Previous studies have examined the merits of IET workouts (2,6,22,27,33). A 5-week elbow flexor IET workout protocol evoked significant isokinetic strength gains (2). Significant time (post > pre) gains occurred for concentric and eccentric peak torque measured at 1.05 rad·s−1, as well as eccentric peak torque at 2.09 rad·s−1 (2). In a comparison of concurrent kinematic and electromyographic (EMG) data collected from phasic and tonic IET elbow flexor actions, 12 subjects performed workouts against loads that ranged from 0 to 9.1 kg (33). As repetitions were performed, EMG data were collected from agonist and antagonist muscle groups (33). Higher peak angular velocities, peak platform accelerations, and average and peak antagonistic EMG activity, as well as less range of motion, resulted from phasic actions (33). In an investigation that examined the same muscle group as the current study, applied loads and EMG activity were compared in eight seated quadriceps exercises done against four different types of resistance (22). Vastus medialis/lateralis EMG ratios from eccentric IET actions evoked higher muscle activity than the other quadriceps exercises examined (22). Thus, for rehabilitative purposes directed toward greater quadriceps activity, the IET may be preferable to other training modalities. Recently, performance measures derived from exercise done on two gravity-independent equipment modalities were compared for their ability to predict the variance in blood lactate levels (6). The results show that the IET performance measures were far superior in the prediction of blood lactate variance than another inertial resistance device (6). The merits of IET workouts were also directed toward improvement of high-speed, sports-specific tasks such as a baseball pitch, whereby a small mass is moved as rapidly as possible (27). Thus, the merits of IET have been examined in both rehabilitative and athletic performance settings; the reliability of performance measures derived from this device is of utmost importance to persons in the aforementioned areas.
The IET permits multiplanar exercise at high velocities and accelerations over a large range of motion. Thus, the potential exists for large amounts of performance variability. To limit current study variability, a second familiarization session was employed, as was a greater sample size compared with similar investigations (1,17,18,21,25,26,29,34,36,37). Prior study test-retest reproducibility values were likely enhanced through single-joint and/or plane exercise (1,17,18,21,25,26,29,34,36,37). Other methodologies that likely limited variability included the use of closed-chain devices (17), exercise done over a smaller range of motion (7,34), or exercise done across fewer repetitions and/or fixed velocities (1,18,21,26,29,34,36,37). Given the current study instrumentation and IET operation, the results have yielded new information on the reproducibility of data obtained from a high-speed, low-resistance device that permits multijoint/planar movement without velocity restrictions. Most of the results shown in Table 2 exceeded our hypothesized outcome and yielded reproducibility values that met or surpassed those from other devices (1,17,18,21,25,26,29,34,36,37).
Current results yielded stark performance contrasts (Table 1) from the tonic and phasic workouts. The differences were likely attributable to the manner in which repetitions were performed. Unlike tonic sets, phasic actions caused the nylon cord to visibly slacken with each repetition. As the cord slackened, the involved muscle groups accelerated as they continued to move until the weight sled resistance was incurred. At that time, motor unit recruitment increased to overcome the resistance, and this produced an impulse of muscle activity as well as the higher PF values from phasic workouts (10). In contrast, both tonic and phasic TW values observed large SEM and SRD values (Table 2). This likely occurred because some error indices typically increase as absolute values become larger, whereas data reproducibility normally improves when within-subject variability is reduced (14,23). In the current study, several test-retest measures were examined so that greater knowledge of our sample's within-subject variability could be ascertained.
Our heterogeneous sample included large spreads in individual performance. Although within-subject variability impacts data reproducibility, it also is important to note that test-retest values are sensitive to between-subject variability or the spread of scores among the sample examined (14). A large performance spread may have contributed to current ICC results (12,14). The ICC is defined as a ratio of the covariance between paired values and total variance (11), or a ratio of the true variance of interest to error variance collected with the measurement (24). Some may regard intersubject ICC values as the variance of greatest relevance, yet some place more importance on intrasubject variability (4). Unlike Pearson correlations, ICCs do not 1) inflate the strength of agreement among paired values or 2) show bias toward small samples (15). Prior exercise studies (17,18) done over a single plane for a limited number of repetitions have stated that ICC values of 0.75-0.80 indicate excellent reproducibility, and these studies have referenced earlier research (30) to derive such values. Current ICC values show that all but one performance variable exceeded the 0.75-0.80 threshold. Other strength training investigations yielded comparable (13) or smaller (31) ICC values. Given the manner in which the IET operates, and that most current ICC values exceeded the 0.75-0.80 threshold, intra- and interworkout performance data may be considered to demonstrate excellent reproducibility.
The CV is a measure of test-retest reproducibility that assesses data distribution and dispersion. An early paper notes CV values of 10-15% as typical for biological systems (32). Current CV results fell within that range or were less. Recent studies (16,34,36) have evoked comparable values. Isokinetic knee extensions yielded interworkout CV values of 3.9-5.9, as higher speeds led to greater variability (36). Isokinetic ankle extension tests also yielded similar intrasubject CV values (34). Finally, a recent study has examined data reproducibility from exercises done on an inertia-based device (16). Subjects (n = 16) performed concentric actions against resistance from four submaximal loads (16). Intertrial and session CV values ranged from 2.6 to 16.3 (16). Like ICC, CV is also impacted by sample heterogeneity (14,15). Yet, current CV values were not only comparable with those from prior studies (16,34,36), but, because of the broad range of scores, variability was less than for more homogenous samples (15). Because of their similarity to prior values (16,34,36), the current CV results are thought to demonstrate high test-retest reproducibility.
Bland-Altman plots enable visual inspection for outliers, systematic data changes, and heteroscedasticity (3,5,12,37). Some suggest that it is the most accurate estimator of measurement error, even after examination with multiple test-retest measures (20,24). Bland-Altman analysis plots allow for examination of heteroscedasticity, which occurs when differences among paired data increase as mean values become higher, and this suggests that variability is influenced by the magnitude of the measured value (4,5,12). For each performance variable, Figures 7-12 do not demonstrate heteroscedasticity. Data plotted beyond ± 1.96 SD or outside the dashed lines are considered outliers, as they lie beyond the limits of agreement, or where 95% of a population is expected to reside (3,5,23). In the absence of heteroscedasticity, SEM and the limits of agreement are considered the most popular measures of absolute reliability (3), and, as such, they are included as a test-retest measure in the current study. Figures 7-12 show that most data fell within ± 1.96 SD, and outliers were primarily interworkout measurements.
Prior studies have employed Bland-Altman plots for outlier detection (12,19). Blood pressure value reproducibility was measured with three different devices (19). Absolute intradevice measurement differences at 5, 10, and 15 mm Hg have shown that roughly 8% of the test-retest measurement differences exceeded 15 mm Hg (19). The percentage of blood pressure outliers exceeded those of the current study. However, criteria provided by The British Hypertensive Society deem reproducibility from the three devices as acceptable (19). A study of 50 stroke patients assessed the degree of agreement between test-retest gait performance measures (12). Bland-Altman analysis identified, in each of the six gait measures assessed, roughly 6% of the data as outliers. It was concluded that gait tests showed a high degree of measurement agreement (12). Because Figures 7-12 generally show a smaller percentage of outliers as prior studies (12,19), there seems to be an acceptable degree of test-retest data reproducibility as determined through the Bland-Altman plots.
The SEM is the square root of within-subject variance, or the dispersion of scores when subjects are tested repeatedly (4,28). The SEM values have the added benefit of being expressed in the same units of measurement as the variables under examination (11). Although current SEM % values compare favorably with prior results (4,12), SEM values are a reflection of the absolute magnitude of each performance variable. Current SEM % values allow for variability comparison among the performance variable without the influence of their absolute magnitude. The reproducibility of gait performance tests administered to subjects with hemiparesis after stroke indicated that SEM and SEM % values ranged from 0.07 to 18.6 and 4.8 to 8.2, respectively (12). Sickness impact profiles collected from stroke victims indicated that SEM % ranged from 3.34 to 14.53 (4). Current SEM % results are similar to those from prior studies (4,12). In contrast, the absolute magnitude of the current performance variables likely led to higher SEM values than those observed previously (4,12).
Associated with SEM, SRD is the minimal change required to note a true difference in subjects' performance (12) and is thought to link subjects' responsiveness to data reproducibility (4). Current SRD and SRD % values, which address the degree of systematic error attributable to learning effects (14), compare favorably with similar values obtained from prior studies (4,12). A study that examined isometric strength performance reproducibility in healthy women (n = 10) performed 1 week apart deemed SEM a practical estimate of intrasubject reliability (28). Prior reproducibility research has observed SRD % values of 9.3-40.3 (4) and 13-23 (12), values comparable with those from the current study. In conclusion, it was initially thought that the instrumentation of the IET would pose a considerable challenge. Because of its operation, IET reproducibility values were hypothesized to be less than those for conventional devices. However, because of the current study's instrumentation methodology, whereby data collection occurred at a high sampling frequency, the reproducibility of IET performance variables was deemed acceptable. Future research investigations in which it is essential to quantify performance from an IET intervention may choose to instrument the device as described in the current study.
Unlike many exercise devices, the novel operation of the IET enables high-speed, low-resistance repetitions to be performed over multiple joints and planes of motion. Because of the low amount of resistance imposed by the IET, its use is popular in rehabilitation programs to aid recovery from injury or to accommodate for pain and limited mobility. In addition, demands for increased rates of force and power development required in sports competition, in which athletes often must move a modest amount of mass as rapidly as possible (27,35), make conditioning programs centered around devices such as the IET an ideal training modality.
We wish to thank our subjects for their participation. J.F. Caruso, P. Hari, M.A. Coday, A. Leeper, and J.K. Monda are TURC (Tulsa Undergraduate Research Challenge) program participants at The University of Tulsa. Financial support of this project was provided by a university-based faculty research grant.
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Keywords:© 2008 National Strength and Conditioning Association
instrumentation; data reproducibility; force transducer