The results show that when side dominance was determined using perceived handedness (LRSD), the differences between the D and ND side AGRFs were not significant (0.2-3.6%; p = 0.107; η = 0.175; β − 1 = 0.361); however, D side ABP was significantly greater than the ND side ABP (1.5-5.4%; p < 0.001; η = 0.785; β − 1 = 1.00). Relative exercise intensity did not significantly affect the pattern or magnitude of the AGRF side differences (p = 0.569; η = 0.039; β − 1 = 0.136), but its effect on ABP side differences approached statistical significance (p = 0.057; η = 0.185; β − 1 = 0.561) with side differences that tended to be greater in the 60 and 90% 1RM condition compared to the 30% 1RM condition (Table 2).
Determining side dominance according to left- and right-side GRF dominance (FSD) resulted in significant differences between the D and ND side AGRFs (1.8-4.3%; p < 0.001; η = 0.713; β − 1 = 1.00) and ABP (0.5-3.4%; p = 0.006; η = 0.428; β − 1 = 0.852). However, relative exercise intensity did not significantly affect the pattern or magnitude of the AGRF (p = 0.216; η = 0.104; β − 1 = 0.313) or ABP (p = 0.215; η = 0.104; β − 1 = 0.314) D and ND side differences.
When side dominance was determined using bar end power output (BSD), the results showed that there were significant differences between the D and ND side AGRFs (5.1-6.4%; p = 0.042; η = 0.264; β − 1 = 0.55) and ABP (3.9-5.6%; p < 0.001; η = 0.883; β − 1 = 1.00). Relative exercise intensity did not significantly affect the pattern or magnitude of AGRF side dominance (p = 0.535; η = 0.044; β − 1 = 0.146) but significantly affected the ABP side differences (p = 0.035; η = 0.213; β − 1 = 0.643); the 60 and 90% 1RM side differences were significantly >the 30% 1RM side differences (p < 0.0001) (Table 2).
The relationships between AGRF and ABP are presented in Table 3. The results of the correlation analysis showed that there were no significant relationships between AGRF and ABP D and ND side differences. However, progressive loading appeared to alter the direction of the relationships from negative to positive (Table 3).
This study set out to establish AGRF and ABP side differences during HPC performance from perceived and measured side dominances and to establish whether progressive loading would intensify these differences. A secondary aim of this study was to establish whether AGRF side differences were related to differences in bar end power outputs.
In agreement with recent research (9,11), there were no differences between the perceived D and ND side AGRFs. Newton et al. (11) recently explained that when perceived side dominance does not correspond with measured side dominance, differences could be nullified. In the present study, all participants reported perceived right side dominance, but only 7 demonstrated this across all conditions; the remainder favored their left and right sides interchangeably, suggesting that side dominance should not be determined from perceived handedness.
Differences between the measured ground kinetic differences (FSD) were lower than those reported in recent research (9,11), remaining below 5%. Further, differences varied both within and between subjects indicating an individualized response to progressive loading. Only 6 demonstrated AGRF side differences that were consistent during the different loading conditions, with 6 demonstrating nonsignificant increases (3.3-14.8%) as load increased, and the remaining three, reductions (2.4-13.6%) in response to load increases.
These differences suggest that movement symmetry should be assessed as part of the strength and conditioning process and that assessment should use key resistance exercises across a range of loads.
The measured AGRF side differences were lower than those recently reported for back squat performance across the same loading conditions (1,9,11). Maximal back squat strength and in turn training loads tend to be considerably greater than their HPC equivalents. This may intensify ground kinetic side differences, although further research using a similar participant population would be needed to clarify this.
Another factor may be the technical demands of HPC performance, in that it requires a more coordinated and relatively symmetrical application of force to achieve the appropriate bar and body displacements. Winchester et al. (14) recently demonstrated that improvements in Olympic lifting technique can improve the efficiency of force application during power clean performance. This suggests that technical proficiency in the HPC is related to the magnitude of ground kinetic side differences and could be an area warranting further research.
Differences in bar end power output were not affected by the way in which side dominance was determined so that both perceived and measured side differences demonstrated statistical significance. However, the pattern of these differences varied, which may explain why AGRF and ABP differences were not related.
Data reported by Lake et al. (9) suggest that transmission of ground kinetic side differences to bar end power outputs is attenuated during back squat performance. For example, ground kinetic side differences of 21% resulted in bar end power output side differences that did not exceed 2.5% (9). In the present study, mean AGRF side differences of 6-8% resulted in ABP output side differences of between 4 and 6%, suggesting that HPC bar end symmetry is more susceptible to ground kinetic side differences than its back squat equivalent. This appears to be a consequence of the way in which HPC and back squat bar displacements differ, suggesting that the bar end power outputs from resistance exercises where the bar is displaced independently of the body should be interpreted with caution.
Therefore, the ABP output side difference across the different loading conditions was 5% with a 95% confidence interval of ±0.9%. The applied relevance of this finding was that in 95% of HPC performances across a range of loads practitioners and investigators should expect differences between bar end power outputs of between 4 and 6%.
These findings highlight the need to assess movement symmetry during the strength and conditioning process, while urging caution in the way in which differences between bar end power outputs are assessed. Practitioners should consider the findings of this study when selecting methods to assess differences in bar end power outputs.
Subsequent investigations should examine joint kinetic responses to ground kinetic side differences and the role that strength imbalances, whether a consequence of leg length discrepancy or past injury, and technical proficiency have in ground kinetic and bar end power output side differences.
Healthy individuals demonstrate significant ground kinetic side differences during HPC performance. Furthermore, power outputs obtained from both ends of the barbell differed by 4-6%. This suggests that relatively small changes in power output should be interpreted with caution unless a D side is identified and considered during power testing. Furthermore, movement symmetry should be assessed as part of the strength and conditioning process.
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Keywords:© 2010 National Strength and Conditioning Association
asymmetry; strength imbalance; weightlifting; ground reaction force