A significant amount of research has been reported investigating the use of augmented feedback (AF); these findings consistently show that when AF is delivered properly, it can facilitate motor skill learning (26). Augmented feedback is defined as information an individual receives that is in addition to the information that is naturally available (19). This naturally available information is often referred to as task-intrinsic feedback and typically is provided through the sense of vision or proprioception. Augmented feedback provides information that is not easily interpretable or provided by task-intrinsic feedback. For example, coaches often observe athletes performing skills incorrectly; a coach might intervene by informing the athlete of error and then providing information about how to correctly perform the skill. The information the coach provides the athlete is an example of AF. Oftentimes athletes, especially beginners, are unaware that they are committing movement-related errors, and it is very common for athletes to be unable to correct movement errors once the errors are brought to their attention (28). This problem demonstrates the value in having coaches who are knowledgeable in the techniques of using AF during practice.
The literature covering AF and motor skill learning commonly refers to 2 types of AF: knowledge of results (KR) and knowledge of performance (KP). Only the latter is discussed in this paper. Knowledge of performance is defined as augmented information provided to a learner that contains specific information about the movement characteristics of the practiced task (18,19). For example, in baseball pitching, a coach may provide KP to a pitcher about specific arm movements during a throw. Research investigating the use of KP in practice has shown that providing feedback too frequently (i.e., after every trial) can slow the learning process and, at times, depress retention (17,31). One common strategy used to provide less than 100% AF during practice is delivering KP in the form of summary feedback. Using this technique, feedback is provided to the learner in summary form after a set of trials. This form of AF delivery seems to facilitate skill acquisition (13).
Several important bar path parameters have been identified and related to body position and kinetic performances in weightlifting movements such as the power snatch exercise (PS) (21,24) (Figure 1). In addition, it has been proposed by certain researchers that there is a relationship between the establishment of certain bar path kinematics by the performer and the level of success attained (22). Three important kinematic factors have been highlighted as important considerations for weightlifting performance: 1) an initial rearward movement of the bar during the first pull (Figure 1, Dx2), 2) a catch position no more than 20 cm behind the most forward bar position (Figure 1, DxL), and 3) the amount of looping (DxL), which should be less than the net rearward horizontal displacement (DxT). The development of maximal bar velocities while performers are in the second pull have also been proposed as a contributing factor to performance in weightlifting movements (12).
Recent kinetic analysis has highlighted the importance of maximal force production during the second pull on performance in the PS (23). These factors are supported by a good deal of evidence from other kinematic and kinetic analyses that have been performed during the past 25 years (2,3,6,8-10). A recent study by Winchester and colleagues (31) tracked changes in the power clean exercise longitudinally and found improvements in power and force production as bar path kinematics improved. However, though reliability measures were reported, a control group was not included in that particular study.
Volumes of non-peer-reviewed material have been published on weightlifting technique (3,5,8-11,12,15). However, those studies that have performed biomechanical analyses of lifts such as the snatch, although certainly worth noting, have used the “traditional” variety as performed in weightlifting competition rather than the “power” variation as is typically used in strength and conditioning settings for improvement in athletic performance (4,14). With the traditional and power varieties of the snatch being very similar, it is still reasonable to assume that they could have significantly different outcomes with respect to bar path, bar velocity, ground-reaction force, and peak power (PP) because of the differences in catch position between the 2 lifts.
It is interesting to note that the results obtained by Hoover et al. (14) demonstrate that fewer than 50% of women weightlifting competitors were able to achieve what would be considered, based on earlier research, an optimal bar path during competition. However, it is important to recognize that neither the Hoover et al. (14) nor the Campos et al. (4) studies were longitudinal in design. In addition, they did not address the manner in which KP outcomes and feedback methods affect improvements in technique.
Significant correlations have been described between variables in successful attempts in weightlifting competition (24), including catch position to amount of loop (Figure 1, DxT vs. DxL), catch position to second pull position (DxT vs. Dx2), and catch position to amount of hipping (DxT vs. DxV). There also is an established link between bar kinematics (such as bar velocity) and kinetic variables (such as ground-reaction force) and subsequent power output (24). This includes relationships between variables such as peak vertical force to PP and PP to peak first-pull velocity. However, these variables have never been examined longitudinally in the context of an athlete attempting to improve performance in the PS.
The PS has typically been used as a training tool for improving athletic performance for reasons such as its transference and close kinematic and kinetic relationship to vertical activities such as jumping (5). In weightlifting movements, the appropriate load to maximize power output is unclear. However, it can be inferred from past data that it may be at a load between 50 and 90% of dynamic 1-repetition maximum (1RM) strength in a PS movement (9,25). In ballistic activities such as jump squats or bench press throws, it is a load, on average, of 30% of dynamic maximum strength in the same movement (16,20,25,30), although more recent research suggests that this may, in fact, be much lower (6). However, proper use of lifting techniques, especially in the PS, is essential to maximize the stimulus applied. Now that bar path kinematic parameters for proper weightlifting technique have been established (24,31), it is important to investigate the progression of these variables with PS training in athletic populations and to observe their relationship to kinetic variables as well. Because bar path is an easy variable to measure and record, this method may be an effective tool for practitioners to use to improve PS technique in athletes.
The purpose of this experiment was to investigate 2 specific hypotheses. First, did PS bar path kinematic parameters change in relation to kinetic variables? We also investigated whether loading intensity influenced this relationship. Second, how does the use of summary AF influence the acquisition of PS technique and the relationship between kinematic and kinetic variables? The answers to these questions will provide practitioners the necessary information to create efficient practice environments that will ultimately lead to enhanced human performance.
Experimental Approach to the Problem
Data collection for this project was divided into 2 sessions for pre- and posttesting. At the time of the first testing session, a 1RM for each participant in the study and a video representation of the subject's technique for evaluation of kinematic variables of bar position were obtained. In addition, peak force (PF) and PP during the second pull were measured using force plate and video data at 50, 70, and 90% of each subject's 1RM. After initial testing, subjects were pair matched for 1RM in the PS and randomly separated into either a control group (CG) (n = 12) or a feedback group (FG) (n = 12). The sessions for data collection were separated by a 4-week training cycle in the PS in which subjects in the FG were given verbal feedback from a certified strength and conditioning specialist and visual cues through 2-D videography to improve individual technique. During the posttesting session, all previously mentioned kinematic and kinetic variables were again measured. There were no significant differences noted in PF, PP, kinematics, or subject characteristics between the FG and CG before the onset of the intervention protocol.
Twenty-four healthy adult men with a mean age of 21.72 ± 1.94 years, mass of 125.16 ± 11.46 kg, and stature of 185.42 ± 15.74 cm volunteered to participate in this investigation. Subjects were National Collegiate Athletic Association Division I football athletes who had moderate (minimum 18 months) experience in the PS exercise. To ensure the safety of all participants, all treatment and testing conditions in this investigation were examined and approved by the Louisiana State University institutional review board (IRB) for protection of human subjects. In compliance with IRB guidelines, subjects were informed of the experimental risks and signed an informed consent document before the investigation. Subjects were allowed to continue their normal physical activities throughout the duration of the study but were instructed to have a minimum of 48 hours of rest before the pre- and posttesting sessions. To minimize factors outside of this particular study, data were collected during a break between academic sessions during the off-season. This allowed us to avoid conflicts such as practice and games that could have altered our results.
One-Repetition Maximum Protocol
The subjects' 1RM for the PS was used to measure concentric muscle performance for pre- and posttesting sessions for this study. Warm-up trials based on a percentage of the subjects' estimated 1RM (E1RM), determined by previous training loads and an Epley chart, were given before 1RM testing in both the pre- and posttesting sessions (7). The Epley chart is one of the more commonly used methods of estimating 1RM in strength and conditioning settings for athletes as well as the general population. Furthermore, use of prediction equations for determination of E1RM with American football athletes has been widely reported in the literature (28,29). The percentage and number of reps for the warm-up protocol are as follows: 4-6 reps at 30% E1RM followed by 2 minutes of rest, 3-4 reps at 50% E1RM followed by 2 minutes of rest, 2-3 reps at 70% E1RM followed by 3 minutes of rest, and 1 rep at 90% or E1RM followed by 4 minutes of rest. From this point, subjects were allowed 3-4 maximal trials to establish their actual 1RM. Five minutes of rest were given between each maximal-effort trial. All participants were able to reach 1RM within the 4 trials allotted.
Subjects trained in the PS for 3 d·wk−1 for 4 weeks. The training days were broken up by level of intensity from low to high to moderate on Monday, Wednesday, and Friday, respectively. For the low-intensity (Monday) training session, subjects performed a warm-up set of PS at 50% 1RM and then performed 5 training sets of 5 reps at 50% 1RM. Subjects in the CG performed identical training protocol minus the feedback provided to the FG. Subjects were allowed 2 minutes of rest between all sets on the low-intensity day. On the high-intensity day, subjects performed 1 warm-up set of 5 reps at 50% 1RM and 1 set of 3 reps at 70% 1RM. Subjects then performed 3 training sets of 1 rep at 90% 1RM with 4 minutes of rest allowed between each set. During the moderate-intensity training sessions, subjects were allowed 1 warm-up set of 5 reps at 50% 1RM before performing the training sets. The training protocol for the moderate day was 4 sets of 3 reps at 70% 1RM. Three minutes of rest between each set was allowed during the training session. During the 4-week training cycle, visual and verbal feedback were provided for warm-up and training sets for those subjects in the FG. However, performance with respect to kinematic variables, PF, and PP was only determined for actual training sets. There was 100% compliance with the training protocol for the duration of the study. The training protocol described above was the only resistance exercise our subjects participated in for the duration of this study.
After each set of lifts, participants in the FG received verbal and visual AF. This information was provided as summary KP. More specifically, after participants had completed a set of attempts, they received visual KP by watching on a computer monitor the bar paths of each PS in the previous set. In conjunction with this visual KP, participants were provided verbal KP containing information on how to adjust their technique to produce the desired bar path. The frequency with which feedback is provided greatly affects rate of learning and coordination (19,27). In an attempt to control this variable, all sets of the PS (warm-up and training) were followed with verbal and visual KP. Participants in the CG received no AF.
The kinematic and force plate data collection were internally synchronized and collected simultaneously using Eva 6.0 software (MotionAnalysis Corp., Santa Rosa, Calif). Kinematic data were collected at 60 Hz using a CDC high-speed digital camera system (COHU, San Diego, Calif) and Eva 6.0 software (MotionAnalysis) to monitor the motion of a 2.5-cm reflective marker attached to the middle of the barbell. The motion of this marker was used to calculate bar velocity. Vertical ground-reaction forces were collected at 960 Hz using an oversized (400 × 800 mm) OR6 force platform (Advanced Mechanical Technologies, Inc., Newton, Mass). Data were then smoothed using a low-pass Butterworth digital filter at 5 Hz.
A repeated-measures analysis of variance was used to determine between- and within-group differences as well as differences between testing sessions with respect to percentage of change. Post hoc ANOVA analysis involved, where appropriate, the use of the Tukey protected t-test. All statistical calculations were performed using SPSS version 11.0 for Windows (SPSS Inc., Chicago, Ill). For all procedures, statistical significance was set at p ≤ 0.05.
Reliability for force and power measurements was determined in the CG using interclass correlations. Reliability for force and power measurements were both high, with interclass correlation values of 0.987 and 0.984, respectively. Reliability data concerning bar path kinematics were assessed using videography in our CG, which was tested pre and post. The following variables were calculated, and no significant difference was observed via 1-way ANOVA between values between testing sessions: DxL, −0.113 ± 0.050, −0.095 ± 0.062; DxT, −0.084 ± 0.033, −0.088 ± 0.073; Dx2, −0.072 ± 0.080, 0.091 ± 0.072; DxV, 0.101 ± 0.023, 0.085 ± 0.052; F(1, 22) = 1.903 (p ≤ 0.05).
Fifty-Percent Testing Load
The PF was improved at the 50% load for the FG from 567 ± 202 to 769 ± 230 N. The PP also improved at the 50% testing load from 2061 ± 562 to 2538 ± 498 W in FG, F(1, 22) = 14.12 (p ≤ 0.05). Measurement of kinematic variables saw improvements in the catch position (DxT) from −0.052 ± 0.012 to −0.224 ± 0.073 m, in the position for the second pull (Dx2) from −0.009 ± 0.052 to −0.082 ± 0.048 m, and in the amount of loop (DxL) from −0.104 ± 0.052 to −0.212 ± 0.076 m, and saw significant improvements between the pre- and posttesting sessions, F(1, 22) = 12.21 (p ≤ 0.05). Figure 2 shows a visual representation of the average change in bar path in the FG before (pre) and after (post) training with all 3 testing loads. Specific visual data for the 50% testing load can be found in Figure 3. Data for both the FG and CG can be found in Tables 1 and 2. No changes in kinetics or kinematics were noted in the CG.
Seventy-Percent Testing Load
For the 70% testing load, PF was improved from 725 ± 186 to 890 ± 199 N in the FG, F(1, 22) = 12.25 (p ≤ 0.05). The PP was improved at the 70% testing load in the FG from 2321 ± 743 to 2754 ± 629 W, F(1, 22) = 13.65 (p ≤ 0.05). Kinematics for the FG improved as follows: DxT improved from −0.085 ± 0.021 to −0.197 ± 0.041 m, and Dx2 also showed improvement pre to post from −0.036 ± 0.027 to −0.102 ± 0.053 m, F(1, 22) = 14.13 (p ≤ 0.05). No changes were noted in the CG for either kinetics or kinematics in the 70% testing load. Data for the FG and CG can be found in Tables 3 and 4. A visual representation of kinematic changes noted in the 70% testing load can be found in Figure 4.
Ninety-Percent Testing Load
At 90% 1RM, subjects in the FG saw an increase in PF from 822 ± 197 to 1008 ± 201 N, F(1, 22) = 13.53 (p ≤ 0.05). Peak power also was increased for the FG at 90% from 2076 ± 437 to 2491 ± 526 W, F(1, 22) = 13.26 (p ≤ 0.05). Kinematic measurements in the FG such as Dx2 improved from −0.053 ± 0.086 to −0.095 ± 0.024 m, and DxT showed significant improvement from −0.071 ± 0.081 to −0.201 ± 0.062 m, F(1, 22) = 14.29 (p ≤ 0.05). Improvement in position for DxV from 0.140 ± 0.09 to 0.078 ± 0.069 m was also noted F(1, 22) = 14.64 (p ≤ 0.05). No improvements were noted in the CG. Data for both the FG and CG can be found in Tables 5 and 6. A visual representation of the changes noted for the 90% testing load can be found in Figure 5.
The primary findings of this study are that 1) during performance of the PS exercise, as kinematic variables with respect to bar path improved, the subjects were able to generate increased levels of PP and PF, thus improving bar kinetics, and 2) the use of summary feedback with respect to lifting performance is an effective method of providing information to athletes performing weightlifting movements. As mentioned earlier, it has been established in previous research that there are several factors that contribute to a successful lift in weightlifting movements such as the PS: first, horizontal (rearward) displacement of the bar in the first pull with respect to the starting position (Dx2); second, the amount of looping of the bar in the catch phase (DxL); and, third, the ratio of looping to the net rearward displacement of the bar (DxL ratio to DxT). The fourth factor is increased peak bar velocity in the second pull (DV2). The fifth factor is increased PF production during the second pull phase of the PS (8,24).
Rearward movement of the bar in from the first to the second pull (Dx2) has been established as an important indicator of the likelihood of success of the lift in weightlifting movements (14,24). As noted in the Results section, Dx2 improved for all 3 testing loads in this study. Establishment of a good starting position in which the lifter's knees are positioned in front of the bar and the chest is positioned over the bar before the first pull is an important first step in establishing good bar kinematics. In addition, keeping a flat-footed stance for as long as possible while keeping the hips directly over or a little behind the ankles and instructing the lifter to extend first at the knee joint would seem to allow for rearward motion of the bar between the first and second pulls. Maintaining this body position and bar path will allow for increased vertical velocities and increased vertical force production through the second pull as more of the force generated is directed vertically (15,24).
The second kinematic variable that is closely correlated with success in weightlifting movements such as the PS is the amount of displacement between the most forward position during the second pull to the catch position commonly referred to as the amount of looping at the top of the movement (DxL) (24). In this particular study, DxL was significantly changed at all 3 testing loads. This reflects earlier research suggesting that that at near-maximal loading conditions, there may be less horizontal displacement of the bar (15,24,31). It is also worth noting that when examining pretest bar path characteristics, for the 70 and 90% testing loads lifters were jumping under the bar to complete the lift. This led to either no rearward displacement from the start to catch position, as in the 70% load, or, as in the case of the 90% load, to a slightly forward net displacement. This may be attributable to the lifter allowing the bar to drift forward slightly or coming up on the balls of the feet too early and causing the athlete to change the direction of applied force to a more horizontal component, commonly referred to as “hipping” the bar. This action will cause an exaggerated DxL and force the lifter to catch the bar in front of the starting position, allowing for very little net horizontal displacement (DxT) or even forward displacement rather than rearward, as in our 90% condition. In previous research, 68% of lifts with an exaggerated DxL were failed attempts (15,24). In this study, as noted in previous work (31), DxL typically increased through use of feedback while training; however, the amount of “looping” never exceeded the recommended amount of 20 cm at any of the testing loads. Also, in both the 70 and 90% testing loads, net horizontal displacement was slightly rearward rather than a net of zero or forward displacement.
Another important kinematic variable is the total amount of horizontal displacement from the beginning of the lift to the catch position (DxT). Previous studies done by Stone et al. (24) found a significant correlation where 76% of successful attempts in weightlifting activities had a rearward displacement of the bar and 64% of failed attempts had no horizontal displacement or the lifter caught the bar in front of the starting position. As noted in earlier work, in the present study DxT was improved for all testing loads, which may indicate an improved performance success rate (31).
The PS has been used for many years to improve performance in athletic endeavors for multiple reasons such as transference to the event performed and the ability to provide stimulus for production of power when training athletes. It is important to note that in this study, no explicit effort was made to improve force or power production during the training sessions, and feedback concerning kinetics was given during feedback sessions. The sole focus of the investigation was on improving the bar path kinematics of each lifter through the use of summary feedback. As such, the reported results are consistent with the findings of other studies showing that the use of summary feedback can facilitate motor skill learning (13,32,33). The results obtained in this study demonstrate that athletes performing whole-body movements at submaximal and near-maximum loads improved bar kinematics as a result of provided summary KP.
During the final testing phase of the study, a new 1RM was not measured, and the 50, 70, and 90% loads were based on the original 1RM measured at the beginning of the investigation. It is, therefore, interesting to note that PP increased at all testing loads. This would seem to suggest that focusing on correct bar kinematics, with particular attention being paid to Dx2, DxL, and DxT, not only prompts the lifter to improve technique in the PS and possible other weightlifting movements but also can translate to increased power and force production. These findings confirm our earlier results, which suggested that even while performing the lift with the same amount of loading, power could be improved through alterations in lifting technique.
It is also worth noting that, much like results observed in earlier work using weightlifting movements (23,31), PP in the PS was higher in the 70% testing load than in either the 50 or 90% loading conditions. It could be hypothesized that a reduction in bar velocity and what were visually observed lower stances for catch positions at the 90% testing load led to the reduction in PP when compared with the 70% testing condition. Further research is warranted to examine whether this phenomenon is observed consistently in the PS exercise or in other weightlifting movements and their derivatives.
Coaches and practitioners should use visual and verbal feedback to track bar paths with athletes learning to use the PS for training. By making adjustments in bar paths, practitioners will ensure that the proper body mechanics are occurring that should translate into improved force and power outputs during the exercise and that maximize the stimulus placed on the body for an optimal training response. The bar path outlined by Stone et al. (24) and validated by Winchester et al. (31) may be used as a reference or template by which comparisons can be made. In addition, knowledge of the effective use of feedback methodology will be important when attempting to implement these results in a real-world setting. Careful attention should be paid to the skill of the athlete and the relevant motor learning literature when deciding how and when to provide feedback to the performer (19).
At this time, there has been little attempt to link research-derived feedback protocols with practical application of weightlifting skills in the literature, in textbooks, and in certifications dealing with resistance training from professional organizations. For example, the most recently published edition of the preparation text for the Certified Strength and Conditioning Specialist certification from the National Strength and Conditioning Association pays little attention to how and when practitioners should use instruction and feedback in their professional practice (1). In addition, an examination of other scientifically based and recently released textbooks related to strength training and conditioning reveals that the authors of those texts have largely ignored the issue of how and when to provide feedback to performers in the context of teaching weightlifting movements. Considering the volumes of peer-reviewed data suggesting that improper use of feedback can hinder learning and transfer of motor skills, further attention to this relationship should be given in the literature in relation to weightlifting and resistance exercise performance as well as athletic skills training in general. In the future, in textbooks discussing the teaching of weightlifting movements, and in certification examinations designed to prepare practitioners in the field of strength and conditioning and/or coaching, there should be an increased focus on the use of instruction and feedback in a manner that is supported by the body of evidence on the topic.
The authors would like to thank LSU Head Strength Coach James “Tommy” Moffitt and members of the 2007-2008 National Collegiate Athletic Association Division I National Championship Football Team for their assistance with this study.
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Keywords:© 2009 National Strength and Conditioning Association
feedback; peak power; technique; weight lifting