There are various types of stretching exercises used according to individual preference of athletes or trainers. Ballistic, proprioceptive neuromuscular facilitation (PNF), static and dynamic stretching are the most used stretching techniques (Hedrick, 2000). Since its easy and safe application, static stretching is the first ranked and broadly preferred stretching method among athletes (Alter, 1997; Hedrick, 2000). Recently, a number of comprehensive review articles have indicated that prolonged static stretching can compromise isometric and isokinetic force output (Behm and Chaouachi, 2011; Behm et al., 2016; Kay and Blazevich, 2012; Simic et al., 2013), whereas dynamic stretching may have either no adverse effect or improve subsequent muscle strength performance (Sekir et al., 2010; Yamaguchi et al., 2007; Yamaguchi and Ishii, 2005), although the overall increases were of small to moderate magnitude (Behm and Chaouachi, 2011; Behm et al., 2016). When the studies were scrutinized, it is obvious that the strength measurements were performed only in the stretched muscles (Behm and Chaouachi, 2011; Behm et al., 2016; Kay and Blazevich, 2012; Simic et al., 2013). However, it is known that during sport activities antagonist muscle co-activations are common to prevent overloading to a joint (Aagaard et al., 1998). Stated in other words, the final external force is directly proportional to the force generated from the agonist muscles and indirectly proportional to the force generated from the antagonist muscles (Baratta et al., 1988; Draganich et al., 1989). On the basis of the evidence that static stretching decreases and dynamic stretching increases muscle strength performance, it can be hypothesized that alterations of muscle strength in the antagonist muscles, either increase with dynamic or decrease with static stretching exercises, could have impacts on the agonist muscles by decreasing or increasing their strength, respectively. Few studies have investigated the effects of antagonist muscle stretching on performance of the agonist muscles (Sandberg et al., 2012; Wakefield and Cottrell, 2015). Sandberg et al. (2012) aimed to investigate the effects of static stretching of antagonist musculature on multiple strength and power measures. They showed that static stretching of the hip flexors and ankle dorsiflexors may enhance vertical jump height and power during countermovement vertical jump, and static stretching of the hamstrings may generate greater isokinetic quadriceps torque production during high angular velocities (300[degrees]/sec), with no differences during slower angular velocities (60[degrees]/sec). Wakefield and Cottrell (2015) investigated the effects static stretching of hip flexor muscles on vertical jump height. The authors concluded that vertical jump height decreased by 1.74% following static stretching of the agonist muscles (hip extensors) and increased by 1.74% following static stretching of the antagonist muscles (hip flexors). Even though the effects of antagonist static stretching on jump performance provided from the literature, no study to date has investigated the effects of static and dynamic stretching of the antagonist muscles on strength performance of the agonist muscles.
Therefore, the aim of this study was to investigate if static and dynamic stretching exercises of the knee muscles (quadriceps and hamstring muscles) have any effects on concentric and eccentric isokinetic peak torques and electromyographic amplitudes (EMG) of the nonstretched antagonist muscles.
Twenty healthy male recreational athletes (24.8 [+ or -] 2.8 years; 1.77 [+ or -] 0.06 m; 72.7 [+ or -] 7.8 kg) voluntarily participated in this study. All subjects were recreational athletes, participating in regular sports activities like, running, soccer, basketball or tennis one time per week. The test procedure was conducted in the dominant leg of the subjects, which was the right leg in 14 and left leg in 6 subjects. The dominant leg was determined according to the declaration of the subjects which leg they are using naturally to kick a ball. Any subject with a current or recent ankle-, knee-, hip-, or low-back-related injury, complaining of swelling, pain or functional limitations in these joints, or having an obvious range of motion limitation in the knee were excluded from the study. Afterwards, the subjects read and signed the informed consent form about the test procedures, and any possible risks and discomfort that might ensue that was approved by the University's Institutional Ethical Board for Protection of Human Subjects (Approval number: 2009-3/76).
Before the procedures, each subject was asked to be present in the laboratory to give him information about the stretching exercises and strength measurements. Thereafter in the same day, they participated to a familiarization trial to practice isokinetic knee extensor and flexor strength measurements in concentric and eccentric modes at the selected two angular velocities (60[degrees]/sec and 240[degrees]/sec). When the subjects attended the laboratory on the next time, they carried out the following intervention protocol in a randomized order on 5 separate days; (a) non-stretching (control), (b) static stretching of the quadriceps muscles, (c) static stretching of the hamstring muscles, (d) dynamic stretching of the quadriceps muscles, and (e) dynamic stretching of the hamstring muscles. Since static stretching exercises either for the quadriceps or the hamstring muscles were carried out in the standing and sitting positions two different stretching modes were designed for dynamic stretching exercises. Concentric and eccentric isokinetic peak torque for the non-stretched antagonist quadriceps or hamstring muscles at angular velocities of 60[degrees]/sec and 240[degrees]/sec and their concurrent electromyographic (EMG) activities recorded with a portable 8-channel EMG device were measured before the intervention protocol (pre) and immediately after (post). Each subject performed a warm-up on a stationary cycle ergometer for 5 minutes at 50W to enable the subjects be ready before the first isokinetic test procedure. Furthermore, since the duration of static and dynamic stretching exercise took 7 [+ or -] 1 minutes, the subjects rested for 7 minutes between the strength measurements in the nonstretching (control) intervention.
Static stretching exercises
The hamstring or quadriceps muscles of the dominant lower extremity were stretched with two unassisted methods. Each unassisted static stretching exercise was performed four times for 30 seconds to the level of mild discomfort, but not pain, as acknowledged by the subject. The rest interval between the four repetitions and between the two unassisted stretching routines was 20 and 30 seconds, respectively. Accordingly, the total static stretching time interval for the hamstring or quadriceps muscle was 7 [+ or -] 1 minutes. A detailed description of the same hamstring (Figure 1) or quadriceps (Figure 2) static stretching exercises in the sitting and standing positions used in this study has been published previously by Sekir et al. (2010).
Dynamic stretching exercises
To be similar with the static stretching exercises, two different stretching methods both for the hamstring and quadriceps muscles were composed for the dynamic stretching routines. The subjects were instructed to contract the opposite muscle of the target muscle (either hamstring or quadriceps) intentionally in the standing upright position and extended or flexed the hip or knee joints once every 2 seconds so that the target muscle was stretched. Before the dynamic stretching exercise, each subject was informed which muscle group they should be contract. Every stretching exercise was repeated four times, slowly at first, and then 15 times as quickly and powerfully as possible without bouncing. The rest interval between the four repetitions and between the two stretching routines was 20 and 30 seconds, respectively. Accordingly, the total dynamic stretching time interval for the hamstring or quadriceps muscle was 7 [+ or -] 1 minutes. A detailed description of the same hamstring (Figure 3) or quadriceps (Figure 4) dynamic stretching exercises in two different stretching methods used in this study has been published previously by Sekir et al. (2010).
The Cybex NORM isokinetic system (Lumex...