Skeletal muscle function can be evaluated with the use of dynamic or isometric force-time curve assessments (5,10,12,19,27). Of particular importance when examining force-time curves are the quantification of the athlete's maximal force-generating capacity and the rate at which force can be applied during maximal contractions. These 2 attributes are generally determined by assessing the peak force (PF) and the rate of force development (RFD) achieved during an isometric force-time curve assessment protocol. Traditionally, these assessments are performed with the use of an isometric knee extension (1,35), midthigh clean pull (12,27), deadlift (6), squat (27), or bench press exercise (37). However, the isometric midthigh clean pull appears to be the most commonly used isometric assessment when attempting to evaluate the force-time curves of athletic populations (18).
When used as an assessment tool, the PF and RFD achieved during the isometric midthigh clean pull have been related to markers of performance in weightlifting (5,10), sprint cycling (33), track sprinting (33), throwing (32), golfing (22), and jumping (10,12,19,27). Although there is a body of scientific literature that suggests that the results of isometric force-time curve assessments relate to dynamic performance capacity, there appears to be conflicting literature regarding the relevance of isometric testing's relationship to dynamic performance capacity (18,24,25,36). One possible rationale for the conflicting relationship between isometric midthigh clean pull results could be related to the overall reliability of the measurement.
Several factors that could impact the reliability of isometric midthigh clean pull assessments could be related to the actual testing process including the positional specificity of the assessment (12,18) and the instructions given to the athlete during testing (30). When performing the isometric midthigh clean pull test, the position that is used mirrors the second pull in weightlifting (12), which is also sometimes referred to as the third pull in the scientific literature (13). Typically this position has a knee angle of between 130 and 140° (7,11) with an upright trunk position (12). Often referred to as the power position, this position represents the point during weightlifting movements where the highest forces and power outputs are achieved (8). To maximize the effective application of forces during the isometric midthigh clean pull, it is essential that the instructions include reference to “pulling as hard and as fast as possible” as this ensures a maximal effort during the performance of the pulling motion (12,18).
Probably the most important factor impacting the reliability of the midthigh pull and contributing to the current conflict in the scientific literature about the applicability of the isometric midthigh clean pull assessment and its effectiveness as a monitoring or performance test is the method of analyzing the various force-time curve variables used as outcome measures for the assessment. Currently, there are several methods reported in the literature that can be used for calculating the RFD from an isometric force-time curve including sampling various preset time intervals (5,20–22), using the slope of the force curve from the initial rise to the maximum force expression (average RFD [avgRFD]) (20) and determining the peak RFD (pRFD) across various sampling windows ranging from 2 to 50 milliseconds (5,10,12,19,22). Additionally, Zatsiorsky (38) suggests calculating the index of explosiveness (IES), reactivity coefficient (RC), S-gradient, and A-gradient when evaluating force-time curve characteristics of isometric muscle actions. It is possible that these various methods result in divergent degrees of reliability that may partially explain the lack of agreement about the reliability and value of the assessment of the RFD measurement present in the contemporary body of scientific knowledge.
To date, the authors know of no study that has performed a comprehensive comparison of the various methods of determining the RFD and examined the reliability of each of the methods typically used in the scientific literature. Therefore, the primary purpose of this investigation was to compare the various methods for assessing the RFD reported in the scientific literature and determine which methods offer the greatest reliability. Additionally, the different methods for evaluating PF-generating capacity were evaluated to determine their reliability.
Experimental Approach to the Problem
This study is a cross-sectional study designed to assess the reliability of the various methods for calculating the RFD and the determination of PF. All subjects participated in 3 testing sessions, which were performed at the same time of day (i.e., morning to correspond with the subjects typical training time) to control for diurnal effects, over the course of a 2-week time period at the beginning of the preparatory phase of the annual training plan for a Division I College Volleyball program. All subjects were instructed to maintain their normal diet and refrain from training for 48 hours before each testing session.
Session 1 was used to gain informed consent and screen the subjects for contraindications to exercise and determine the subjects' body composition. Session 2 was then used to establish the isometric midthigh clean pull position and familiarize the subject with the testing procedures. Seven days later, the subjects participated in session 3, which required the performance of a 10-minute standardized warm-up followed by a series of submaximal dynamic midthigh pulls, and 2 maximal effort isometric midthigh clean pulls. Data collected were then analyzed with several different methodologies to determine which method produced the most reliable results.
Twelve female division I collegiate volleyball players (age: 19.5 ± 1.2 years; weight: 68.6 ± 8.2 kg; height: 1.75 ± 0.08 m) who regularly perform resistance training that included weightlifting movements (i.e., power cleans, dynamic midthigh clean pulls, etc.) and had previously performed the isometric midthigh pull as part of their regular training monitoring program were recruited to be subjects in this study. All subjects voluntarily read and signed an informed consent form in accordance with the West Virginia University Institutional Review Board (H-22385) and The Code of Ethics of the World Medical Association (Declaration of Helsinki). All subjects were engaged in a supervised strength and conditioning program, which included weightlifting movements, before participating in this study. No subjects were under 18 years old. After completing the informed consent, all subjects were screened for contraindications to exercise with the use of a health history questionnaire. Subjects body mass was determined to the nearest 0.01 kg using a calibrated electronic scale (Bod Pod; Life Measurement Instruments, Concord, CA, USA). Stature was then measured to the nearest 0.1 cm with the use of a stadiometer. Body density was then determined with the use of air displacement plethysmography (Bod Pod; Life Measurement Instruments) and body fat percentage was then estimated with the use of the Siri equation (31).
Preisometric Assessment Warm-up
All subjects preformed a standardized warm-up based on previously published literature looking at force-time curves generated during midthigh clean pulls (12,22). Initially each subject performed 5 minutes of cycling (Cyclops; Saris, Inc., Madison, WI, USA) at a cadence of 70 rpm at a resistance that yielded an intensity of 90–100 W. After this was completed, each subject performed a 5-minute dynamic stretching warm-up (12). Subjects then performed dynamic midthigh clean pulls from standard lifting blocks in a position that corresponded to the position established in the familiarization session for the isometric midthigh clean pull. Three sets of 3 dynamic pulls separated by 1 minute of recovery were performed with 1 set performed at 30, 50, and 70% of the subjects' 1 repetition maximum power clean established as part of their regular strength and conditioning program for collegiate volleyball.
Isometric Force-Time Curve Assessment
All isometric testing was conducted on a custom isometric testing system (Sorinex, Inc., Irmo, SC, USA) that used a combination of pins and hydraulic jacks to fix a bar at any desired height above the floor (5,12). A force plate (Rice Lake Weighing System, Fairmont, WV, USA) was integrated into the testing system, and the bar was positioned to correspond to the subjects' power clean second pull position, where the knee and hip angles were 140.0 ± 6.6° and 137.6 ± 12.9°, respectively (Figure 1). This position was selected for assessment because it corresponds to the portion of the clean and snatch where the highest forces and velocities are generated (8). All force plate data were sampled at 1,000 Hz using a BNC-2010 interface box with an analog-to-digital card (NI PCI-6014; National Instruments, Austin, TX, USA) and a custom Labview (version 8.1; National Instruments) acquisition software package.
The acquisition of data was performed in accordance with previously established methods (5,12). Briefly, once the bar position was set and subjects completed the standardized warm-up, they were strapped to the bar with standard lifting straps and athletic tape (12,19). Subjects were then placed into the appropriate position and their knee and hip angles were measured with the use of a handheld goniometer. Five minutes after completing the dynamic midthigh pull warm-up, each subject received standardized instructions, which were to “pull the bar as hard and as fast as possible until being told to stop” to ensure that maximal forces and RFD were achieved (12,22). The isometric midthigh clean pull was initiated after a countdown “3, 2, 1, Pull” with athletes applying a maximal effort for 5 seconds (10). Two minutes were given between the 2 maximal pulls. If there was a greater than 250-N difference between trials or the lead investigator deemed that the pull was less than maximal, the trial was repeated (21,22). All analyses were performed on the 2 best isometric midthigh clean pull trials.
Isometric Force-Time Curve Analyses
All force-time curves were analyzed with the use of custom Labview (version 8.1; National Instruments) analysis software. Each force-time curve underwent rectangular smoothing with a moving half-width of 12 before being analyzed for specific force-time curve characteristics (22).
The maximum force generated during the 5-second isometric midthigh clean pull trial was reported as the absolute PF. Additionally, force at 30, 50, 90, 100, 150, 200, and 250 milliseconds from the initiation of the pull were determined for each trial (5,22).
The RFD was analyzed with the different methods that have been reported in the scientific literature. Specifically, the RFD was calculated with the following equation:
The RFD equation was applied to specific time bands including 0–30, 0–50, 0–90, 0–100, 0–150, 0–200, and 0–250 milliseconds (5,22). The pRFD(10) was then determined as the highest RFD during a 2-millisecond (pRFD2), 5-millisecond (pRFD5), 10-millisecond (pRFD10), 20-millisecond (pRFD20), 30-millisecond (pRFD30), and 50-millisecond (pRFD50) sampling windows (12,21,29,34,35). The avgRFD, which is identical to the IES, was calculated from the PF achieved and the elapsed time between the initiation of the pull (0) and the PF values (20,38).
To further examine the force-time curve, the RC, S-gradient, and A-gradient were calculated based on the methods of Zatsiorsky (38). The RC was calculated using the PF, time to PF (TPF), and the subject's body weight using the following equation:
The S-gradient was calculated using half the PF (PF0.5) and the time to achieve it (TPF0.5) with the following equation:
The A-gradient was then used to determine the RFD in the late stages of the isometric muscle action by using the PF0.5, TPF, TPF0.5 using the following equation:
Paired comparisons with significance set at p ≤ 0.05 were used to determine if differences existed between the PF and RFD bands calculated for the 2 isometric midthigh clean pull trials. A 2 × 6 (trial × sampling window) repeated-measures analysis of variance was used to determine if there were significant differences between the any of the pRFD values and the isometric midthigh clean pull trials. Mauchly's test of sphericity was used to determine if sphericity was violated and a Greenhouse-Geisser correction was used when this occurred. When significant F values were determined (p ≤ 0.05), paired comparisons were used in conjunction with a Holm's Sequential Bonferroni method for controlling for type I error (14) to determine significant differences.
Reliability was calculated by determining the coefficient of variation (CV), the intraclass correlation alpha (ICCα), and the 90% confidence interval (90% CI) for each force-time curve variable analyzed (3,4,16,17). The CV was calculated based on the mean square error term of logarithmically transformed data (15). Acceptable reliability was then determined as an ICCα >0.70 and a CV <15% (2).
All data are reported as mean ± SD values and were analyzed with SPSS (version 22.0; SPSS, Inc., Chicago, IL, USA). All reliability assessments were performed with the use of a custom spreadsheet (15).
There were no statistically significant differences between the 2 isometric midthigh clean pulls for the absolute PF or time-dependent force variables analyzed in this study (Table 1). Additionally, all force variables analyzed met both criteria (CV% and ICCα) for acceptable reliability (Table 2). Overall, the absolute PF demonstrated the highest overall reliability (CV = 1.7%, 90% CI = 1.2–2.9%; ICCα = 0.99, 90% CI = 0.95–1.00).
There were no statistically significant differences between the 2 isometric midthigh clean pulls for any of the RFD bands examined (Figure 2). Additionally, with the exception of the avgRFD (ICCα = 0.74, 90% CI = 0.32–0.92), all RFD bands achieved acceptable reliability on both of the criteria used to assess reliability (Figure 3).
There were no statistically significant differences between the 2 isometric midthigh clean pull trials for any of the pRFD measures analyzed (p = 34, η2 = 0.083, 1 − β = 0.149). Conversely, there were significant differences between the pRFD sampling windows assessed (p < 0.001, η2 = 0.63, 1 − β = 1.00). Follow-up tests revealed that pRFD50 was significantly lower than pRFD2, pRFD5, pRFD10, and pRD20 (Figure 4). Additionally, pRFD30 was significantly lower than pRFD2. When reliability was assessed, only the pRFD20 sampling period met the 2 reliability criteria (ICCα = 0.90, 90% CI = 0.73–0.97; CV = 12.9%, 90% CI = 0.5–20.7) (Figure 5).
There were no statistically significant differences between the 2 isometric trials for the A-Gradient, S-Gradient, or RC (Table 3). However, all 3 of these measures only met the ICCα criteria for acceptable reliability (Figure 6).
The primary finding of this study was that the method used to assess the RFD during an isometric midthigh clean pull impacts the overall reliability of the measure. Specifically, using selected time bands for the quantification of the RFD offers greater reliability when compared with the quantification of the pRFD. Additionally, if the pRFD is quantified, the sampling window used in the calculation significantly impacts the results determined and the reliability of the RFD measure. Therefore, when using the midthigh clean pull assessment in research and as an athlete monitoring tool, it is important that the methods used to quantify the RFD are precisely chosen and clearly articulated as this will impact the overall interpretation of the results achieved in the test and possibly impact the ability of strength and conditioning professionals to diagnose performance characteristics.
Generally, one of the first steps when examining the reliability of any measure is to compare the 2 trials to see if they are statistically different (3). In this study, there were no statistical differences between the 2 isometric midthigh clean pull trials for any of the predetermined RFD bands (Figure 2), pRFD sampling windows (Figure 4), A-Gradient, S-Gradient, or RC (Table 3). Although the paired comparison can be used to compare the mean values of 2 repeated tests to assess if there is any statistical bias between tests, it is not recommend as a sole measure of reliability because it does not give any indication of random variation between tests (3). Therefore, based on this study, it is evident that when only examining the ICCa the highest reliability is found with predefined RFD time bands and the pRFD20, whereas the other pRFD sampling windows (i.e., 2, 5, 10, 30, and 50 milliseconds) and the aRFD only result in acceptable reliability.
When examining the literature in sport science, the ICCα is commonly reported when determining the reliability of a measurement (3,26). This practice has traditionally been used when examining the reliability of the RFD during the isometric midthigh pull (5,10,19,21,23,27,32,33). In this study, the RFD was deemed to be reliable based on an ICCα >0.77, which was higher than the preset threshold of an ICCα >0.70 (Figures 3B and 5B) and has been suggested as a cutoff for reliability in the sport science literature (2). Similarly, the RFD has also been reported to be reliable in the scientific literature based on an ICCα >0.75 (5,10,19–21,23,27,32,33).
Although it appears that the RFD measure is reliable based solely on the ICCα, it is possible that the degree of reliability for the measure may be impacted by how it is calculated. For example, there is an increased reliability noted when using predetermined RFD bands during the isometric midthigh clean pull analysis (5) as compared with the avgRFD (20). Specifically, Beckham et al. (5) have reported ICCα values for RFD time band reliabilities between 0 and 100 milliseconds (ICCα = 0.89), 0–150 milliseconds (ICCα = 0.92), 0–200 milliseconds (ICCα = 0.95), and 0–250 milliseconds (ICCα = 0.95) that are on average between an ICCα of 0.86–0.95. Similarly, when specific RFD time bands (i.e., 0–50, 0–90, 0–100, 0–150, 0–200, and 0–250 milliseconds) are evaluated in this study, the overall reliability of the RFD increases as indicated by an ICCα >0.95 (Figure 2B). However, when examining the pRFD, the ICCα ranged between 0.77 and 0.90, with the pRFD20 (ICCα = 0.90) resulting in the highest reliability (Figure 5B). Additionally, when the avgRFD was quantified, it produced one of the lowest ICCα (ICCα = 0.74) values. This was not unexpected based on the work of Khamoui et al. (20) who reported an ICCα of 0.75 for the avgRFD. Therefore, based on this study, it is evident that when only examining the ICCα the highest reliability is found with predefined RFD time bands and the pRFD20, whereas the other pRFD sampling windows (i.e., 2, 5, 10, 30, and 50 milliseconds) and the avg RFD only result in acceptable reliability.
Although it is a common place to only report the ICCα when trying to depict the reliability of the RFD during the isometric midthigh pull (5,10,19,21,23,27,32,33), it may also be important to report CIs in conjunction with the ICCα value (3,26). Through the inclusion of the CI, a more informative depiction of the reliability of a measure can be made (26). When determining the CIs for the ICCα in this study, it becomes evident that using predetermined RFD time bands results in better overall reliability. Specifically, the various RFD time bands (i.e., 0–30, 0–50, 0–90, 0–100, 0–150, 0–200, and 0–250 milliseconds) resulted in a very high degree of reliability based on the lower limit of the CI falling above an ICCα of 0.90 (Figure 3B). Conversely, when looking at the pRFD, it is apparent that for some of the calculation methods, the lower limit of the CI falls way below the 0.70 cutoff for reliability (Figure 5B). In fact, only the pRFD20 resulted in the lower limit of the CI being above 0.70. Additionally, when looking at the avgRFD, RC, S-Gradient, and A-Gradient, the lower limit of the ICCα did not meet the minimum threshold for reliability set in this study (Figures 2B and 6B). Taken collectively based on the ICCα and CIs, the most reliable methods for evaluating the RFD during isometric midthigh pull are achieved with the use of predetermined RFD time bands or the pRFD20.
Although the ICCα is widely accepted as a general measure of reliability that can be enhanced through reporting CIs, including other measures of reliability (3) can make for a better overall depiction of a measures reliability. Specifically, Atkinson and Nevill (3) suggest that the use of a CV that is calculated with logarithmically transformed data may be useful when interpreting reliability. Although this method is commonly used for expressing the typical error of a measurement, it has not been commonly used when describing the reliability of the RFD during isometric midthigh clean pull assessment. In this study, the predetermined RFD time bands resulted in a CV and CI that met the criteria for acceptable reliability. Specifically, the CV was generally less than 5.0% and the upper limit of the CI was less than 7% (Figure 2A). Conversely, only the pRFD20 achieved a CV less than the 15% criteria established as being acceptable for reliability. Although the pRFD20 did meet this criterion, its upper limit for the CI fell above the 15% threshold established for this study (Figure 5A), which makes this measure questionable from a reliability perspective. All the other pRFD measures were well above the 15% CV threshold, which suggests that they lack the adequate reliability necessary to be used as a performance diagnostic tool (Figure 5A). Additionally, the avgRFD, S-gradient, A-gradient, and RC each failed to achieve the minimal threshold for reliability (Figure 6A).
An additional consideration when looking at the various pRFD sampling frequencies is related to the actual value determined by the analysis. For example, some studies use a 2-millisecond (12,19), 5-millisecond (5,34), 20 millisecond (30), 30-millisecond (37), or 50-millisecond sampling window (9) when calculating the pRFD during isometric assessments. Conversely, several studies do not report the sampling window used when analyzing the pRFD (21,23,24), which could be problematic when interpreting the data presented. In this study, it was determined that the sample window used when calculating the pRFD resulted in significant differences in the values quantified (Figure 4). Specifically smaller sampling windows (i.e., 2, 5, or 10 milliseconds) resulted in greater pRFD values when compared with longer sampling windows (i.e., 30 or 50 milliseconds). The findings of this study confirm our hypothesis that the pRFD sampling window can impact the overall results of the pRFD assessment. These divergent results may also impact the relationships that exist between isometric and dynamic muscle actions presented in the literature. Therefore, when quantifying the pRFD, care must be taken to clearly define how the pRFD has been calculated.
Although not a primary focus of this study, the reliability of the methodologies typically used in the scientific literature for the establishment of PF and force values at specific predetermined time points were also examined. Overall, there were no significant differences noted between the 2 isometric midthigh pull trials for any of the absolute PF or time-specific force values quantified during each of the testing trials (Table 1). Additionally, the ICCα and the lower limit of the CI were above 0.95 indicating very high overall reliability (Table 2). The high degree of reliability noted for the absolute PF determined during the isometric midthigh clean pull was inline with the ICCα values (ICCα >0.94) that are consistently reported in the literature (5,10,11,21,24,25,28,33). Although the CV and its confidence limits are not typically reported for the absolute PF in the literature, this study demonstrates that the absolute PF has a CV = 1.7% and the upper limit for the CI is <3.0%, which is well below the cutoff for reliability used in this study (Table 2). Taken collectively, the present data suggest that the absolute PF is a very reliable measure when collected during the isometric midthigh clean pull test.
Although the absolute PF is typically measured during the isometric midthigh clean pull, there has been a recent trend where time-specific force values are analyzed (5,21,22). Typically, these measurements include quantifying the PF at 30, 50, 90, 100, 150, 200, and 250 milliseconds from the initiation of the pulling motion. Based on ICCα being >0.85, these measures are generally considered to be reliable (5,21,22). In this study, the ICCα and lower limit for the CI were above 0.94 indicating very good reliability for each of the time-specific measurements. Additionally, when examining the CV and its upper limit for the CI, these measures were again well below the cutoff for acceptable reliability. Specifically, the CV was <3% and the upper limit of the CI was <4.6%. Overall, the time-specific force values met both criteria for reliability (ICCα and CV).
Based on the data collect in this study, it is clear that strength and conditioning professionals need to carefully consider the method used to quantify the RFD during the isometric midthigh clean pull as this study demonstrates that the method chosen directly impacts the reliability of the measurement. This becomes an increasingly important consideration when attempting to monitor athlete performance and track changes in performance over time. To maximize the overall reliability of the RFD measurement, strength and conditioning professionals should analyze the force-time curves of the isometric midthigh pull with predetermined time zone RFD bands such as 0–30, 0–50, 0–90, 0–100, 0–150, 0–200, and 0–250 milliseconds. These measures have been demonstrated to offer the highest reliability based on the ICCα (>0.70) and CV (<15%) reliability criteria as well as have CIs that fall within the acceptable reliability zones established for this investigation. Based on these findings, it is recommended that strength and conditioning professionals preferentially use predetermined time zone RFD bands when analyzing the force-time curve data collected during an isometric midthigh clean pull.
If the strength and conditioning professional wants to quantify the pRFD, it is recommended that a sampling window of 20 milliseconds is used. Although the pRFD20 meets the ICCα (>0.70) and CV (<15%) criteria, it is important to note that the upper limit of the CV and CI fails to fall under the threshold for acceptable reliability. Because of this, there may be incidents where the pRFD20 is not reliable and it may be difficult to effectively interpret this measure. Additionally, it is very important that when using the pRFD, the method of analysis is clearly defined as the various sampling windows result in different RFD values. Therefore, if the pRFD20 measure is used by the strength and conditioning professional when analyzing the pRFD during the isometric midthigh clean pull, it should be used with caution and preferably in conjunction with predetermined time RFD time band analyses.
A common method for analyzing the RFD is to divide the absolute PF by the time it takes to achieve this force value, which we have termed the avgRFD. Conceptually, this seems like a good measure; however, if you examine the force-time curves of various athletes, it is clear that this measure does not accurately represent the true RFD achieved during the isometric muscle action. Additionally, this measure fails to meet any of the reliability measures examined in this study. Similarly, the A-Gradient, S-Gradient, and RC each fail to achieve acceptable reliability. Therefore, when analyzing the force-time curves of isometric midthigh clean pulls, the quantification of the avgRFD, A-Gradient, S-Gradient, and RC should be avoided in favor of predetermined time zone RFD bans.
Finally, when examining the forces achieved during an isometric midthigh clean pull, the strength and conditioning professional can examine a variety of assessments that consistently produce reliable results. The absolute PF and time-specific force values (i.e., 30, 50, 90, 100, 150, 200, and 250 milliseconds) all meet the 2 criteria standards for reliability. Therefore, strength and conditioning professional can use these variables during the force-time curve analysis procedures for data collected with the isometric midthigh clean pull.
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Keywords:Copyright © 2015 by the National Strength & Conditioning Association.
force-time curve; peak force; reliability; isometric muscle action