Effects of different stretching techniques on muscle and tendon properties

Stretching is generally divided into pre-participation stretching (in preparation to exercise) to induce acute effects and habitual stretching to induce long-term adaptations. The main aim in habitual stretching is the increase of flexibility and functional range of motion, as observed e.g. in ballet. An additional aim in pre-participation stretching is to decrease muscle-tendon-unit stiffness and thus, theoretically, the risk of injuries. Commonly used methods of stretching are static stretching, ballistic stretching, and proprioceptive neuromuscular facilitation (PNF) stretching. Some comparative studies have reported that all stretching techniques increase the range of motion of a joint but PNF stretching appears to be the most efficient method. However, there is no general consensus about the general mechanism. Typical parameters to assess the effect of stretching on the muscle-tendon-unit are functional parameters like active forces or torques, passive torques, power output, joint range of motion, muscle activity, etc. Only the development and recent enhancements of modern imaging techniques have allowed investigating whether the change in muscle-tendon-unit properties is due to alterations in the properties of the muscle or the tendon or some combination of the two. Only a handful of studies have recently investigated the changes in structural parameters of the muscle-tendon-unit like tendon lengths, muscle fascicle lengths, and pennation angles due to pre- participation or habitual stretching. However, these studies had always a single focus either on a specific stretching method or on acute or chronic effects, respectively. In our novel project we will study systematically the effects of the most common pre-participation and habitual stretching methods on muscle and tendon properties. Additional to the determination of classical functional parameters we are going to determine structural parameters like tendon length and stiffness, muscle lengths and stiffness, muscle fascicle length, and pennation angle prior to and following stretching exercises and stretch training programs. This gives us a unique insight into the mechanisms of acute and long-term muscle - tendon reactions induced by stretching exercises. We hypothesize that different stretching methods have different effects on muscle architecture and mechanical tendon properties due to the different character of stimulus given by the different stretching methods. We furthermore hypothesize that the knowledge gained will explain some of the discrepancies in the literature on functional parameters.

The three most common stretching methods are static, ballistic, and proprioceptive neuromuscular facilitation (PNF) stretching. Many studies have investigated a yield in range of motion of a joint following a single stretching intervention and following a stretching training for several weeks. However, until now it was unclear which part of the muscle tendon unit is responsible for such a yield in range of motion. Therefore, the purpose of this study was to investigate if there are functional (e.g. range of motion) and/or structural (e.g. muscle stiffness, tendon stiffness) changes or adaptations in the muscle tendon unit following a single and a six-week stretching training, respectively. A further goal was to determine if there are differences in the effects between the three stretching methods. We hypothesized a gain in range of motion and adaptions in the MTU (e.g. more compliant tendon and/or muscle tissue) in all three stretching methods. Moreover, we expected a different amount of functional and structural adaptations in the three stretching methods.Before and following the single stretching intervention (4x30 sec) und following the six week stretching intervention (4x30 sec/5x week) we determined the functional and structural parameters with a dynamometer (engine controlled machine) and ultrasound of 122 subjects.Following the single stretching intervention all three stretching methods showed a yield in range of motion, a decrease in passive stretching resistance, and a decrease in muscle stiffness. Moreover, there was a decrease in maximal voluntary contraction force in the PNF stretching group only.Following the six week stretching training a yield in range of motion was found in all three stretching methods. However, only in the PNF stretching group the yield in range of motion could particularly explained by a decrease in tendon stiffness.Further analysis showed that there were no clinically relevant differences in the effects between the three stretching techniques. It can be summarized that a single stretching intervention changes muscle function and structure. However, these changes are reversible and return to the initial state quickly. Furthermore, repeated stretching training for six weeks increases the range of motion, however our results suggest that this is more likely due to an adapted perception of pain, rather than adaptations in the muscle and tendon tissue.