Journal of Strength and Conditioning Research, 2003, 17(1), 88–94 q 2003 National Strength & Conditioning Association
The Effect of Short-Term Isokinetic Training on Force and Rate of Velocity Development LEE E. BROWN1
AND
MICHAEL WHITEHURST2
Human Performance Laboratory, Arkansas State University, State University, Arkansas 72467; 2Florida Atlantic University, Boca Raton, Florida 33431. 1
ABSTRACT This study determines the effects of short-term isokinetic training on rate of velocity development (RVD) and force. Three groups were pre- and posttested for knee extension RVD and force at 1.04 (slow) and 4.18 rad·s21 (fast) on a KinCom dynamometer. The slow and fast groups completed 2 days of velocity-specific training, whereas the control group did not train. Four-way analysis of variance results demonstrated significant (p , 0.05) decreases in RVD between preand posttests for the slow group at the slow velocity (RVD— 1.25 6 0.048 vs. 1.08 6 0.038) and for the fast group at the fast velocity (RVD—14.24 6 0.338 vs. 13.59 6 0.298). Force exhibited no significant differences between testing days for any group. These results demonstrate that short-term isokinetic training results in velocity-specific RVD improvements. These acute RVD improvements may serve to offset strength deficits in power environments on the basis of the mutable relationship between force and velocity.
Key Words: strength, resistance training, acceleration Reference Data: Brown, L.E., and M. Whitehurst. The effect of short-term isokinetic training on force and rate of velocity development. J. Strength Cond. Res. 17(1):88–94. 2003.
Introduction
A
pproximately 8 weeks of heavy resistance training can produce significant gains in strength. The consensus position supported by experimental studies is that gains in strength are the primary result of muscle tissue changes collectively called hypertrophy (12). What is less well known is the phenomenon that results in the manifestation of increased strength after only a few strength-training sessions. The suggestion is that initial changes in strength after training occur at a rate too fast to be accounted for by morphological changes. Therefore, neurological changes must play a role in acute strength expression. Moritani and deVries (20) investigated the relative contributions of neural and hypertrophic factors to muscle strength gains. After 8 weeks of training, all subjects had increased their 88
maximal strength and the electromyographic changes clearly illustrated that changes in electrical activity at the elbow flexor were primarily responsible for early strength increases, whereas hypertrophic responses gradually increased as a contributory factor over time. Therefore, the short-term acute strength gains in untrained subjects appeared to be due to neural adaptations. Much of the previous strength research has focused on slow-velocity, high-resistance movements. However, the effect of high-velocity, low-resistance training has not been thoroughly investigated. Additionally, research has focused almost exclusively on torque production as the outcome, with little consideration given to the possible training adaptations related to a subject’s ability to produce greater limb acceleration. Previous studies have not emphasized acceleration adaptations accompanying short-term training. Such shortterm acceleration gains could affect human performance by the mutable nature of force and velocity. A decline or deficit in force production of a limb may be balanced by an increase in the velocity of that limb (25). However, proper measurement of this phenomenon can prove to be problematic. Prevost et al. (24) had volunteers train for 2 days in either a slow group (;0.52 rad·s21) or a fast group, (;5.22 rad·s21) performing 3 sets of 10 repetitions between pre- and posttests. Mean peak torque did not change at the slow-test limb velocity for either group, whereas the fast group exhibited approximately 22% increase in mean peak torque between pre- and posttests. They attributed this to neural facilitation, and it is well established that performance gains after shortterm training are primarily a function of neural factors (1, 2, 14, 17, 21). However, a 22% increase in force production after low-load, high-velocity exercise appears excessive. Because isokinetic training involves an acceleration phase of movement (8) and the failure to control for this phase will artificially increase force production, (26) a neurally mediated increase in acceleration or the rate of velocity development (RVD) may be responsible
Short-Term Training, Acceleration, and Power 89
for reported force improvements. Therefore, this study quantified the short-term strength and acceleration effects of a high-velocity, low–resistance-training regimen.
Methods Experimental Approach to the Problem Apparently healthy university students (30 men and 30 women) participated in this study. All subjects were right-hand dominant to maximize RVD and force production (7, 8). Each subject read and signed a physical activity readiness questionnaire (PAR-Q) and a written informed consent document approved by the University Human Subjects Institutional Review Board. A subject answering yes to any query on the PAR-Q was further questioned to ascertain their suitability for participation. Elimination criteria included a right-knee injury within the last year or a physician’s instructions not to exercise. No subjects in this study were precluded from participation based on positive responses. Dynamometer Setup A single chair Kin-Com isokinetic dynamometer (Model AP; Windows compatible software version 1.0; Chattanooga Corp., Chattanooga, TN) was used for all testing and training. It is a reliable instrument for collecting isokinetic kinematic data (10, 11). The dynamometer shaft was aligned with the assumed axis of rotation (lateral femoral condyle) of the dominant right knee with the subject in a seated position and the back reclined at approximately 1108. The right thigh was secured with a strap, as were the waist and thoracic torso. Arms were placed across the chest with hands grasping the straps. The lever arm pad was positioned to place the inferior aspect immediately superior to the medial malleolus. Range of motion (ROM) mechanical stops were set at 84 and 68 of knee flexion (08 at full extension) so that each subject would pass through the identical total and physiological ROM. The Kin-Com software (Windows version 1.0) allows manipulation of lever arm minimum force preload and turnaround ROM. For this study, minimum force was set to zero to allow instantaneous movement of the lever during limb acceleration without neural preactivation (19). The lever arm turnaround was set to ‘‘high,’’ which allows the least amount of damping by the computer and the greatest independence of the subject to freely accelerate. Likewise, the high turnaround restricts computerinitiated deceleration until the very end of the ROM. Both these settings permit the greatest amount of individual subject freedom while restricting computer intervention during exercise, which theoretically should allow for maximum expression of human variability.
Testing Systemic warm-up was accomplished by cycling for 5 minutes on a Monark (Varberg, Sweden) cycle ergometer at 50–60 pedal revolutions per minute with a resistance of approximately 25 W. Specific limb warmup on the isokinetic device consisted of 3 submaximal reciprocal concentric extension and flexion repetitions at each test velocity with increasing intensity (i.e., first repetition at 25% perceived effort, second repetition at 50% perceived effort, etc.). In addition the subject completed 2 maximal intensity repetitions at each velocity, then rested for 1 minute before testing. Testing began from a dead stop with the subjects’ leg at 848 of flexion and consisted of 5 maximal concentric reciprocal knee extension and flexion repetitions. Velocities were presented in a fixed order at 1.04 and 4.18 rads per second (rad·s21) with 1-minute rest between bouts (28). The identical testing procedure was repeated at the conclusion of training. Each subject was encouraged to contact the mechanical end stops during both the extension and flexion motions. Consistent and identical verbal encouragement was provided during each test session, but no visual feedback of any kind was used (3). Training Subjects were randomly assigned to either a control (N 5 20; 10 men and 10 women), fast (N 5 20; 10 men and 10 women), or slow (N 5 20; 10 men and 10 women) group. The control group did not train but was retested after 1 week. The training groups performed 2 workouts, separated by 48–72 hours, consisting of 3 sets of 8 maximal intensity repetitions at either 1.04 (slow) or 4.18 (fast) rad·s21. Upon completion of training, all subjects in each training group were retested. All setup information was duplicated for repeat sessions and the primary investigator administered every test. Data Collection Data were collected by a separate data collection system by diverting the signal from the Kin-Com dynamometer to an analog to digital board attached to a computer capable of sampling at the rate of 1,000 Hz. Raw ASCII data were exported to spreadsheet software as time, force, velocity, and position values. In this manner, it was possible to determine the kinematic data of lever arm force, position, and velocity every 1/1,000 of a second relative to the prescribed and controlled ROM. Using the velocity data, extension (positive) and flexion (negative) values were used to identify 2 phases of motion (see Figure 1). Rate of velocity development was movement before matching the predetermined velocity, and load range ROM was velocity maintained at the predetermined velocity (3, 6–8). Peak force during the load range phase only was recorded for analysis. Only repetitions 2, 3, and 4 were
90 Brown and Whitehurst Table 2. Results of control group analysis of variance by time on means 6(SE) of rate of velocity development (8) and force (N) as well as intraclass correlation coefficients for reliability between pre- and posttest scores by velocity. Pre
Post
ICC
p
1.04 rad路s21 RVD 1.14 6 0.03 Force 528.45 6 26.2
1.11 6 0.02 535.21 6 28.1
0.40 0.94*
0.474 0.598
4.18 rad路s21 RVD 13.44 6 0.27 Force 345.58 6 22.9
13.47 6 0.31 355.41 6 22.3
0.87* 0.97*
0.866 0.187
* Significant at p , 0.05.
Figure 1. Velocity phases of an isokinetic repetition.
recorded, and the mean of these 3 was used for data analysis because repetition 1 exhibits dissimilar RVD characteristics when compared with subsequent repetitions (7). Although the dynamometer is a rotary system outfitted with a lever arm, the Kin-Com uses a force transducer at the point of contact with the limb and, therefore, records linear force data not torque. Also, because ROM during the RVD phase was measured, any positive training effect would be demonstrated as a decrease in RVD. Statistical Analyses Two univariate (RVD and force) 4-way (2 velocities 3 2 genders 3 2 times 3 3 groups) mixed factorial analysis of variance (ANOVA) was performed to analyze the data for interactions that involved time and group. These interactions were followed up by 3-way, 2-way, and 1-way ANOVA as indicated by the data to determine relevant significant differences between dependent variables. An a priori alpha level of p # 0.05 was used for significance, and all data analyses were performed using SPSS V.10.0 software. Intraclass correlation coefficients (ICC) were also performed for RVD and force of the control group to determine test-retest reliability.
Results Table 1 depicts the physical characteristics of all subjects by group and gender. One-way ANOVA demon-
strated that men and women were similar in age, but men were significantly taller and heavier. There were no differences between combined gender cells. Reliability At 1.04 rad路s21, (see Table 2) a high significant ICC was found for force but not for RVD. Also, ANOVA revealed no significant differences between mean scores of either variable between pre- and posttests. At 4.18 rad路s21, high significant ICC values were found for both RVD and force, but again ANOVA revealed no significant differences between mean scores of either variable between pre- and posttests (Table 2). Analysis of Variance Both RVD and force demonstrated a main effect for velocity and gender. On the basis of these results, gender was removed as a variable. There were no 4-way interactions for either variable, but RVD demonstrated a 3-way interaction, which required further statistical measures, whereas force exhibited no interaction of interest. There was a significant (p , 0.05) 3-way interaction for RVD between velocity, time, and group whereby the model was decomposed to 3 separate 2 by 2 repeated measures of ANOVA for velocity by time for each group. The results exhibited a significant interaction between velocity and time for the slow and fast groups but not for the control group. A 1-way ANOVA was then performed on pre- and posttest RVD scores at each velocity for each group (see Table 3). Significant
Table 1. Mean 6 SD physical characteristics of subjects by gender and group (male and female combined cells, n 5 20). Male (N 5 30) Control Age (y) Height (cm) Weight (kg)
26.1 6 4.09 179.32* 6 7.20 80.81* 6 9.09
Slow 26.8 6 5.92 178.81* 6 6.01 87.18* 6 8.70
* Significantly greater than females.
Female (N 5 30) Fast 27.4 6 5.99 177.55* 6 7.42 74.64* 6 8.57
Control 24.1 6 4.14 167.89 6 8.42 62.31 6 9.50
Slow 22.9 6 3.14 165.86 6 7.92 57.36 6 7.35
Fast 26.4 6 5.71 164.46 6 7.50 59.72 6 5.67
Short-Term Training, Acceleration, and Power 91
Table 3. Results (mean 6 SE) of rate of velocity development (8) and force (N) 1-way analysis of variance by time at both velocities for the slow and fast groups. Pre
Post
p
Slow 1.04 rad·s21 RVD 1.25 6 0.04 Force 527.83 6 38.7
1.08 6 0.03 514.61 6 30.2
0.004 0.343
4.18 rad·s21 RVD 13.73 6 0.36 Force 328.77 6 26.1
13.94 6 0.38 345.98 6 22.4
0.221 0.087 Figure 3. Rate of velocity development (RVD) by group at 4.18 rad·s21.
Fast 1.04 rad·s21 RVD 1.20 6 0.04 Force 457.54 6 25.1
1.12 6 0.02 446.60 6 23.7
0.107 0.310
4.18 rad·s21 RVD 14.24 6 0.33 Force 308.13 6 18.7
13.59 6 0.29 325.83 6 19.2
0.020 0.109
Figure 2. Rate of velocity development (RVD) by group at 1.04 rad·s21.
decreases in RVD were found during posttesting at the slow velocity for the slow group (see Figure 2) and at the fast velocity for the fast group (see Figure 3). Peak force demonstrated significant main effects for velocity and gender but no significant differences between times (see Tables 2 and 3).
Discussion This study determines the effect of short-term isokinetic training on limb acceleration or RVD. The primary results indicate that explosive limb acceleration may be increased in the absence of force improvements and that these increases are velocity-specific. Furthermore, this may be explained as a neural adaptation, and these improvements are similarly expressed across genders. An acceleration phase at the beginning of an isokinetic movement has been described in detail because
it relates to dynamometry (9). Osternig et al. (23) tested college athletes’ knee extension and flexion isokinetically through limb velocities of 0.87–6.96 rad·s21. The portion of ROM that was spent at a constant limb velocity decreased from 92 to 16% from slow to fast, respectively. Thus, it is known that as velocity increased a smaller portion of the movement was isokinetic. Further studies have quantified RVD and load range. Present dynamometry software enables the researcher to differentiate portions of an isokinetic repetition, thereby allowing phase-specific analysis of human movement (3). Brown et al. (8) described the ROM during constant velocity where resistance is encountered resulting in overload to the limb as load range. Load range is characterized as the constant velocity phase and is entirely dependent on the RVD. Previous work performed using the Kin-Com system (11) has reported ICC values for static force of 0.99, whereas dynamic isokinetic testing on other dynamometers (4, 5) at velocities between 0.52 and 3.65 rad·s21 has revealed torque ICC values ranging from 0.95 to 0.99. At both velocities in the present study, control subjects exhibited a nonsignificant absolute force increase between 7 and 10 N or approximately 2 lbs, thereby exhibiting very little variability in knee extension strength over approximately 7 days in the absence of a training stimulus. Farrel and Richards (11) documented the velocity error of the Kin-Com to be within 2%, whereas Taylor et al. (27) calculated an error rate of 3.5% of the preselected velocity at speeds up to 7.83 rad·s21 on the Biodex. Although previous research has detailed maximum velocity error rates, it has failed to calculate the phase-specific reliability essential to measurement of accelerative or RVD movements. This investigation, however, detailed low-to-moderate reliability of RVD at 1.04 rad·s21 with an ICC of 0.40, whereas high reliability was demonstrated at 4.18 rad·s21 with an ICC of 0.87. Thus, phase-specific velocity measurements at low velocities (1.04 rad·s21) exhibit error levels that may
92 Brown and Whitehurst
hamper any attempt to measure training adaptations associated with isokinetic training by masking small increases in limb acceleration. In contrast, high-velocity, phase velocity measurements exhibit high reliability from test to retest and may serve as critical indicators of training adaptations. One explanation for greater reliability of RVD at high velocities when compared with low velocities may be that the measurement apparatus was not sensitive enough to detect the minute variability among human participants because of the very small RVD at slow velocities. The absolute RVD values exhibited in this study compare well with previous research on similar dynamometers. Measures on a Biodex system have documented RVD levels ranging between approximately 1.5 and 178 when collected using shoulder (6) and knee protocols (7, 8), whereas the current study identified a range of approximately 1 and 148. As expected, RVD demonstrated a strong positive relationship with velocity and significant differences between genders. These results have been well documented in previous research investigations (6–8) involving similar experimental design, subject populations, and data collection methodology. Given that no significant gender by group by time interactions were manifest, any further discussion of these specific results is unnecessary. This study resulted in significant decreases in RVD in a velocity-specific manner for each training group. Thus, short-term isokinetic training may be sufficient to elicit a neural response demonstrated as increased limb acceleration. An unexpected finding of this study was that RVD not only increased for the fast group but also for the slow group at their relative training velocities. Apparently the intent to move quickly at a slow velocity was sufficient to elicit a neural adaptation even in the absence of actual high-velocity limb movement. This is in agreement with previous work documenting fast velocity adaptations because of intended movement (2). This investigation relies heavily on the theory underlying specificity of training (12, 21). This is based on research supporting the concept that resistancetraining effects are most robust when performed at the velocity in which the actual activity occurs. It is clear that strength adaptations are greatest when exercises are performed in a velocity-specific training manner (17). The decrease in RVD exhibited in this study is evidence that velocity-specific training can serve to increase limb acceleration in the absence of force alteration. As such, trained subjects are able to produce force more rapidly even though their absolute peak force remains unaffected. The ability to produce force rapidly may have implications in a power paradigm where the rate of force production may be deemed to be of greater importance than the magnitude of force. The acute effects of 1 low-velocity, high-overload
group and 1 high-velocity, low-overload group demonstrate that neither had a significant effect on peak force production. Peak force data presented here were predictable in that it demonstrated a significant inverse relationship with velocity, as well as significant differences between genders. These relationships have been shown in similar studies (4, 13, 18). However, an interesting finding was that both genders appeared to adapt to isokinetic training in the same way. That is, although women exhibited less peak force when compared with men, they reacted to the short-term training equally by displaying no significant increases in leg strength. One of the most interesting results of this study was that force failed to demonstrate sensitivity to velocity-specific training for a short time period. In contrast, a similarly designed study by Prevost et al. (24) involving 2 days of isokinetic training, reported strength improvements of approximately 22% in the fast-training group (;5.22 rad·s21) only. A possible explanation for this discrepancy between studies is that Prevost failed to perform proper reduction techniques in order to account for extraneous data collected during the acceleration and deceleration repetition phases. The spurious artifacts associated with nonload range data, such as torque overshoot and impact artifact, increase linearly with velocity (6, 8, 26, 29). Therefore, the significant torque improvements reported at the fast velocity of 5.22 rad·s21 may be a function of limb accelerative improvements. In this way, a faster RVD may have resulted in greater artifactual data when compared with slower velocities secondary to the forceful effect between the accelerating lever arm and the dynamometer’s velocity restricting servomotor. Another recent study (1) demonstrated short-term improvements in isokinetic torque at velocities between 1.04 and 4.18 rad·s21 but used a protocol consisting of 4 times the frequency and twice the exercise volume (9 days; 10 sets of 5 repetitions) reported in this study (2 days; 3 sets of 8 repetitions). This increase in the amount of exercise performed undoubtedly contributed to the neuromuscular adaptation demonstrated by their subjects. However, when the quadriceps cross-sectional area of each subject was measured by magnetic resonance imaging at the conclusion of training, muscle fiber area revealed no changes. In contrast, the electrical activity of the quadriceps, as a percentage of total cross-sectional area, increased significantly. These results point to a learned facilitation of motor unit recruitment as a function of the training stimulus. Force, being a derived variable (F 5 MA), is influenced equally by a change in either mass or acceleration. Furthermore, increases in acceleration are associated with the ability to produce force quickly, which is an essential element in the performance of many sporting events and daily activities. This rapid rate of force development (RFD) is analogous to a power mea-
Short-Term Training, Acceleration, and Power 93
surement. Although a detailed discussion of power is beyond the scope of this article, a training program that results in an acute increase in RFD while maintaining absolute force has wide ranging applications to exercise prescription designed to affect explosive work produced under time constraints. A recent review of the biomechanics associated with falls (25) reported a strong relationship between step length, magnitude of force generated at ground contact, and step execution time, defined as total elapsed time from foot placement under the body to floor contact after stepping forward. Their results demonstrated that very short-step execution times were associated with low force generation, whereas long-step times required greater force output. A model generated from the study calculated that increasing step time by 75% while maintaining step length would require a force increase of approximately 82%. In contrast, increasing step velocity by approximately 65% (therefore decreasing step time) would require approximately 50% less force output. An 82% strength increase is unrealistic in an elderly population who are at the greatest risk of falling; however, short-term increases in RFD, and similarly in RVD, may attenuate the need for greater strength. Therefore, relative to this study, elderly persons might be better served by training to move quickly as opposed to a protracted strength-training regimen with concomitant hypertrophic adaptations. Previous work has also observed increases in explosive-type activity, such as vertical jumping, because of explosive exercise. A training study (15) consisting of 16 weeks of knee-extension exercise demonstrated a strong correlation between increased dynamic strength and the time to reach a submaximal isometric force level (RFD). After detraining RFD was maintained, whereas absolute force decreased significantly. A follow-up study (16) over 24 weeks using jumping exercises with light loads resulted in significant improvements in RFD with little or no improvement in absolute force. Furthermore, this increase in RFD was associated with an increase in muscular electrical activity with little or no hypertrophic changes. Newton et al. (22) also conducted an investigation using squat jumps in which the subjects were divided into 3 training groups consisting of body weight only, body weight plus a 20-kg load, and body weight plus a 40kg load. Their results described a significant 47% increase in RFD for the body weight–only group, whereas the 2 overload groups demonstrated no changes. In short, considerable neural adaptations may occur under high-velocity, low-load training conditions that are responsible for performance enhancement. It is clear from the previous discussion that highvelocity, low-load training is related to an ability to produce force quickly and that this ability has implications for activities in everyday life as well as athletic
endeavors. Increases in limb acceleration after shortterm velocity-specific training, as measured by RVD increases detailed in this study, may provide an insight into the analogous behavior of force and velocity.
Practical Applications This study was unique in that it documented acute limb acceleration changes after short-term velocityspecific training with the maintenance of force production. This RVD accelerative increase points to a more rapid RFD that may be important in activities necessitating explosive movement, including sporting events requiring power. Furthermore, an increase in RVD may offset a force decrement with increased velocity. This information may assist in designing highvelocity, low-load resistance exercise programs to maximize human performance.
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Address correspondence to Lee E. Brown, leebrown@ fullerton.edu.