Reduced blood flow to muscles during resistance exercise impacts tissue metabolism and fatigue (3,30,31). Concomitant with fatigue, reduced blood flow may affect perceptions of effort, force, and possibly pain (2,20,29). One technique for investigating the role of limited oxygen supply to muscles is to manipulate peripheral sensations through vascular occlusion of an exercising limb (3,28,34). Occlusion of vessels supplying blood to skeletal muscle during resistance exercise has been shown to enhance growth hormone release, increase lactate levels, and diminish disuse atrophy (23,27,30,31). These responses and adaptations suggest that training with light resistance and occlusion may be more efficient than moderate or high intensity resistance exercise in rehabilitation of partially deconditioned muscles and joints (28). Vascular occlusion during resistance exercise also has been shown to elicit an overestimation of force production to moderate or heavy intensity exercise without occlusion (29). Less is known about how partial occlusion environments impact sensations accompanying standard types of dynamic resistance training movements. Perceptual cues are important tools for forms of occlusion training (known as KAATSU) because they have been correlated to metabolic changes during resistance exercise and increased perception of pain (9,21).
In pain research, when a stimulus and anticipated experience are compared, it is important to contextualize perceptual changes by using a criterion or midpoint at which perception is altered in a significant way (1). Although this type of contextualization has not been routinely performed in resistance exercise, a model exists in aerobic exercise. A perceptual “break point” has been validated with aerobic exercise that equates to a “13-15” on the 6-20 Borg ratings of perceived exertion (RPE) scale or a “6” on the category-ratio 10 (CR-10) scale (11,19,24). This break point is important because it marks the point at which perception of effort will begin to accelerate more rapidly toward failure or fatigue; an exponent model has been applied to this phenomenon with category-ratio scaling (2), suggesting that RPE may rise at a value of 1.2 or greater when compared with a linear regression plot (instead of 1.0). Thus, effort sense rapid ascent may be related to other sensations such as pain (2). Previous studies have demonstrated that RPE and pain are distinct perceptions (4) with different sensory cues (18). Frankenhaeuser (8) proposed that affective changes during work may be related to perceived distress (such as pain), whereas effort may be related to changes in intensity. There are sparse data concerning effects of vascular occlusion on effort sense and pain. One study demonstrated an overestimation of force by subjects performing a handgrip task with occlusion of blood flow to working muscles compared with no occlusion (29). Another study found that total occlusion during leg extension elicited increased pain at 30, 40, and 50% 1 repetition maximum (1RM) loads but did not affect perceived exertion (33). However, total vascular occlusion elicits a limitation for resistance exercise performance, whereas employment of partial vascular occlusion allows an extended exercise duration. Collectively, these data suggest that there is a psychological break point for RPE and perhaps pain, but this phenomenon has limited replication with resistance exercise.
Lactate accumulation in muscle has been associated with increased effort (15) and pain sensation (12). Lambert et al. (18) speculated that altered local changes in lactate may elicit afferent signals to the brain, altering glucose metabolism and perceptions of fatigue. Thus, use of the partial occlusion model would be instructive to better understand alterations of perception of effort and pain in the experience of muscular ischemia during resistance exercise that ultimately could affect ability to continue the exercise.
We recently reported the effects of partial vascular occlusion during resistance exercise on hormonal responses (23) and oxidative stress (10). As an extension of these studies, we sought to determine the effects of partial vascular occlusion on perceptual and related physiological markers. This information potentially could be used to determine the viability of this form of exercise training because it relates to psychological stress. We hypothesized that perceptual and associated physiological responses to light resistance exercise with partial vascular occlusion and moderate resistance exercise without occlusion would be similar but significantly different from the partial occlusion trial without exercise. Moreover, we hypothesized that these measures would change with successive sets of each exercise.
Experimental Approach to the Problem
Subjects performed 3 sessions employing a within-subjects study across 3 weeks. At least 1 week separated each session. The sessions included both single-armed biceps curl and single-legged calf extensions for 3 sets to failure (in the first 2 sessions). The 3 sessions consisted of (a) light resistance exercise (3 sets to failure) at 30% of 1RM with partial occlusion (LRO), (b) moderate resistance (3 sets to failure) at 70% of 1RM with no occlusion (MR), and (c) partial occlusion without exercise (OO). The first 2 sessions were performed in randomized counterbalanced fashion. The final session was partial occlusion control to maintain partial occlusion without exercise for the same length of time as the partial occlusion, exercise session. Heart rate (HR), perceived exertion, and pain were measured across sessions to compare perceptions of effort, pain, and cardiovascular stress during low-load resistance exercise with partial vascular occlusion and during moderate resistance exercise with no occlusion.
Seven adult males with recreational weight training experience of at least 1 year completed the study. They were a subset of subjects from previous studies that investigated endocrine (23) and oxidative stress responses (10) to resistance exercise with partial vascular occlusion. Characteristics for age, weight, height, and percent body fat were 21.7 ± 1.5 years, 87.2 ± 5.8 kg, 180.2 ± 6.3 cm, and 10.1 ± 3.5%, respectively. Exclusion criteria have been reported elsewhere (23). Subjects gave written consent, and the study was approved by the Institutional Review Board of Southeastern Louisiana University. Some of the procedures have been reported previously (10,23) and thus will be described in truncated form.
Body fat was determined using a 3-site (chest, abdomen, and thigh) skinfold measurement with skinfold calipers. A 1RM for the single-arm biceps curl and single-leg calf extension were assessed on the dominant side; a good lift was only acknowledged when the subject completed the repetition at a constant cadence through the full range of motion. The biceps curl was performed with a free weight dumbbell using the dominant arm. To ensure that proper form was executed and that momentum was minimized, subjects stabilized their upper torso by holding onto a stationary structure with the nondominant hand. Calf extension was performed with the dominant leg using a leg press machine (Body Masters, Rayne, LA, USA). Careful attention was paid to proper lifting technique to minimize any assistance from the quadriceps muscles. Choice of muscles was based upon the biceps and gastrocnemius representing muscles of the upper and lower limbs that have comparable cross-sectional areas, involve unilateral movement, and have convenient occlusive application areas proximal to the muscle. Submaximal weight approximating 40, 60, and 80% of the subject's estimated 1RM served as warm-up sets consistent with previous research (12,13). A 30% 1RM for the LRO session and 70% 1RM for the MR session were calculated for the experimental session workloads. A 30% 1RM represented a low workload that could be used in rehabilitation settings.
Experimental and Control Trials
Light resistance occlusion session
To ensure that caloric deficit did not impact results, subjects consumed a standardized caloric beverage (Naturite, Jacksonville, FL, USA) 3 hours before each experimental session (per serving: 250 kcal; 40 g carbohydrates, 9 g protein, and 6 g fat:). Additionally, subjects refrained from exercise, caffeine, and alcohol consumption 12 hours before data collection. Blood pressure readings were taken from the arm after 15 minutes of rest (Figure 1). Fifteen minutes before session onset, a blood sample was taken from an antecubital vein in the nondominant arm. A customized inflatable cuff was then placed around the dominant arm in specific position below the deltoid, inflated to a pressure of 20 mm Hg below acute systolic pressure determined approximately 15 minutes prior, and remained in place for the duration of session. The subject then performed 3 sets of single-arm biceps curls at 30% 1RM to exhaustion. One repetition per second was employed to rapidly fatigue the biceps and the gastrocnemius. Rest periods lasted 1 minute, whereas the occlusion cuff remained in place throughout and 1 minute after completion of the third set. Heart rate, RPE, and pain rating were assessed after each set. To prevent total vascular occlusion, a pulse oximeter (Model 3301; Smiths Medical PM, Inc., Waukesha, WI, USA) measured O2 saturation immediately after each set. If a pulse was not detected (a rare occurrence), cuff pressure was reduced approximately 5-10 mm Hg until blood flow was detected at the finger. Time duration of cuff occlusion was replicated in the occlusion-only (OO) condition (mean arm occlusion time was 341 ± 4.5 seconds). After cuff removal, the subject moved to the leg press device; the inflatable cuff was applied to a specific proximal portion of the lower leg on the dominant side and inflated 5 minutes after removal from the arm, to a pressure of 40 mm Hg above the arm occlusive pressure (accounting for larger vasculature and muscle mass). Pressure was maintained for the duration of 3 sets of calf extensions to failure and the subject's corresponding interset rest periods. As before, a pulse oximeter affixed to the toe of the subject ensured blood flow between sets. The inflated cuff remained in place through final set completion and a minute of recovery (mean leg occlusion time was 387 ± 13.1 seconds). After deflation and cuff removal, the subject sat for a second blood draw within a minute of testing completion and remained seated for another 15 minutes. The final blood draw occurred 15 minutes after exercise. There was a minimum of 1 week between sessions.
The OO session followed the exact time intervals, body positions, and partial vascular occlusion pressures as in the LRO session but without any actual loads lifted.
The MR session was conducted using a protocol similar to the LRO session but at 70% 1RM without partial vascular occlusion. The session was analogous to that of a traditional weight lifting protocol for strength and muscle mass gains.
After listening to instructions concerning the way to rate the pictorial OMNI resistance exercise scale (OMNI-Res) developed by Lagally and Robertson (17), subjects were asked to report the repetition that represented an RPE of 6 on a 0-10 scale. Previous studies have shown that a midrange RPE of 12 and 13 on the 6-20 scale (2,7) or a 6 on the 0-10 scale (24,25) equates to a metabolic break point after which RPE rapidly ascends (2). This phenomenon has also been observed in exercising children (19). We chose a postexercise recall of the repetition for an RPE of “6” rather than subjects completing the number of repetitions to reach an RPE of “6” (production method) for several reasons. First, we were concerned that if subjects were instructed to complete repetitions until they perceived an RPE of “6,” they would prematurely anticipate that repetition and thus complete a lower number of repetitions for an RPE of “6” as has been reported in previous research. Second, a similar methodology has been used successfully for determining resistance exercise session RPE and recall after each set (5,6,16). Third, we collected separate pilot data from 51 college students (28 men and 23 women) to confirm that the recall method was most appropriate for our study. Subjects completed biceps curls using both the recall and production methods and indeed reported lower repetitions for the production method vs. the recall method (7.13 vs. 8.8 repetitions for a 12RM, respectively). In the present study, an RPE and pain of “6” were reported between 54 and 57% of the total number of repetitions, which was close to the expected 60% of repetitions completed to failure during the exercise. Finally, past techniques of assessment (magnitude estimate and production RPE methods that assess effort during exercise) would have required our subjects to anchor perceptions throughout a range of intensities and create a prefatigue condition that could have compromised our hormonal and local metabolic results. Thus, justification for postexercise recall of RPE was warranted.
The CR-10 pain scale was employed to maintain consistency with previous research that used overall pain ratings for single and multiple joint movements (12). Subjects were asked to rate at what repetition they felt a pain rating of “6” on a scale from 0 to 10 (0 = no pain to 10 = worst pain ever felt) using a CR-10 scale. Subjects were directed to rate their pain experienced in relation to the 3 most severe pain experiences they had ever experienced. Subjects were also informed that they could respond in half values such as 0.5 or 2.5 or in decimals. They were told that they could report a pain scale rating as greater than 10 if they experienced the sensations as such. Additionally, the subject instructions on perceptual assessment included a paragraph of basic and separate scaling instructions for OMNI-Res and CR-10 pain. RPE was anchored by following the instructions of Lagally and Robertson (17). Category-ratio scales for both pain and RPE were selected based on high concurrent and predictive validity.
A series of 3 (condition) by 6 (time) analysis of variances with repeated measures were performed for OMNI-Res, pain, and HR. Planned post hoc independent t-test analyses were conducted when significance was found. It was clear that the OO condition, when no exercise was being performed but partial occlusion was applied, did not alter RPE, pain, and HR. For these measures, only the 2 groups (LRO and MR) were compared. Partial eta-squared and power estimates were calculated to compare variance accounted for by the main and interaction effects.
OMNI Resistance Exercise Scale and Pain
Because subjects reported a rating of “0” for OMNI-Res and pain in the OO trial, analysis of OMNI-Res and pain only included the MR and LRO trials. When analyzed together, significant differences among the OO, LRO, and MR conditions were present for all the perceptual data. Additionally, separate analyses were performed for the biceps curls and the calf raises because upper-body vs. lower-body movements have demonstrated different ratings of perceived exertion (13,14). Tables 1 and 2 represent the total number of repetitions completed and the proportion of repetitions elicited an OMNI-Res or pain rating of “6”. The percentage of total repetitions to reach an OMNI-Res of 6 across the trials was significantly lower than that for pain (Table 2); approximately 55% of total repetitions were performed before an RPE of 6 was achieved vs. approximately 59% of total repetitions were completed before a pain rating of 6 was reported (p < 0.001). Because Mauchly's test of sphericity was significant for the OMNI-Res, Greenhouse-Geisser adjustments were employed. Significant main effects for time were demonstrated for biceps curls and the calf raises, respectively, Fs(2,24) = 22.75, 20.86, ps < 0.0001. Conversely, neither main effects for condition, Fs(1,12) = 1.45,0.07, ps > 0.05, nor interaction effects, F(2,24) = 1.03, were significantly different. Planned post hoc contrasts for the time effect demonstrated significant differences such that the LRO condition elicited more repetitions during the first set of biceps curls when compared with the MR condition. However, no other significant differences were noted. The biceps curl main effect for time was significant with each progressive set in the MR and LRO trials; that is, a “6” on the OMNI-Res was rated at a lower number of repetitions when compared with the previous set (Figure 2) (p < 0.001).
Partial eta-squared values for the time analyses were moderately strong (η2s = 0.64, 0.66), whereas those for condition and interaction effects were much lower (η2 ranged from 0.006 to 0.11). Observed power ranged from 0.06 for the condition effect for calf raise to 1.00 for the time factors for both movements, suggesting that the time variable accounted for the greatest amount of variance in the present study.
Again, Mauchly's test of sphericity was significant for all analyses of pain, and Greenhouse-Geisser adjustments were employed. Significant main effects for time were demonstrated for biceps curls and calf raises, respectively, Fs(2,24) = 18.95, 24.52, ps < 0.01. Conversely, neither main effects for condition, Fs(1,12) = 1.22, 2.90, ps > 0.05, nor interaction effects, Fs(2,24) = 0.10, 0.15, were significantly different (Figure 3).
Partial eta-squared values for time analyses were moderately strong (η2 ranging from 0.62 to 0.70), whereas those for condition and interaction effects were much lower. Observed power ranges were similar to those demonstrated in the OMNI-Res analysis, suggesting that the time variable accounted for the greatest amount of variance.
For the HR analysis, only a significant condition effect was revealed, F(2,18) = 20.12, p < 0.001, η2 = 0.69. Planned post hoc comparisons demonstrated that the LRO condition elicited significantly higher HR than MR and OO conditions for biceps curls and calf raises (Figure 4). However, no significant differences were revealed between MR and LRO conditions.
Given recent data suggesting resistance exercise with partial vascular occlusion may be beneficial for rehabilitation in populations with strength limitations (32); we compared perceptual changes with moderate resistance exercise and light resistance exercise with partial occlusion with an OO trial as a control condition. The major finding of the study was that perception of pain and effort rose similarly in the moderate resistance exercise trial and the light resistance exercise with partial occlusion trial but was greater than that of the OO trial, confirming our first hypothesis. This finding reveals the important role of peripheral sensation in determining RPE and pain during resistance exercise. This is the first study to demonstrate that partial vascular occlusion during dynamic resistance exercise using light loads elicits similar perceptual responses to dynamic resistance exercise with heavier loads without occlusion. These results also indicate that partial vascular occlusion with light loads may provide a valuable method for rehabilitation in individuals with cardiovascular or orthopedic limitations.
Another significant finding was the cumulative effect of exercise duration on changes in repetitions that produced perception of pain and effort to a level of “6” during dynamic exercise with partial vascular occlusion. Both LRO and MR conditions elicited perceptual ascent to an effort sense and pain score of 6 (indicating a moderate pain level) in a similar and rapid fashion, confirming our second hypothesis. Moreover, during these conditions, volume of work, a strong indicator of fatigue, was a better indicator of perceptual change than HR response. Heart rate response was higher in the LRO than in the MR conditions, but perceptions of effort and pain were reached with successively lower volume of work (i.e., decline in number of repetitions). Because HR did not show the same pattern of change as perceptual responses, local fatigue rather than cardiac responses appeared to be an important factor explaining perceptual responses. This is the first study to compare the progressive alterations in RPE and pain during dynamic light resistance exercise with partial occlusion to moderate resistance exercise without occlusion. Several observations can be made from the present results when compared with previous research. A recent investigation demonstrated that when occlusion was paired with light isometric handgrip exercise, perception of force was greatly overestimated (ranging from 6 to 22%) (29). Moreover, this overestimation was greatest in the lowest force condition. Similar to those static exercise findings, the present study demonstrated that with dynamic resistance exercise using low loads with partial vascular occlusion, there was an enhanced estimation of effort that was similar to that of higher loads with no vascular occlusion. One mechanism for this action could be that compression of cutaneous sensory neurons inhibited the response of somatic efferent neurons (29). Another may be that ischemia induced by the tourniquet-impeded conduction of peripheral nerves via mechanical deformation (29).
Data from the present study also demonstrated that partial occlusion (LRO condition) produced a pain threshold of 6 at a lower repetition number in the last set for calf extensions compared with MR condition despite the MR condition eliciting similar lactate levels reported previously (23). Prior research has demonstrated in light isometric exercises bouts that hypoxia and depressed lactate clearance are likely causes of increased metabolites in distal portions of the occluded limb (28,29). One might speculate that both hypoxia and decreased lactate clearance coupled with artery deformation and diminished arterial circulation could create an enhanced perception of pain. Takarada et al. (28) observed that at intensities of <40% of 1RM the ability of the natural pumping action of the muscles to clear lactate was diminished and perceptual force overestimation occurred. These investigators suggested that enhanced acidic environment coupled with hypoxia elicited an increase in motor unit activity as suggested by increases in electromyographic activity. Coupled with the shared pathway with type IV afferents, this could account for the pain sensations in the LRO condition in the present study via possible accumulations of substance P, bradykinins, histamines, or prostaglandins in the occluded limb (9). However, more research is needed before these speculations can be confirmed.
Heart rates were significantly higher for the LRO condition when compared with the MR condition, 93 vs. 78 beats·min−1; however, there was no statistical difference between conditions in percentage of repetitions completed before an OMNI-Res of 6 reported in the LRO and MR trials. This observation suggests that perception of effort was locally driven for the most part because if HR had strongly affected effort sense, then the LRO condition would have also revealed a significantly different number of repetitions at which a “6” was achieved when compared with the MR condition, which was not the case. Although studies have examined how occlusion impacts HR after exercise (26), less is known about the relation to other meaningful metabolic responses during resistance exercise with partial vascular occlusion. In a study by Stanley et al. (26), application of occlusion after exercise elicited slightly higher HR than the nonoccluded control group; but after occlusion release, there was a brief period of HR acceleration before returning to baseline, suggesting a blunted cardiac preload response. Another study corroborated these results (27). Specifically, when partial occlusion was applied during a leg extension exercise in healthy untrained males in that study, HR rose during exercise sessions (4 sets of leg extensions to failure; 20% 1RM; 20 seconds interest rest). It was concluded that increased HR during occlusion compared with nonocclusion was induced by compensation for reduced venous return and subsequent cardiac preload. It is likely that this mechanism could also explain elevated HR (22) in the LRO trial in the present study; however, our research design did not allow us to compare the LRO HR responses with a nonocclusion trial at the same resistance exercise load.
The psychological data revealed that RPE and pain perceptions of 6 were reached at consistent time points relative to the cessation of exercise for each set regardless of resistance environment. This provides compelling evidence that sensory feedback helps determine exercise cessation and could lead to application of partial occlusion in rehabilitation of affected limbs to create reduced joint stress through reduced load while equaling or bettering the metabolic responses of higher loads. This could enhance the effectiveness of rehabilitation and reduce risk of injury.
Limitations of the present research include sample size and use of unilateral movements that use smaller muscle groups than classic rehabilitation or conditioning exercises. However, the strength of the study was the within-subjects design that reduced between subjects variability. Additionally, randomization, counterbalancing, and the degree of control (use of pulse oximetry, lifting to a metronome, frequent measurement of perception, and so on) enabled us to confirm our hypotheses with confidence. It should also be noted that recall of RPE and pain were performed rather than in task ratings as have been commonly practiced. The rationale for this methodology was to avoid disruption of the physical exertion through questioning of subjects during the repetitions. This form of disruption could create alteration in the measures we were attempting to examine and add an “in task” evaluative component to the exercise environment causing subjects to lose focus on the exercise or distract them from pain.
The data clearly indicate that when partial vascular occlusion with a light load is applied during dynamic resistance exercise, both pain and effort sense are altered to a similar degree as with dynamic resistance exercise using greater (moderate) loads but no occlusion. Moreover, in partial occlusion and nonocclusion exercise, there is a cumulative effect of successive sets in which fewer repetitions can be performed. Use of effort sense and pain ratings appear to be a valuable monitoring tool during light resistance partial vascular occlusion protocols to monitor novel or unexpected load changes during exercise. Coaches can use this information to determine pain and effort ratings that reveal the perceptual discomfort thresholds (approximately a “6” on a CR-10 pain scale or OMNI-Res scale) so that during future repetitions, motivation or distraction can be provided to develop pain tolerance, increase performance, or improve management of fatigue with volume load during a training session. Moreover, these findings have implications for individuals who seek exercise with muscle loading proscriptions. The results also imply that when partial occlusion is applied, lower loads can produce similar perceptual results as moderate loading with no occlusion. Thus, individuals may be better able to tolerate perceptual change at low loads with occlusion because joint stress may be minimized and local muscle metabolism is increased.
We wish to thank the subjects for their cooperation and patience.
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