Dynamic Stretching does not Change the Stiffness of the Muscle-Tendon Unit

Abstract

The purpose of this study was to identify changes in ankle range of motion and passive mechanical properties of the muscle-tendon unit after dynamic stretching. 12 healthy subjects participated in this study. Displacement of the muscle-tendon junction was measured using ultrasonography while the ankle was passively dorsiflexed at 1°/sec to its maximal dorsiflexion angle. Passive torque was also measured using an isokinetic dynamometer. Measurements were conducted pre-intervention, immediately after the intervention and 5, 10, 15 and 30 min post-intervention. The dynamic stretching consisted of four 30-s periods of ankle dorsiflexion and plantarflexion. Ankle range of motion was significantly increased immediately (from 18.3±1.8° to 21.4±1.7°) and 10 min (20.9±1.9°) after dynamic stretching, but this change disappeared within 15 min. However, stiffness of the muscle-tendon unit and displacement of the muscle-tendon junction at the submaximal dorsiflexion angle did not differ between the experimental conditions. These results demonstrate that dynamic stretching by contracting an antagonist muscle group increases ankle range of motion temporarily without changing the passive mechanical properties of the muscle-tendon unit. The increased range of motion of the ankle after dynamic stretching might be caused by enhanced stretch tolerance.

Dynamic stretching is not detrimental to neuromechanical and sensorimotor performance of ankle plantarflexors

“…Recently, Pamboris et al 38 found a decrease in muscle fascicle strain combined with an increase in muscle stiffness (measured by shear wave elastography) attributing the increase to ankle dorsiflexion RoM to increased tendon strain. Mizuno and Umemura 16 reported an increase in maximum passive ankle dorsiflexion RoM without changing the passive stiffness of the MTU attributing it to an increase in pain tolerance. Our results cannot refute that alteration in pain tolerance (ie, discomfort or pain perception at a given ROM) is a contributing mechanism to the increased RoM changes after stretching.…”

“…Direct stimulation is considered necessary to change the mechanical behavior of the MTU . Thus, studies in which DS involved “agonist” muscle group contractions demonstrated decreased in mechanical behavior, while in Mizuno and Umemura, whose DS protocol involved “antagonist” muscle group contractions, there was no change in MTU behavior.…”

“…15 Contrasting views on the potential effect of DS on MTU properties are, however, reported. Mizuno and Umemura 16 reported that a DS technique did not change the mechanical properties of the MTU, attributing the change in RoM to enhance stretch tolerance.…”

“…Dynamic stretching protocols used in the studies above were varied in their respective methodology greatly. For example, one involved contracting the muscle group "antagonist" to the stretched target muscle group, 16 while the others involved contracting the muscle group "agonist" to the stretched target muscle group. 13,14 Discrepancies among the results of studies which employed DS necessitate further examination of this commonly employed warm-up intervention given the different neuromuscular activation effects of the above contraction modalities.…”

Stretching combined with repetitive small length changes of the plantar flexor muscles enhances their passive extensibility for longer duration than conventional static stretching, while not compromising strength

“…Possible factors involved in DS include dynamic stretching and shortening of actively contracting muscles, which might be responsible for the smaller negative effect of DS on muscle functions. This can be due to a retained neural drive and/or a negligible change in MTU stiffness after DS [10,11].…”