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Original Research

Lower-Limb and Trunk Muscle Activation With Back Squats and Weighted Sled Apparatus

Maddigan, Meaghan E.; Button, Duane C.; Behm, David G.

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Journal of Strength and Conditioning Research: December 2014 - Volume 28 - Issue 12 - p 3346-3353
doi: 10.1519/JSC.0000000000000697
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Abstract

Introduction

In recent years, there has been emphasis toward less traditional strength training methods. The weighted sled apparatus is one of the recent popular additions to resistance training exercise alternatives. All previously published sled research has examined pulling a weighted sled (5,31), primarily as a tool for developing speed (1,2,12,25,28,35). There are no studies examining the pushing action on a weighted sled.

The squat is a traditional resistance exercise that is used to develop strength and power in the lower body (11). Athletes such as football, rugby, hockey players, and wrestlers, among others, use the squat as a predominant exercise in their strength training program. Wisloff et al. (34) reported strong correlation between maximal strength in half squats with sprint performance and jump height in high-level soccer players. Similarly, McBride et al. (22) reported a trend to improved sprint times with jump squats using 30% of their squat 1 repetition maximum. As the squat exercise is a ground-based resisted exercise using similar muscle groups as running and jumping, athletic performance might be enhanced through this training-specific movement (13). However, 9 weeks of machine-based squat training regardless of depth did not improve vertical jump height (30).

According to the principle of training specificity (5), the vertical movements of the squat in a stationary position would not coincide with the predominant movement patterns of the aforementioned athletes. These athletes move more on a horizontal plane (e.g., sprinting, change of direction, blocking, tackling, skating, hockey checking) with their body at various inclinations depending on the sport or action. Okkonen and Häkkinen (25) concluded from their data that sled pulling was more velocity specific and movement specific than a half squat. A number of studies have reported that pulling a weighted sled did not compromise running technique (2), was beneficial for the sprint block start (25), and sled pull training studies improved sprinting acceleration (28,35). On the contrary, Clark et al. (12) reported that unresisted training was more effective for improving sprint performance in the 18.3- to 54.9-m interval. As the body is inclined when pushing a sled, the movement pattern would correspond more closely to the predominant movements (e.g., sprint starts and acceleration, change of direction agility, tackling, blocking, skating, checking) of the previously mentioned sports in comparison with a squat action.

Although there are no studies directly comparing squat and sled resisted exercises, sled pulling has been shown to alter trunk and knee angles compared with unresisted sprinting (16). Cronin et al. (16) examined the kinematics of sled towing and reported greater trunk angles (less vertical) and greater thigh extension vs. weighted vests and nonresisted sprinting technique. They suggested that sled towing might be more appropriate for the early stages of the acceleration phase of sprinting. Conversely, Zafeiridis et al. (35) found a decreased trunk angle (57° vs. 62°) after resisted sled sprint training vs. unresisted training. Furthermore, resisted sled sprint training (10% of body mass) did not significantly alter upper body kinematics or hip extension while moderately increasing hip flexion angles (28). Differences in joint angles can affect force-length relationship of muscles significantly affecting force and power output (20). There are no studies comparing the extent of muscle activation when performing resisted sled pushing exercises vs. squats. If resisted sled exercises can provide similar or greater intensities of muscle activation compared with the squat, then sled exercises may be a more preferable form of training because of the specificity of the resistance training movements for certain athletes. The proposed research would provide an evidence-based rationale for or against this form of training because there is no literature published to date on muscle activation levels (exercise intensity) when pushing the sled apparatus for strength gains.

Hence, the objective of the study was to compare the activation of various trunk and lower-limb muscles when pushing a resisted sled vs. a similar relative resistance when performing a squat. It was hypothesized that pushing a weight loaded sled apparatus would provide a similar or greater extent of muscle activation as compared with a traditional bilateral back squat.

Methods

Experimental Approach to the Problem

Participants took part in a randomized crossover design study consisting of 2 preparation sessions and 2 testing sessions separated by a minimum and maximum duration of 2–5 days each. During the initial session, anthropometric measures (height, mass, and age) were recorded. Subsequently, the participant randomly selected a slip of paper from a box with either sled or squat written on the paper, to determine which of the 2 testing conditions (sled vs. squat) they would complete first. This was also the order in which they completed the preparation sessions. During the initial 2 sessions, participants came to the fitness center to be familiarized with the testing protocols and equipment, in particular the sled apparatus. On these first 2 preparation sessions, experimenters determined (on separate days) the participant's 20-step maximum (20SM) push with the weighted sled apparatus and the 10 repetitions maximum (10RM) with a bilateral back squat. The 20SM sled push and the 10RM bilateral back squat provided similar number of repetitions per limb as the sled push involved 10 steps per limb, whereas the squat involved 10 repetitions per limb. In the 2 testing sessions, each of the participants returned to the fitness center where they performed each of the 2 techniques again. During the bilateral squat or the sled push testing sessions, muscle activation of the lower quadriceps, gastrocnemius, lower abdominals (transversus abdominis/internal obliques [TrA/IO]), and lower erector spinae was recorded by electromyography (EMG) electrodes during the concentric contraction phase of the limb movement. Subjects were tested at approximately the same time of day to minimize diurnal variations.

Subjects

Ten healthy resistance-trained men (age 24.6 ± 4.2 years, mass 84.5 ± 6.9 kg, height 178.3 ± 6.2 cm) from the university community volunteered to participate in this study. All the participants had at least 2 years of resistance training and squat experience. They were also familiar with performing the sled exercises; however, the volume of squat experiences exceeded sled training volumes or training durations. The specific volumes of squat vs. sled training were not documented. They filled out a Physical Activity Readiness Questionnaire form from the Canadian Society for Exercise Physiology (10) to determine their general health status. The participants read and signed a consent form before start of the study. Subjects were not informed of the hypotheses of the study. The institutional Human Research Ethics Authority approved the study (HREB 13.082).

Testing Procedure

Preparation Sessions

After anthropometric measures were recorded during the initial session, the preparation and experimental sessions each began identically with a 5-minute warm-up on a cycle ergometer at 60 rpm with a resistance of 1 kp.

Sled 20-Step Maximum Determination

Participants were first oriented to the sled apparatus (Figure 1), and the researcher (certified with the National Strength and Conditioning Association and the Canadian Society for Exercise Physiology) gave detailed instructions and demonstrations on proper sled pushing technique. After the cycle ergometer warm-up, the participants were asked to push the sled, without added resistance, for 20 steps. At this time, the experimenter monitored and recorded hand position on the sled and gave feedback on proper technique. Participants were then asked to push the sled with 40 kg of resistance for 10 steps, and once more they were asked to push the sled with 90 kg of resistance for 10 steps. After this task-specific warm-up, the participant was given a 3-minute rest period. After completion of the rest period, the experimenters proceeded with a trial-and-error procedure to determine the maximum resistance the participant could push for 20 steps. Participants were asked to take 10 steps with an estimated resistance (relative to the 90 kg warm-up resistance) and assess whether the resistance was appropriate to require a maximum effort on their final step. If the resistance was too light or too heavy, they were asked to stop after the 10th step so that the experimenter could adjust the resistance accordingly. If this adjustment occurred, the participant was again given a 3-minute rest period. This procedure was also replicated if the participant tried to complete the 20 steps and was unable to do so. The number of trial-and-error attempts ranged from 2 to 4 repetitions. The stride rate for the subjects was approximately 1 second per stride. Once the maximum resistance was recorded, the session was complete.

Figure 1
Figure 1:
A picture illustrating the weighted sled apparatus being pushed by a subject during the sled push preparation session.

Squat 10 Repetition Maximum Determination

Participants were first oriented to the squat rack, and the researcher gave detailed instructions and demonstrations on proper back squat technique. After the cycle ergometer warm-up, the participants were asked to perform 10 squat repetitions, with the barbell on their back but with no added weight on the barbell. At this time, the experimenter monitored the depth of the squat and placed a box behind the participant to give the participant tactile feedback that they had reached a 90° knee angle, the depth that was deemed proper technique for the purposes of this study. The experimenters also monitored and gave feedback about proper back position at this time. Participants were then asked to perform 5 squat repetitions with 40 kg on the barbell, and once more they were asked to perform 5 squat repetitions with 60 kg resistance. After this task-specific warm-up, the participant was given a 3-minute rest period. After completion of the rest period, the experimenters proceeded with a trial-and-error procedure according to the American College of Sports Medicine guidelines (3) to determine the maximum weight the participant could hold and perform 10 squat repetitions. For the 10RM trial-and-error process, participants were asked to perform 5 repetitions with the estimated weight and assess whether the resistance required a maximum effort on their final repetition. If it was too light or too heavy, they were asked to stop after the fifth repetition so that the experimenter could adjust the resistance accordingly. If a new resistance was necessary, the participant was again given a 3-minute rest period. This procedure was also replicated if the participant continued after the fifth repetition and tried to complete the 10 repetitions and was unable to do so. Subjects were informed to perform the repetitions at a rate of approximately 1 second eccentric and 1 second for the concentric contraction while the data from the surface electrodes were recorded. Once the maximum weight was recorded, the session was complete.

Testing Sessions

After the cycle warm-up, EMG electrodes were attached. Thorough skin preparation for the electrodes included shaving areas where electrodes would be placed, removing dead epithelial cells with an abrasive paper around the designated areas, and then cleansing with an isopropyl alcohol swab. Two surface EMG recording electrodes (Meditrace Pellet Ag/AgCl discs and 10 mm in diameter; Graphic Controls Ltd., Buffalo, NY, USA) were placed 2 cm apart (mid-electrode to mid-electrode) over 5 muscles: (a) the midpoint of the right quadriceps, the muscle belly of rectus femoris, half way between the anterior superior iliac spine and the top of the patella with a ground electrode placed on the head of the fibula, (b) the midpoint of the right hamstrings, the muscle belly of biceps femoris, half way between the gluteal fold and the popliteal fossa, (c) the midpoint of the right calf, the muscle belly of gastrocnemius, half way between the musculotendinous junction and the popliteal fossa, (d) the TrA/IO 3 cm toward the midline from the right anterior superior iliac spine on a 45° angle downward (this surface EMG site has been reported to detect activity from both the TrA and IO (23)), (e) lower lumbar erector spinae, 2 cm to the right of L4 and L5. These electrode placements have been used reliably in a number of published articles from this laboratory (6,26,29). All electrode placements were marked with indelible ink to ensure accurate and consistent surface electrode placement in the subsequent session. Surface electrodes were secured to the skin with adhesive tape and further secured to the leg with a Tensor elastic bandage to try and eliminate any movement artifact. Participants then took part in further task-specific warm-up depending on the nature of the specific testing session.

Sled 20-Step Maximum Test

First, the participant completed the task-specific warm-up that included pushing the sled with no additional resistance for 20 steps, pushing the sled with 25% of the individual's 20SM weight for 10 steps, pushing the sled with 50% of the individual's 20SM weight for 10 steps, and finally pushing the sled with 75% of the individual's 20SM weight for 10 steps. After each of the weight increases and before the final test, the participant was given a 3-minute rest period. After completion of the rest period, the experimenters proceeded to place the individual's specific 20SM weight on the sled, and the participant completed the 20-step sled push.

Squat 10 Repetitions Maximum Test

First, the participant completed the task-specific warm-up that included 10 squats with no additional resistance on the barbell, 5 squats with 25% of the individual's 10RM weight, 5 squats with 50% of the individual's 10RM weight, and 5 squats with 75% of the individual's 10RM weight. After each of the weight increases and before final test, the participant was given a 3-minute rest period. After completion of the rest period, the experimenters proceeded to place the individual's specific 10RM weight on the barbell and the participant completed 10 squat repetitions while the data from the surface electrodes was recorded and subsequently analyzed.

Electromyographic Data Analysis

Data from the surface EMG electrodes were recorded with a commercially designed software program (AcqKnowledge 4.1; Biopac Systems, Inc., Holliston, MA, USA) and stored on a personal computer. Electromyography activity was sampled at 2000 Hz, with a Blackman −61 dB band-pass filter between 10 and 500 Hz, amplified (bipolar differential amplifier, input impedance = 2 mΩ, common mode rejection ratio ≥110 dB minute (50/60 Hz), gain × 2000, noise ≥5 μV), and analog-to-digitally converted (12 bit). Using the AcqKnowledge 4.1 software program, the EMG signal was filtered (20–500 Hz) and smoothed (averaged over 3 samples), and the mean amplitude of the RMS EMG signal was calculated over a 1-second segment of the concentric contraction phase of each step or repetition (determined from the goniometer) for each muscle. The goniometer and EMG signal were synchronized so that the different contraction phases could be identified. The unilateral stepping action of the sled exercise did not involve resisted eccentric contractions during the recovery phase of the step compared with the bilateral squat that had to contend with resisted eccentric contractions during the lowering of the weight. Hence the EMG activity was compared between sled and squat during the concentric phase of the contractions. As the lower erector spinae and TrA/IO muscles acted more as stabilizers and may not have had similar concentric durations as the limbs, these muscles were analyzed over the same period as the concentric contraction of the rectus femoris. Previously published articles from this laboratory using similar EMG procedures have reported on reliability with intraclass correlation coefficients of 0.72–0.99 (4,8,29).

Electronic Goniometry

An electronic goniometer was attached to the knee joint with the arms of the goniometer secured to the lateral midline (frontal plane) of the thigh and shank. Data were sampled at 2000 Hz through the BioPac system and analyzed with the AcqKnowledge software. The goniometer was used to determine the knee angular range of motion for each step/repetition. The eccentric vs. concentric phases of the movements were also determined with the goniometer.

Statistical Analyses

All statistical analyses were conducted using SigmaPlot (version 10.0; Systat Software Inc., San Jose, CA, USA) and Microsoft Office Excel spreadsheets. All muscle activation measures were analyzed separately with 2-way repeated measures analysis of variance (2 conditions [sled vs. squat] × 3 phases [mean of (a) first 2 SM/RM, (b) middle 6 SM/RM, and (c) last 2 SM/RM) to determine whether there were significant main effects or interactions. Differences were considered significant at p ≤ 0.05. If significant main effects or interactions were present, a Bonferroni (Dunn) procedure was conducted. A repeated t-test was used to analyze the differences in the knee joint range of motion. Effect sizes were also calculated to provide qualitative descriptors of standardized effects using these criteria: trivial <0.2, small: 0.2–0.5, moderate: 0.5–0.8, and large: >0.8 (14). All data are presented as mean ± SD. A linear regression equation was used to determine the line of best fit and correlation coefficient.

Results

Rectus Femoris Electromyography

There was a main effect for time (phase) (p < 0.001; effect size [ES]: 3.28) with rectus femoris EMG activity increasing 22.9% from phase 1–3 (Figure 2). There were no significant condition × time interactions.

Figure 2
Figure 2:
Average root mean square (RMS) EMG recorded in exercise phase 1, 2, and 3 from (A) rectus femoris, (B) biceps Femoris, (C) gastrocnemius, and (D) erector spinae during the concentric portion of the sled and squat exercises. Data points represent the mean ±SD. Figures A′, B′, C′, and D′ represent individual RMS EMG data points from the rectus femoris, biceps femoris, gastrocnemius, and erector spinae, respectively, for each subject, illustrating the entire spread of the data. Open circles represent the sled condition, whereas the filled squares represent the squat condition. Asterisk (*) represents a significant difference at p ≤ 0.05 between the sled and squat groups.

Biceps Femoris Electromyography

There was a main effect for time (p = 0.003; ES: 0.32) with biceps femoris EMG activity increasing 13.2% from phase 1–3 (Figure 2). There were no significant condition × time interactions.

Gastrocnemius Electromyography

There was a main effect for condition (p = 0.01; ES: 1.65) with an overall 61.2% greater gastrocnemius EMG with the sled exercise. The sled exercise provided 61.3% (ES: 1.8), 60.8% (ES: 1.64), and 61.5% (ES: 1.65) greater EMG activity during the 3 phases, respectively (Figure 2). There was a main effect for time (p = 0.008; ES: 0.21) with gastrocnemius EMG activity increasing 10.5% from phase 1–3.

Erector Spinae Electromyography

There was a main effect for condition (p = 0.002; ES: 1.1) with the squat providing 33.8% greater erector spinae EMG activity overall. A significant interaction (p < 0.0001) demonstrated 29.1% (ES: 0.7), 46.4% (ES: 1.26), and 50.1% (ES: 1.44) greater erector spinae activity with the squat exercise during phases 1–3, respectively (Figure 2).

Transversus Abdominis/Internal Obliques

There were no significant main effects or interactions.

Knee Range of Motion and Angular Velocity

The squat exercise (98.9° ± 14.03) demonstrated 9.5% significantly (p ≤ 0.05, ES: 0.86) greater knee joint range of motion than the resisted sled (89.5° ± 7.8). However, the sled (74.5°·s−1 ± 9.6) had 14.7% significantly (p ≤ 0.05, ES: 1.1) higher knee angular velocities than the squat (64.9°·s−1 ± 7.3).

Loads

The sled load (240.5 ± 31.2 kg) was 124.3% significantly (p < 0.0009, ES: 4.76) greater than the squat load (107.2 ± 25.6 kg). A linear regression equation was calculated between the maximum loads of the squat and sled. The resultant equation (y = squat load) was as follows: y = 0.5996x −34.355, with an R2 = 0.55279 (Figure 3).

Figure 3
Figure 3:
A scatter plot with a line of best fit and linear regression equation and correlation provided for the maximum loads of the squat and sled exercises.

Discussion

The main findings of this study were that (a) the sled exercise elicited significantly higher gastrocnemius EMG activity, whereas (b) the squat provoked significantly higher erector spinae EMG activity. There was no significant difference with rectus femoris, biceps femoris, and TrA/IO EMG activity between the exercises.

There are no comparable studies examining muscle activation levels of a squat vs. a resisted sled exercise. However, the squat has been shown to elicit relatively high EMG activity compared with other activities. Behm et al. (8) reported higher quadriceps activation with squats compared with single and double knee extensions. The authors suggested that the contractions of multiple lower-body muscle groups enhanced the activation of the quadriceps and that greater levels of activation may be necessary to cope with the stabilization necessary for bilateral and multi-articular contractions. Therefore, the similar EMG activity of the rectus femoris and biceps femoris with the squat and sled exercises indicate that although the sled push and the squat use different kinematics (i.e., trunk angles, knee range of motion) (16,25), both exercises can enhance lower-body strength.

The greater erector spinae activity with the squat in this study has been reported with other exercise comparisons. Hamlyn et al. (19) demonstrated that the erector spinae EMG activity with a squat performed at 80% 1RM significantly exceeded an 80% 1RM deadlift by 34.5%. Similar to this study, there were no significant differences in the TrA/IO activity. When stability ball exercises were compared with squat and deadlift exercises at 50, 70, and 90% 1RM, the squat produced equal or more trunk muscle activation than stability ball exercises (24). A number of studies comparing stable vs. unstable squats have reported similar or greater activation with the stable squats for the vastus lateralis, biceps femoris (4,21), and trunk muscles (21,33) when using maximal (higher loads with stable squats) rather than similar loads to the unstable squats (7). The squat has also been shown to display greater muscle activation compared with a Smith machine squat (27), leg press, and leg extension exercises (32). Lower lumbar activity has been demonstrated with both stable and unstable, dynamic, and static trunk-forward flexion postures (15). Thus, the stable, more forward inclination of the trunk with the sled push, contributed to the lower erector spinae activity. Hence, this capability for achieving high limb and trunk activation with the free squat is likely one of the reasons for its ubiquitous use.

An alternative reason for the popularity of the squat is the resemblance of the squat position to a number of sport actions. The typical athletic ready position of bilateral hip and knee flexion (mid-range of the squat position) with feet approximately shoulder-width apart is adopted in numerous sports such as football, basketball, volleyball, wrestling, hockey, and others. Hence the initial movement specificity of the closed kinetic chain squat provides a training-specific advantage (9) over other lower-limb resistance exercises such as the open kinetic chain knee extension and flexion or seated leg presses for example. However, the squat movement is stationary and with primarily vertical movement, whereas the sled is a translational movement with the upper body at a forward inclination (oblique position) similar to many sports (i.e., football blocking and tackling positions, hockey skating and checking, wrestling, sprint acceleration phase, and others).

The results of this study indicate that the resisted sled exercise provided large magnitude (phases 1–3, ES: 1.64–1.8) and significantly greater gastrocnemius EMG activity. The rectus femoris, biceps femoris, and TrA/IO differences were nonsignificant. Thus, based on statistical significance, the sled and squat exercises provide relatively similar activation of the rectus femoris, biceps femoris, and TrA/IO. The significantly greater erector spinae activity with the squat ranged from moderate to large magnitudes (phase 1–3: ES: 0.7, 1.26, and 1.44). As suggested by Behm et al. (8), higher erector spinae activation may be necessary to provide adequate stabilization. With the sled exercise, the individual pushed against a stable device that would not fall over, whereas with the squat, the core muscles must control the sway associated with the freely moving squat bar located atop the inverted pendulum-like anatomical structure of the body (17,18). Thus, in terms of eliciting greater lower-back activation, the back squat would be the preferred exercise.

The higher gastrocnemius activity associated with the sled would be attributed to its role as a dynamic motive muscle and the last segment of the kinetic chain attempting to maintain linear momentum. Thus, it was involved with repeatedly overcoming the inertia of the sled load. With the squat, the gastrocnemius as a component of the plantar flexors acts as a stabilizer and contributes to postural integrity. Hence, the gastrocnemius is not primarily involved in helping to move the load vertically but to help control the extent of postural sway. The intensity (activation) of the gastrocnemius to accomplish this task was significantly less than needed to actively move the resistance with the sled.

Although the squat provided a significantly greater knee range of motion (9.5%), instructions to the subjects to perform their concentric and eccentric phases in approximately 1-second segments resulted in a slower angular range of motion (14.7%). These statistically significant but moderate differences would not be expected to be the main impetus for the significant mean EMG differences that were detected. However, according to the force-length relationship (20), muscle force capabilities at the greatest knee flexion position of the squat would elicit lower forces than the greatest sled knee flexion position. In addition, although the knee angles of both the squats and the sled actions provide approximate movement specific range of motions to sprint running knee angles (25), there was lack of sprint velocity specificity with both exercises.

An examination of the correlation between sled and squat weight illustrates that for almost all subjects except one; 2-fold or more of the load (approximately 124% greater load) could be pushed with the sled vs. the load lifted during a squat. The regression equation provides an opportunity to calculate an appropriate squat load from the sled weight. However, the sled load is based on the coefficient friction of the surface (5), and thus higher or lower loads may be used depending on the surface and device friction coefficients. The substantial loads and high muscle activation ensure that both exercises provide an optimal environment for improving lower-body muscle strength.

Although it was difficult to recruit more than 10 experienced resistance-trained individuals, a greater number of subjects may have reduced the relative variability affecting the extent of significant findings. The lack of a telemetry EMG system meant that the subjects had to contend with leads attached to the acquisition system. Although there were no complaints of restrictions, a telemetry system may have provided less restricted movement. The gluteus maximus plays an important role with hip extension and thus is another muscle that should be evaluated in subsequent research.

In conclusion, the sled and squat exercises provided relatively similar EMG activity for the rectus femoris, biceps femoris, and TrA/IO. The squat provided higher activation of the lower erector spinae muscles, whereas the sled had superior activation of the gastrocnemius. In terms of exercise intensity as represented by the extent of muscle activation, both exercises may provide similar training stimuli for the quadriceps and hamstrings. Further studies are encouraged to compare resisted sled pushing exercises with squats and other lower-body exercises with varying loads, velocities, and measures as well as over a prolonged training period.

Practical Applications

Individuals, especially athletes who wish to ensure movement-training specificity for lower-body resistance exercises should consider using the resisted sled exercise. The sled exercise can provide similar muscle activation as the squat for the quadriceps, hamstrings, and TrA/IO with superior gastrocnemius activity. If lower-back strength/activation is a primary objective, then the squat would provide an advantage for this muscle group. Both exercises should be incorporated into a periodized resistance training program.

Acknowledgment

The authors wish to thank Peter Barbour and the other fitness and conditioning specialists of Max Fitness and Conditioning Centre of St. John's Newfoundland for their participation and the use of their resistance equipment, sled, and facilities.

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Keywords:

electromyography; core muscles; fatigue; quadriceps; hamstrings

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