Because BLa− affects improvements to training and competition, and higher rates of movement dictate success in many sports, our study's purpose is to examine acceleration as a BLa− predictor from IET workouts. We hypothesize acceleration will account for a significant degree of BLa− variance incurred from RE. We also hypothesize, because of the way repetitions are performed, phasic actions will elicit higher acceleration values (30). Results may benefit exercise prescription for those who seek improved tolerance for metabolic acidosis and BLa− acquired through increases in their rate of movement.
Experimental Approach to the Problem
To assess how well acceleration predicts BLa− variance, subjects (n = 45) performed 4 workouts, each comprised solely of phasic or tonic repetitions, on an instrumented IET. Subjects performed 2 phasic and tonic workouts each, their sequence randomized to prevent an order effect. Each workout examined BLa− that resulted from seated knee- and hip-extension RE done with subjects' left legs. The workout protocol entailed two 1-minute sets separated by a 90-second rest period. Both average and peak acceleration values were collected from each set, which served as current study predictor variables. Both before and 5 minutes after the conclusion of workouts, subjects submitted to finger pricks to assess post and delta (post-pre) BLa−, which served as the 2 current criterion variables. Thus, our experimental approach shows how acceleration may predict the BLa− variance and enables us to test each of our hypotheses.
Healthy college-age volunteers (32 women, 13 men) gave written informed consent for the project, which received Institutional Review Board approval from a university-based human subjects committee. General subject characteristics (mean ± SEM) were as follows: height 1.73 ± 0.01 m, weight 74.5 ± 2.5 kg, and body mass index 24.6 ± 0.5 kg m−2. Each subject made 6 visits to our laboratory spaced at least 48 hours apart. Per subject, all 6 visits occurred within a 30-day period. Our data collection timetable accommodated for the daily schedules of our volunteers (classes, work, etc.) such that laboratory visits per subject were made at the same time of day.
Current subjects averaged 2 years prior RE experience, yet none had performed IET workouts. Thus, each subject's first 2 laboratory visits were familiarization sessions, whereby they were introduced to RE on the IET. Preceded by a 5-minute stationary cycle ergometer warm-up performed against 9.8N of resistance at a self-selected velocity, familiarization sessions developed motor patterns specific to tonic and phasic actions. Once familiarization sessions were completed, the final 4 laboratory visits entailed 2 tonic and phasic workouts each, with their sequence randomized to prevent an order effect. Each workout was comprised solely of 1 type of repetition. The current study RE, which enabled performance of both tonic and phasic repetitions, was a seated hip- and knee-extension maneuver with the left leg. A unilateral maneuver was chosen in lieu of the difficulty of a similar RE done in a bilateral fashion on the IET. The RE chosen involves the largest muscles of the body and those responsible for acceleration in numerous high-velocity sports and greater BLa−.
At the start of workouts subjects were first weighed and then provided a drop of whole blood to determine their pre-RE BLa−. The blood drop was obtained via a finger prick under aseptic conditions. Blood sample analyses were done with a calibrated device (Accutrend Sports Resource Group, Hawthorne, New York) that demonstrates a high degree of data reliability (3). After pre-RE BLa−, subjects performed a warm-up identical to that used for familiarization sessions. During warm-ups subjects were randomized to a tonic or phasic workout. For tonic repetitions resistance was incurred through a full range of motion because the cord that connects subject to the weight sled was kept continuously taut. In contrast, phasic actions involved a cyclic pattern to cord movement in which it both slackened and tightened over the course of each repetition, which allowed intermittent application of the weight sled load. Subjects performed the RE seated in a chair that faced the opening of the U-shaped IET (Figure 1). Chair position was held constant per subject. Workouts entailed two 1-minute sets separated by a 90-second rest period. With 8.1 kg added to the sled and a Velcro strap wrapped around the distal portion of their left foot, subjects performed as many repetitions as possible per set. Subjects were told to move the sled as rapidly as possible, not to pace themselves, and received vocal encouragement. After completion of the second set, subjects remained seated. At 5 minutes post-RE BLa− was again measured with the same methods used at the start of workouts.
Per set, peak and average acceleration values were calculated from data collected online. Peak accelerations were the highest increase in the rate of movement observed from a single repetition from each 1-minute set. Per repetition, acceleration was measured from the start of each movement until attainment of a peak velocity. In contrast, average accelerations were the mean rate of movement increase per set. To derive peak and average acceleration values, IET instrumentation required continuous calculation of weight sled position and force output throughout sets. Whereas the current RE was a unilateral maneuver, IET right and left halves (Figure 1) were instrumented in an identical manner. Each half was equipped with a TLL-2K load cell (Transducer Techniques, Temecula, California) attached to its lowest pulley and an infrared position sensor (model CX3-AP-1A, Automationdirect.com) located midway on the underside of each 1.9-m track. As the sled traversed the track, the load cell and position sensor concurrently recorded force output and displacement, respectively. DI-158U signal conditioners (DATAQ Instruments, Akron, Ohio) received load cell and position sensor data measured by a 4-channel analog data acquisition card at 4,000 Hz. Instrumentation included a power source (Model Dual 0-30VDC/3A & 5VDC/3A; Jones & Associates, Lake Park, Florida). A macro was written to perform the numerical integration of force data and was calculated on an Excel spreadsheet. Per repetition, acceleration was calculated as the force:mass ratio.
Per workout mode (phasic, tonic), pre, post and delta RE BLa− were initially assessed for data reproducibility with intraclass correlation coefficients (ICC). Average and peak acceleration values, collected from each phasic and tonic workout set, were assessed for both intraworkout (set 1 vs. set 2) and interworkout (set 1 workout 1 vs. set 1 workout 2, etc.) data reproducibility with ICC. High average and peak acceleration ICC values were then pooled for multivariate regression. Both post-RE and delta BLa− served as criterion variables. Per workout mode, multivariate regression included 4 predictor variables-namely, average and peak acceleration values from each of the 2 workout sets. An α ≤ 0.05 determined statistical significance for the multivariate regression analyses.
BLa− ICC values (Table 1) reveal a higher data reproducibility post-RE, likely because the current RE treatment led subjects to become more similar with respect to BLa−. Pre-RE BLa− were not used in multivariate regression. Intraworkout and interworkout acceleration ICC values appear in Tables 2 and 3, respectively. Prior RE studies (18,19), in which workouts were done over a single plane of motion and far fewer repetitions, claim ICC values of 0.75 to 0.80 indicate excellent reproducibility. Given IET repetitions permit multiplanar and joint movement at high speeds, current acceleration data appear to be reproducible. Acceleration data thus were pooled from the 2 phasic and tonic workouts for use as predictor variables in multivariate regression.
Criterion and predictor raw data appear in Table 4. Differences in gender sample size led male and female data to be pooled for multivariate regression. From tonic workouts, the 4 predictor variables explained insignificant amounts of post and delta BLa− variance. Low tonic workout BLa− ICC values may in part be responsible for the insignificant effect. However, with post-RE BLa− as a criterion variable, phasic data showed a trend (p = 0.08) to explain the variance. Yet with delta BLa− as our criterion, the 4 predictor variables collected from phasic workouts explained a significant (p < 0.05) degree of variance. Results yielded the following prediction equation: delta BLa− = 1.40 + 1.116 (average acceleration set 1) - 0.011 (peak acceleration set 1) - 0.634 (average acceleration set 2) + 0.005 (peak acceleration set 2). The multiple R 2 value shows roughly 25% of the BLa− variance was explained by this equation.
High-speed workouts like those done on the IET may induce more sport-specific adaptations in athletes than conditioning programs centered around standard RE equipment because of stark differences in movement velocities. For instance, whereas standard RE velocities for the squat average 2.0 rad · sec−1 (10), vertical jump countermovements typically attain knee angular velocities of 10.5 rad · sec−1 (29). Far greater knee angular velocities (20.7-39.4 rad · sec−1) occur from running and kicking motions by athletes (22,23,31). Although standard RE equipment workouts improve force output, without inclusion of a high-speed physical conditioning program, the athletes may inadvertently become slower, which undermines their performance in actual competition. Thus, because of sport-specific demands of modern-day competition, athletes may be better served by physical conditioning programs done at higher speeds, such as IET workouts, than those centered around standard RE equipment.
The current study examined acceleration as a predictor of BLa− variance from IET workouts. Although acceleration-induced BLa− increases have received little attention, the novelty of the IET design and operation has yielded surprising workout results. For instance, despite the light load imposed on users, 5 weeks of IET elbow flexor workouts caused significant concentric and eccentric peak torque gains (1). A comparison of phasic and tonic IET actions, which entailed kinematic and electromyographic (EMG) data collection, showed higher velocities, accelerations, antagonistic EMG activity, and smaller ranges of motion with phasic actions (30). Applied loads and EMG data were compared in 8 seated quadriceps RE done against 4 levels of resistance (21). Vastus medialis/lateralis EMG ratios from eccentric IET actions evoked higher values than the other RE, even those that imposed higher loads (21). Thus, with IET force and EMG data, it appears higher speeds and accelerations compensate for lighter loads (1,21,30). The IET is well suited for gains to high-speed sports-specific tasks like a baseball pitch, whereby a small mass is accelerated (25). Finally, a recent study compared RE workout variables from 2 inertia-based devices on their ability to predict BLa− variance (5). Results showed IET variables were far more correlated to BLa− variance than the other device examined (5). Continued inquiry into IET workouts is warranted.
Although prior studies have not specifically examined the impact of acceleration on BLa−, its effect on performance was assessed in earlier work (4,13,17,20,29). Acceleration produced by impact exercise was revealed as a determinant of bone mineral density changes in premenopausal women (13). The effects of RE repetitions done at accelerated rates were compared to those done in a normal fashion on upper-body power changes to 2 groups of American college football players. Results revealed accelerated repetitions improved some upper-body power measures (17). Acceleration-accentuated conditioning programs also were examined for their effect on speed development (4,29). In addition to regular workouts, 8 weeks of additional resisted sprint training by rugby, soccer, and football players improved acceleration kinematics and gait (29), yet a similar group that also performed workouts without resisted sprints incurred comparable gains (29). Similar gains in acceleration occurred from elastic cord exercises geared toward faster sprint performance (4). A study that compared rotator cuff acceleration among 2 groups of athletes showed no significant differences existed among baseball players and controls (20). Thus, for greater acceleration in sport-specific skills, programs for athletes should emphasize bigger muscle groups normally recruited for gross motor tasks (4,20,29).
Studies done with standard RE equipment note factors such as the load imposed, work volume, and rest period duration affect BLa− (2,7,32). Whereas repetition rates for standard equipment allow little acceleration, high-speed RE, albeit at constant angular velocities, was examined to note its affect on BLa− (11,26). Experienced weight trainers subjected to 60-second bouts of knee extensor and flexor RE at 3 (0.52, 2.09, and 5.23 rad · sec−1) angular velocities showed BLa− increased significantly at higher speeds (11). BLa− increases corresponded to EMG changes incurred from RE (14). Other studies compared BLa− changes from isokinetic RE done by strength and endurance athletes (24,26). Strength athletes consistently showed higher BLa− post-RE vs. the endurance group (24,26). Reagan and Potteiger (26) showed higher post-RE BLa− occurred at slower angular velocities, which is in disagreement with earlier work (11). Differences may result from the RE protocols used; Reagan and Potteiger workouts entailed 20 repetitions per velocity, whereas the Douris paradigm involved 60-second sets per velocity examined (11,26). Current results are most like those of Douris, perhaps as a result of similarities of the RE workout paradigms used (11).
In the current study, roughly 25% of the delta BLa− variance from phasic workouts was accounted for by average and peak acceleration values and deserves further inquiry in future work. Acceleration may have led to greater reliance on anaerobic glycolysis that yielded higher BLa−. Current results support our hypotheses whereby acceleration (a) described a significant degree of BLa− variance and (b) phasic workouts evoked higher accelerations. Differences in the way phasic and tonic repetitions are performed likely accounts for higher phasic workout accelerations. Resistance is continually imposed over a full range of motion with tonic repetitions, whereas phasic workouts entailed oscillatory cord actions whereby the load was alternately imposed and removed over a given range of motion. IET weight sled mass is imposed on subjects by a nylon cord; when it is taut resistance is incurred, but as it becomes slack the load is removed. The portion of phasic repetitions that offer no weight sled resistance allows a large rate of movement increases and higher peak accelerations with the current phasic workouts.
With delta BLa− as a criterion measure, phasic workout average acceleration values explained more variance than did peak acceleration. The strong delta BLa−-average acceleration correlation from phasic workouts best represents the increase in the metabolite incurred from bouts of RE. Whereas peak accelerations represent the highest single value attained per 1-minute set, average accelerations are a more accurate indicator of the overall mean rate of movement increase, and the intensity of the RE effort, incurred from IET workouts. Thus, delta BLa− and average acceleration values each provide a greater overall representation of changes incurred from phasic workouts, which likely explains why the 2 variables are highly correlated. Continued inquiry into the effects of acceleration and its impact on IET workouts is warranted.
The ability to accelerate, or increases in the rate of movement, is paramount for success in modern-day athletic competition. Rate of movement increases may cause greater reliance on anaerobic glycolysis that yield higher BLa− and greater fatigue that subsequently impairs an athlete's ability to accelerate. This relationship is affirmed by current results, which show average acceleration values from 1-minute sets serve as a good predictor of delta BLa− variance. For improved athletic performance, results suggest IET workouts as described herein may possibly improve an athlete's lactate threshold. Although such an adaptation could improve an athlete's ability to accelerate at higher BLa−, it is important to note the current study did not measure lactate threshold. For those who rely on anaerobic glycolysis for adenosine triphosphate resynthesis, their coaches and trainers should tailor an athlete's workouts with respect to the muscles involved and the duration of exercise bouts for their chosen sport. When this occurs, workouts become more sport-specific to the athlete's needs and will lead to greater improvements.
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Keywords:© 2009 National Strength and Conditioning Association
metabolism; inertia; velocity