Static stretching is widely used in sports practice during warm-up, with the goal being to acutely increase the range of motion (RoM) of a joint. The consistently observed increased RoM is accompanied with an increased tolerance to stretch and mechano-morphological changes in the muscle-tendon unit (MTU). The mechano-morphological changes following a single stretching exercise are often related to a decrease in overall muscle-tendon stiffness, which is sometimes also referred to as "joint stiffness" (Kay et al., 2015; Konrad et al., 2017a; 2017b) or "passive resistive torque" (PRT) (Konrad et al., 2017a; 2017b; Nakamura et al., 2013). Both measures describe the resistance to stretch of the MTU. Most of the studies that distinguished between the muscular and tendinous tissue of the whole MTU have reported a decrease in the muscle stiffness (Kay and Blazevich, 2009; Kay et al., 2015; Konrad et al.2017a; 2017b; 2019a), while only some have reported a decrease in tendon stiffness (without changes in the muscular component) (Kubo et al., 2001; Kato et al., 2010). These controversial results can be explained by the different stretching durations or intensities applied. While shorter stretch durations and lower stretch intensities are related to changes in the muscle structure, longer stretch durations (> 10 min) and/or greater stretch intensities (e.g. including maximum active contractions) seem to predominantly affect the tendon structure.
Static stretching might also affect muscle performance with a clear "dose-response relationship", as analyzed by several reviews (Behm and Chaouachi, 2011; Kay and Blazevich, 2012; Behm et al., 2016). These authors pointed out that stretches that last longer than 60 s probably have a detrimental effect on muscle performance, while this is not expected for stretches of less than 60 s.
Although several studies have investigated the acute effect of stretching on RoM, muscle-tendon structure, and performance, less is known about the time course of these measures following stretching. Depending on the stretch duration, some authors have reported an increased RoM, which lasts between 10 min (Ryan et al., 2008a, Konrad and Tilp, 2019; Konrad et al., 2019) and up to 120 min after stretching (Power et al., 2004).
Following 5 min of static stretching, changes in muscle-tendon stiffness have been observed both immediately (Mizuno et al., 2012; Konrad et al., 2019) and up to 5 min after stretching (Mizuno et al., 2012), but these changes recovered within 10 min. The changes are associated with a decreased PRT, muscle stiffness, or increased muscle elongation for up to 5 min (Mizuno et al., 2012; Konrad et al., 2019). All the structural changes that occurred in the muscle had recovered within 10 min. The effects of shorter stretching durations (3 min) were analyzed by Kay and Blazevich (2009), who reported a decrease in muscle stiffness immediately after stretching, which recovered after 30 min. However, the development of the responses up to 30 min after the stretching were not investigated. A shorter stretching duration of 1 min of static stretching, however, did not lead to changes in muscle-tendon stiffness and muscle stiffness immediately, 20 min, and 40 min after the stretching exercise, respectively (Konrad and Tilp, 2019).
A decrease in muscle performance (measured as maximum voluntary contraction (MVC)) was observed after 5 min of static stretching, for at least 10 min (Konrad et al., 2019), but not after 1 min of static stretching (Konrad and Tilp, 2019).
Hence, for the full picture of the time course of the dose-response relationship of stretching, there is still a knowledge gap with regard to the precise time course of the response of muscle and tendon properties (e.g. passive muscle and tendon stiffness, active tendon stiffness) and function responses (e.g. RoM, MVC) within the first minutes after a static stretching exercise of medium (between 1 and 5 min) duration.
Therefore, the objective of this study was to analyze the time course (immediately, 5 min) of the properties and functional responses of the plantar flexor muscle-tendon system following a 3-min stretching exercise. Based on previous results, we hypothesized an increase in RoM and a decrease in PRT and MVC, both immediately and 5 min after the stretching. We further assumed that these changes would be associated with a decrease in muscle stiffness, and that these changes would last up to 5 min after the stretching.
On the first day, subjects were familiarized with the laboratory equipment, the assessments (RoM, passive, active), and the stretching regime. Moreover, participants visited the laboratory for a further two sessions on different days (with a 2- to 7-day break in between) at the same time of day to assess the effects of stretching immediately (0minpost) and 5 min (5min-post) after the stretching, in a randomized order. Before and after the two conditions (0 min_post, 5 min_post), we determined the RoM, PRT, MVC torque, muscle-tendon stiffness, muscle stiffness, and passive and active tendon stiffness of the gastrocnemius medialis (GM) muscle.
Three healthy female (mean [+ or -] SD; 24.6 [+ or -] 2.3 years, 1.71 [+ or -] 0.02 m, 64.2 [+ or -] 5.7 kg) and 11 healthy male (mean [+ or -] SD; 24.8 [+ or -] 3.8 years, 1.83 [+ or -] 0.05 m, 75.6 [+ or -] 9.0 kg) physically active volunteers with no history of lower leg injuries participated in this study. Subjects were informed about the testing procedure, but were not informed about the study's aim and hypotheses. According to a sample size calculation (primary outcome variable muscle stiffness) for a univariate linear model based on the literature and on our own data (mean change = 4% (i.e. Kay and Blazevich, 2009); SD = 5%, alpha = 0.05, beta = 0.9) suggests a necessary group size of 14 subjects.
The study was approved by the local research ethics board (GZ. 39/77/63 ex 2013/14), and written informed consent was obtained from all volunteers before the onset of the experimental procedures.
The temperature in the laboratory was kept constant at around 20.5 [degrees]C. Measurements were performed without any warm-up and in the following order: pre-tests: RoM (1-min rest), PRT (1-min rest), MVC (1-min rest); intervention: stretching for 3 x 60 s; post-tests: immediately following stretching, or following 5 min of rest in the same order (RoM (1-min rest), PRT (1-min rest), MVC).
RoM measurement: RoM was determined with an isokinetic dynamometer (CON-TREX MJ, CMV AG, Duebendorf, Switzerland) in a seated position with a hip joint angle of 110[degrees], with the foot resting on the dynamoeter foot plate and the knee fully extended. Two oblique straps on the upper body and one strap around the thigh were used to secure the participant to the dynamometer and exclude any evasive movement. The estimated ankle joint center was carefully aligned with the axis of the dynamometer and the foot was fixed barefooted with a strap to the dynamometer foot plate to avoid any heel displacement. Participants were moved to the neutral ankle joint position in the dynamometer (90[degrees] between foot sole and tibia), and were subsequently asked to regulate the motor of the dynamometer with a remote control to get into a dorsiflexion (stretching) position until they reached their individual maximum tolerable stretch. The angular velocity of the dynamometer during this procedure was set to 5[degrees]/s. The difference between the neutral ankle position and the maximum dorsiflexion was defined...