Resistance training has been found to have a great number of benefits relating to human performance, such as increased muscular hypertrophy, strength, power, and speed and positive effects on bone mineral density (1). As with other forms of exercise, resistance training eventually leads to fatigue and impaired performance, particularly when the rest periods are short, as seen by Ratamess et al. (22), who found that subjects were able to lift less weight as rest periods were reduced. Not only may fatigue negatively impair performance via the amount of weight lifted or the velocity of movements but fatigue may also alter movement technique and predispose an individual for injury.
Several studies have reported impaired balance and kinesthesia after an exercise protocol that causes fatigue, whether it be from resistance exercise (12), cardiovascular exercise (16,18), or anaerobic exercise such as jumping (17). Johnston et al. (12) induced muscular fatigue to 50% initial strength level with the use of an isokinetic dynamometer at varying speeds (20– 60 cm·s−1) for 10 minutes and observed impaired balance. Lattanzio et al. (16) demonstrated that subjects were less accurate when trying to recreate knee joint position after 3 different cardiovascular exercise fatiguing protocols. Miura et al. (18) also discovered discrepancies in the ability to reproduce knee joint position after cardiovascular exercise but not after resistance exercise. The resistance exercise involved an isokinetic exercise protocol that was not fully described, but a significant reduction in peak torque of knee flexors and extensors was observed after the protocol was followed. The author suggested that perhaps the resistance exercise did not induce dysfunction of the muscle mechanoreceptor. In a more dynamic movement, McNeal et al. (17) indicated changes in jumping technique, including a reduction in knee flexion and ankle dorsiflexion, which progressed toward the end of a fatiguing 60-second repeated jump assessment. These studies show that various types of exercise can lead to functional impairments; however, changes in movement after resistance training has not been studied.
Besides having a negative effect on balance and kinesthesia, fatigue from light load resistance exercise also appears to influence gross movement technique. Trafimow et al. (23) found that as the quadriceps became fatigued during a box lifting task, subjects began using a stoop technique where the hips and spine are more flexed when compared with a proper squat technique. Hagen et al. (5) conducted a similar box lifting study and made the same conclusions that as the quadriceps become fatigued the lifter reduces the range of motion in the legs during the lift and increases the range of motion in the lower trunk.
These changes in joint biomechanics that occur with fatigue may be a concern from an injury perspective. Neumann (19) suggests that abnormal performance of the hip muscles may alter the distribution of forces across the joint's articular surfaces, potentially causing, or at least predisposing degenerative changes. If this can occur at the hip, as Neumann (19) states that it is also reasonable to suggest that this could be a mechanism for injury at any joint affected by changes in movement as a result of fatigue.
More specifically, Hattin et al. (7) observed the effect of fatigue on knee forces and found that during a set of 50 half-squats, anterior-posterior shear force increased significantly during the second half of the set. The authors also found that fatigue increased all articular force components. Therefore, fatigue might increase compressive and shear forces at the joints, suggesting another possible route to injury with excessive fatigue.
Furthermore, the increase in trunk flexion seen during box lifting tasks as previously described by Hagen et al. (5) and Trafimow et al. (23) might also have injurious repercussions because Potvin et al. (21) found that greater trunk flexion significantly increases shear force in the lumbar spine, thus increasing the risk of injury. Also, If fatigue has been shown to make unfavorable changes in technique in box lifting tasks involving 17 kg (5) or 30 kg (23), it is reasonable to suggest that in demanding resistance training protocols using much heavier loads that the risk of injury would be greater.
Fatigue from various forms of exercise has consistently been shown to cause technique changes, which could predispose individuals to injury via various possible mechanisms. Although resistance exercise has been used to produce fatigue, to our knowledge, no studies have examined the fatigue caused from typical resistance training exercises. When resistance exercise is performed in a circuit-style fashion, average blood lactate concentrations of 13.87 mmol·L−1 has been observed in previously untrained subjects (6), suggesting that this type of exercise can be highly fatiguing. The purpose of this study is to examine the effects that fatigue from resistance exercise on joint biomechanics to determine what residual movement effects may exist after the workout.
Experimental Approach to the Problem
This study will attempt to add greater practical application by observing the effects of a workout including typical resistance exercises (squat, bench press, and deadlift) on the technique of an athletic movement (body weight squat). The resistance training workout is designed to induce as much fatigue as possible, thus creating a worst-case scenario in terms of the negative effects that resistance training fatigue could have on exercise technique. We employed a repeated measures design to study the effects of this resistance training workout on exercise technique during a body weight squat.
To analyze the squat movement, the primary dependent variables were knee flexion, trunk flexion, hip flexion, hip rotation, and hip adduction. Proper squat technique has been previously described by the National Strength and Conditioning Association in a position stand (3). Knee flexion was analyzed as the position stand recommends squatting until the thigh is parallel to the ground. Hip and trunk flexion was analyzed because it is also recommended that the trunk stay as close to vertical as possible. Hip rotation and hip adduction were analyzed as both have been associated with injury (8,10). Also, the position statement (3) highlighted the potential for injury from femoral rotation, especially in a flexed position, which lead us to analyze hip rotation. Furthermore, it has been suggested that the athlete descends in a controlled manner during the squat; therefore, the rate of all movements was also analyzed.
Twelve men (age 24 ± 4.2 years, height 173.1 ± 3.6 cm, weight 76.9 ± 7.8 kg, body fat percentage 9.0 ± 2.2) volunteered for the study. All the risks and benefits were explained to the subject before the investigation. Each subject then signed an informed consent document that was approved by the University's institutional review board for us of human subjects in research. All the subjects had at least 6 months of previous resistance training experience and were very familiar with the 3 resistance exercises used in this study: barbell squat (1 repetition maximum [1RM] 125.76 ± 16.82 kg), barbell bench press (1RM 95.27 ± 18.27 kg), and deadlift (1RM 144.70 ± 23.47 kg). All the subjects were free from injury at the time of testing.
Barbell squat and barbell bench press 1RM were measured at least 48 hours after the familiarization session. After another minimum of 48 hours, barbell deadlift 1RM was measured. Finally, after a minimum of 72 hours, a 3-dimensional analysis of 5 body weight squats was conducted before and after a highly fatiguing resistance training workout. Because of limitations of the 3-dimensional analysis equipment, it was not possible to analyze the squats during the workout itself. However, the use of the body weight squat as a movement screen has been previously identified as an effective way to assess an athlete's preparedness for the barbell squat (14).
Before all experimental visits, the subjects were assessed for hydration level using a urine specific gravity (USG) refractometer (Reichert, Lincolnshire, IL, USA). The subjects were excluded from participating in that day's testing unless USG was ≤1.025. If USG >1.025, the subjects were instructed to drink water, and USG was measured repeatedly until the desired level was reached. Familiarization and strength testing were performed at any time of the day that was convenient for the subject, but every subject performed the resistance training protocol in the morning. All the subjects were fed breakfast approximately 2 hours before the protocol to standardize the diet before the workout.
Before a familiarization session, body composition was measured using the Jackson-Pollock method (11) with skinfold calipers (Bodycare, United Kingdom) and body mass was measured using calibrated electronic scales (Ohaus, Florham Park, NJ, USA). The subjects were then familiarized with the warm-up protocol to be used before all experimental visits, which included 5 minutes using a cycle ergometer (Precor, WA, USA ) at resistance level 5 with a speed of 60 rpm. This was followed by dynamic stretches including body weight squats, forward and lateral lunges, knee hugs, quadriceps stretches, and a straight leg march. The 3 lifts used in the workout (barbell back squat, barbell bench press, and barbell deadlift) were demonstrated, and the subjects performed 2 sets of 8–10 repetitions with light loads and 2 minutes of rest between sets.
One-Repetition Maximum Testing
The subjects performed the back squat and bench press 1RM tests on the same day, with the 1RM deadlift test taking place 48 hours later to ensure that the squat and deadlift exercises would not compromise each other. Each 1RM test followed the same protocol. Before testing, the subjects performed the standardized warm-up. A warm-up weight of 50% of preicted 1RM was then performed with 8–10 repetitions. After 3 minutes of rest, a second warm-up set of 80% predicted 1RM was performed. After another 3 minutes of rest, a weight was selected that the subjects believed they could lift once. After each successful lift, the weight was increased, and another lift was attempted. The 1RM was considered the most amount of weight the subject could lift at one time with appropriate technique. Appropriate technique was defined as completing the full range of motion for each lift; for the squat exercise this was considered as the knee flexing until upper leg was parallel to the ground, for the bench press the barbell was required to make contact with the chest and for the deadlift the subject was required to stand fully erect while maintaining their grip on the barbell. The 1RM was reached within 5 attempts for all subjects.
Lower extremity movement during a squat task was assessed before (pretest) and after (posttest) a fatiguing resistance training protocol. Kinematic data for the lower extremity were collected using a 3-dimensional electromagnetic motion analysis system (TrakStar, Ascension Technologies, Inc., Burlington, VT, USA) controlled by Motion Monitor software (Innovative Sports Training, Inc., Chicago, IL, USA) at a sampling rate of 150 Hz. Electromagnetic sensors were placed on each subject over the spinous process of C7, apex of the sacrum, midpoint of the lateral thigh, and shank of the tibia. Sensors of the thigh and tibia were placed on the dominant limb in areas consisting of the least amount of muscle mass to minimize potential artifact induced by muscle contraction. The dominant limb was the limb used. The sensors were affixed to the body with double-sided tape and an elastic wrap (Figure 1).
Once the electromagnetic sensors were attached, the subjects were asked to stand in a neutral posture with their arms relaxed at their sides. The following bony landmarks were digitized using a mobile electromagnetic sensor attached to a stylus (Figure 2): spinous process of T12, medial femoral condyle, lateral femoral condyle, medial malleolus, lateral malleolus, left anterior superior iliac spine, and right anterior superior iliac spine. Digitization of bony landmarks defined the segment end points and joint centers of the lower extremity segments. The ankle and knee joint centers were located at the midpoints between the medial and lateral malleoli and femoral condyles, respectively. The hip joint center was determined by the Bell method (2). Once the subjects were digitized, they were instructed to stand relaxed with their arms at their side allowing the computer to calibrate the subject's neutral position. The subject was then asked to perform 5 body weight squats, with arms extended out in front of the body.
Fatiguing Resistance Training Protocol
The 75% 1RM was used on each of the 3 lifts; back squat, bench press, and deadlift. The subjects began with 10 repetitions of each lift and then reduced the number consecutively by 1 until they reached only 1 repetition. The subjects were instructed to perform all of the prescribed repetitions as quickly as possible. If the prescribed repetitions for a set were not able to perform in one single set (i.e., rest was taken midset), the following set was performed at 5% RM less (i.e., 70%). The subject was instructed to perform the routine with as little load reduction as possible.
At the completion of the workout, the electromagnetic sensors were reattached, and the digitization procedure was repeated as previously described. The subjects then performed 5 more body weight squats with the arms extended out in front of the body.
Measures of Fatigue and Lactate Analysis
Blood lactate concentration was measured immediately before and after the workout. For the resistance exercise protocol session, an indwelling cannula (catheter) was inserted into the antecubital vein. The cannula was kept open with a saline solution. Before each blood draw, 3 ml of blood was extracted to avoid inadvertent saline dilution of the blood sample. Blood was collected, centrifuged at 3,000 rpm, and serum and was separated to sample tubes and was stored at −80° C until subsequent analyses. The liquid lactate assay was performed in duplicate on human serum samples using methods reported by Gutman et al. (4). The assay wavelength was read at 546 nm on a molecular devices VERSAmax tunable microplate reader. The intraassay coefficient of variation (CV) was 2.8%, whereas the interassay CV was 4.4%.
Ratings of perceived exertion (RPEs) were measured using the CR-10 scale (20). The RPE data were obtained after each set of the squat exercise was completed and the average RPE for the entire workout was analyzed.
Data were collected on the last 4 squats and averaged. Pretest squats were compared with posttest squats with the pretest serving as a control. The squats were normalized to 101 time points. Peak angles for each variable were assessed by taking the average maximum value. Angle displacement was measured by subtracting the average minimum value from the average maximum value. To calculate the rate, the time between the minimum and maximum value was divided by the displacement value.
Dependent 2-tailed paired t-tests were performed to compare pretest (before the workout) and posttest (after the workout) values for peak angle for each variable, the displacement for each variable and the rate of change for each of the following variables: knee flexion, trunk flexion, hip flexion, hip rotation, and hip adduction. An a priori alpha level of 0.05 was used maintained for all analyses.
We observed a significant effect on knee flexion peak, displacement and rate; hip flexion peak, displacement and rate; hip adduction peak, displacement and rate; and hip rotation displacement and rate (Table 1) (p ≤ 0.05). These findings indicate that the subjects squatted with less knee flexion, hip flexion, and hip adduction and at a slower rate of knee flexion, hip rotation, hip flexion, and hip adduction (Figures 3–5).
Ratings of Perceived Exertion and Serum Lactate
The mean (±1SD) preexercise concentration of serum lactate was 1.48 ± 0.30 mmol·L−1. After the workout, the postexercise serum lactate was 14.21 ± 2.19 mmol·L−1. The mean (±1SD) RPE for the entire workout with all of the sets combined was 7.61 ± 1.68. To provide some context to the dramatic perceptual fatigue associated with this style of workout, the mean RPE in this study was similar to the mean RPE (i.e., ∼7.5 on the CR-10 scale) of the first study that evaluated such high-intensity metabolic workouts (14). Thus, the lactate and perceptual data give context for the movement conditions that were examined in this study.
The primary findings of this study are that resistance exercise fatigue leads to substantial alterations in exercise technique. Considering that the subjects were given no instruction as to exactly how to perform the squat exercise, the changes seen can be attributed to the fatigue induced by the demanding workout. Reductions in movement after fatigue might be expected considering that the further the subject descends in to the squat, the more the lever arms extend and the greater the torque that is required to ascend back to the standing position. If a subject is fatigued, it would be reasonable to suggest that the subject would perform less movement and reduce the amount of force that they are required to produce during the squat. However, it has been previously stated that when performing the squat exercise that every effort should be made to maintain a consistent pattern of motion to load the muscles in a consistent manner and help prevent injury (3).
Reducing the amount of force that the subject is required to produce might explain the changes in the sagittal plane movements (hip flexion, knee flexion), but there may also be an alternative explanation for the changes in the frontal plane, such as the increase in hip adduction (Figures 3 and 4). Hoy et al. (9) suggested that as hip flexion increases, there is an increase in the contribution of hip adductors to assist in hip extension. However, this study found that despite a decrease in hip flexion, surprisingly, an increase in hip adduction was still seen (Figures 3 and 4). This suggests that even though less hip flexion occurred, the subjects appeared to recruit hip adductors to function as hip extensors and to eccentrically control the descent of the squat and to produce the concentric force to ascend back the neutral position. This would likely occur because of fatigue of the muscles that provided the movement in the sagittal plane during the workout, such as the hamstrings and gluteus maximus during the squat and deadlift.
Besides observing a decrease in hip and knee sagittal plane motion, we also observed a decrease in hip abduction motion. In other words, the subjects performed the squat with a relatively greater hip adducted position after the resistance training exercises. Hip adduction has been discussed as a contributor to the dynamic position of knee valgus (10), which has been shown to prospectively increase risk of knee injury (8). This finding may indicate that subjects were less effective with activating their gluteus medius to resist hip adduction motion and became increasingly reliant on using the hip adductors as hip extensors. This study found increased hip adduction in the body weight squat movement, a much more simple task. As a result, it could be inferred that even greater changes in hip adduction might be seen in a cutting task after a fatiguing resistance training workout, suggesting a possible route to injury.
When comparing the blood lactate concentrations from previous studies, it is clear that the resistance training protocol used in this study caused substantial fatigue. With the use of lighter loads (40–60% 1RM) but similarly short rest, Harber et al. (6) found average blood lactate concentrations of 13.87 mmol·L−1 in previously untrained subjects. This study found comparatively higher blood lactate concentrations of 14.58 mmol·L−1 in a subject population that has extensive resistance training experience. In a study comparing 2 volume controlled workouts, including 5RM loads with 3 minutes of rest and 10RM loads with 1 minute of rest, Kraemer et al. (13) observed blood lactate concentrations of 4.39 and 8.61 mmol·L−1 respectively in men. In another study by Kraemer et al. (14), a workout much more similar to the one used in this study, including 10RM loads with only 30 seconds of rest periods produced blood lactate concentrations of >21 mmol·L−1. In this study, trained bodybuilders were used with the workout designed to mimic their typical workout. Therefore, these results show that well-trained individuals are able to physically tolerate even more demanding workouts than the one used in this study, emphasizing the importance of understanding the technique changes that occur in such workouts.
Other studies have observed the effects of fatigue on technique change in box lifting tasks to observe the potential injury risk (5,23). After a protocol designed to fatigue the quadriceps, both studies found an increase in trunk movement, which has been highlighted as a potential route to injury (21). Although differences were found in variables in several movements (Figures 3–5), this study failed to find any differences in trunk flexion after a fatiguing protocol. This might be explained by the use of a body weight squat as the method of assessment. In the aforementioned studies, the lifting technique was assessed while lifting a box; thus, the subjects were required to carry a load in the front of the body. In a body weight squat task, there is no load for the trunk musculature to tolerate; therefore, it is unsurprising that the trunk movement was not significantly different. In future studies, it would be necessary to analyze the squat movement with a load to improve the comparison to real-life situations.
Although the specific movement effects of fatigue on the body weight squat have not been previously studied, changes in proprioception (16,18), motor control (12) and jumping technique (17) have all been previously shown to result from fatigue. However, these studies all used contrasting fatigue protocols. Lattanzio et al. (16) used a variety of cycling protocols, and Miura et al. (18) and Johnston et al. (12) used an isokinetic dynamometer, and McNeal et al. (17) used 60 seconds of repeated jumping. Also, all these fatigue protocols differing significantly in physical demands, varying from cardiovascular exercise, to strength, to jumping, they do not replicate a program of exercises that would be performed during a typical exercise regimen. By using common exercises such as the squat, bench press, and deadlift and also performing the exercises in conventional repetitions ranges, the protocol used in this study was able to create a fatigue more specific to resistance training and show the effects of resistance training on joint biomechanics in a near worst-case scenario by using such a physically demanding protocol. In addition to providing a more specific fatigue, the use of the body weight squat as the measure to analyze technique changes also provided a way of assessing the effects of fatigue in a real-life situation.
When analyzing the movement of the body weight squat before and after a highly fatiguing resistance exercise protocol, it is clear that many of variables were significantly different. Overall, movements were abbreviated with fatigue, suggesting that individuals are not capable of using the same proprioceptive mechanisms as they would in a nonfatigued state, thus altering the efficiency of the movement and potentially increasing the risk of injury through various mechanisms. As a result, it is important for strength coaches to provide a level of fatigue that is necessary to stimulate strength adaptations, but in turn, the effects that fatigue has on joint biomechanics must be considered to prevent possible injury. Coaches should be aware that fatigue can continue to affect the body after the workout itself, so several considerations should be made with this in mind. For example, care should be taken when scheduling conditioning after resistance training workout as athletes may be at an increased risk of injury. Also, when using complex training that involves combining heavy lifting with explosive exercises, attention should be paid to the quality of movement in the explosive exercise. Finally, in the so-called extreme exercise protocols that combine short rest with complex movements, care should again be taken with regards to exercise technique to ensure injury risk is kept at a minimum.
The authors would like to thank all the subjects for their participation in the study. Also, they thank the undergraduate and graduate research assistants along with their medical staff in the laboratory for their help in the conduct of this study. This study does not reflect the endorsement by the NSCA and is the sole result of an independent research by the team of investigators on the project and no external grant funding supported this investigation.
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