 stances, foot angles, and foot placements. We hypothesized that knee force and electromyographic (EMG) measurements would be significantly different among the squat, LP with high foot placement, and LP with low foot placement while employing these varying foot positions. This information will provide valuable insights to athletes, physicians, therapists, and trainers concerning which exercises and technique variations would be most effective for athletic training or knee rehabilitation.
Ten male lifters experienced in performing the squat and LP served as subjects. All subjects had previously performed the squat and LP regularly in their training regimens and employed varying stances, foot angles, and foot placements throughout a periodization yearly training cycle. The subjects had 10.1 ± 7.7 yr experience performing the squat and 9.0 ± 8.3 yr experience performing the LP. To accurately measure knee forces while performing squat and LP variations, it was important to have subjects who had experience in performing these exercises with varying techniques. The subjects had a mean height of 177.0 ± 8.5 cm, a mean mass of 93.5 ± 14.0 kg, and a mean age of 29.6 ± 6.5 yr. All subjects had no history of knee injuries or knee surgery. Before subjects participated in the study, informed consent was obtained.
A pretest was given to each subject 1 wk before the actual testing session. The experimental protocol was reviewed, and the subjects were given the opportunity to ask questions. During the pretest, the subject’s stance, foot angle, foot placement, and 12-repetition maximum (12 RM) were determined and recorded for the squat and LP. The subjects were first asked to perform the squat and LP with their preferred narrow and wide stances that they normally used in training. Because the subjects’ preferred narrow stance for the squat and LP ranged between 24 and 31 cm when measured between their medial calcanei, and the mean distance between their anterior superior iliac spines (ASIS) was 28.6 ± 2.5 cm, the distance between each subject’s ASIS was used to normalize and define the narrow stance. Because the subjects’ preferred wide stance for the squat and LP ranged between 53 and 65 cm, and twice their mean ASIS distance was 57.2 cm, twice the distance between each subject’s ASIS was used to normalize and define the wide stance. The subjects’ mean foot angles measured during their preferred narrow stance squat and LP were 7.7 ± 7.6° and 7.2 ± 7.1°, respectively, whereas their mean foot angles measured during their preferred wide stance squat and LP were 36.6 ± 8.6° and 32.5 ± 6.3°, respectively. Because most of the subjects employed foot angles that ranged between 0 and 30° of forefoot abduction, the two foot positions defined for all exercises and stances were a) the feet pointing straight ahead, which was defined as 0° of forefoot abduction; and b) 30° of forefoot abduction.
To define high and low foot placements on the LP foot plate, each subject was asked to perform the LP with their preferred high and low foot placements. For their high foot placement, each subject’s preference was to position their feet near the top of the foot plate so that their leg was near parallel with the back pad and near perpendicular to the foot plate at approximately 90–100° knee flexion (Fig. 1, bottom). For their low foot placement, the subjects preference was to move their feet down on the foot plate a mean distance of 20.1 ± 1.4 cm from their high foot placement (Fig. 1, top).
Performing the narrow stance leg press with a low foot placement (top) and a high foot placement (bottom).
Each subject’s 12 RM was determined for both the squat and LP utilizing the most weight they could lift for 12 consecutive repetitions. Because it was predetermined that the same 12 RM weight would be employed for all technique variations within an exercise, each subject’s 12 RM was determined for the squat and LP by using a foot position halfway between their defined narrow and wide stances, halfway between their two defined foot angle positions (i.e., 15°) and halfway between their defined low and high foot placements on the LP foot plate. The mean 12 RM loads that were employed during testing were 133.4 ± 37.0 kg for the squat and 129.1 ± 26.8 kg for the LP.
The subjects reported for testing 1 wk after the pretest. Spherical plastic balls (3.8 cm in diameter) covered with reflective tape were attached to adhesives and positioned over the following bony landmarks: a) medial and lateral malleoli of the left foot, b) upper edges of the medial and lateral tibial plateaus of the left knee, c) posterior aspect of the greater trochanters of the left and right femurs, and d) acromion process of the left shoulder. In addition, a 1-cm2 piece of reflective tape was positioned on the third metatarsal head of the left foot. Four electronically synchronized high-speed charged couple device video cameras were strategically positioned around each subject, and centroid images from the reflective markers were transmitted directly into a motion analysis system (Motion Analysis Corporation, Santa Rosa, CA).
EMG was utilized to quantify muscle activity and help estimate internal muscle forces (11). EMG data from quadriceps, hamstrings, and gastrocnemius musculature were quantified with an eight channel, fixed cable, Noraxon Myosystem 2000 EMG unit (Noraxon U.S., Inc., Scottsdale, AZ). The amplifier bandwidth frequency ranged from 15 to 500 Hz, with an input voltage of 12 VDC at 1.5 A. The input impedance of the amplifier was 20,000 kΩ, and the common-mode rejection ratio was 130 Db. The skin was prepared by shaving, abrading, and cleaning. A model 1089 mk II Checktrode electrode tester (UFI, Morro Bay, CA) was used to test the contact impedance between the electrodes and the skin, with impedance values less than 200 kΩ considered acceptable (11). Most impedance values were less than 10 kΩ.
Blue Sensor (Medicotest Marketing, Inc., Ballwin, MO) disposable surface electrodes (type N-00-S) were used to collect EMG data. These oval-shaped electrodes (22 mm wide and 30 mm long) were placed in pairs along the longitudinal axis of each muscle or muscle group tested, with a center-to-center distance between each electrode of approximately 2–3 cm. One electrode pair was placed on each the following muscles in accordance with procedures from Basmajian and Blumenstein (5) : 1) rectus femoris, 2) vastus lateralis, 3) vastus medialis, 4) lateral hamstrings (biceps femoris), 5) medial hamstrings (semimembranosus/semitendinosus), and 6) gastrocnemius.
A standard 20.5-kg Olympic barbell, disks (Standard Barbell), and a Continental squat rack were used during the squat. Each subject squatted with his left foot on an Advanced Mechanical Technologies, Inc. (AMTI) force platform (Model OR6–6-2000, Advanced Mechanical Technologies, Inc., Watertown, MA) and his right foot on a solid block (Fig. 2). A variable-resistance LP machine (Model MD-117, Body Master, Inc., Rayne, LA) was used during the LP. An AMTI force platform for the left foot and a solid block for the right foot were mounted on a customized LP foot plate (Figs. 1 and 3). The force platform, solid block, and LP foot plate all remained stationary throughout the lift, while the body moved away from the feet.
Performing the wide stance squat with 30° forefoot abduction (top) and with 0° forefoot abduction (bottom).
Performing the wide stance leg press with 30° forefoot abduction (top) and with 0° forefoot abduction (bottom).
EMG, force, and video collection equipment were electronically synchronized, with EMG and force data sampled at 960 Hz and video data sampled at 60 Hz. Because bilateral symmetry was assumed, force, video, and EMG data were collected and analyzed only on the subject’s left side (11). Each subject performed four variations of the squat (Fig. 2): a) narrow stance, 0° forefoot abduction; b) narrow stance, 30° forefoot abduction; c) wide stance, 0° forefoot abduction; and d) wide stance, 30° forefoot abduction. These same four variations were also performed during the LP (Fig. 3), with the feet placed both high and low on the LP foot plate (Fig. 1). Therefore, each subject performed a total of eight LP variations.
The order of performing the four squat variations and the eight LP variations was randomly assigned for each subject. All subjects performed two to three warm-up sets in preparation for testing. For all lifting variations, each subject used their 12 RM weights previously established for the squat and LP. To help the subjects determine their defined stance and foot placement for each exercise variation, a numerical grid was overlaid on the squat and LP force platforms (Figs. 2 and 3). A tester used a goniometer to help the subjects determine 0° and 30° of forefoot abduction. Once the feet were appropriately positioned for the squat and LP, a tester gave a verbal command to begin the exercise. The starting and ending positions for the squat and LP were with the knees in full extension, which was defined as 0° knee angle (KA). From the starting position, the subject flexed their knees to maximum KA (approximately 90–100°) and then extended their knees back to the