-hoc comparison indicating faster RFD for unshod v shod conditions. § Indicates main effect of shoe condition – post-hoc comparison indicating greater COP excursion with shod v unshod conditions
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Figure 1: Placement of Gym Aware Power Tool 5 and position of participant on force plate.
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Figure 2: Mean (± SE) shoe main effects for rate of force development (N.s-1) between shod (S) and
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unshod (US) conditions. * Indicates significantly higher RFD in the US condition (P = 0.045).
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Figure 3: Mean (± SE) shoe main effects for medio-lateral (ML) peak center of pressure (COP) displacement (mm) between shod (S) and unshod (US) conditions. * Indicates
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significantly greater displacement in the shod condition (P < 0.041).
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ORIGINAL RESEARCH
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Shod versus barefoot effects on force and power
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development during a conventional deadlift
Mark E. Hammer, 1 Rudi A. Meir, 1 John W. Whitting, 1 Zachary J. Crowley-McHattan
School of Health and Human Sciences, Southern Cross University, Lismore, New South Wales, Australia
Running title: Shod and un-shod deadlift
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Correspondence:
R. Meir School of Health and Human Sciences Southern Cross University Lismore Australia Email: [email protected] Tel: +2-66203911 Fax: +2-66203022
Word count: 3,361 Submitted to: The Journal of Strength and Conditioning Research Date: July, 2017 Copyright ª 2017 National Strength and Conditioning Association
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ABSTRACT
The kinetics of a conventional deadlift in shod (S) versus unshod (US) footwear conditions in 10 male participants (mean ± SD, age = 27.0 ± 5.8 years; weight = 78.7 ± 11.5 kg; height = 175.8 ± 8.2 cm; 1RM deadlift = 155.8 ± 25.8 kg) was assessed in two testing sessions. A counterbalanced, crossover
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experimental design was used with different loads (60% and 80% 1RM). Four sets of four repetitions
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were prescribed per session with two sets per shoe and with each shoe condition involving one set per load. Peak vertical force (PF), rate of force development (RFD), time to peak force (TPF), anteriorposterior (COP-AP) and medio-lateral (COP-ML) center of pressure excursion, and barbell peak power (PP) data were recorded during all repetitions. Except for RFD (F = 6.389; p = 0.045; ƞp2 = 0.516) and
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ML-COP (F = 6.696; p = 0.041; ƞp2 = 0.527), there were no other significant main effects of shoe. There were significant main effects of load for PF (p < 0.05), COP-AP (p = 0.011), TPF (p = 0.018) and COPAP (p = 0.011). There were no significant interactions found between session, shoe and load (p range from 0.944 to 0.086). While the unshod condition may have produced changes in RFD and ML-COP compared with the shod condition, there is only limited evidence in the current study to support this
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lifting technique for the conventional deadlift. Further investigation is required to clarify any possible
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implications of this result and its benefit to lifters.
Word count: 250/250
Keywords: lifting technique, force production, footwear
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INTRODUCTION The deadlift is a closed chain, multi-joint resistance exercise that involves moving a stationary barbell from the floor until extension is established at the hips and knees. Within the strength and conditioning fields, the deadlift is seen as an important part of an athletic training program for the development of strength and power in the posterior chain i.e. back, hips and hamstrings (10, 34, 35, 37, 40). In
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Powerlifting competitions, the deadlift is the final of three contested lifts and is essential in developing a
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competitive total weight lifted (16). In the rehabilitation community, the deadlift and its variations are a common protocol in the post-operative and non-operative rehabilitation of the lower back, anterior cruciate ligament (ACL) and hamstrings (25).
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As with the squat, there is debate among weightlifting equipment forums about the best shoes to wear while deadlifting. Strength and conditioning researchers, and a range of industry practitioners (e.g. 7, 9, 14, 19, 20), have stated that weightlifting shoes, non-compressive soled shoes, or unshod (socks only or barefoot) conditions are essential in providing a stable platform and effective force transfer from the ground to the bar. In contrast, it could be argued that soft soled shoes, such as traditional running shoes,
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delay force transmission and create instability, altering the direction of the resultant ground reaction force
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(GRF), thus compromising lifting performance.
There appears to be no research published on how different footwear affect the kinetics or kinematics of the deadlift. However, with respect to the barbell back squat Shorter et al. (30) found that when using loads at 80% of 1RM shod lifters wearing lower profile, soft sole training shoe produced higher peak power and peak GRF than lifters wearing minimalist shoes or no shoes. Furthermore, Shorter et al. (30) found that the shod condition displayed a larger excursion in the center of pressure (COP) than the minimalist or no shoe conditions. Interestingly however, research conducted by Whitting et al. (38) reported a significantly larger anterior-posterior COP excursion during the barbell back squat for Copyright ª 2017 National Strength and Conditioning Association
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participants wearing a solid, rear-foot elevated weightlifting shoe when compared to a standard sports shoe. Although one might reasonably surmise that the stable weightlifting shoe could elicit similar effects in COP excursion to minimalist or barefoot conditions, it seems that the findings of these two studies are at odds with one another. Further research comparing a shod condition with an unshod condition in the barbell back squat has not provided kinetic data to help elucidate this small body of research (14, 28). As
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a result, it appears that shoe effects on kinetic parameters in common weightlifting modes remains poorly
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understood.
The deadlift and its variations have been studied extensively, with a number of biomechanical analyses of the deadlift reported in the literature (4, 34-37). Several studies have focused on joint and segment angles
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in deadlift variations (29, 39), with two studies specifically focused on the comparison of kinematics and kinetics of the sumo deadlift and the conventional deadlift (10, 12). Further research has been completed on muscle activation patterns via electromyography in both stable and unstable conditions (6, 11, 18, 24). However, none of the aforementioned studies made reference to the types of footwear worn during testing conditions, and nor was COP data provided.
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Research (16) has also found that there are significant kinematic differences between the back squat and deadlift, indicating that the data compiled for the squat may not be entirely relevant for the deadlift.
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Therefore, given current and yet to be supported beliefs that deadlifting performance may be enhanced by wearing minimalist shoes or by being barefoot, it was the purpose of this study to determine possible kinetic effects of different footwear conditions (shod and unshod) while performing a conventional deadlift. Specifically, this study aimed to determine whether the peak vertical ground reaction force (PF), peak vertical power (PP), rate of force development of the peak force (RFD), time to peak force (TPF), and center of pressure excursions in the anterior-posterior axis (COP-AP) and medio-lateral axis (COPML) were affected by these shoe conditions. It was hypothesized that a deadlift in an unshod condition
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would provide a higher magnitude of PF, PP, RFD and a reduced COP excursion on the ML and AP axis when compared to a shod condition.
METHODS Experimental Approach to the Problem
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A within-subject, counter-balanced, crossover experimental design was used to quantify the impact of the two lifting conditions (S = shod in regular training shoes, US = Unshod) on the kinetics of the
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conventional deadlift at different %1RM loads. Data was collected for each participant over 3 sessions (initial 1RM testing and 2 trial sessions), each separated by a minimum of 72 hours to avoid any possible influence of fatigue. All sessions were conducted at the institutional biomechanics laboratory. During the
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trial sessions, participants completed 4 repetitions of the conventional deadlift with pronated grip using 60 and 80% of their predetermined 1RM. A total of 4 sets were completed on each of the 2 trial sessions. All lifts were completed in both shod and unshod conditions with the order of footwear condition reversed for the second testing session. Participants were required to wear those shoes that they normally wore (all soft soled “trainers”) during training in the shod condition, and to be barefoot in the unshod
Subjects
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condition.
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A total of 10 male participants (group mean ±SD: age = 27.0 ± 5.8 years, weight = 78.7 ± 11.5 kg, height = 175.8 ± 8.2 cm and 1RM = 155.8 ± 25.8 kg) volunteered for this study. To be eligible to participate, participants were required to have a minimum of 2 years training age using the conventional deadlift and to have used maximal and near maximal loads in this lift as part of their normal training program. Participants had to be free from injury and illness and complete a pre-screening exercise and health questionnaire to identify any contraindications to their participation. Prior to agreeing to their involvement participants were provided with an information sheet that described all aspects of the
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research, including any potential risks and benefits. Having agreed to participate each participant attended a familiarization session where the test process was explained verbally and written informed consent was obtained. At this session, each participant had their 1RM deadlift score established under the supervision of an accredited Australian Weightlifting Federation coach. Participants were instructed to avoid both caffeine and lifting maximal loads in the 24 hours immediately prior to each data collection
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session. Participants were randomly assigned to Group 1 (G1) or Group 2 (G2) to determine the ordering of their first shoe test condition (S or US respectively) in their first testing session. All participants
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reversed the order of shoe condition during their second session to control the potential effect of shoe order on the kinetic outcomes. All procedures used in this research were approved by the institutional
Procedures Establishing deadlift 1RM
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ethics committee (ECN-16-072) and complied with the Declaration of Helsinki.
Upon agreeing to participate, baseline measurements (age, height and weight) were recorded for each participant. Participants then performed a standardized warm-up of approximately 8 minutes that comprised of 2 minutes on a Monarch cycle ergometer (utilizing a low load while maintaining a cadence
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of ~70-80 rpm), and a series of prescribed dynamic stretches. Participants were then tested for their deadlift 1RM wearing their normal training shoes. Deadlift technique was visually assessed by an
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accredited Australian Weightlifting Federation coach prior to establishing their 1RM. Participants performed the following conventional deadlift sequence to attain their 1RM: i) 5-10 repetitions on a 20kg bar; followed by time to load the bar; ii) 4-5 repetitions at 70kg (approximately 40-60% of predicted 1RM) followed by 3 minutes rest; iii) 1 repetition at 70-80% of predicted 1RM followed by 3 minutes rest; iv) 1 repetition at approximately 90% of their predicted 1RM followed by a minimum of 3 minutes rest. Participants were then given six attempts to reach their 1RM with a minimum of 3 minutes rest between attempts as established in previous protocols (5, 14, 18, 21). The final 1RM was considered as
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the load successfully lifted immediately prior to the failed attempted load. A successful lift was considered as observation of full extension of the hip and knee. Chalk was the only supportive aid used to prevent hands from slipping on the bar due to sweat.
Deadlift trial conditions
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On Arrival for session 2, participants performed the standardized warm-up and then performed the following deadlift sequence: i) 5-10 repetitions with a 20kg bar; followed by time to load bar; ii) 5-6
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repetitions at 50% of their first experimental load (i.e. approx. 30% of 1RM) followed by 1 minute rest; iii) 4 repetitions at 60% of 1RM followed by 3 minutes rest; iv) 4 repetitions at 80% of 1RM followed by 5 minutes rest. Following this rest period, participants completed the lifting sequence as identified in
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points iii-iv above, in the second experimental condition. All deadlifts were performed as a conventional deadlift with both hands pronated. No feedback was given during data capture; however, prior to lifting, all participants were instructed to perform the concentric phase of the lift as fast as possible.
GRF data were sampled (1000Hz) for each trial as participants stood with both feet on a single Kistler force plate (Type 9287; Winterthur, Switzerland) using Nexus software (version 1.8.4, 2013). GRF were
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filtered (ƒc = 100 Hz) with a Butterworth fourth-order low-pass filter. Filtered data were used to calculate the PF, RFD and COP excursions in the anterior-posterior (AP) and mediolateral (ML) directions. Data
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for PP were collected using a Gym Aware Power Tool 5 (Kinetic Performance Technology, Canberra, Australia) as seen in Figure. 1. All variables of interest were determined from data extracted from the concentric phase of the lift, corresponding to the period of time where force and power is developed during the lift. Insert Figure 1 about here
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Statistical Analysis A three-factor (session x shoe x load) repeated-measures ANOVA was used to determine whether there were main effects of session, shoe condition (S and US) or load (60 and 80% 1RM), and whether there were any interactions (session x shoe, session x load, session x shoe x load, shoe x load). Upon identification of significant effects, post-hoc analyses with a Bonferroni adjustment were used to
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minimize the chance of type I errors. Processed data were identified and entered into an Excel spreadsheet
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for the development of all descriptive values. A priori sample size calculation using G*Power with an α level of 0.05, and a power of 0.8, indicated that a sample size of 8 was sufficient for statistical analysis. All statistical analyses were conducted using SPSS version 20 for Windows with an α level set at 0.05. Effect size interpretation was in accordance with Cohen (1988), which states that for partial eta squared a
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small effect is 0.01 – 0.059, medium effect is 0.06 – 0.139 and large is 0.14 and above.
RESULTS
Means and standard deviations (±SD) for all variables are displayed in Table. 1. Shoe condition main
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effects were found to be significant for RFD (F = 6.389; p = 0.045; ƞp2 = 0.516) and ML-COP (F = 6.696; p = 0.041; ƞp2 = 0.527), with post-hoc comparisons indicating the unshod condition producing a
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greater rate of force development (Figure. 2) and lower ML-COP displacements (Figure. 3) than the shod condition respectively. Non-significant shoe effects were found for PF, TPF, PP, or AP-COP (F range from 0.270 to 4.567; p range from 0.618 to 0.076). No significant main effects of session (F range from 0.103 to 2.861; p range from 0.758 to 0.142) were found for any of the independent variables. Main effects of load were significant for PF (F = 83.090; p < 0.05; ƞp2 = 0.912), TPF (F = 9.411; p = 0.018; ƞp2 = 0.573), and COP-AP (F = 10.914; p = 0.011; ƞp2 = 0.577), with pair-wise post-hoc comparisons indicating a significant increase in these variables at 80% compared to 60% of 1RM. No significant main effects of load were found for PP (F = 0.005; p = 0.944; ƞp2 < 0.05), RFD (F = 3.632; p = 0.105; ƞp2 = Copyright ª 2017 National Strength and Conditioning Association
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0.377), or COP-ML (F = 4.055; p = 0.091; ƞp2 = 0.403). There were no significant interactions found between session, shoe and load (F range from < 0.05 – 4.222; p range from 0.944 to 0.086).
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Insert Table 1 and Figures 2 & 3 about here
DISCUSSION
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The purpose of this study was to determine possible kinetic effects of different footwear conditions (shod versus unshod) while performing a conventional deadlift; specifically during the concentric phase of the lift using submaximal training loads. Biomechanical effects of the deadlift exercise have been studied
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extensively (2, 3, 5, 10-12, 16, 17, 29, 34, 35). However, to the best of our knowledge there has been no research published with reference to the kinetic effects of shod or unshod conditions during this exercise.
Data analysis showed significant main effects of shoe on RFD and ML-COP; with unshod lifters able to apply more force to the floor at a faster rate. RFD was assessed as one of the main variables to investigate
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anecdotal beliefs in the strength and conditioning community that there is a perceived ‘disconnect’ between the ground and the feet in a soft soled shoe (9, 13, 19, 20). This lifting strategy is based on an
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assumption that the soft sole in these shoes is likely to produce instability, since the shoe sole must be compressed before any effective force transfer can occur between the ground and the feet. The extension of this belief is that an impeded force transmission will result in a delayed and reduced vertical GRF, thereby compromising lifting performance. Upon initial analysis of the data in the present study, a significantly higher RFD in the unshod compared to shod condition lends some weight to these anecdotal comments. However, this finding needs to be viewed with caution for a number of reasons. Firstly, the statistical strength appears weak to moderate (p = 0.045; ƞp2 = 0.516; observed power = 0.563). Secondly, given Figure 2, it is questionable as to whether the means (±SE) of each data set are functionally different. Copyright ª 2017 National Strength and Conditioning Association
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Lastly, and perhaps most importantly, there were no shoe x load interactions, and no other significant main effects of shoe for the other primary indicators of power and force output (PP and PF). As such, this study does not provide substantive evidence to support the hypotheses developed to prove current anecdote and practice i.e. that there is an advantage for producing force and power when deadlifting
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unshod.
It should also be noted, that kinematic data were not collected during the present study. As a result, it
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could not be determined whether compression of the shoe sole might be a factor correlated with a possibly slower RFD in the shod condition. Furthermore, with TPF only significantly different in main effects of load, it would appear that the duration of the concentric phase of the deadlift may be unaffected
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by shoe condition, and that any possible delay in GRF at the start of the lift may have been overcome during the concentric phase. Future biomechanical investigations of the deadlift, concerning different footwear conditions, should collect kinematic data to determine any possible links between performer and shoe movements, and kinetic outputs such as RFD.
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As expected, peak vertical ground reaction forces (PF) were found to significantly differ between the load
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conditions. Post-hoc analyses indicates a greater magnitude of peak force at 80% of 1RM than at 60% of 1RM, thereby confirming the results of previous studies (3, 5, 34, 39). Peak power was similar between loads of 60% and 80% of 1RM, respectively. Previous studies that focused on PF, velocity and PP (3, 5, 34) for the deadlift and upper body (33), have reported a larger magnitude of power at 60% than at 80% of 1RM. Power, being a product of force and velocity (P = F.V) (15, 26, 33) may be generated more easily with submaximal resistance from lighter loads, predominantly due to substantially greater velocities, despite slightly lower peak forces. While not reported, velocity data taken from the Gym Aware showed a 20% increase in bar velocity at 60% of 1RM compared to loads lifted at 80% of 1RM,
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while GRF data showed a 9% increase in PF when lifting at 80% of 1RM. These values show a somewhat proportional increase in force to the decrease in velocity, thereby explaining the similarity in our absolute PP values. While we are unable to confirm the cause of the discrepancy in our PP data when compared to the previously mentioned studies, the majority of participants noted several grip issues with the pronated grip, which was standardized in this analysis. A majority of the lifters in this study performed the deadlift
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with an alternating grip during training sessions, and so it is speculated that this may have had an effect
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on lift velocity.
The control of bipedal stance involves the complex integration of sensory inputs, perceptual processing of those inputs, and the production of appropriate motor commands in response (23). The measurement of
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the COP trajectory changes during the deadlift in the current study showed that COP-AP displayed significant main effects of load, and COP-ML showed an effect of shoe. Post-hoc analyses of COP-AP indicated larger COP displacement at 80% versus 60% of 1RM while the COP-ML exhibited larger COP displacements when shod versus unshod. This suggests that the motor control systems appears to alter independently the motor behavior of the postural control system when experiencing changes in either load
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or surface conditions. To our knowledge however, this is the first study to investigate COP excursions in the deadlift and the potential effect of differing loads and shod conditions on COP behaviour, and
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therefore direct comparisons of findings is difficult.
One possible way to interpret these findings could be to explain the results through the Equilibrium Point (EP) hypothesis. This hypothesis states that the central nervous system (CNS) initiates and modifies movement by using sensory input to shift the equilibrium states of the motor system (22). Since the motor control system controls the COP movement and directs it towards steady state behaviors, under heavier
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loads or more compliant plantar surface conditions, the motor system may be achieving its final planned behaviour through flexibility in its control strategies and hence leading to larger COP movements.
Tahayori et al. (36) investigated the effects of differential loading on the COP behaviour in quiet stance and found that a load equal to 15% of body mass to the back of the torso was effective in increasing COP
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movement, especially in the ML direction. Interestingly, the application of the same load to the front of the torso led to no change in COP movement behaviour, which is contrary to the COP changes found in
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the current study during dynamic frontal loading (i.e. the deadlift). These differences may be the result of the considerable methodological differences between the studies, including static v dynamic actions and relatively low loads v relatively high loads. Therefore it is suggested that further research be undertaken
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to elucidate the potential mechanism of the changes in COP behavior with differing loads.
The deadlift is performed with a variety of techniques (e.g. sumo, clean deadlift, snatch deadlift, alternating grip) and with a variety of assistive equipment, such as straps, belts and deadlifting suits. As such, limiting preferred technique and assistance equipment may influence an individual’s deadlifting
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performance, which may explain large variances between subjects in this and other studies.
PRACTICAL APPLICATION This study was developed from an observed practice within the strength and conditioning field regarding possible or perceived effects of different footwear on performance during the conventional deadlift. Motives for the participation in the deadlift vary and it is performed using a variety of techniques. Thus it is essential that rehabilitation and strength and conditioning practitioners employ evidence-based practices to reduce the chance of injury and to provide optimal outcomes. The findings of this current
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investigation do not provide strong support for the practice, by some sections of the strength and conditioning community, of being unshod during the deadlift exercise as a strategy for significantly improving deadlifting performance. REFERENCES
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Table 1: Mean (±SD) of recorded variables displayed for each shoe (S, US) and load (60 and 80% 1RM) condition (N = 10). 80% 1RM US
Average PF (N)†
2060.1 (236.4)
Average PP (W) Average RFD (N/s)‡ Average TPF (s)†
S
US
2072.4 (240.5)
2259.3 (269.3)
2257.4 (289.0)
1129.0 (142.5)
1135.5 (148.4)
1093.6 (74.6)
1117.7 (75.2)
1840.4 (794.2)
2099.7 (915.9)
1593.7 (524.0)
2008.1 (772.3)
0.5 (0.2)
0.7 (0.3)
0.6 (0.3)
45.6 (16.4)
55.0 (16.7)
56.4 (22.0)
15.8 (7.2)
23.3 (7.9)
18.6 (5.4)
0.5 (0.3)
Average COP-AP (mm)†
41.7 (13.6)
Average COP-ML (mm)§
18.8 (7.0)
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S
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60% 1RM
C
1RM = 1 repetition maximum; S = Shod; US = Unshod; COP = center of pressure; AP = anterior-posterior; ML = medio-lateral.
A
† Indicates a main effect of load – post-hoc comparison showing larger magnitude of PF, TPF and COP-AP. ‡ Indicates main effect of shoe condition – post-hoc comparison indicating faster RFD for unshod v shod conditions. § Indicates main effect of shoe condition – post-hoc comparison indicating greater COP excursion with shod v unshod conditions
Copyright ª 2017 National Strength and Conditioning Association
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TE
D
18
A
C
Figure 1: Placement of Gym Aware Power Tool 5 and position of participant on force plate.
Copyright ª 2017 National Strength and Conditioning Association
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Figure 2: Mean (± SE) shoe main effects for rate of force development (N.s-1) between shod (S) and
A
C
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unshod (US) conditions. * Indicates significantly higher RFD in the US condition (P = 0.045).
Copyright ª 2017 National Strength and Conditioning Association
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Figure 3: Mean (± SE) shoe main effects for medio-lateral (ML) peak center of pressure (COP) displacement (mm) between shod (S) and unshod (US) conditions. * Indicates
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C
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significantly greater displacement in the shod condition (P < 0.041).
Copyright ª 2017 National Strength and Conditioning Association
