Athletic Training and Sports Health Care

Original Research 

Development and Validation of a Leg Press Force Measuring Device to Assess Limb Strength Asymmetry

Adam B. Rosen, PhD, ATC; Russell Buffum, BS; Brian A. Knarr, PhD

Abstract

Purpose:

To develop and validate a low-cost modular instrument that interfaces with a leg press device to measure limb strength asymmetry.

Methods:

Fifteen participants completed double-limb leg presses at several weights on a leg press device that was fitted with modular force measuring devices. Asymmetry ratios were calculated for the limbs during concentric, transition, and eccentric phases and compared across phases and weights.

Results:

Participants demonstrated more symmetrical leg press patterns during higher loads compared to lower loads, particularly during the concentric phase.

Conclusions:

As weight increases toward a one-repetition maximum, participants would be expected to demonstrate more symmetrical patterns. This preliminary assessment indicates this device may be valid to assess limb asymmetries in patients.

[Athletic Training & Sports Health Care. 20XX;X(X):XX–XX.]

Abstract

Purpose:

To develop and validate a low-cost modular instrument that interfaces with a leg press device to measure limb strength asymmetry.

Methods:

Fifteen participants completed double-limb leg presses at several weights on a leg press device that was fitted with modular force measuring devices. Asymmetry ratios were calculated for the limbs during concentric, transition, and eccentric phases and compared across phases and weights.

Results:

Participants demonstrated more symmetrical leg press patterns during higher loads compared to lower loads, particularly during the concentric phase.

Conclusions:

As weight increases toward a one-repetition maximum, participants would be expected to demonstrate more symmetrical patterns. This preliminary assessment indicates this device may be valid to assess limb asymmetries in patients.

[Athletic Training & Sports Health Care. 20XX;X(X):XX–XX.]

For patients with an injury to their lower limb, common rehabilitation exercises often use a standard leg press device to strengthen muscles of the lower extremity. Because the leg press is a bilateral strength device and is not equipped to measure force production, it is unknown how individual limbs contribute to force generation. This could be of particular importance in unilateral injuries because the uninjured limb may be performing much of the force generation, limiting the potential benefits of the exercise.1–3

Similarly, limb strength asymmetry is a factor in injury recurrence and is often implicated in different lower extremity joint pathologies such as osteoarthritis, ACL injuries, and chronic ankle instability.4–6 Despite many athletes passing return to sport criteria, many often demonstrate persistent limb asymmetry that may increase their risk of reinjury.7 Furthermore, it has been shown that individuals who have quadriceps strength asymmetry have decreased knee function compared to those with higher symmetry.8

The leg press is one of the most common exercises performed throughout rehabilitation programs, as well as by athletes during strength and conditioning training to improve quadriceps strength.9 Measuring limb asymmetry during bilateral strength measures such as a leg press device can therefore provide insight into individual limb deficiencies and throughout rehabilitation progressions. However, no devices are commercially available to accurately measure limb strength asymmetry during a leg press. To address these limitations, we have developed a low-cost modular instrument designed to interface with a standard leg press device to individually measure the force provided by each leg. Thus, the purpose of this study was to fabricate, validate, and assess the ability of a low-cost modular instrument that interfaces with a standard leg press device to measure limb strength asymmetry ratios in healthy individuals.

Methods

Instrumentation

Each of the leg press attachments was created out of an extruded aluminum 80/20 T-slot building system and the prototype was assembled for less than $500 in components. The structure was designed to resist torsion in x, y, and z directions with the mounting of the load cells in parallel. Two load cells were mounted to an aluminum frame and designed with two 9.5 × 8 × 0.75 inch pressure-treated plywood with textured anti-slip traction tape attached to each respective load cell (Figure 1). Because each load cell is a transducer that converts a force acting on it into an electronic signal, they were placed a quarter distance from the top and bottom of the frame to capture anterior/posterior pressure differences for a single foot. This was to keep in line with the two major contact points, the calcaneus and the padded portion of the sole between the toes, arch, and metatarsals. Load cells were positioned to collect heel to toe, and left to right with forces collected in parallel orientation (Figure 2). Each attachment was strapped to the leg press device (Life Fitness Leg Press Signature Series; Life Fitness Co) via tow straps that were tightened to maintain maximum contact between the device and aluminum frame.

Schematic of the modular leg press device.

Figure 1.

Schematic of the modular leg press device.

Leg press force measuring device attached to the leg press device.

Figure 2.

Leg press force measuring device attached to the leg press device.

Data were collected through H30A bar load cells and independently amplified HX711 amplifiers. The data were processed through the chip's low digital header to be output through the USB serial interface. This data stream was picked up by the computer's USB serial interface and written in real time at 60 Hz. Prior to attaching to the leg press device and performing the study, known weights were placed on the load cells to ensure proper calibration and accuracy.

To validate the fabricated instrumentation, each leg press attachment was placed on known and calibrated Bertec force plates (Bertec Co). The Bertec force plates used for initial validation were not portable units; portable Bertec units are significantly larger and heavier than the leg press devices, and not able to interface with a leg press device. The validation was completed in the same environment as the force plates for consistency, which is considered the gold-standard validation method.10,11 Each of the four strain gauges was calibrated independently with known laboratory weights that were placed one at a time and stacked to increase the load in 7.5-kg increments up to 60 kg. This process was repeated to create more accurate calibrations until error resolved under 0.05%. The final difference between the force plates and fabricated plates is 0.0003% and 0.00003% for left and right, respectively.

Participants

This study was approved by the local human subjects review board and all participants consented prior to testing. Participants were included if they were between the ages of 19 and 35 years and had no limitations in physical activity. Participants were excluded if they had a pathology that directly affected the musculoskeletal system (eg, rheumatoid arthritis, neuropathy, myopathy, vertigo, joint replacement, or diabetes) or a history of surgery or joint pain in the lower extremity. To perform data collection, participants were asked to wear shorts and required to wear shoes with laces.

Procedures

Participants first completed health history questionnaires and inclusion/exclusion criteria screening forms. Limb dominance was determined by asking participants which leg they preferred to kick a ball with. A one-repetition maximum was determined for the leg press exercise using normative data charts based on self-reported fitness level, age, and sex.12 Prior to testing, practice trials were done to ensure testing procedures were followed. Individuals when testing were instructed to set the weight to 25 lbs to be able to handle lightweight without swinging the pendulum out too hard. This was then followed by one repetition as an example and instruction to “push and release the load at the speed of one second for each direction, while being consistent and smooth attempting to not let the pendulum hit the base point.” If they failed this initial instruction, it was repeated one more time by an investigator to demonstrate the procedures. During the trial, individuals were provided a metronome on a laptop screen to give consistent visual and auditory cues. Repetitions were performed at each weight to the beat of a 60 beats per minute metronome (ie, 1 second up, 1 second down) to standardize across individual participant, as in previous studies.13 A 1-minute rest period was provided between repetitions, which was determined to be adequate during initial pilot testing with the lower loads. Six repetitions were recorded at each weight, starting at no weight and increasing in 10% increments to 50% of their determined one-repetition maximum.

Data and Statistical Analysis

An ATmega328P at 60 Hz was used to process and transfer force data collected from each of the load cells for logging. The peak load was averaged between the two sensors per foot by an Arduino Uno, sent via Bluetooth, and saved on a laboratory computer for analysis. The 20%, 30%, 40%, and 50% repetition maximum trials were used for data analysis and normalized to 200 data points. Data were then split into three phases for the concentric, transition, and eccentric phases of the leg press exercise for each of the trials. Asymmetry ratios were calculated for comparison between the right and the left limb, as well as the self-reported dominant and non-dominant limbs. An asymmetry ratio of one indicted perfect limb symmetry, whereas an asymmetry ratio of greater than one indicated more reliance on the right or dominant limb, respectively.

Although an a priori power analysis was not completed prior to the study, validation studies of similar design used comparable sample sizes.14,15 Repeated-measures analyses of variance (P < .05) were used to assess differences in phases across each of the four trials (20%, 30%, 40%, and 50%) for each of the three phases. Follow-up pair-wise comparison testing and Cohen's d effect sizes were used to assess specific differences between trials. Cohen's d effect sizes were interpreted as 0.2 to 0.5 = small, 0.5 to .8 = moderate, and greater than 0.8 = large, respectively.16 IBM SPSS software (version 24.0; IBM Corporation) was used for all statistical analyses.

Results

Fifteen healthy participants (4 women/11 men; 9 right-limb dominant, 6 left-limb dominant; age = 23.8 ± 3.0 years; mass = 81.3 ± 17.9 kg; height = 170.3 ± 29.0 cm) were included. Significant differences in the concentric phase were found between weighted trials (F = 3.71, P = .019). Specifically, greater asymmetry ratios were found in the 20% repetition-maximum trial (Table 1, asymmetry ratio = 1.79 ± 1.56) compared to the 40% (asymmetry ratio = 0.84 ± 0.75, P = .007, d = 0.78) and the 50% (asymmetry ratio = 0.89 ± 0.54, P = .032, d = 0.77, Figure 3) trials. There were no significant differences in asymmetry ratios for the transition (F = 0.30, P = .83) or eccentric (F = 0.45, P = .72) phases.

Asymmetry Ratios (Means ± SD) Across the Participants for Each of the Leg Press Phases and Weights for Right vs Left and Dominant vs Non-dominant Analyses

Table 1:

Asymmetry Ratios (Means ± SD) Across the Participants for Each of the Leg Press Phases and Weights for Right vs Left and Dominant vs Non-dominant Analyses

Differences in right versus left limb asymmetry ratios in the concentric phase between 20%, 30%, 40%, and 50% one-repetition maximum testing. *Indicates significant differences.

Figure 3.

Differences in right versus left limb asymmetry ratios in the concentric phase between 20%, 30%, 40%, and 50% one-repetition maximum testing. *Indicates significant differences.

On average, all trials favored the participants' dominant limb, but there were no significant differences across the phases found between weighted trials. However, as the trials increased in weight, asymmetry ratios tended to become closer to one, indicating more symmetrical patterns (Figure 4).

Asymmetry ratios for the dominant versus non-dominant comparison of the concentric, transition, and eccentric phases for the 20%, 30%, 40%, and 50% one-repetition maximum testing.

Figure 4.

Asymmetry ratios for the dominant versus non-dominant comparison of the concentric, transition, and eccentric phases for the 20%, 30%, 40%, and 50% one-repetition maximum testing.

Discussion

The purpose of this study was to fabricate and validate a low-cost modular instrument that interfaces with a standard leg press device to measure limb strength asymmetry in healthy individuals. Based on the data collected, it appears that we were successful in our aims. As the weight increased, participants tended to have a more symmetrical leg press exercise. As weight increases toward a one-repetition maximum, we would expect participants to demonstrate more symmetrical patterns as each limb bears a greater responsibility.17

Asymmetry is characterized in a variety of ways in the literature. Previous work indicates that 10% to 15% asymmetry during strength and power movements and exercises is normal physiologically.18,19 To our knowledge, only one previous study has attempted to assess bilateral asymmetry during a leg press exercise.20 This study used insole devices to estimate pressure values with their main purpose to identify perceptions of asymmetry during the leg press exercise in individuals with varying training.20 They found that trained individuals demonstrated lower asymmetry compared to untrained individuals.20 Although they assessed slightly higher workloads (50% to 70%), our results align, demonstrating a similar range of asymmetry levels and values.

Limb symmetry indexes are often used for several clinical outcomes, including hop testing, landing forces, joint kinetics, kinematics, and strength testing.21–25 For strength testing, although a variety of methods are available, the most commonly used are either handheld dynamometry or isokinetic dynamometer metrics.26–28 However, although the leg press is one of the more commonly employed strength testing exercises to maintain and increase quadriceps strength, force production data are limited to the amount of weight lifted. Similarly, although the leg press device has shown promise for unilateral assessments and their relationship to hop testing and isokinetic torque values,29 current assessments are not capable of assessing individual limb contributions during a bilateral leg press exercise. Furthermore, previous work has indicated that individuals who off-load an ACL-reconstructed limb during double-limb landings may be more at risk for graft failure.30 Thus, the current device may fill a critical need within clinical practice and be able to monitor patients during strength training to ensure proper loading is occurring.

Assessing limb dominance and limb symmetry, as well as their impact on performance, is often disputed in the literature. In particular, due to the reliance on self-report measures (eg, which limb do you kick a ball with), researchers and clinicians often make assumptions based on faulty information because limb dominance has been found to be dependent on the type of task being performed.31 Similarly, the impact of self-reported limb dominance on functional performance in healthy individuals is unclear. In fact, a recent systematic review with meta-analysis concluded that there was no statistical effect of limb dominance for any strength testing, quadriceps/hamstring ratios, functional performance tests, and ground reaction force data from single-limb landings.32 Despite these findings, clinicians and researchers may still rely on limb-dominance tests to provide intra-limb comparisons during rehabilitation.33 Therefore, objective measures for limb dominance, particularly during functional movements such as the leg press, may provide more reliable and task-specific data than self-reported information from patients.

We must acknowledge some limitations with our data. We used a healthy cohort of 15 individuals with an unequal sampling of men and women. Therefore, the generalizability of this data set may be limited and a larger sample with more diverse populations is necessary to investigate the efficacy of the device. However, we acknowledge that the intent of this is a validation study with the subsequent goal to use the device in pathological populations. Particularly, we see this device as having significant promise to inform clinical decision-making for patients who have had ACL reconstruction. In addition, we acknowledge that the use of a one-repetition maximum to determine weight percentages would have been a more accurate research design compared to normative values. Finally, although we validated the leg press device using known weights and without concurrent human subject validation, previous studies have not shown a discrepancy between the two methods.10,11

Implications for Clinical Practice

This preliminary assessment indicates this device may be valid to assess limb asymmetries in patients. This device may provide important objective asymmetry strength measurements to guide evidence-based decision-making throughout the injury evaluation process, rehabilitation, and return to play for a variety of lower extremity injuries. Future research should explore bilateral limb strength asymmetry in pathological populations (eg, ACL reconstruction and osteoarthritis), as well as test-retest reliability. Researchers and clinicians may also consider integrating leg press symmetry measures as a standard outcome.

References

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Asymmetry Ratios (Means ± SD) Across the Participants for Each of the Leg Press Phases and Weights for Right vs Left and Dominant vs Non-dominant Analyses

Parameter20%30%40%50%
Right vs left
  Concentric1.79 ± 1.56a1.41 ± 1.640.84 ± 0.75a0.89 ± 0.54a
  Transition1.16 ± 0.691.12 ± 0.691.07 ± 0.611.02 ± 0.70
  Eccentric1.11 ± 0.960.80 ± 0.740.77 ± 0.930.77 ± 0.61
Dominant vs non-dominant
  Concentric1.50 ± 1.341.42 ± 1.601.11 ± 1.311.23 ± 0.97
  Transition1.36 ± 1.351.36 ± 0.801.12 ± 0.641.23 ± 0.92
  Eccentric1.77 ± 1.871.86 ± 1.951.79 ± 2.261.26 ± 1.11
Authors

From the School of Health and Kinesiology (ABR) and Department of Biomechanics (RB, BAK), University of Nebraska at Omaha, Omaha, Nebraska.

Supported by the National Institutes of Health (P20 GM109090 and R15 HD094194).

The authors have no financial or proprietary interest in the materials presented herein.

Correspondence: Adam B. Rosen, PhD, ATC, University of Nebraska at Omaha, 6001 Dodge Street, H&K207Y, Omaha, NE 68182. Email: arosen@unomaha.edu

Received: February 27, 2020
Accepted: September 23, 2020
Posted Online: February 17, 2021

10.3928/19425864-20201116-01

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