Free weight and machine exercises that engage the same major muscle groups and have similar lower-body joint actions, such as the back squat and the leg press exercises, are often used interchangeably. However, such exercises are not necessarily equivalent (34). For example, the primary movers are the same for the squat and the leg press exercise (e.g., quadriceps and gluteus), but the squat is a closed kinetic chain exercise, whereas the leg press is an open kinetic chain exercise. More importantly, in contrast to the leg press, the squat requires the individual to balance on two feet and thus requires substantial engagement of stabilizing and core musculature (e.g., abdominals and back) (3). Research on muscle activation has also shown that, even among the primary movers, free weight exercise results in greater muscle activation than machine exercise (29). Thus, free weight exercises seem to induce a larger overall muscle mass involvement than “similar” machine weight exercises. This consideration could be important for subsequent physiological adaptations (i.e., musculoskeletal strength and hypertrophy), as exercises involving greater muscle activation or volume of work can elicit a larger acute hormonal response than exercises using smaller quantities of muscle mass (18).
Resistance exercise can provide a potent stimulus for acute increases in concentrations of circulating neuroendocrine factors including testosterone (T), growth hormone (GH), and cortisol (C). Although resistance exercise often induces increases in these circulating anabolic and catabolic hormones (15,16,21,22,30), not all resistance exercise protocols induce an acute hormonal response (6,27,30). The appearance and magnitude of an acute resistance exercise-induced increase in hormones depends, in large part, on the selection among the acute program variables: intensity, sets, order of exercise, rest period duration, and exercise selection (16,20,30,33). The independent and combined (i.e., volume) effects of intensity and sets and the effect of rest period duration on the neuroendocrine response have received considerable attention. However, little research exists on the effects of resistance exercise selection on the acute hormonal response, especially with respect to machine vs. free weight exercise. In the only study that seems to have been published on this topic, Kang et al. (12) examined the GH release patterns after the back squat and the leg press performed using 3 different intensity/repetition schemes (3-, 10-, and 25-RM). In that study, the 10-RM resulted in the greatest increase in GH after performing the leg press, whereas 25-RM produced the greatest increase in GH for the squat. Kang’ study did not directly compare the effect of the exercise modality, but the greatest GH response to the squat (25-RM) was ∼10% larger than the greatest response for the leg press (10-RM).
The acute hormonal increase after resistance exercise can modulate training adaptations (28); thus, investigations of appearance and magnitude of the acute hormonal response in response to the different selection among acute program variables are important. Although the neuroendocrine response to manipulation of most of the acute program variables (e.g., intensity and sets) has received substantial attention, no study seems to have directly compared the effect of the exercise modality using similar multijoint free weight and machine weight exercises on the acute hormonal response. This gap in the literature related to the physiological effects of commonly interchanged exercises (e.g., squat and leg press) presents a potential dilemma for strength and conditioning practitioners in terms of effective resistance exercise program design. If a difference in the acute hormonal response exists between free weight and machine weight exercises, substitution of exercise, such as the leg press for the squat, might lead to suboptimal resistance training adaptations. Therefore, the purpose of this study was to examine the effect of resistance exercise selection (free weight vs. machine weight involving similar multijoint movements) on the acute hormonal response to exercise.
Experimental Approach to the Problem
This investigation used a within-participants design to compare the acute hormonal responses to the back squat (free weight) and leg press (machine weight) exercises to determine the effect of exercise selection (free weight vs. machine weight involving similar multijoint movements). The back squat and the leg press exercises are well suited for a comparison of exercise mode since they involve similar lower-body multijoint movements, engage a large muscle mass, can elicit a substantial hormonal response, and are often used interchangeably in training program design and implementation. Participants reported to the laboratory for 1 visit per week for 4 consecutive weeks. During the first and the second visits, participants were familiarized with the squat and leg press exercises, and their 1-repetition maximum (1-RM) was determined (only 1 exercise mode was performed per visit). The order of exercise modes was assigned using a randomized and counterbalanced design. For the third and fourth visits, participants performed an acute heavy resistance exercise test (AHRET) designed to elicit a large acute hormonal response with the squat or leg press exercise. The AHRET consisted of 6 sets of 10 repetitions of the squat or leg press with an initial load of 80% of 1-RM and 2 minutes of rest between sets. The order of exercise mode for visits 1 and 2 was repeated for visits 3 and 4. Fasted blood samples were collected before the warm-up (PRE), immediately postexercise (IP), and 15 minutes (P15) and 30 minutes (P30) into recovery from exercise, and analyzed for T, GH, C, and lactate concentrations.
Ten apparently healthy, recreationally resistance trained men (21–31 years, 25 ± 3 years, 179 ± 7 cm, 84.2 ± 10.5 kg) completed this study. Participants were resistance trained for at least 6 months (≥2 times a week) before the study and performed the squat exercise on a regular basis. Participants were screened for potential confounding medical conditions using a medical history questionnaire. Potential participants were excluded if they had a musculoskeletal, cardiovascular, metabolic, endocrine, or neurological disorder that could inhibit involvement in exercise; previous orthopedic injuries that limited the range of motion of the hip, knee, or ankle joint; back injuries, such as herniated discs; steroid use; or inability to meet the demands of the exercise protocol (e.g., inability to attain proper squat depth with good technique or inability to perform the leg press without the hips lifting off the seat). Before inclusion in the study, participants were informed of the procedures and risks associated with the study and subsequently provided written informed consent to participate. The study was approved by the University Institutional Review Board.
Visits 1 and 2: Anthropometric Measurements, Familiarization, and One-Repetition Maximum Determination
Upon arrival at the laboratory, participants' height and weight were measured, and their body composition was assessed using dual-energy X-ray absorptiometry (DXA) (Lunar Prodigy General Electric Company, Madison, WI, USA). Participants wore only light athletic clothing and no shoes for these measurements. Height and body composition were measured only in visit 1. Participants then performed a standardized dynamic warm up (lunges, heel kicks, high knees, body weight squats, and shoulder circles) and were familiarized with the proper technique for performing the back squat or leg press exercise following the National Strength and Conditioning Association's guidelines (1). The specific leg press exercise and device are presented in Figure 1. The participant performed the squat with feet placed at approximately shoulder width and descended until the femur was parallel to the ground (full squat position). Vertical barbell displacement was measured from the fully standing position to the full squat position. To ensure that the participants performed identical displacement for each repetition in the squat, the participant descended until a researcher observed proper squat depth and gave the participant a command to return to the standing position. The leg press exercise was performed using a standard hip sled machine with the sled set on tracks at a 45° angle to the ground (Body Masters, Rayne, LA, USA). The participant lowered the sled until 90° angle of knee flexion was attained (at which point the thighs were parallel with the foot platform), and then extended their knees to full extension (∼180°). Sled displacement at 45° was recorded and marked on the leg press; the vertical sled displacement was determined using vector decomposition of the 45° sled displacement. To ensure participants performed identical displacement for each repetition in the leg press, the participant lowered the sled until a researcher observed proper sled displacement and gave the participant a command to return to the full extension position. Once participants demonstrated proper technique in the exercise, their 1-RM was measured using the methods described by Kraemer (13).
Visits 3 and 4: Acute Heavy Resistance Exercise Test
Participants reported to the laboratory in the morning (0800–1000 hours) after a 12-hour fast and 48 hours without exercise; the arrival time was the same for both visits for a particular participant. Hydration status was measured using urine refractometry (Reichert, Depew, NY, USA), and participants with urine specific gravity ≥1.020 g·mL−1 were provided cold water to drink. To aid participants reporting in a euhydrated state (urine specific gravity <1.020g·mL−1), they were instructed to drink 0.5–1.0 L of water the night before and the morning of visits 3 and 4. Once hydration status was addressed, a teflon catheter was inserted into an antecubital forearm vein and kept patent with a 0.9% sodium chloride saline solution. The catheter remained in the vein for the duration of the visit for rapid collection of blood samples.
The standardized dynamic warm-up (same as for visits 1 and 2) was performed shortly after the insertion of the catheter. After the warm up, participants performed 6 sets of 10 repetitions of the squat or leg press with 2 minutes of rest between sets. The initial load for both exercises was set at 80% of their 1-RM. If participants could not complete 10 repetitions on their own, they were assisted in completing the remaining repetitions by a spotter (side spotters for the leg press and a back spotter for the squat). If a participant was assisted in completing repetitions, the load was subsequently lightened with the aim of allowing for the completion of 10 repetitions in the following set.
Blood samples were obtained at PRE, IP, P15, and P30. Before blood collection, 3 mL of blood was extracted and discarded to avoid inadvertent saline dilution of the blood sample. Heart rate (HR) was measured continuously using a HR monitor (Polar, Lake Success, NY, USA), and the values were recorded at PRE, immediately after each set, and at P15 and P30. Rating of perceived exertion (RPE) was recorded at PRE and immediately after each set using the 0–10 category-ratio scale of perceived exertion (25).
Calculation of Work Performed
For each exercise, the (positive) work performed (“external work”) during each set was calculated based on the external load (e.g., the mass of bar/sled and weight plates), as (external mass × 9.81 m·s−2 × vertical displacement × number of repetitions). Conventionally, the work in resistance exercise is calculated using the preceding equation and using only the external load. However, in this study, because the mass of the body segments moved differed substantially between the 2 exercises, the “total work” performed was also calculated, using the total load (external mass + body mass moved) moved during the exercise, as ([external mass + body mass moved] × 9.81 m·s−2 × vertical displacement × number of repetitions). Body mass above the knee (determined by DXA) was used in total work calculations for the squat, whereas leg mass (determined by DXA) was used in total work calculations for the leg press. The vertical displacement for the leg press was calculated as (sled displacement × Cos 45°). Work performed for each AHRET was calculated as the sum of the work performed in each of the 6 sets of that session.
Additional Experimental Controls
Participants were asked to refrain from ingesting alcohol and performing resistance or exhaustive exercise for the 48 hours before each visit. Participants were also advised to avoid engaging in sexual activity the night before or the morning of testing. For 3 days before visit 3, the participants recorded their nutritional intake (e.g., all food and beverages consumed) and repeated the same diet for visit 4. The participants were instructed to have consistent sleep the night before both visits. All participants attested that they conformed to these guidelines. Visits 3 and 4 were performed at the same time of day for each participant to minimize any potential confounding effects of circadian rhythms.
Blood samples were allowed to clot at room temperature and were subsequently centrifuged at 1,500g. The resultant serum was divided into several aliquots and stored at −80° C until analysis. Samples were analyzed for serum T, GH, and C using commercially available enzyme-linked immunosorbent assays (Alpco, Salem, NH, USA). The sensitivity and coefficient of variance for each assay was T, 0.08 nmol·L−1 and 7%; GH, 0.5 μg·L−1 and 12%; and C, 11.0 nmol·L−1 and 6%, respectively. Hemoglobin and blood lactate concentrations were measured in duplicate using automated analyzers (Hemocue, Angelholm, Sweden and Nova Biomedical, Waltham, MA, USA, respectively). Hematocrit was measured in triplicate after centrifugation of heparinized microhematocrit capillary tubes (Fisherbrand, Pittsburgh, PA, USA). From hemoglobin and hematocrit values, plasma volume shifts were calculated using the methods of Dill and Costill (7). Compared with PRE, plasma volume was decreased at IP (squat: −14.3 ± 5.1%; leg press: −11.9 ± 9.6%) and gradually returned toward baseline at P15 (squat: −5.1 ± 5.5%; leg press: −2.3 ± 12.9%) and P30 (squat: −2.5 ± 4.8%; leg press: −0.3 ± 14.1%). Concentrations of circulating hormones were not corrected for plasma volume changes.
Data for each variable were analyzed using 2-way analysis of variances (exercise mode × time point) with repeated measures on both factors. Where appropriate (significant analysis of variance), Fisher's LSD post hoc was used for specific predetermined pair-wise comparisons. Data for C were log10 transformed before statistical analysis. For GH and C, the assumption for sphericity was not met; therefore, the degrees of freedom were adjusted. For hormonal data, the area under the curve was calculated using the trapezoidal method and compared between modes using a paired means t-test. Alpha level was set at p ≤ 0.05. All statistical analyses were performed using SPSS version 20 (SPSS, Chicago, IL, USA). Data are presented as mean ± SD unless otherwise indicated.
The T concentrations are shown in Figure 2. A significant main effect was found for time. In both the squat and leg press, T concentrations were significantly increased (p ≤ 0.05) at IP, P15, and P30 compared with PRE. A significant main effect was found for mode; T was higher for the squat than for the leg press. There was a significant interaction between mode and quadratic trend for time; post hoc analysis revealed that T was significantly higher for the squat than for the leg press at IP. The area under the curve was significantly greater for the squat (1527 ± 258 nmol·L−1·min) than for the leg press (1362 ± 143 nmol·L−1·min).
The GH concentrations are shown in Figure 3. A significant main effect was found for time. Growth hormone was significantly increased at IP, P15, and P30 compared with PRE. A significant main effect was also found for mode; GH was higher for the squat compared with the leg press. After adjusting the degrees of freedom, there was a significant exercise mode × time interaction effect; post hoc analysis revealed that GH was significantly higher for the squat than for the leg press at IP, P15, and P30. The area under the curve was significantly greater for the squat (433 ± 315 μg·L−1·min) than for the leg press (163 ± 122 μg·L−1·min).
The C concentrations are shown in Figure 4. A significant main effect of time was found for C; C concentrations were significantly increased at IP, P15, and P30 compared with PRE. A significant main effect was also found for mode; C was higher for the squat compared with the leg press. If sphericity was assumed, there would be a significant (exercise mode × time) interaction effect; however, this assumption was not met. Therefore, the degrees of freedom were adjusted after which only a statistical trend (p = 0.07–0.10) for an (exercise mode × time) interaction effect was found. The area under the curve was significantly greater for the squat (34,344 ± 10,179 nmol·L−1·min) than for the leg press (29,193 ± 8456 nmol·L−1·min).
Lactate concentrations are shown in Table 1. A significant main effect was found for time. Lactate concentrations were significantly increased at IP, P15, and P30 compared with PRE. A significant main effect was also found for mode; lactate was higher for the squat than for the leg press. There was a significant (exercise mode × time) interaction effect; post hoc analysis revealed that lactate was significantly higher for the squat than for the leg press at IP, P15, and P30.
A significant main effect of time was found for HR (Table 1). The HR was significantly increased at each time point following the onset of exercise as compared with PRE. A significant main effect was also found for mode; HR was higher for the squat than for the leg press. There was a significant (exercise mode × time) interaction effect; post hoc analysis revealed that HR was significantly higher for the squat than for the leg press for each time point with the exception of PRE and immediately following set 1.
For RPE (Table 1), a significant main effect was found for time. Rating of perceived exertion was significantly increased following each set compared with PRE. There was no main effect for mode. There was a significant (exercise mode × time) interaction effect; post hoc analysis revealed that RPE was significantly higher for the squat than for the leg press following set 4.
Results for work are presented in Table 2. The external work (calculated using external load only) performed did not differ between exercise modes; however, when the body mass moved during each exercise (squat: total body mass—shank mass; leg press: leg mass) was included in the load (added to external load), the resultant total work was significantly greater for the squat compared with the leg press exercise.
This investigation seems to be the first study to directly compare the T, GH, and C response with similar multijoint free and machine weight resistance exercise: the free weight squat and machine weight leg press. The primary finding of this investigation was that differences exist between the acute hormonal and metabolic (based on HR and lactate) responses to the 2 exercise modes; in general, the squat induced greater hormonal and metabolic responses than the leg press. The total external work performed in the 2 exercises was similar, but when the work of moving body mass during each exercise mode was included in the calculation, the total work was greater for the squat exercise. Greater total work in the squat exercise might explain, at least in part, the observed differences in the acute hormonal and metabolic response to exercise between modes. Regardless, the differences found in acute physiological responses between free weight and machine weight exercise in this investigation provide unique insight into the utility of their inclusion in resistance exercise programs.
Testosterone is important for muscle growth and strength because of its anabolic (23) and anticatabolic effects (5). Kvorning et al. (21) found that when T was reduced to castrate levels in young men, isometric strength did not increase after resistance exercise training, and muscle hypertrophy was attenuated. In this study, both the squat and leg press acutely increased T. This finding is consistent with previous investigations, which have reported that a high volume bout of heavy resistance exercise acutely increases circulating T (10,18,22,30,32). In this study, the T concentration was significantly higher after performing the squat than after performing the leg press. The magnitude of increase in T concentration is affected by the muscle mass involved during exercise (18) and by the total work performed (31). The squat incorporates more muscle mass than the leg press (34), in part because the squat involves the activation of a greater amount of upper body musculature than the leg press. Furthermore, the partially flexed starting hip position in the leg press allows for less hip range of motion and thus, less displacement of the mass that could lead to less work being performed per repetition. In this study, the squat involved greater total work performed (e.g., when work was calculated including body mass) than the leg press. Combined, these factors (greater muscle mass involvement and more work performed) might explain the greater acute T response for squat than for the leg press exercise.
Growth hormone is a complex superfamily of polypeptides and is an anabolic hormone that supports muscular and skeletal growths (15). The most abundant form of GH is the 22 kDa isoform, although over 100 isoforms of GH exist (2). The biological activities of many of these non-22 kDa GH isoforms (e.g., GH aggregates such as dimers) appear to be greater than that of the 22 kDa GH isoform; however, the acute response of such GH aggregates to resistance exercise has not been fully elucidated. In accordance with most previous investigations (14,19,22,26,28), this study investigated only the immunoreactive 22 kDa isoform of GH. Thus, the current findings for GH do not necessarily represent the entire acute GH family response to resistance exercise modes. In this study, 22 kDa GH concentration was significantly elevated after both the modes of exercise; the increase was significantly greater for the squat exercise. As in previous studies (14), the increase in GH followed a pattern similar to that of lactate. Elias et al. (8) found that the anterior pituitary secretes GH in response to the reduction in pH that is associated with increased lactate concentration in the blood. Increases in circulating lactate and GH have also been observed when the rest period duration is abbreviated (16). This suggests that increases in GH may be dependent on the metabolic demand of the exercise. In the present study, the greater increase in GH after performing the squat was likely influenced by the larger work performed and the greater metabolic demand (exemplified by the greater HR and blood lactate concentration) (14).
It is also likely that the greater amount of work performed in the squat affected the GH response. Total volume (work) influences the acute GH response to resistance exercise, and high volume protocols that use a large muscle mass are effective for acutely increasing circulating GH concentrations (10,20,22,24,30). Craig and Kang (4) reported greater increases in GH after performing multiple sets of squats than for a single set of squats using the same intensity and suggested that total work was more important than intensity for stimulating a GH response. However, a greater acute GH response to higher volumes of total work has not been demonstrated in all studies. Kang et al. (12) found a ∼33% larger GH concentration 16 minutes postexercise for the leg press than for the squat when the intensity was set at 10-RM, even though the squat protocol involved more external work (external mass × 9.81 m·s−2 × vertical displacement × number of repetitions).
Cortisol can modulate muscle metabolism by inhibiting components of the AKT-mTor pathway and subsequently inhibiting protein synthesis (31) and is important in blood glucose mobilization and preservation. After a bout of heavy resistance exercise, C increases acutely (11,17). Like T and GH, the magnitude of the C response is related to the total volume of work performed; a greater volume of work produces a greater acute increase in circulating C (9,10). As expected, in this study, exercise significantly increased C at all postexercise time points. Importantly, it was found that C was significantly greater for the squat than for the leg press. The greater acute C response for the squat than for the leg press exercise could be explained by the greater total work performed (30,35), muscle mass involved (28), and metabolic demand (9,17).
Consistent with previous findings (12), HR was elevated during the squat and leg press exercise tests; however, the increase was greater for the squat than the leg press. Similarly, lactate was elevated following both exercise protocols, but the magnitude of increase was greater after performing the squat. The greater exercise-induced HR and lactate response suggests a greater metabolic demand in the squat than the leg press and is consistent with the finding of greater total work performed in the squat. It is possible that the lower HR in the leg press than the squat might be, at least in part, because of the partially supine (∼30° incline) body position during exercise. In this position, venous return might not be hindered by gravity to the extent it is hindered in the upright standing (e.g., squat) position, resulting in increased stroke volume and thus, a reduced HR for the same cardiac output. Interestingly, despite that the squat resulted in a greater physiological demand (HR and lactate) and involved more total work than the leg press, the participants reported similar RPEs for both exercise modes. From this, it seems that there is a divergence between perceived and actual demand of the exercise across the 2 exercise modes.
Our study presents a few limitations that might reduce the transferability of the current findings to comparisons between other free weight and machine exercises. First, the amount of total work was somewhat crudely estimated. During exercises such as the squat, the individual must lift not only the loaded barbell but also the portion of the body mass that is moved vertically during the exercise. Therefore, the mass of the moving body parts were included in the work calculations to account for the effect of body mass on total work performed. Although this method should be more accurate than using only the external loads of the exercises, the estimation of body mass involvement was limited by the lack of direct measurement of vertical displacement of the center of gravity for the body. It is possible that the present method overestimated the vertical displacement of the body's center of gravity by using simply the vertical displacement of the barbell or sled. However, considering the large difference in body mass moved during the exercises and thus, large difference in vertical displacement of the body's center of gravity combined with a similar external work performed, it seems reasonable to conclude that the total work performed was greater for the squat than for the leg press. Second, the range of motion in the hip and knee joints differed between the 2 exercises because of the nature of the equipment used. In the leg press, less knee flexion and hip extension are performed than in the squat because the hip is partially flexed at the start position. The squat exercise allowed for a greater range of motion and a greater vertical displacement of the center of mass, and consequently a greater amount of work at a similar relative intensity than for the leg press. Although this difference in range of motion is a limitation when comparing the 2 exercises, it cannot be avoided when using the proper technique and equipment for each exercise. Finally, this study was performed in the early morning with participants in the fasted state. It is possible that the magnitude of the acute hormonal response would have been different in the fed state or at a later time of day, but it is unlikely that the observed pattern of responses between exercise modes would have been different.
In summary, the hormonal milieu and the acute hormonal response to resistance exercise are important considerations in resistance exercise program design. The acute program variables, such as exercise selection, directly influence the acute hormonal response to exercise. Consequently, one must evaluate the efficacy of each exercise included in a training program to ensure the optimal benefits from that program; one such measure of efficacy might be the magnitude of the hormonal response. At similar relative intensities and RPE, the free weight (squat) exercise produced a greater acute hormonal response than the machine weight (leg press) exercise. Although participants reported similar RPEs between exercise modes, markers of metabolic demand (lactate and HR) were also significantly greater following the squat. The findings of greater hormonal response and the markers of metabolic demand are likely because of the greater quantity of muscle mass involved and higher amount of total work in the squat. Thus in conclusion, one should consider choosing free weight exercises over machine weight analogs to induce a greater acute hormonal response, as this might subsequently induce superior physiological adaptations.
The hormonal milieu and the acute hormonal response to resistance exercise are important considerations in resistance exercise program design. The acute program variables, such as exercise selection, directly influence the acute hormonal response to exercise. Consequently, the strength and conditioning professional must evaluate the efficacy of each exercise included in a training program to ensure the optimal benefits from that program. Interchanging what appear to be similar multijoint free weight and machine exercises (e.g., the squat for the leg press) might seem acceptable for the development of muscle size and strength, as both exercises use the same large muscle groups. But, based on the results of the present study, it is clear that these exercise modes do not produce the same acute physiological response. At similar intensities and RPE, the free weight (squat) exercise produces a greater acute hormonal response than the machine weight (leg press) exercise. The strength and conditioning professional should therefore consider choosing free weight exercises over machine weight analogs to induce a greater acute hormonal response, as this might subsequently result in superior physiological adaptations.
This study was supported in part by a university thesis award grant (A.A.S). The authors declare that they have no conflict of interest.
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