Individuals in recreational and athletic settings frequently participate in concurrent training, the simultaneous performance of resistance, and endurance training. A trend commonly seen is performing 1 bout of aerobic exercise immediately before a bout of resistance exercise in a single exercise session. Some of the research on concurrent training has found negative influences on strength, power, and anaerobic performance (7,8,10), whereas other studies have shown no interference effect on the development of aerobic capacity or strength (2,5).
One factor that may affect the compatibility of concurrent strength and endurance training is the order of exercises within a training session. Presumably, acute effects of concurrent training in a single exercise session may affect the chronic adaptations over weeks or months of training. One study found a reduction in the number of repetitions completed in the back squat when preceded by a high-intensity run on the treadmill (6). Another study used exercise on a cycle ergometer before performing repetitions to failure on the leg press and bench press, with a reduction in the number of repetitions completed for the leg press (15). There was no effect found for the bench press.
Although several previous studies focused on the effect of a lower-body aerobic exercise preceding upper-body and lower-body resistance exercise (6,15), we wanted to examine the origin of fatigue—whether or not it was from a peripheral origin, such as phosphagen depletion or changes in pH, or from a centralized origin, such as changes in the recruitment pattern of different types of motor units. This would help in determining whether the negative effects of previous endurance exercise are limited to the same muscle groups used during that exercise (fatigue is peripheral in nature), or the effects are more general and affect other muscle groups (from a central origin).
To the best of our knowledge, no studies have looked at the effects of using the elliptical machine as a means of aerobic exercise outside of the clinical setting or among the disabled population. The elliptical machine is oftentimes seen in the gym setting, yet research about the elliptical machine as part of an exercise regimen has not had as much attention as the treadmill or the cycle ergometer. The elliptical machine has been shown to place a greater emphasis on activating the quadriceps femoris muscle during exercise (14), and this may particularly affect performance in exercises such as the back squat. Therefore, the purpose of this study was to examine the effects of a single bout of aerobic exercise using the elliptical machine on muscle activation, using electromyography (EMG), and the number of repetitions performed during upper- and lower-body resistance exercises.
Experimental Approach to the Problem
College-aged men with resistance training experience performed resistance exercise workout with and without preceding lower-body aerobic exercise on an elliptical machine. Both upper-body (bench press) and lower-body (back squat) exercises were performed to determine whether the effects were specific to the muscles used during the aerobic exercise. Electromyographic data were collected and analyzed to determine whether any negative effects of aerobic exercise were of a central or peripheral origin. For example, a reduction in the amplitude of the surface EMG signal may be interpreted as a reduction in motor unit recruitment or firing rate, and therefore would be an indicator of central fatigue (4).
Fourteen male subjects (mean age = 24.1 ± 2.3 years, height = 180.8 ± 6.9 cm, body mass = 91.9 ± 16.4 kg) with experienced resistance training volunteered for this study. Resistance training experience was defined as having performed the back squat and bench press at least once per week for the past 6 months. Subjects also needed to have performed aerobic exercise at least 2 times per week in the past 6 months. Subject's familiarity with elliptical exercise varied. A repeated measures design was used, where every subject participated in each treatment condition. Subjects had at least 3, but not more than 7, days between experimental trials to provide sufficient recovery. Subjects were instructed not to participate in vigorous physical activity for at least 36 hours before the time of the trials. Testing was conducted in both morning and afternoon sessions depending on subjects' availability, but the time of day was held constant for any particular subject, ±1 hour. Subjects were also instructed to have at least 8 hours of sleep the night before an experimental trial to help reduce the effects of fatigue. Each participant was also asked to drink 1 L of water the night before and 1/2 L of water on the day of testing, in addition to their normal water intake to assure ample hydration before testing. The study was approved by the University Institutional Review Board, and all participants signed an informed consent form before participating in the study.
Familiarization and One Repetition Maximum Testing
The 1 repetition maximum (1RM) of the back squat and then bench press were assessed. All testing was conducted using the technique and procedures recommended by the National Strength and Conditioning Association (1). Before attempting a 1RM, a warm-up protocol was given. The warm-up consisted of 8–10 repetitions at 50% of the estimated 1RM, 2 minutes of rest, 3–5 repetitions at 75% of the estimated 1RM, 2 minutes of rest, 1–2 repetitions at 90% of the estimated 1RM, followed by 3–5 minutes of rest. The subject then attempted the estimated 1RM. With each successive attempt, the subject rested 3–5 minutes before attempting an increased load. The load was progressively increased until the subject was no longer able to complete the repetition with proper technique. During the back squat 1RM testing, the subject fitted a Safety Squat (Bigger Faster Stronger, Salt Lake City, UT, USA) on their left vastus lateralis to monitor the thigh coming down parallel to the ground. A beep sounded off once the Safety Squat was down to parallel, and that was when the subject came up to the starting position. During 1RM testing, subjects were fitted before exercise with EMG electrodes on the upper body (pectoralis major), and lower body (vastus lateralis and rectus femoris). After the 1RM testing, subjects were familiarized with the elliptical machine to make sure that the subjects were aware of the function and how to operate the machine.
Aerobic Exercise Protocol
Subjects used the Elliptical Fitness Cross Trainer EFX 544 (Precor) for the aerobic exercise. During the familiarization visit, subjects practiced using the elliptical machine until proper technique was achieved. Subjects used the function with no ramp incline and with a set resistance at level 4. Subjects were able to manipulate their heart rate (HR) by increasing or decreasing the stride frequency and were asked to do so to maintain a HR of 70% of their age-predicted maximum HR. This intensity was chosen because it is consistent with ACSM guidelines for the development of aerobic fitness (16) and is widely used in fitness and athletic settings. Maximum HR was estimated using the formula 220—age. A Polar HR monitor (Polar FS1, Poloar Electro Oy, Kempele, Finland) was fitted to each subject before elliptical exercise. Subjects used the elliptical machine for 30 minutes with their hands on the rail for balance only. This way, the upper body was not fatigued, and the exercise was focused mainly on the lower body. Afterwards, a 10-minute seated break was given. Electromyography electrodes were fitted on the subject during this time.
Resistance Exercise Protocol
Before the resistance exercise protocol, subjects performed a warm-up of 8–10 repetitions with 50% of their 1RM, followed by a 2-minute rest. After the rest, another warm-up set of 6–8 repetitions was completed at 60% of their 1RM. Two minutes of rest were given, then subjects proceeded to perform 3 sets to failure at 75% of their 1RM. Once the subject was unable to complete an additional repetition, the weights were reracked, and the 2-minute rest period was commenced.
Each participant's skin was prepared before the placement of the EMG electrodes. Hair at the site of electrode placement was shaved off, and the skin was rubbed with an abrasive pad to remove dead skin cells. The area was then cleaned with isopropyl alcohol. The EMG data were collected and stored on a personal computer (Dell Latitude D610; Dell, Round Rock, TX, USA). Three separate bipolar (3.5 cm center to center) surface electrode (BIOPAC EL500 silver-silver chloride; BIOPAC Systems, Inc., Goleta, CA, USA) arrangements were placed over the longitudinal axes of the vastus lateralis and rectus femoris (squat visits) or the pectoralis major (bench press visits) (9). Electrodes for the vastus lateralis were placed on the most distal segment of the vastus lateralis. Electrodes for the rectus femoris were placed halfway between the inguinal crease and the top of the patella. Electrodes for the pectoralis major were placed horizontally, 4 cm medial to the axial fold. All measurements were taken from the right side of the body. The EMG signals were pre-amplified (gain 1,000×) using a differential amplifier (bandwidth = 1–500 Hz, EMG 100C; BIOPAC Systems, Inc., Santa Barbara, CA, USA). Surface EMG was used to measure muscle activation during the entire range of motion for each repetition in every set for both the back squat and the bench press.
The EMG signals were band-pass filtered (fourth-order Butterworth filter) at 10–500 Hz. The amplitude of the signals was expressed as root mean square values. All analyses were performed with custom programs written with LabVIEW software (version 7.1; National Instruments, Austin, TX, USA). Before statistical analysis, each participant's EMG amplitude values were normalized to their highest recorded values during the 1RM testing.
A 3-way (exercise [bench press, squat] × condition [elliptical, no elliptical] × set [1, 2, 3]) repeated measures analysis of variance (ANOVA) was used to compare the number of repetitions completed. To examine the EMG data for the pectoralis major, a 3-way (condition [elliptical, no elliptical] × set [1, 2, 3] × rep [first, final]) repeated measures ANOVA was used. For the vastus lateralis and the rectus femoris muscles of the leg, a 4-way (condition [elliptical, no elliptical] × muscle [vastus lateralis, rectus femoris] × set [1, 2, 3] × repetition [first, final]) repeated measures ANOVA was used. When appropriate, follow-up analyses for the ANOVAs included Tukey's post hoc comparisons or paired t-tests. An alpha of 0.05 was used to determine statistical significance.
There was no significant 3-way interaction (exercise × condition × set). There was, however, a significant (p ≤ 0.05) 2-way interaction (condition × exercise). Previous elliptical exercise did not affect the number of repetitions completed for the bench press (p > 0.05; Figure 1). However, the number of repetitions completed in the back squat after the elliptical exercise was significantly (p ≤ 0.05) less than when the elliptical exercise did not precede the squat exercise.
Electromyographic Amplitude Bench Press
There was no significant 3-way (condition × set × repetition) interaction. There was, however, a significant 2-way interaction (set × repetition). Normalized EMG amplitude values increased significantly (p ≤ 0.05) from the first to last repetition for each of the first 2 sets. There was no significant (p > 0.05) increase in normalized EMG amplitude between the first and last repetition for the third set (Figure 2).
Electromyographic Amplitude Back Squat
There were no significant 4-way (condition × muscle × set × repetition) or 3-way interactions (p > 0.05). There was, however, a significant 2-way interaction (set × repetition). Normalized EMG amplitude values increased significantly (p ≤ 0.05) from the first to last repetition for each of the first 2 sets. There was no significant (p > 0.05) increase in normalized EMG amplitude between the first and last repetition for the third set (Figure 3).
The primary finding of this study was a reduction in the total number of repetitions completed in the back squat exercise (elliptical and back squat, 8.2 ± 1.7 repetitions; back squat only, 9.5 ± 1.7 repetitions) when preceded by 30 minutes of aerobic exercise on an elliptical machine (using the lower body only). This could be attributable to peripheral muscle fatigue, as the number of repetitions completed in the bench press trials was unaffected (elliptical and bench press, 8.5 ± 1.7 repetitions; bench press 8.7 ± 2.1 repetitions) by previous performance of lower-body aerobic exercise. In this study, subjects were asked to refrain from using their upper bodies during the elliptical exercise to isolate the lower body. This was done to see whether the lower-body aerobic exercise had an effect on the subsequent upper-body workout. Other research studies have found that performing aerobic exercise before resistance exercise reduced the number of repetitions completed for the resistance exercise (11–13). However, it should be noted that these investigators did not examine the effects of a lower-body aerobic exercise task on the upper-body musculature.
De Souza et al. (6) investigated the effects of lower-body exercise on upper- and lower-body resistance exercise and found that a high-intensity run reduced the number of repetitions completed for a leg press protocol performed to failure but not a bench press protocol. However, a 5-km run had no effect on the repetitions completed for the lower or upper body. The authors attributed the decrease in leg strength endurance after the higher intensity intermittent runs, as being because of fast twitch motor units activated during the run. Because fast twitch motor units are purported to fatigue more easily, they concluded that the 10 minutes of rest allowed in between exercises might not have been enough time to allow the muscles to recover. However, in this study, the fatigue was present (as shown by the reduced number of repetitions for the lower body) at a lower intensity (70% of age-predicted HR). This may have been because of the greater activation of the rectus femoris while using the elliptical machine. For example, a previous study by Prosser et al. (14) found that there was greater activation in the rectus femoris muscle when using the elliptical compared with the treadmill. Because the rectus femoris is one of the primary muscles used while performing squats, it is possible that the greater activation of the rectus femoris muscle during this study while using the elliptical led to the decrease in the number of repetitions completed, despite a lower endurance intensity than in previous studies.
In this study, the bench press trial was performed 10 minutes after the aerobic exercise. Other studies included a lower-body resistance exercise in between the lower-body aerobic exercise and upper-body resistance exercise (6,15). Sporer and Wenger (15) examined the effects of aerobic exercise on strength performance after various periods of recovery. After performing a bout of aerobic exercise, subjects performed 4 sets of leg press immediately followed by 4 sets of bench press at 75% 1RM. The findings of this study showed that the total number of repetitions in the leg press was diminished 4 and 8 hours after aerobic exercise. Although Sporer and Wenger (15) did not use EMG to determine muscle activation, there seemed to be no signs of central fatigue involving the upper body because the number of bench press repetitions completed were the same across all trials. The execution of the leg press in between the aerobic exercise and bench press exercise may potentially have diminished any fatiguing effects for the upper body.
In this study, surface EMG was used to measure the extent of muscle activation. Muscle activation is a function of the nervous system and is reflected by the amplitude of the surface EMG signal. The results of this study indicated that EMG responses were the same for the back squat and bench press exercises. That is, normalized EMG amplitude increased between the first and last repetitions for the first 2 sets of exercise performed for both the squat (set 1: first repetition = 0 0.99 ± 0.16, final repetition = 1.24 ± 0.20; set 2: first repetition = 1.04 ± 0.16, final repetition = 1.19 ± 0.18) and bench press (set 1: first repetition = 0 0.63 ± 0.05, final repetition = 1.05 ± 0.07; set 2: first repetition = 0.75 ± 0.04, final repetition = 1.08 ± 0.08). There was no significant (p > 0.05) difference in EMG amplitude between the first and final repetitions in the third set for the squat (set 3: first repetition = 1.09 ± 0.13, final repetition = 1.18 ± 0.16) or bench press (set 3: first repetition = 0.82 ± 0.06, final repetition = 1.02 ± 0.07). These results suggest that muscle activation was not affected by first performing a lower-body aerobic exercise. An increase in EMG amplitude during a fatiguing work task likely represents progressive recruitment of motor units, as previously recruited motor units fatigue (4).
In this study, there was no difference in EMG response between conditions. This suggests a peripheral origin of fatigue because the amplitude of the EMG signal reflects the recruitment pattern of motor units by the central nervous system. However, others have found evidence of central fatigue mechanisms resulting from acute bouts of aerobic exercise. For example, Bentley et al. (3) examine muscle activation of the knee extensors after a high-intensity bout of endurance exercise. Trained cyclists performed maximum voluntary contraction force for the knee extensors, and EMG and force were significantly reduced immediately post and 6 hours after exercise. The authors of this study suggested that the reductions in torque were both from central and peripheral mechanisms. This difference in findings compared with this study might be attributed to the higher intensity aerobic exercise used by Bentley et al., which included 30 minutes of cycling at 80% V[Combining Dot Above]O2max, followed by 4 bouts of 60-second exercise at 120% of V[Combining Dot Above]O2max.
These results suggest that to optimize the quality of a lower-body resistance-training workout, the workout should not be preceded by lower-body aerobic exercise. The quality of an upper-body resistance workout is unaffected if preceded by a lower-body aerobic exercise. A strength and conditioning practitioner or fitness professional might consider performing resistance exercise before aerobic exercise, if the goal is to maximize the quality of the resistance-training workout, or even performing aerobic exercise on a separate day from resistance exercise.
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