Concurrent activation potentiation (CAP) has been proposed to enhance prime mover performance via the simultaneous contractions of muscles remote from the prime mover (4). The simultaneous contractions of muscles remote from the prime mover have been described as remote voluntary contractions (RVCs), which increase lower body reflexes (1,12,14), performance during isometric testing (7,15), and the countermovement jump (6).
Remote voluntary contractions have been shown to increase lower body reflexes in clinical populations during the Jendrassik maneuver (1,12,14), potentially as a result of motor overflow (3). The relationship between some RVCs, such as jaw clenching and muscular performance, has its historical roots in the 1970s and 1980s sports dentistry literature, though the results of this work and the methods employed have been questioned (8). Subsequent studies using nonathlete or clinical subjects revealed that RVCs such as jaw clenching produced an increased rate of force development (RFD) during hand gripping tasks (10), and increased isometric, but not isokinetic plantar flexion torque (15).
A recent review of the literature introduced the concept CAP and outlined potential methods for optimizing this method of training (4). Since then, researchers studying the effect of RVCs determined that an aggregate of jaw clenching, hand gripping, and the Valsalva maneuver was more effective than jaw clenching or hand gripping alone (7). In this study, the aggregate RVC condition produced isometric average torque and peak torque that was 14.6 and 14.8% higher in the RVC compared to the NO-RVC condition (7). However, given the specious history of the effect of RVCs on muscular performance (8) and the fact that RVCs have been demonstrated to be effective during isometric (7,15), but not during some dynamic tasks (15), the effect of RVCs on dynamic performance requires further investigation.
Only one published study has examined the effect of RVCs during a dynamic athletic event (6). In this study, subjects produced a 19.5% higher RFD and 20.2% faster time to peak force during the countermovement jump while jaw clenching, compared to in the nonjaw clenching condition (6). However, these subjects did not produce greater peak force in the jaw clenching condition. Thus, the potential of RVCs as a potentiation phenomenon for dynamic athletic tasks remains uncertain. The effectiveness of a comprehensive aggregate of RVCs has been demonstrated during isometric testing (7). However, the effect of RVCs on the performance of dynamic tasks including commonly used resistance training exercises such as the squat and loaded squat jump is yet to be investigated. The squat and loaded jump squat were included for analysis because they are commonly used resistance training exercises, performed in the closed kinetic chain, and provide differing force and velocity requirements.
Therefore, the purpose of this study was to compare RVC and NO-RVC conditions of the back squat and jump squat and assess the resultant ground reaction forces (GRFs), RFD, and jump height to determine if there are differences between the conditions during dynamic closed kinetic chain strength tasks.
Experimental Approach to the Problem
This study evaluated the hypothesis that there are differences between RVC and NO-RVC conditions during ground-based closed kinetic chain resistance training and loaded jumps. Independent variables included the contraction condition. Dependent variables included peak GRF, RFD during the first 100 milliseconds of the concentric phase (RFD-100), RFD to peak force (RFD-P), and jump squat height (JH), which were included to provide a global measure of force development, to assess the RFD during fast and slow stretch shortening cycle phases, and to obtain a functional measure of lower body power, respectively.
Subjects included 13 men (mean ± SD, age 21.4 ± 1.5 years, body mass 87.1 ± 15.74 kg, 5RM back squat 132.8 ± 22.4 kg, countermovement jump height 62.6 ± 8.6 cm) who participated in intercollegiate or recreational athletics and lower body resistance training with exercises that included knee extension with a frequency of at least 2-3 training sessions a week, for at least 2 months. Subjects included 3 college football players, 2 college track athletes, and 1 college hockey player. Subjects who were college athletes were all participating in their off-season resistance training programs. Six of the subjects previously participated in high-school track, 4 each participated in basketball and football and 1 each participated in wrestling, soccer, hockey, golf, and crew. Subjects who were former high-school athletes were participating in their own self-designed resistance training programs at various phases of their periodization scheme. Exclusion criteria included any history of lower limb pathology that resulted in functional limitation of the exercises to be assessed in this study. Subjects refrained from participating in resistance training for at least 48 hours before the test session. The subjects were informed of the risks associated with the study and signed informed written consent. The study was approved by the institution's internal review board.
A pretest habituation session was conducted to determine the subjects' 5 repetition maximum (RM) back squat load and countermovement jump height. During this session, subjects performed each of the test exercises in the RVC and NO-RVC conditions. In the RVC condition of the back squat and jump squat, subjects were instructed to maximally clench their jaw on a dental vinyl mouth guard (Cramer Products Inc., Gardner, KS, USA), grip forcefully on the barbell and pull it down into their trapezius and perform a brief Valsalva maneuver during the concentric phase of the exercise. The NO-RVC condition included the subjects using their preferred method of gripping the barbell, performing the exercises with an open mouth and pursed lips to limit the likelihood of jaw clenching, and consistent cycling between inspiratory and expiratory flow to reduce the Valsalva effect. These methods were similar to those previously used (7).
During the test session, subjects warmed up for 5 minutes by doing light exercise on a rowing ergometer and performed dynamic stretching exercises for approximately 15 seconds for each major muscle group. Subjects then performed 2 repetitions each of the back squat with their previously assessed 5RM load and the jump squat with an added load equivalent to 30% of the subjects previously estimated 1RM of their back squat. The test exercises and the order of the RVC and NO-RVC conditions were randomized, and 5 minutes of rest was provided between the exercises to reduce fatigue and order effects. Subjects were instructed to perform maximally and were encouraged equally for all test sets.
The test exercise was assessed with a 60 × 120-cm force platform (BP6001200, Advanced Mechanical Technologies Incorporated, Watertown, MA, USA) that was bolted to the laboratory floor according to manufacturer specifications and mounted flush in the center of a 122 × 244-cm weightlifting platform. The force platform was calibrated with known loads to the voltage recorded before the testing session. Kinetic data were collected at 1,000 Hz, real time displayed, and saved with the use of computer software (BioAnalysis 3.1, Advanced Mechanical Technologies, Inc.) for later analysis. Peak GRF, RFD-100, RFD-P, and JH were calculated from the force-time records consistent with methods previously used (13). All values were determined as the average of 2 trials for each exercise. Peak GRF during the concentric phase was defined as the highest value attained. The RFD-100 and RFD-P were defined as the first peak of GRF minus the initial GRF during the concentric phase divided by the time to the first peak of GRF minus the time of initial GRF, and normalized to a second.
All data were analyzed with SPSS 16.0 using an analysis of variance to evaluate the differences between the RVC and NO-RVC conditions. The reliability of the 2 trials was assessed using intraclass correlation coefficient (ICC), for each of the dependent variables. Assumptions for linearity of statistics were tested and met. Statistical power (d) and effect size (η2 p) are reported, and all data are expressed as mean ± SD. The a priori alpha level was set at p ≤ 0.05.
Peak GRF were significantly higher in the RVC compared to in the NO-RVC condition for both the back squat (p ≤ 0.05, η2 p = 0.45, d = 0.83) and jump squat (p ≤ 0.05, η2 p = 0.27 η2 p = 0.27, d = 0.48). The RFD-100 was significantly higher in the RVC compared to in the NO-RVC condition for both the back squat (p ≤ 0.05, η2 p = 0 .18, d = 0.33) and jump squat (p ≤ 0.05, η2 p = 0.32, d = 0.60). The RFD-P was not significantly higher in the RVC compared to in the NO-RVC condition for the back squat (p = 0.82) but was significantly higher in the RVC compared to in the NO-RVC condition for the jump squat (p ≤ 0.05, η2 p = 0.51, d = 0.90). Finally, the JH was significantly higher in the RVC compared to in the NO-RVC condition for the jump squat (p ≤ 0.05, η2 p = 0.34, d = 0.55). Data for the back squat and jump squat are presented in Tables 1 and 2, respectively. Results of the ICC of each dependent variable are presented in Table 3.
This is the first study to investigate the effects of CAP during ground-based exercises such as the back squat and jump squats, demonstrating that subjects in the RVC condition, compared to in the NO-RVC condition, accrued statistically significant higher performances of 2.9-32.2% for all outcome variables, with the exception of the RFD-P during the squat. Previous research examining the effect of CAP during ground-based closed kinetic chain exercise demonstrated 19.5% higher RFD during the countermovement jump while in the RVC compared to in the NO-RVC condition (6). This measure can be compared to RFD-P from the present study because both values were calculated using the same methods.
In the present study, the GRF for the jump squat and back squat was 2.9-4.0% higher in the RVC and NO-RVC conditions. This finding differs from previous research examining the countermovement jump in RVC and NO-RVC conditions, which found no significant difference in GRF between conditions (6). The differences in GRF between these studies may be because of a more comprehensive aggregate of RVCs used in the present study. Previous research demonstrated that comprehensive RVCs, such as those used in the present study, produced up to 4.0% more average torque than jaw clenching alone, which was used in the study by Ebben et al. (6).
The differences between the RVC, compared to the NO-RVC condition, were larger for the RFD-100 than for the RFD-P for both the back squat and the jump squat. This finding raises the possibility that CAP may be more effective during the earlier portion of the concentric phase of ground-based exercises and may be optimized during fast stretch shortening cycle activities that are often defined as those occurring within 100 milliseconds (16). The idea that the CAP effect may decrease during movements with a longer concentric phase, and thus a longer time to peak force, is seen in the fact that the RVC compared to NO-RVC difference in RFD-P was larger during the countermovement jump (6) compared with the squat jump and not present at all for the back squat in the present study. Previous research revealed that men demonstrated an average time of peak force of 303 milliseconds during the countermovement jump (5). In the present study, the average time to peak force for the jump squat and back squat was 740 and 1,668 milliseconds, respectively. The belief that the CAP effect may be multiphasic has previously been reported (4) and is consistent with the idea that RVCs such as the Jendrassik Maneuver demonstrate a triphasic response characterized by initial maximal intensity because of supraspinal facilitation, followed by a phase of H-reflex or local facilitation and a concomitant decrease in reflex intensity (1,2,9).
Subjects in the present study demonstrated a significant increase in jump squat JH, providing additional practical evidence that CAP may be effective during ground-based exercise. This study demonstrated that RVCs enhance the performance of ground-based closed kinetic chain tasks, in addition to the countermovement jump (6) and exercises performed with isometric muscles actions (7,15), and stands in contrast to previous research demonstrating that RVCs did not work during dynamic isokinetic exercises (15). This finding may be partly because of comprehensive RVCs used in the present study, compared to the jaw clenching alone used by Sasaki et al. (15), which has been demonstrated to be less effective (7). Findings of the present study add the research demonstrating the effectiveness of RVCs and the CAP phenomenon and support the early beliefs that RVCs are effective even if the studies used to evaluate them were not (8). To date, data from studies examining CAP demonstrate ergogenic effects that typically exceed the magnitude of those demonstrated for other potentiation phenomenon such as post activation potentiation (11).
As previously reported (6), the role of RVCs and their effects on oral structures and the temporomandibular joint, teeth, and gums are unknown. Anecdotal observations suggest that some exercisers use RVCs such as jaw clenching naturally, with little documented evidence of injury to oral structures. The addition of mouth guards likely decreases the potential for trauma to these oral structures and should be used.
Strength and conditioning practitioners often seek strategies that maximize the acute effects of the resistance training session. Based on the results of this study, strength and conditioning practitioners should encourage their athletes to perform RVCs such as jaw clenching on a mouth guard, a brief Valsalva maneuver, and forcefully gripping the barbell and pulling it down into the trapezius during the concentric phase of exercises such as the back squat and jump squat. This study demonstrates that these practices produced higher GRFs and RFD in most test conditions, and increased JH during the jump squat. Thus, these RVCs are likely to acutely enhance the quality of the resistance training workout for exercises such as those employed in this study.
1. Delwaide, PJ and Toulouse, P. Jendrassik maneuver vs. controlled contractions conditioning the excitability of soleus monosynaptic reflexes. Arch Phys Med Rehabil
61: 505-510, 1980.
2. Delwaide, PJ and Toulouse, P. Facilitation of monosynaptic reflexes by voluntary contraction of muscle in remote parts of the body: Mechanisms involved in the Jendrassik manoeuvre. Brain
104: 701-709, 1981.
3. Donoghue, J and Sanes, J. Motor areas of the cerebral cortex. J Clin Neurophysiol
11: 382-396, 1994.
4. Ebben, WP. A brief review of concurrent activation potentiation: Theoretical and practical constructs. J Strength Cond Res
20: 985-991, 2006.
5. Ebben, WP, Flanagan, EP, and Jensen, RL. Gender similarities in the rate of force
development and time to takeoff during the vertical jump. J Exerc Physiol
10: 10-17, 2007.
6. Ebben, WP, Flanagan, E, and Jensen, RL. Jaw clenching results in concurrent activation potentiation during the countermovement jump. J Strength Cond Res
22: 1850-1854, 2008.
7. Ebben, WP, Leigh, D, and Geiser, C. The effect of remote voluntary contractions
on knee extensor torque. Med Sci Sports Exerc
40: 1805-1809, 2008.
8. Gelb, H, Mehta, NR, and Forgione, AG. The relationship between jaw posture and muscular strength
in sports dentistry: A reappraisal. Cranio
14: 320-325, 1996.
9. Hays, KC. Jendrassik maneuver facilitation and fractionated patellar reflex times. J Appl Physiol
32: 290-295, 1972.
10. Hiroshi, C. Relation between teeth clenching and grip force
production characteristics. Kokubyo Gakkai Zasshi
70: 82-88, 2003.
11. Hodgson, M, Docherty, D, and Robbins, D. Post activation potentiation. Underlying physiology and implications for motor performance. Sports Med
35: 585-595, 2005.
12. Hortobagyi, T, Taylor, J, Petersen, N, Russell, G, and Gandevia, S. Changes in segmental and motor cortical output with contralateral muscle contractions and altered sensory inputs in humans. J Neurophysiol
90: 2451-2459, 2003.
13. Jensen, RL and Ebben, WP. Quantifying plyometric intensity via rate of force
development, knee joint and ground reaction forces. J Strength Cond Res
21: 763-767, 2007.
14. Pereon, Y, Genet, R, and Guiheneuc, P. Facilitation of motor evoked potentials: Timing of Jendrassik maneuver effects. Muscle Nerve
18: 1427-1432, 1995.
15. Sasaki, Y, Uneo, T, Taniguchi, H, and Ohyama, T. Effect of teeth clenching on isometric and isokinetic strength
. J Med Dental Sci
45: 29-37, 1998.
16. Schmidtbleicher, D. Training for power
events. In: Strength and Power in Sport
. Komi, PV, ed. Boston, MA: Blackwell Scientific, 1992. pp. 381-395.