The loss of muscle and bone strength due to long-duration spaceflight is well-documented (6,13,20). To combat these deleterious changes, astronauts on the International Space Station (ISS) perform resistance exercise (7,19,20). In-flight resistance exercise programs consist of upper, lower, and full body exercises similar to those performed by athletes on Earth (14).
A limitation to the performance of full body exercise in microgravity is the need to compensate for the loss of gravitational weight. Loads used during exercises such as the squat, deadlift, and their variants are typically increased by some percentage of the lifter's body weight (14). Although this approach helps to increase total resistance loading to equal that experienced on Earth, exercise loads are often limited not by lower body strength but rather by the ability of the body to handle the loads due to grip or back strength.
Although astronaut strength is evaluated during clinical and functional testing before flight and after return (5,12), there is currently no method to assess strength during flight (7). It is possible to perform 1 repetition maximum (1RM) testing in flight, but this is avoided because of injury risk and the limitations discussed above. The isometric midthigh pull (IMTP) offers a potential alternative that could enable quick, reproducible maximal strength measurements throughout a mission.
Many researchers have documented the relationship between IMTP peak force and squat 1RM and other athletic testing variables (1,8–10,17,18,21). However, there are few data available that characterize the relationship between the IMTP and deadlift 1RM. The deadlift is a functional lift closely resembling those performed in tactical or operational settings as it involves displacing a mass from the ground.
The purposes of this study were to (a) assess the reliability of IMTP force and rate of force development (RFD) measures and (b) characterize the relationship between IMTP and deadlift 1RM to establish whether astronauts can use the test during spaceflight to reliably assess maximal full body strength. We hypothesized that IMTP outcomes would be reliable and significantly related to deadlift 1RM.
Experimental Approach to the Problem
All subjects participated in 3 sessions, including 1 familiarization session and 2 testing sessions, each separated by at least 72 hours. The familiarization session was used to acquaint each subject with the study methods and to instruct them on proper performance of the IMTP. During the testing sessions, subjects performed a standard warm-up consisting of cycling, dynamic stretching, and performance of multiple sets of a normal deadlift at submaximal loads. Subjects were strapped to the bar with standard weightlifting wrist straps and used an overhand/underhand grip that remained constant across trials and between sessions. Subjects then performed 3 maximal IMTP trials, followed by an assessment of 1RM deadlift strength.
Nine non-astronaut subjects (5 men and 4 women; 40.4 ± 7.8 years; 31–49 years; 1.72 ± 0.10 m; 75.6 ± 13.4 kg, mean ± SD), who self-reported experience with resistance training and the deadlift exercise, volunteered to participate in the investigation. Subjects were recruited from the human test subject pool at NASA Johnson Space Center. Before participating in the project, all subjects provided written informed consent, and the investigation was reviewed and approved by NASA Johnson Space Center's Institutional Review Board. All testing occurred at NASA Johnson Space Center.
During the familiarization session, subjects were briefed on the experimental procedures and practiced the deadlift exercise and the IMTP. During the familiarization session, the bar was set at a height midway between the knees and hips so that the knees were flexed at 36 ± 3 degrees and the hips were flexed at 43 ± 3 degrees; a manual goniometer was used to verify joint angles. These values are similar to those reported in previous studies that evaluated the IMTP (9,10). The position of the bar was noted and used for subsequent testing.
All tests were performed on the advanced resistive exercise device (ARED) ground unit (15). The device is identical to the resistance exercise device used on the ISS (see Figure 1). Resistance is created using a mechanical lever system attached to 2 vacuum cylinders. A standard barbell bar is used as the load interface. Thus, for the current experiment, the data were collected as they would be using normal free weights (i.e., with a fixed standard barbell). Bilateral ground-reaction force data were collected at 1,000 Hz by 2 force plates (P6000; BTS Bioengineering, Milan, Italy). A single video camera synchronized with the force plates recorded all lifts and was used to visually verify joint angles (Vixta; BTS Bioengineering). Identical testing configurations were used for all tests.
To obtain body weight, an initial static trial was completed in which the subject stood with 1 foot on each platform in the same stance to be used for the IMTP. After the static trial, a standard warm-up was completed in which subjects performed deadlifts at 30, 50, and 75% of their estimated 1RM.
Isometric Midthigh Pull Testing and Analysis
Immediately after the deadlift warm-up, subjects completed the IMTP trials. The bar was adjusted to the proper height based on the familiarization sessions. To lock the bar in place, the device load was set to the maximum resistance possible (273 kg). During the familiarization session, this load was verified as greater than a subject's maximal lifting strength. Two potential subjects were excluded from the study because they were able to lift the maximum 273 kg resistance during the familiarization session.
Participants performed warm up IMTP with instructions to pull at 50 and 75% of their maximal effort. Subsequently, the 3 trials of maximal IMTP were performed, separated by 2 minutes of rest. The participants were instructed to relax before one of the operators gave the verbal signal to start. Subjects were instructed to pull as hard as possible. Each IMTP trial lasted 5 seconds. All testing sessions were supervised by a certified strength and conditioning specialist trainer.
Data were postprocessed using custom scripts. Body weight determined during the static trials was subtracted from the IMTP data. Resultant ground-reaction force was found from both plates and summed to a single data stream. Peak force and force at 30, 50, 90, 100, 150, 200, and 250 milliseconds were determined for each trial according to the methods of Haff et al. (10). The rate of force development was also found using the equation RFD = ΔForce/Δtime, where ΔForce is the change in force from the start of the pull to the instant of interest, and Δtime is the change in time. Rate of force development was found for the periods from 0 ms to 30 ms, 50 ms, 90 ms, 100 ms, 150 ms, 200 ms, and 250 ms (1,21). Peak RFD was found as the maximal rate over successive 20 ms windows (9).
Isometric midthigh pull reliability was determined by the intraclass correlation coefficient (ICCα) and confidence intervals (CIs) calculated using the variance components from a one-way analysis of variance. Intraclass correlation coefficient α was found using the formula:where σ2(w) is the pooled variance within subjects, and σ2(b) is the pooled variance between subjects.
Acceptable reliability was determined as an ICCα ≥0.80 (3,4). Reliability was determined within subjects across all IMTP trials and test sessions (n = 6), for all trials within test sessions 1 and 2 (2 × n = 3) and for the peak force trial between test sessions (n = 2). Pearson product-moment correlation analysis was used to assess relationships between IMTP outcome variables and deadlift 1RM. Correlation coefficients were evaluated as suggested by Hopkins et al. (11) and Cohen (2), where thresholds of 0.1, 0.3, 0.5, 0.7, and 0.9 were attributed to small, moderate, large, very large, and extremely large correlation coefficients. Bivariate linear regression was used to relate peak force and RFD variables to deadlift 1RM. Significance was defined a priori at p ≤ 0.05. All data are reported as mean ± SD and were analyzed using R Studio (version 0.99.491; R Studio, Boston, MA, USA) executing R (ver 3.2.4; University of California, Berkeley, CA, USA).
Descriptive statistics for IMTP outcome variables and deadlift 1RM are shown in Table 1. The deadlift 1RM for our subjects, expressed in terms of body weight, was 1.46 ± 0.29 kg/kg of body weight. Table 2 lists the correlation coefficients between deadlift 1RM and each IMTP outcome variable. Isometric midthigh pull peak force had a very large association with deadlift 1RM (p < 0.01). Other IMTP outcome variables demonstrated small-to-moderate associations with deadlift strength but were not significant.
Reliability for all trials was acceptable for peak force (ICCα = 0.92; 95% CI = 0.84–0.99). For test session 1, reliability was acceptable for peak force (ICCα = 0.98; 95% CI = 0.95–1.00), force output at 200 ms (ICCα = 0.90; 95% CI = 0.80–1.00), and force output at 250 ms (ICCα = 0.89; 95% CI = 0.76–1.00). For test session 2, reliability was also acceptable for peak force (ICCα = 0.97; 95% CI = 0.94–1.00). When using only the maximum peak force trial from each test session, reliability was acceptable for peak force (ICCα = 0.89; 95% CI = 0.74–1.00) and force output at 250 ms (ICCα = 0.80; 95% CI = 0.56–1.00).
Figure 2 shows the significant relationship between IMTP peak force and deadlift 1RM across all subjects. Linear regression analysis revealed that peak force significantly predicted deadlift strength (r = 0.88, r2 = 0.77; p ≤ 0.05). Figure 3 depicts the frequency of Pearson product-moment correlation between IMTP peak force and deadlift 1RM using all combinations of single trials between subjects.
The primary purposes of this study were to investigate the reliability of the IMTP and the relationship between IMTP peak force and RFD with deadlift 1RM. Multiple IMTP trials were collected over 2 testing sessions, and combinations of all trials were used to assess whether trial selection influences relationships between peak force and deadlift 1RM. The following were the primary findings of this study: (a) isometric midthigh pull peak force is reliable both within and across test sessions; (b) the peak force generated during the IMTP is significantly associated with deadlift 1RM and can thus be used to estimate maximal dynamic full body strength; (c) the lack of association between deadlift 1RM and RFD and intermediate force measures indicates that maximal dynamic deadlift strength is not related to the ability to generate isometric explosive force; and (d) provided proper warm-up is completed, a single IMTP can be performed to assess maximal strength.
Isometric midthigh pull peak force measures were reproducible both within and between sessions; these findings are consistent with previous reliability results (9,10) and demonstrate the utility of the IMTP test to quickly and efficiently assess longitudinal changes in strength.
Our data indicate that peak force explains 77% of the variance in deadlift 1RM. This is a similar finding to that of Wang et al. (21) who found that peak force explained 75% of the variance in squat 1RM. Isometric midthigh pull peak force has been shown to correlate with powerlifting performance (1,8,18), and McGuigan and Wincheseter found similar relationships between peak force and power clean 1RM (17).
The data presented in this article along with those of other authors indicate that IMTP peak force can be useful to estimate maximal strength from a variety of dynamic lifts. We chose to use the deadlift as our dynamic strength test for 2 main reasons. The first is that the literature relating IMTP to deadlift 1RM are sparse. The second is that in our intended application as a method to measure maximal strength in astronauts during spaceflight, the deadlift represents a more functional motion requiring total body strength. An issue of concern for astronaut health is the maintenance of necessary strength to perform critical mission tasks in microgravity, reduced gravity, and on return to Earth. Functionally, the deadlift is the displacement of a mass from the ground—a task that resembles moving an incapacitated crewmember or an object. By relating IMTP peak force to deadlift maximal strength, coaches and trainers can use the IMTP to estimate subject's readiness for functional tasks common to tactical activities.
Past researchers have used the IMTP to evaluate relationships between strength and RFD capabilities of their subjects. During the design of our investigation, we were less interested in understanding IMTP relationships to explosiveness than to strength, primarily because of limitations in exercise capabilities of current hardware at the ISS. Peak forces were correlated with deadlift 1RM, but none of the RFD variables or intermediate force output levels showed significant correlation. This finding is similar to the reports of others who investigated relationships between IMTP peak force and RFD and squat 1RM (16–18). Several factors may explain the lack of relationship. First, maximal strength and explosive strength are independent qualities as noted by McGuigan et al. (16). Another possible explanation is that while the deadlift and other dynamic lifts can be performed explosively, when completed with maximal loads, movement velocity is typically low because of the force-velocity characteristics of the involved musculature. The RFD during the IMTP may be indicative of explosive ability but not of maximal strength capacity.
The collection of multiple trials of a particular test is often necessary to ensure that the data represent actual physiologic values and, as much as possible, to exclude learning and motivational effects. However, in the field setting, especially when working with large numbers of athletes, additional data collection may not be feasible because of a lack of time or resources. When collecting multiple trials, especially with performance data, the trial with the maximal value is commonly used as the athlete's measure. We were interested in determining how much influence trial selection had on the correlation between IMTP peak force and deadlift 1 RM. If a large influence was found, then multiple trials would need to be performed in the field. Our experimental design allowed us to perform a secondary analysis to determine how trial selection influenced the results. We were interested in understanding whether a single trial would suffice or multiple trials were necessary. Our analysis of over 10 million combinations of trials between subjects influenced our major finding less than 0.001% of the time. This result demonstrates that, with proper warm-up, the number of trials performed has little influence on the relationship between IMTP peak force and deadlift 1RM.
One limitation to our findings is that we used a sample of subjects with deadlift 1RM values ranging from 61.4 to 163.6 kg. The subject pool tested was not comprised of dedicated strength athletes and had an average age of 41 years, which is similar to the current astronaut corps. This affected the deadlift 1RM values that were attained. Coaches of elite athletes may work with individuals of much greater strength. Although our results may be valid only for individuals within the strength ranges of our subjects, we do not expect that the correlation between IMTP peak force and deadlift 1RM would be affected for athletes of greater strength.
The results from this study demonstrate that the peak force developed during the IMTP is strongly correlated with deadlift 1RM strength. In addition, although the use of multiple trials is always optimal to reduce potential error, a single IMTP trial is sufficient if using peak force as the outcome measure of interest.
Coaches and scientists are frequently in need of maximal strength measures for their athletes and subjects to assess training programs and to provide periodized prescriptions. One repetition maximum testing is not always appropriate because of various constraints, including lack of time and injury concerns. However, during a training program, strength levels change, and modification of the prescription is necessary. The IMTP provides a quick assessment of maximal strength with a theoretically low risk of injury. Coaches and trainers of tactical athletes could use the IMTP as a quick and efficient method to assess maximal strength in the field with limited equipment, and thus create more effective training programs for their athletes.
The authors thank the NASA Human Research Program for funding this project.
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