Adaptations of short-term high-velocity isokinetic training vs. short-term plyometric training on vertical jump and isokinetic performance in physically active people

1. Introduction

Performance in several sport disciplines is related to the ability to produce large amounts of mechanical power or the rate of doing force [1, 2]. Isokinetic assessments have been extensively used for assessing change in performance conduct rehabilitation process or to examine muscle balance in untrained [3] or trained individuals [4, 5]. Furthermore, training with isokinetic devices have been effective to improve strength and mechanical power in different studies [4, 5, 6]. Short term isokinetic based intervention using velocity controlled movement resulted in significant isokinetic strength gains [2, 3, 7, 8]. Coburn et al. [8] examined the effects of 3 days isokinetic training programs (4 sets of 10 maximal repetitions at 30 ∘ ⋅ s - 1 or 270 ∘ ⋅ s - 1 ) on leg extension peak torque. Training with at 30 ∘ ⋅ s - 1 resulted in peak torque increases at both low and high velocities (30 and 270 ∘ ⋅ s - 1 ), whereas exercising at 270 ∘ ⋅ s - 1 improved performance at 270 ∘ ⋅ s - 1 alone. Using a very similar protocol Prevost et al. [2] reported no increase in peak torque at 30 ∘ , 150 ∘ or 270 ∘ ⋅ s - 1 for the low-velocity group, while significant increases were determined for the high-velocity group when performed at 270 ∘ ⋅ s - 1 . Moreover, short duration [2, 8] (2–3 days) isokinetic training programs have been able to induce significant changes in peak torque. They also investigated the relation between the increase in leg extension peak torque and the electromyographic (EMG) signal [2, 8].

Differently from isokinetic exercise, such as leg extension, which involves an open kinetic chain movement performed at controlled velocity, jumps are closed kinetic chain exercises, requiring the co-activation of several muscles acting over different joint velocities [9]. As plyometric exercises impose higher rate of force development along with superior level of muscle tension compared with conventional resistance training [10, 11], a variety of plyometric exercises have been widely used to increase performance in power related sports disciplines (e.g. jump or sprint). Several investigations have reported positive effect of plyometric protocols to increase vertical jump performance for both elite and recreationally trained athletes [12]. However, the effect of short-term high-velocity isokinetic training versus short-term plyometric training has not been thoroughly investigated. Therefore, the purpose of the present study was to compare the effects of a 3-week isokinetic training vs. plyometric training on isokinetic knee flexion and extension strength in physically active, though not specifically trained subjects. We hypothesized that following training, both plyometric and isokinetic training groups will demonstrate greater increases in isokinetic strength compared to the control group. Furthermore, the isokinetic training group will achieve greater increases in the concentric knee extension and flexion peak torque, average power and total work compared to the plyometric group.

2. Methods

3. Results

Pre and post values for the dependent variables were analyzed to determine if the distributions were normal using Kolmogorov-Smirnov test ( $p>$ 0.05).

Isokinetic tests: In ISO (i) ANOVA analysis revealed significant differences in Knee Flexion Average Power in the right leg ( 0.05, ES = 0.041). Moreover, post-hoc tests revealed an increase in knee extension peak torque in both legs, and average power ( 0.001) only on the right side. However, the rest of variables decreased after the training period (Table 3). Regarding PLY (ii), all variables significantly decreased after the training period, except for knee extension peak torque in both legs, and average power in the right leg. In the control group (iii) no significant differences were observed (Table 3).

With regard to the jump variables (Table 3), ANOVA revealed that absolute power in ABK jump increased significantly ( 0.001) for the ISO group (i). In PLY (ii), take off velocity and jump height increased in CMJ and ABK jumps ( 0.05), and so did absolute power in all SJ, CMJ and ABK jumps ( 0.01). Conversely, vertical velocity at takeoff and jump height in ABK ( 0.05) decreased in the control group (iii).

Table 1 describes the results of the Isokinetic test determined at pre and post intervention for the three analyzed groups.

4. Discussion

The purpose of this study was to compare the effects of a 3-week lower body protocol using an isokinetic or a plyometric exercise program on strength and jump performance.

After the 3-week intervention period, no significant differences in isokinetic strength and jumping performance were determined between groups. However, our results indicated that knee extension peak torque increased in both legs in ISO but not in PLY or C. In line with our results, some previous studies reported increases in isokinetic strength after short periods of training (2–5 total sessions) [7, 10]. The short-term strength gains in untrained individuals appeared to be due to neural adaptations, as described by Beck et al. [14]. Neural adaptations predominate in short-term isokinetic training, as shown by Prevost et al. [2] and Coburn et al. [8].

Prevost et al. [2] reported significant peak torque increases after 2 days of high-velocity isokinetic in healthy students. Likewise, Corburn et al. [8] found that a very short-term isokinetic training program, consisting of 3 training sessions during 1 week using a low velocity (30 ∘ ⋅ s - 1 ) resulted in an increase in peak torque at low and high velocities (30 and 270 ∘ ⋅ s - 1 respectively), whereas training at the high-velocity (270 ∘ ⋅ s - 1 ) increased peak torque only at 270 ∘ ⋅ s - 1 . Considering the results of previous studies that confirmed positive changes, we expected optimistic effects from short-term training protocols. In our study, the participants trained twice per week during 3 weeks. The higher number of sessions in our study might explain the increase in peak torque in ISO. Our hypotheses pronounced that more significant changes in isokinetic and jumping performance would occur in PLY and ISO vs. the C group due to training stimuli. In addition, ISO group would achieve superior increases in isokinetic strength than the PLY group. Nevertheless, our results do not support our hypotheses. Only the participants included in ISO reached significant increases in knee extension peak torque in right and left legs while no changes in the isokinetic strength was observed in the PLY group (Table 3).

Most researchers and practitioners appear to agree that plyometric training is the method of choice when aiming to improve athletes’ explosiveness and is considered as an important strategy for training the stretch-shortening cycle. It is the belief of some sports physiologists that neuromuscular adaptations contributing to explosive power occur early in the specific preparation of the periodization phase of training [21]. Indeed, ample evidence demonstrates that plyometric training is effective for improving tasks that rely on the stretch-shortening cycle [19, 20, 21], such as the CMJ, but no peak torque [12] even training at high velocities (300 ∘ ⋅ s - 1 ). This investigation heavily relies on the theory underlying specificity of training. However, PLY training did not improve jump performance, possibly because the time needed to produce adaptations was not enough. On the other hand, Morris et al. [3] showed that strength gains in peak torque subsequent to isokinetic resistance training for 7 weeks do not influence a simple functional performance task such as the standing long jump. Their findings highlight the importance of the principle of specificity as another type of muscular contraction occurs during isokinetic training. Positive effects of a resistance-training program are most robust when performed at the velocity at which the actual task occurs [7]. In our study the plyometric exercise program does not improve isokinetic strength in knee flexion and extension at high velocity. These results are in contrast to the findings of Remaud et al. [11] who found that the isotonic training group produced greater isotonic and isokinetic power than the isokinetic training group. In our study, the ISO group improved isokinetic strength whereas PLY group did not show improvements in isokinetic strength. The lack of enhancements found in this study also differs with the positive results of Prevost et al. [2], Evetovich et al. [6] and Morris et al. [3]. The limited number of training sessions in our study and/or the total amount of training load on each session could partially explain the findings.

Unexpectedly, the total work and average power decreased in the PLY group and in the ISO group. The decrease in total work and mean power revealed in this study proves the importance of velocity-specific training. These results are in contrast to the study of Brown and Whitehurst [7], who documented acute limb acceleration changes after short-term velocity specific training on a dynamometer with the maintenance of force production. This may be important in activities requiring high intensity impulsive movement, such as change of directions, accelerations or sprints in team sports. Although speculative, significant changes in jump performance would have been observed if, in addition to the three standard assessed tests, we had also included a more specific stretch-shortening cycle jump exercise, such as rebound drop jump. These types of actions have been shown to better reflect the ability to change quickly from an eccentric to concentric muscle action as occurred plyometric training [24].

The lack of enhancements found in total work and average power could also be attributed to the sample physical fitness characteristics (physically active people) in contrast with other studies whose participants were athletes. Pääsuke et al. [25] found a significant correlation between SJ and CMJ ( 0.05) with isometric strength in knee extension ( r , 0.62–0.83), and with isokinetic peak torque at 60 ∘ ⋅ s - 1 ( r , 0.72–0.80) in Nordic athletes. Also, Bosco et al. [17] found a significant correlation ( r , 0.51–0.64) between isokinetic knee extension torque and SJ in male volley-ball players. However, a low correlation between isokinetic strength variables and jumps test were found in female soccer players [26].

Another explanation could be that isokinetic dynamometers measure the quadriceps-hamstring strength during free knee extension and flexion, whereas jumping is a closed kinetic chain activity in which many muscles are implicated [27]. However, in male students, isokinetic torques could predict jumping work for 75% (CMJ) and 69% (SJ) when the data were normalized by sample body weight [28].

The results from this research could be useful to coaches and athletes, as a recommendation to opt for isokinetic strength training over plyometric training to improve peak force in physically active people. Our findings indicated that isokinetic training can be used effectively as a training method for rapidly improving peak force.

Our results suggest that plyometric training fails to provide any additional advantage over the isokinetic method in improving isokinetic strength in sports science and physical education students when conducted over short-term (3-week) intervention programs (i.e., 2 days per week for 3 weeks).

Short-term isokinetic training appears to improve the strength of the knee extensor in physically active people due probably to predomination of neural adaptations. Further investigation is required, though, to determine more precisely the specific physiological adaptation. No significant differences were found in SJ, CMJ or ABK compared with the baseline values. In our study the short-term plyometric training protocol have not induced changes in the vertical jump. This information may assist coaches, athletes and physical therapist in designing other short-term exercise programs to maximize performance.