Isometric Force–Time Characteristics of Different Positions in the Clean in Competitive Weightlifters

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For use in a weightlifter monitoring program.

Abstract

This study examined isometric force–time characteristics of weightlifters at three key positions of the clean and their ability to predict competition performances. The three key positions were the isometric mid-thigh pull (IMTP), the isometric pull at the start of the transition (IPST), and the isometric pull at the start position (IPSP). Seventeen collegiate-level competitive weightlifters (10 males and 7 females) with varying weightlifting achievements (10 of the 17 have medaled at sanctioned USAW national meets) performed isometric strength tests that measured peak force (IPF), rate of force development (RFD), Impulse (IMP), and allometrically scaled variables. The reliability for all measures was high (ICC ≥ 0.86). The IMTP produced the largest absolute forces; however, the IPSP and IPST showed the largest correlations with snatch, clean and jerk, and total, with multiple near-perfect correlations (r ≥ 0.90). RFD and Impulse demonstrated more significant correlations at later time bands (≥200 ms). These findings suggest that measuring multiple isometric positions may provide valuable insight into a weightlifter’s positional strength. Including IPSP and IPST testing protocols with RFD and IMP measurements can augment athlete monitoring and inform training strategies.

1. Introduction

In the sport of weightlifting, often referred to as Olympic weightlifting, two lifts are contested. The first lift contested is the snatch (SN) and the second lift contested is the clean and jerk (CJ). The goal of a weightlifting competition is to lift more weight than your competitors in a combined total (TOT) that is measured in kilograms. The SN is performed in one fluid motion, while the CJ has two motions. Both lifts require a large focus on technique throughout the entirety of the lift [1,2,3]. The “pull” in a weightlifting movement is vital for success of the lift from both a technical and physical aspect. The pull can be broken down into different phases, which are the first pull, transition, and second pull.

In the sport of weightlifting, the weightlifter must develop their physical and technical attributes of the pull through consistent strength training and performing specific weightlifting movements [4,5,6]. It is well-established that maximal strength largely contributes to success in weightlifting performance [7,8,9]. Alongside this, the pull in weightlifting largely contributes to the success of the lift and consists of the first pull, transition, and second pull phases [10,11]. Thus, the technique of the pull is of high importance to weightlifters and weightlifting coaches. To maintain optimal technique throughout the weightlifting movement, a weightlifter must produce large amounts of force at various positions and time points throughout the SN and CJ, which further demonstrates the importance of strength. During the transition phase specifically, many experienced weightlifters have achieved great proficiency, typically making it the shortest phase of the lift [12]. This quick transition causes the knees to re-bend and is commonly referred to as “the double-knee bend”. The double knee bend causes a rapid stretch in the agonist muscles, which will utilize elastic properties in the muscle for greater overall force production in the following phase. Rate of force development (RFD) may be important during the transition, as a faster double knee bend is likely due to a higher rate of eccentric force development [13].

The technical attributes can be assessed kinematically, either in a lab setting or in a competition setting. The attributes of strength and power can be measured using force plate testing [14,15]. It is important for the coach to monitor these kinetic and kinematic variables over time, to assess the progress of the weightlifter. The coach can use a variety of tools to assess the strength characteristics of weightlifters, such as one-repetition maximum testing and isometric strength testing.

In weightlifting, isometric strength testing is important for the monitoring of fitness characteristics over time, which can tell the coach or sport scientist how an athlete is adapting to the stress of the training program [7]. Additionally, monitoring the strength characteristics over time can inform the coach or sport scientist about the overall efficacy of the training program [16,17]. A common method for isometric testing is the isometric mid-thigh pull (IMTP). Common strength characteristics typically measured with the IMTP are peak force (PF), RFD, and Impulse (IMP). Thus, measuring and monitoring the physical adaptations to training through strength testing can be insightful on how the weightlifter is progressing through training [18].

Additionally, the IMTP is becoming more popular for strength testing with many other sports [19]. The IMTP is often used because it is the position that is most advantageous to force production, and it positions the athlete in a way that mimics the position at the start of the second pull. Recent studies have shown significant correlations of strength characteristics in weightlifters performing an isometric pull from the start position in a clean [20,21]. This start position (IPSP) is commonly found to have stronger correlations to weightlifting performance than the IMTP. [20] observed significant correlations of SN, CJ, and TOT with the IPSP and noted r values of 0.94, 0.95, and 0.95, respectively. This would suggest that strength at the start position may largely contribute to the success of the lift.

A more recent study included youth weightlifter performances and compared strength at the start position and at the mid-thigh position [22]. The youth weightlifter performances were found to be correlated to both positions, but with the start position having the stronger correlation, but only significant at the relative and allometrically scaled levels. To date, a paucity of studies exist that have investigated different positions of the pull, and it may prove insightful to further explore isometric pulls from other positions in the clean, like a pull from the beginning of the transition phase (IPST). This position was studied by both [23,24], in addition to the start position and the IMTP. Ref. [23] observed significant correlations of SN, CJ, and TOT with the IPST, with r values of 0.95, 0.94, and 0.95, respectively. Ref. [24] determined through a jack knife t-test that the IPSP test was the only position that was statistically significant. The authors noted that using the IPST and IMTP tests does not improve the prediction of performance beyond what the IPSP provides. However, there is a need to provide further evidence that the strength characteristics of these positions may be crucial pieces of information to include in a weightlifting monitoring program, as it is well-known that strength at different positions is important for the success of the lift [25]. The authors hypothesize that there will be strong correlations between all three isometric pull positions and weightlifting performance. The aim of this study is to determine how well force–time characteristics from three different key positions of the clean can predict weightlifting competition performance.

2. Materials and Methods

In this study, 17 male and female collegiate- and high school-level weightlifters (10 males; age: 20.0 years ± 1.8; weight: 90.0 kg ± 163; height: 174.86 cm ± 4.9; bodyfat: 22.0% ± 8.0 and 7 females; age: 19.8 ± 1.7; weight: 63.2 kg ± 6.0; height: 159.0 cm ± 5.9; bodyfat: 24.2% ± 3.8) were recruited to participate. This study’s protocol was approved by both the ETSU IRB (ID# 0623.8s) and WVU IRB (Protocol #: 2302733143). Participants in this study consisted of both collegiate- and high school-level weightlifters. To better describe the participant’s weightlifting ability, 10 of the 17 had medaled at sanctioned USAW national meets during the time of March 2021 through March 2024. During this time, there were 46 total medals won across the SN, CJ, and TOT categories. Of the 46, there were 15 gold, 11 silver, and 19 bronze medals won. Participants in this study were all members of a USAW affiliated weightlifting club throughout the duration of the study. Participants in this study competed in an official USAW weightlifting meet within 3 months of isometric testing. Participants who were injured or who experienced any physical pain in joints or muscles immediately prior to or during the testing session were excluded from the remainder of the study. This study was granted permission from both West Virginia University’s and East Tennessee State University’s Institutional Review Board.

Isometric force–time data was collected using force plates (Rice Lake Weighing Systems, Rice Lake WI, USA; 1000 Hz). The force plate collects data at a sampling frequency of 1000 Hz. This is the standard for force plate measurement because important data may be missed at lower collection rates. In conjunction with force plates, a custom-designed power rack was used for each isometric pull trial. The reason that the iso-pull rack was used is that it is commonly used for the IMTP to examine isometric force–time characteristics in athletes and can accurately determine their isometric maximum strength levels [14]. Furthermore, the iso-pull rack is a safe testing apparatus and the IMTP is less fatiguing than performing a one-repetition maximum strength test. It is with this reasoning that isometric tests at other positions in the iso-pull rack were used as well. Prior to isometric testing, subjects provided a urine sample to determine their hydration status through urinary specific gravity (USG). This was measured using a refractometer (ATOGO, Tokyo, Japan). The purpose of testing hydration is to prevent dehydration from negatively affecting the isometric pull performances [26]. After passing the hydration test, participants had their body composition measured through a SECA bioelectrical impedance device (SECA, Hamburg, Germany). The purpose of the body composition scan was to better describe the participants’ physical qualities, which provides clarity on their relative force outputs.

Before subjects began testing, they provided a urine sample to determine their hydration status through USG. Subjects who had a USG greater than 1.020 were considered dehydrated and drank water until their USG was less than 1.020. This is because dehydration can negatively impact performance [27]. After achieving an acceptable hydration status, participants had their body composition measured through a bioelectrical impedance device. Subjects performed a warm-up with one set of twenty-five jumping jacks, one set of mid-thigh pulls at 20 kg, and three sets of five mid-thigh pulls at 60 kg for men and 40 kg for women. After this, the participants performed the IMTP. The testing order for each isometric position was determined based on efficiency, as set-up was easiest starting with the mid-thigh pull position, while adjusting the rack down is both easier and faster than adjusting the rack up. Each participant was placed in their normal position that mimics the power position in the clean. This position has the participant’s knee angle at roughly 125–145 degrees with the hip angle roughly 140–150 degrees [28]. The participants had their hands strapped to the bar using wrist straps and athletic tape to ensure that grip strength did not limit their ability to produce force. The subjects performed two warm-up pulls at 50% and 75% effort. The participants were told to pull “fast and hard”, in accordance with [29], and received a verbal countdown from three. The subjects applied force against the isometric pull rack into the force platforms beneath until the force output plateaued or began to decrease. At this point, the testers told the subjects to stop. The weightlifter was given two minutes of rest between trials to allow for adequate recovery. Two trials were performed unless the difference between the two pulls was greater than 250 newtons of force or if errors in the pull were detected that effected results, such as a countermovement before the start of the pull or significant changes in body position [28].

Following the IMTP, the weightlifters performed an IPST. A 50% and 75% effort pull from this position was used for warm-up. Trials of the IPSP followed the IPST. The same procedures previously used for the IMPT were used for both the IPFK and IPSP isometric tests. The main variables of interest were IPF, RFD, Impulse, and allometrically scaled versions of these variables to account for relative force–time characteristics. Each trial was collected through a custom-made analysis program written in LabView (National Instruments Co., Austin, TX, USA). Newtons of force were derived from voltage measured from the load cells of the force plates and were quantified via the LabView program.

This study followed a cross-sectional time horizon. The reason for this was that the isometric pull data was collected once per subject group. Because of time constraints, data was collected on multiple days. Assessing one subject required roughly 25 min to perform pulls from each of the three key positions. The study focus was cross-sectional and was not meant to ascertain change over time, but to observe individual performances.

For data analysis, the data was presented by mean ± standard deviation. A Shapiro–Wilks test was used to assess if peak isometric strength characteristics and weightlifting performance were normally distributed. Test–retest reliability between trials was assessed for all positions through an Intra-Class Correlation Coefficient (ICC). A Pearson Correlation Coefficient with 95% confidence intervals was used to assess the relationship between isometric strength characteristics and weightlifting performance. Weightlifting experience was not used as a covariate. A one-way analysis of variance (ANOVA) was used to assess differences between the three isometric pull protocols. An independent samples t-test was used to assess differences in isometric pull characteristics between males and females, using 95% confidence intervals and effect sizes. The isometric strength variables for all three isometric positions that were measured were isometric peak force (IPF), rate of force development (RFD), and allometrically scaled isometric peak force (IPFa). Competition data that was used in the analysis were SN, CJ, and TOT. Correlations were interpreted using the following criteria: 0 = Trivial, 0.1 = Small, 0.3 = Moderate, 0.5 = Large, 0.7 = Very Large, 0.9 = Nearly Perfect [30] Statistical analyses were performed using SPSS 29.0 (IBM Corp., Armonk, NY, USA).

3. Results

The mean SD for isometric testing and weightlifting performances are presented in

Table 1. Descriptive statistics for force–time characteristics and weightlifting performances (M and SD).

Group	SN (kg)	CJ (kg)	TOT (kg)	IMTP IPF (N)	IPST IPF (N)	IPSP IPF (N)
Combined (n = 17)	91.06 ± 28.29	115.35 ± 36.44	206.42 ± 64.43	4575.95 ± 1399.51	2226.39 ± 557.32	2128.73 ± 589.16
Male (n = 10)	109.7 ± 20.96	139.1 ± 28.32	248.8 ± 49.05	5419.84 ± 1230.74	2579.97 ± 482.23	2542.02 ± 437.59
Female (n = 7)	64.43 ± 8.2	81.43 ± 7.44	145.86 ± 13.78	3370.38 ± 261.45	1771.78 ± 194.47	1597.37 ± 163.86

. The ICC showed a high degree of reliability between trials for all positions of isometric peak force (IPF) and RFD at 200 ms (RFD200).

The results of the one-way ANOVA showed statistically significant differences in IPF between the IMTP, IPST, and IPSP (F = 35.69, p < 0.001) for the combined group. Post hoc analysis revealed that the IMTP IPF was statistically significantly, greater than that of the IPST (p < 0.001, 95% CI = 1551.19–3147.93) and IPSP (p < 0.001, 95% CI = 1648.84–3245.59). However, there were no statistically significant differences in IPF between the IPST and IPSP. Similar results were found for male (F = 38.59, p < 0.001) and female (F = 150.75, p < 0.001) groups. The post hoc analysis for the male group revealed statistically significant differences between the IMTP and IPST (p < 0.001, 95% CI = 1895.44–3784.30), and statistically significant differences in the IMTP and IPSP (p < 0.001, 95% CI = 1933.39–3822.26). Similarly to the combined group, no statistically significant differences in IPF were found between the IPST and IPSP. For the female group, post hoc analysis revealed statistically significant differences between the IMTP and IPST (p < 0.001, 95% CI = 1311.34–1885.86), and statistically significant differences in the IMTP and IPSP (p < 0.001, 95% CI = 1485.75–2060.28). No statistically significant differences were found between the IPST and IPSP.

The results of the Shapiro–Wilks test for normality showed that all force–time characteristics in the combined group were normally distributed, apart from the IMTP, IPF, and CJ. In the male group, all force–time characteristics were normally distributed. In the female group, all characteristics were normally distributed, apart from the IPST and IPF. Of the allometrically scaled variables, in the combined group, the IPST IPFa and IPSP IPFa were not normally distributed. In the male group, the IPST IPFa and IPSP IPFa were, again, not normally distributed. All allometrically scaled variables were normally distributed. The variables that were not normally distributed were corrected by bootstrapping, with a sampling of 1000.

The correlations of IPF and weightlifting performance between the IMTP, IPST, and IPSP for the combined, male, and female groups can be observed in

Table 2. Correlations of IPF between isometric pull tests and weightlifting performance.

	IMTP		IPST		IPSP
Group	r value	95% CI	r value	95% CI	r value	95% CI
Combined
SN	0.88 **bt	0.876–0.876	0.88 **	0.687–0.959	0.95 **	0.850–0.982
CJ	0.90 **bt	0.903–0.903	0.92 **bt	0.915–0.915	0.97 **bt	0.970–0.970
TOT	0.90 **bt	0.896–0.896	0.91 **	0.744–0.967	0.97 **	0.900–0.988
Male
SN	0.78 **	0.285–0.944	0.80 **	0.295–0.957	0.94 **	0.715–0.987
CJ	0.83 **	0.424–0.959	0.89 **	0.338–0.960	0.96 **	0.803–0.991
TOT	0.81 **	0.372–0.954	0.82 **	0.332–0.960	0.95 **	0.788–0.990
Female
SN	0.69	−0.129–0.950	0.03 bt	0.882–0.882	0.02	−0.745–0.761
CJ	0.56	−0.340–0.923	0.62 bt	0.915–0.915	0.36	−0.536–0.877
TOT	0.71	−0.09–0.954	0.35 bt	0.906–0.906	0.21	−0.646–0.831

** Indicates a p-value of <0.01. bt indicates that the values were bootstrapped.

. In the combined group, most notably, there were statistically significant and Near-Perfect (>0.90) correlations between the IPST IPF and TOT (0.91), the IPST IPF and CJ (0.92), the IMTP IPF and CJ (0.90), the IPSP IPF and SN (0.95), the IPSP IPF and CJ (0.970), and the IPSP IPF and TOT (0.965). Smaller correlations, however, were still considered as Large (>0.50) and Very Large (>0.70), and statistically significant correlations were noted between many other force–time characteristics (Table 2).

The results of the Pearson Correlation Coefficient within the male group were shown to be statistically significant, and Near-Perfect correlations were found between the IPSP IPF and SN (0.935), the IPSP IPF and CJ (0.957), and the IPSP IPF and TOT (0.953). Similarly to the combined group results, there were statistically significant and both Large and Very Large correlations between the weightlifting performance variables (SN, CJ, and TOT) and the force–time characteristics of the three key positions (IPSP, IPST, and IMTP). These correlations are shown in Table 2, with the results of the Pearson Correlation Coefficient for RFD in

Table 3. Correlation matrix of RFD variables at 50 ms, 90 ms, 200 ms, and 250 ms, for the IMTP, IPST, and IPSP compared to weightlifting performance.

Combined	IMTP RFD50	IPST RFD 50	IPSP RFD 50	IMTP RFD 90	IPST RFD 90	IPSP RFD 90	IMTP RFD 200	IPST RFD 200	IPSP RFD 200	IMTP RFD 250	IPST RFD 250	IPSP RFD 250
SN	0.77 **	0.65 **	0.83 **	0.82 **	0.81 **	0.83 **	0.67 **	0.69 **	0.70 **	0.83 **	0.80 **	0.80 **
CJ	0.74 **	0.63 **	0.82 **	0.82 **	0.86 **	0.87 **	0.72 **	0.79 **	0.77 **	0.86 **	0.88 **	0.86 **
TOT	0.76 **	0.65 **	0.83 **	0.83 **	0.84 **	0.85 **	0.70 **	0.75 **	0.75 **	0.85 **	0.85 **	0.84 **
Male
SN	0.57	0.46	0.68 *	0.71 *	0.85 **	0.76 *	0.64 *	0.70 *	0.55	0.80 **	0.85 **	0.70 *
CJ	0.49	0.32	0.63	0.70 *	0.84 **	0.81 **	0.71 *	0.80 **	0.69 *	0.85 **	0.93 **	0.82 **
TOT	0.52	0.38	0.65	0.71 *	0.85 **	0.80 *	0.69 *	0.76 *	0.63	0.83 **	0.90 **	0.77 *
Female
SN	0.34	−0.39	0.04	0.16	−0.38	0.00	0.20	−0.17	0.15	0.24	−0.09	0.23
CJ	0.39	0.27	0.35	0.17	0.30	0.20	0.24	0.52	0.23	0.40	0.58	0.33
TOT	0.42	−0.09	0.21	0.19	−0.06	0.11	0.25	0.18	0.21	0.36	0.26	0.31

* Indicates a p-value of <0.05; ** Indicates a p-value of <0.01.

. Interestingly, the results of the Pearson Correlation Coefficient within the female group showed no statistically significant correlations between IPF and weightlifting performance.

The correlation between allometrically scaled variables in the combined group is shown in

Table 4. Correlations of IPFa between isometric pull tests and allometrically scaled weightlifting performance.

	IMTP		IPST		IPSP
Group	r value	95% CI	r value	95% CI	r value	95% CI
Combined
SNa	0.78 **	0.474–0.916	0.66 **	0.242–0.870	0.84 **	0.584–0.942
CJa	0.82 **	0.569–0.935	0.75 **	0.412–0.910	0.91 **bt	0.746–0.967
TOTa	0.82 **	0.550–0.931	0.84 **	0.356–0.898	0.89 **	0.707–0.962
Male
SNa	0.61	0.100–0.919	0.61	−0.086–0.908	0.85 **	0.412–0.967
CJa	0.72 *	0.282–0.944	0.66	−0.015–0.919	0.91 **	0.624–0.981
TOTa	0.69 *	0.097–0.918	0.65	−0.032–0.917	0.89 **	0.566–0.978
Female
SNa	0.82 *	0.180–0.973	−0.00 bt	−0.754–0.752	0.02	−0.743–0.763
CJa	0.52	−0.383–0.915	0.42 bt	−0.484–0.892	0.00	−0.753–0.753
TOTa	0.76 *	0.008–0.962	0.20 bt	−0.649–0.829	0.01	−0.747–0.759

* indicates a p-value of <0.05; ** Indicates a p-value of <0.01; bt indicates that the values were bootstrapped.

. The results showed that there were statistically significant and Very Large correlations between the CJa and IMTP IPFa (0.82), TOTa and IMTP IPFa (0.82), and Sna and IMTP IPFa (0.78). There were no statistically significant correlations found in any of the variables for IPFK or IPFF. Table 4 shows the results of the Pearson correlation in the male group for allometrically scaled variables. The results showed that there were statistically significant and Very Large correlations between the CJa and IMTP IPFa (0.72). Like the combined group, there was no statistically significant correlation between performance variables and the IPST or IPSP. The results of the Pearson Correlation Coefficient of the allometrically scaled variables in the female group are shown in Table 3. The results indicate that there were statistically significant and Very Large correlations between the Sna and IMTP IPFa (0.82), and the TOTa and IMTP IPFa (0.82). The results of the Pearson Correlation Coefficient between Impulse variables at the three key positions can be observed in

Table 5. Correlations between Impulse (s⁻¹) variables at 50 ms, 90 ms, 200 ms, and 250 ms, for the IMTP, IPST, and IPSP compared to weightlifting performance.

Combined	IMTP IMP50	IPST IMP50	IPSP IMP50	IMTP IMP90	IPST IMP90	IPSP IMP90	IMTP IMP200	IPST IMP200	IPSP IMP200	IMTP IMP250	IPST IMP250	IPSP IMP250
SN	0.72 **	0.72 **	0.82 **	0.87 **	0.75 **	0.74 **	0.87 **	0.81 **	0.89 **	0.88 **	0.83 **	0.81 **
CJ	0.68 **	0.72 **	0.80 **	0.87 **	0.74 **	0.71 **	0.85 **	0.83 **	0.89 **	0.90 **	0.85 **	0.80 **
TOT	0.70 **	0.72 **	0.81 **	0.87 **	0.75 **	0.73 **	0.86 **	0.82 **	0.90 **	0.90 **	0.85 **	0.81 **
Male
SN	0.35	0.34	0.53	0.78 **	0.43	0.41	0.70 *	0.65	0.81 **	0.82 **	0.73 *	0.55
CJ	0.25	0.21	0.40	0.77 **	0.29	0.31	0.65 *	0.55	0.78 *	0.85 **	0.65	0.50
TOT	0.29	0.26	0.46	0.78 **	0.35	0.35	0.67 *	0.60	0.80 **	0.84 **	0.69 *	0.52
Female
SN	−0.28	−0.42	−0.19	−0.25	−0.44	−0.18	−0.09	−0.43	−0.12	−0.03	−0.40	−0.08
CJ	−0.56	0.07	−0.15	−0.53	0.10	−0.11	−0.30	0.22	−0.04	−0.19	0.28	0.00
TOT	−0.47	−0.21	−0.19	−0.43	−0.21	−0.17	−0.22	−0.14	−0.09	−0.12	−0.09	−0.05

* indicates a p-value of <0.01; ** Indicates a p-value of <0.01.

.

4. Discussion

This study aimed to investigate the relationship between isometric force–time characteristics at three key positions in the clean and weightlifting performance. Olympic weightlifting is widely considered a strength and power sport and so it is commonplace to strength train in preparation for a weightlifting competition [5,31]. To measure gains in strength, the isometric mid-thigh pull has been used because of its relationship to weightlifting performance [22,32]. Many studies have found correlations between isometric strength and weightlifting performance variables such as SN, CJ, and TOT [20,23,33]. In this study, the relationship of the isometric force–time characteristics of three key positions and weightlifting performance was examined. The primary finding of this study was that there appears to be a relationship between isometric force–time characteristics at the three key positions and absolute weightlifting performance, with the IPSP having the largest correlation for the combined group. This is consistent with [20].

More specifically, [20] found statistically significant correlations between the IMTP and SN, CJ, and TOT with r values of 0.83, 0.72, and 0.67, respectively. Additionally, they found statistically significant correlations between the IPSP and SN, CJ, and TOT, with r values of 0.94, 0.95, and 0.95, respectively. The authors explain that the primary finding was that the IPSP has stronger correlations to weightlifting performance when compared to the IMTP. Similar findings were observed by a later study by [23], in which the authors observed r values between the IPSP and SN, CJ, and TOT at 0.94, 0.94, and 0.95, respectively. The correlations between the IMTP and SN, CJ, and TOT had r values of 0.89, 0.87, and 0.88, respectively, which is consistent with [20]

The current study expanded upon the [20] article by adding an isometric test at the start of the transition in the clean. The results indicated that IPF at this position in the combined group was statistically significantly correlated with SN, CJ, and TOT, with r values of 0.88, 0.92, and 0.91, respectively. In the female group, there were no significant correlations at any position. However, strong correlations were found at the IMTP position, although none were statistically significant (Table 2).

The current study found similar IPF correlations to those found in [21]. Like the current study, the authors found significant correlations between both the IPSP and IMTP positions and weightlifting performance. For IPF, the IMTP had Large, Very Large, and Large correlations to SN (r = 0.67), CJ (r = 0.71), and TOT (r = 0.69), respectively. The IPSP had Very Large correlations to SN (r = 0.81), CJ (r = 0.85), and TOT (r = 0.84). However, the current study found greater correlations between both positions and weightlifting performance, which can be observed in Table 2.

The correlations of IPF were statistically significant and were Large or Near-Perfect in the combined group. This agrees with findings from [23], who also measured isometric force–time characteristics at the three key positions and observed similar correlations between IPF and weightlifting performance. While the relationship between the IMTP and weightlifting performance is evident in both the Ben-Zeev study and the current study, it appears, based on absolute values, that the IPST and IPSP are stronger predictors of performance. This may be explained by the first pull requiring a great amount of force to overcome the inertia of the motionless barbell off the floor.

From a technical standpoint, the start position of the pull is somewhat simple, in that the lifter should primarily be focused on barbell position over the metatarsophalangeal joint and starting with an advantageous knee and hip position that is somewhat unique to the individual lifter due to anatomical differences [34]. However, once the barbell is lifted from the floor, a large focus on technique is needed to get the barbell to each key position [35,36]. This may be a possible source of the greater correlations observed with the first two positions, because errors in technique at maximal or near-maximal weights may not allow for the bar to efficiently reach the “power position” or the position that the IMTP mimics, thus decreasing its correlation to weightlifting performance.

This study expands on previous findings from the Ben-Zeev study by investigating the relationship between allometrically scaled peak force at the three positions and weightlifting performance. All three performance variables had statistically significant correlations in the combined group, with r values of 0.78, 0.82, and 0.82 for SNa, CJa, and TOTa, respectively. This is contradictory to [20], who observed Large and Very Large statistically significant correlations between the IPSP IPFa and weightlifting performances across all groups. This difference between studies may be attributed to the difference in experience or strength levels of weightlifters who participated in each study.

In the current study, collegiate-level weightlifters were recruited, [20] investigated national and international weightlifters. These lifters would likely be stronger at smaller body masses and would have greater positional strength at the start of the lift, as this position is likely one that has been reinforced through years of performing full lifts in training. Performing a full lift guarantees that the start position is trained, but it is not guaranteed that the weightlifter reaches the other key positions with good technique, resulting in those positions being less trained. It also may be possible that the mid-thigh position is not a priority in their training, thus resulting in smaller correlations to performance. However, this cannot be made certain, as the exercise type and training volume are both unknown to the investigator.

In addition to IPF, this study was an investigation of the relationship between RFD at various time points and weightlifting performance. In Table 3, the results of the Pearson Correlation Coefficient between RFD variables and performance are shown. Statistically significant and Very Large to Near-Perfect correlations can be observed in the combined group across all three isometric positions. This is contradictory to [23], as the only statistically significant correlations in the combined group were at 200 ms and 250 ms for all performance variables for the IPSP. Additionally, the r values for the other two positions ranged from Small to Moderate. The cause of this discrepancy between studies could be in the procedures. It does not appear that Ben-Zeev provided instructions to the participants to pull “fast and hard”, as this would elicit greater voluntary contraction speed [29]. This is important, because greater voluntary contraction influences RFD, especially in a 0–300 ms window [37]. Furthermore, it does not appear that the participants’ hands were taped to the bar. This could negatively affect force transfer, which in turn reduces RFD when the grip slackens due to large forces pulling on the bar.

In the current study, the male group early RFD time bands (e.g., 50 ms) do not show statistically significant correlations. This is somewhat inconsequential, because the reliability of early RFD time bands has come into question in previous research, as there is a large degree of variability [38]. This is most likely due to changes in the pre-activation of the muscle from the participant anticipating the start of the pull. Therefore, it is difficult to make any inferences based on the RFD of an early time band, such as 0–50 ms. On the other hand, later time bands, like 0–200 ms, are reliable and can provide information directly related to the explosiveness of a weightlifter [39,40].

In the current study, RFD at 200 ms showed significant correlations with all positions. This disagrees again with the findings of [23] and may also be a result of differences in weightlifting experience or training methods. It appears that, in the combined group, RFD200 had statistically significant correlations to weightlifting performance at all three positions. This is contradictory to [23], who only found statistically significant correlations between the IPSP and weightlifting performance in the combined group. The current study noted statistically significant correlations of RFD in both the combined and male groups across all positions, barring SN and TOT for the IPSPs.

Another isometric force–time characteristic that was investigated in this study was Impulse (force X time) at various time bands. The results of the Pearson Correlation Coefficient between Impulse at three key positions and weightlifting performance can be seen in Table 5. Much like RFD, there were no statistically significant correlations in the female group. This makes sense, because Impulse is related to RFD, because rapid increases in Impulse are congruent with rapid increases in force; thus, similar correlations to performance were observed. However, there were many statistically significant correlations in the combined group; therefore, Impulse may be a good predictor of weightlifting performance, especially for combined or male groups.

5. Conclusions

The primary aim of this study was to determine the relationship between isometric force–time characteristics at three key positions and weightlifting performance. The results indicated that there is a relationship between all three key positions and weightlifting performance. More specifically, the IPSP appears to have the largest correlation to weightlifting performance for combined and male groups; however, the IPSP does not appear to have a relationship to weightlifting performance in the female group according to the current study. The low number of female participants is a limitation in this study that may have contributed to lower correlations. The IPST appears to have a strong relationship to weightlifting performance across all groups. This indicates that it may be a worthwhile tool to include in a weightlifter monitoring program. The IMTP had somewhat weaker correlations to performance when compared to male and female groups. However, the r values observed were still considered Very Large and a few were Near-Perfect, indicating that the IMTP is also a worthwhile tool for predicting weightlifting performance and may be beneficial to include in an athlete monitoring program. Furthermore, additional variables to include in a weightlifter monitoring program are RFD and Impulse at various time bands. These two variables also showed strong correlations to performance at all isometric positions.

There are many important strength characteristics to be obtained from testing a weightlifter’s isometric strength. These characteristics are vital to the success of weightlifting performance, as they have implications for barbell speed, power, and technique. In measuring characteristics of strength, the IMTP is a valid and reliable tool to aid in the monitoring of the weightlifter, but other isometric tests at differing positions may also be beneficial but have rarely been studied. A coach interested in measures of absolute strength should still include the IMTP protocol, as the position allows for the greatest amount of force application. The IPSP and IPST are isometric strength tests that have not been studied as much as the IMTP and may provide useful information. A recommendation is to use all three isometric testing positions at the beginning and end of a training cycle while also performing the IMTP more frequently to measure strength gains, such as from one block of training to the other. Additional, information that may be learned from the tests would be the weightlifter’s positional strength in the clean. A coach may choose to program differently based on the strengths and weaknesses observed in the isometric testing. If an athlete’s force–time characteristics increased in one position but not the other, this may indicate that more focused training in the latter position is needed.