Randall L. Jensen

Quantifying plyometric intensity via rate of force development, knee joint, and ground reaction forces.

Because the intensity of plyometric exercises usually is based simply upon anecdotal recommendations rather than empirical evidence, this study sought to quantify a variety of these exercises based on forces placed upon the knee. Six National Collegiate Athletic Association Division I athletes who routinely trained with plyometric exercises performed depth jumps from 46 and 61 cm, a pike jump, tuck jump, single-leg jump, counter- movement jump, squat jump, and a squat jump holding dumbbells equal to 30% of 1 repetition maximum (RM). Ground reaction forces obtained via an AMTI force plate and video analysis of markers placed on the left hip, knee, lateral malleolus, and fifth metatarsal were used to estimate rate of eccentric force development (E-RFD), peak ground reaction forces (GRF), ground reaction forces relative to body weight (GRF/BW), knee joint reaction forces (K-JRF), and knee joint reaction forces relative to body weight (K-JRF/BW) for each plyometric exercise. One-way repeated measures analysis of variance indicated that E-RFD, K-JRF, and K-JRF/BW were different across the conditions (p > 0.05), but peak GRF and GRF/BW were not (p > 0.05). Results indicate that there are quantitative differences between plyometric exercises in the rate of force development during landing and the forces placed on the knee, though peak GRF forces associated with landing may not differ.

Reliability of the reactive strength index and time to stabilization during depth jumps

Flanagan, EP, Ebben, WP, and Jensen, RL. Reliability of the reactive strength index and time to stabilization during depth jumps. J Strength Cond Res 22(5): 1677-1682, 2008-The reliability of the reactive strength index (RSI) and time to stabilization (TTS) during maximal-effort plyometric depth jumps was examined. Twenty-two subjects performed three depth jumps from a height of 30 cm. Measures such as height of jump (JH), ground-contact time (CT), RSI, and TTS were obtained and analyzed for reliability using Cronbach alpha reliability coefficient and intraclass correlations. The JH, CT, and RSI were shown to be highly reliable from trial to trial (evidenced by high Cronbach reliability coefficients (α > 0.95) and high single- and average-measures intraclass correlations (>0.9). Time to stabilization was not reliable from trial to trial, as evidenced by a low Cronbach reliability coefficient (α < 0.7) and poor single- (<0.5) and average-measures (<0.7) intraclass correlations. The RSI was observed to be consistent for single measures, suggesting that coaches dealing with large numbers of athletes can conduct only a single trial from each depth jump height when attempting to optimize plyometric depth jump heights for their athletes. Time to stabilization could be a useful tool for strength and conditioning investigators to quantify the landing portion of plyometric exercises, but the protocol used in the current study to measure this variable did not prove to be reliable. Investigators wishing to use this measurement in such a context in future research studies may need to allow subjects appropriate habituation periods and control for arm movement during the landing phase of the exercise.

THE OPTIMAL COMPLEX TRAINING REST INTERVAL FOR ATHLETES FROM ANAEROBIC SPORTS

Comyns, T.M., A.J. Harrison, L.K. Hennessy, and R.L. Jensen. The optimal complex training rest interval for ath- letes from anaerobic sports. J. Strength Cond. Res. 20(3):471- 476. 2006.—Complex training research has indicated that 3-4 minutes may be an optimum intracomplex rest interval. The purpose of this study was to determine if a heavy resistive ex- ercise causes performance enhancement of a slow stretch-short- ening cycle exercise and if there is an optimal rest interval. Eighteen subjects performed countermovement jumps (CMJs) before and after a 5 repetition maximum back squat lifting pro- tocol. This procedure was repeated 4 times over 2 days using rest intervals of 30 seconds and 2, 4, and 6 minutes. Flight time and peak ground reaction force (GRF) were the dependent var- iables. All jumps were performed on a specially constructed sledge and force platform apparatus. Repeated measures anal- ysis of variance found a significant reduction in flight time at the 30-second and 6-minute interval (p 0.05). No significant difference was found between men and women. Only the men showed an enhancement in jump performance after the 4-min- ute interval. The improvement window was different for each subject, and an analysis of the greatest increase and decrease in flight time and peak GRF was conducted, showing a significant decrease for men and women and a significant increase in flight time for men and peak ground reaction force for women. The results suggest that complex training can benefit or inhibit CMJ performance depending on the rest interval. The individual de- termination of the intracomplex rest interval may be necessary in the practical setting.

EVALUATION OF PLYOMETRIC INTENSITY USING ELECTROMYOGRAPHY

The purpose of this study was to investigate the motor unit activation of the quadriceps (Q), hamstring (H), and gastrocnemius (G) muscle groups during a variety of plyometric exercises to further understand the nature of these exercises. Twenty-three athletes volunteered to perform randomly ordered plyometric exercises, thought to cover a continuum of intensity levels, including two-foot ankle hops; 15-cm cone hops; tuck, pike, and box jumps; one- and two-leg vertical jump and reach; squat jumps with approximately 30% of their 1RM squat load; and 30- and 61-cm depth jumps. Integrated electromyographic data were analyzed for each exercise using a one-way repeated-measures ANOVA. Results revealed significant main effects for the Q when all subjects are analyzed, as well as for separate analysis of men, women, subjects with vertical jumps greater than 50 cm, and those with vertical jumps less than or equal to 50 cm (p ≤ 0.05). Significant main effects were also found for the G muscle group in the analysis of all subjects, as well as for men and subjects with vertical jumps greater than 50cm (p ≤ 0.05). No significant main effects were found for the H muscle group. Pairwise comparisons revealed a variety of differences among plyometric exercises. In some cases, plyometrics previously reported to be of high intensity, such as the depth jump, yielded relatively little motor unit recruitment compared with exercises typically thought to be of low intensity. Results can assist the practitioner in creating plyometric programs based on the nature of the motor unit recruitment.

Identifying the optimal resistive load for complex training in male rugby players

Alternating a resistance exercise with a plyometric exercise is referred to as “complex training”. In this study, we examined the effect of various resistive loads on the biomechanics of performance of a fast stretch–shortening cycle activity to determine if an optimal resistive load exists for complex training. Twelve elite rugby players performed three drop jumps before and after three back squat resistive loads of 65%, 80%, and 93% of a single repetition maximum (1-RM) load. All drop jumps were performed on a specially constructed sledge and force platform apparatus. Flight time, ground contact time, peak ground reaction force, reactive strength index, and leg stiffness were the dependent variables. Repeated-measures analysis of variance found that all resistive loads reduced (P < 0.01) flight time, and that lifting at the 93% load resulted in an improvement (P < 0.05) in ground contact time and leg stiffness. From a training perspective, the results indicate that the heavy lifting will encourage the fast stretch–shortening cycle activity to be performed with a stiffer leg spring action, which in turn may benefit performance. However, it is unknown if these acute changes will produce any long-term adaptations to muscle function.

Jaw clenching results in concurrent activation potentiation during the countermovement jump.

Ebben, WP, Flanagan, EP, and Jensen, RL. Jaw clenching results in concurrent activation potentiation during the countermovement jump. J Strength Cond Res 22(6): 1850-1854, 2008-The study assessed the effect of current activation potentiation by evaluating jaw clenching and its effect on the rate of force development (RFD), time to peak force (TTPF), and peak force (PF) during the countermovement jump. Fourteen subjects performed the countermovement jump on a force platform while maximally clenching their jaw on a dental vinyl mouthguard (JAW) as well as without clenching their jaw by jumping with an open mouth (NON-JAW). Results reveal that the RFD was 19.5% greater in the JAW compared with the NON-JAW condition (p < 0.05). The TTPF was 20.15% less in the JAW compared with the NON-JAW condition (p < 0.05). There were no significant differences (p = 0.60) in PF between the JAW and NON-JAW conditions. These findings indicate that concurrent activation potentiation is manifested through jaw clenching during the countermovement jump. As a result, athletes may employ this strategy of maximally clenching their jaws to gain an ergogenic advantage during the countermovement jump.

Use of EMG analysis in challenging kinetic chain terminology

PURPOSE The purpose of this study was to compare mean integrated electromyographic values (EMG(int)) during biomechanically comparable closed kinetic chain/fixed external load, open kinetic chain/moveable no load, and moveable external load exercises. METHODS EMG(int) were obtained for the pectoralis major (PM) and the long head of the triceps (T) during absorption and force phases of three biomechanically comparable exercises: the push-up (PU); the bench press-load (BP-L); and the bench press-no load (BP-NL) equated to the closed kinetic chain/fixed external load; moveable external load; and open kinetic chain/moveable no load conditions respectively. A force plate was used to equate load for the PU and BP-L exercises. Ten males (24+/-4.4 yr) participated in the three randomly ordered exercises. A pronated, closed grip was used for all exercises. The mean integrated EMG values from three isometric maximal voluntary contractions were used to determine a reference EMG (EMG(MVC)) for each muscle. Normalized EMG values (NEMG) were determined by EMG(int)/EMG(Mvc) and analyzed by one-factor repeated measures ANOVA for each muscle (PM and T) during each phase (absorption and force). RESULTS There was no significant difference (P > 0.05) between the equivalently loaded, different boundary exercises (PU and BP). Statistical significance (P < 0.05) was determined between the differently loaded, equivalent boundary exercises (BP and BP-NL) as well as the differently loaded, different boundary exercises (PU and BP-NL). CONCLUSIONS Results from this study support the theory that activities of similar biomechanical motions and mass of loading, regardless of the boundary condition, have comparable EMG values of primary muscle groups. In addition, this study suggests external load is more important than boundary condition in describing human activity.

Strength training for women: debunking myths that block opportunity.

Traditional gender roles and differences in absolute strength have resulted in misconceived approaches to strength training for women. Male physiology, more than hormones, explains mens superior absolute strength. When other measures of strength are used, such as strength relative to cross-sectional area of muscle, the strength of men and women is nearly equal. Women who practice the same well-designed strength training programs as men benefit from bone and soft-tissue modeling, increased lean body mass, decreased fat, and enhanced self-confidence.

Bilateral facilitation and laterality during the countermovement jump.

This study assessed bilateral facilitation and laterality during counter-movement jumps. 23 NCAA Division-I athletes (13 men, 10 women) who participated in track and field volunteered to serve as participants and performed bilateral, right, and left leg countermovement jumps on a force plate. Dependent variables included jump height, reactive strength index, and time to stabilization. A one-way analysis of variance showed significant sex differences in single-leg and bilateral jump height and left leg and bilateral reactive strength index. No sex differences were found for time to stabilization. A repeated-measures analysis of variance indicated significant main effects for jump height and reactive strength index but not for time to stabilization. Bonferroni adjusted pairwise comparisons showed that bilateral jump height and reactive strength index values were different from the unilateral conditions. No differences were found between the right and left legs for jump height, reactive strength index, or time to stabilization. Results indicated no evidence of laterality as assessed by jump height, reactive strength index, and time to stabilization. Bilateral facilitation was found as assessed by jump height and reactive strength index. Time to stabilization demonstrated a bilateral deficit phenomenon.

A Comparison of Laser and Video Techniques for Determining Displacement and Velocity During Running

The reliability of a laser system was compared with the reliability of a video-based kinematic analysis in measuring displacement and velocity during running. Validity and reliability of the laser on static measures was also assessed at distances between 10 m and 70 m by evaluating the coefficient of variation and intraclass correlation coefficient for each distance. Ten participants completed 3 running trials at self-determined fast, medium, and slow speeds. Running velocity was measured simultaneously by a laser and 2 video cameras (1 at 50 Hz, 1 at 100 Hz) within a 3-m measurement zone (νzone). The Root Mean Square errors for static distance measurement using the laser varied with distance, ranging from 4.5 mm at 10 m to 17.8 mm at 70 m. The test-retest reliability of running velocity measurements in the 3-m zone for the 50 Hz and 100 Hz cameras via intraclass correlation were estimated at .984 and .981, respectively. For the laser, test-retest reliability was estimated at ICC = .986. The results of this study indicated that the laser distance measurement device produced valid and reliable estimates of distance. When used within its limitations, the laser system will provide valid and reliable results, which can be made immediately available to the coach, performer, or user without the need to carry out time-consuming digitizing procedures.