Stretching routines are commonly performed before exercise, and they are often included in rehabilitation programs.
One of the most common types of stretching is static stretching. Static stretching involves lengthening the muscle, then holding it at a lengthened position for several seconds. For example, if you bend over to touch your toes and hold the position for several seconds – you are performing a static stretch of the muscles at the back of your legs. When these types of stretches are performed by healthy adults for several weeks, joint range of motion (i.e., flexibility) improves (Kokkonen et al. 2007).
However, when static stretches within a single exercise session are held for longer than 60 seconds, they significantly reduce the muscle’s ability to produce force (Kay & Blazevich 2012). This reduction in force might be due to processes occurring within the nervous system, rather than the muscle itself. In our recent review article (Trajano et al. 2017), we discussed the processes in the nervous system that might explain the reductions in muscle force which are commonly observed after bouts of static stretching.
WHAT DID WE FIND?
We put forth the idea that processes in the spinal cord likely contribute to the loss in force after static stretching.
Muscle spindles are structures in the muscle that sense stretch. For instance, when a calf muscle is quickly stretched these spindles detect the changes in muscle length and transmit signals to motoneurones in the spinal cord. The motoneurones then send signals back to the calf muscle, causing the muscle to contract. This is an example of a stretch reflex.
Also, the stretch signals from the muscle spindles are an important part of an amplification system in the spinal cord. After receiving the stretch signals, this amplification system magnifies the output from the motoneurones (Heckman & Enoka 2012). The increased output from the motoneurones then helps to ensure the muscle contracts.
One of our previous studies discovered that this amplification system is negatively affected by 5 minutes of static stretching (Trajano et al. 2014). The consequence was a 60% reduction in reflex force immediately after stretching, and a 35% reduction 5 minutes after stretching. Thus, the study’s results, together with other data, show that mechanisms in the spinal cord are likely involved in the force loss caused by stretching.
Also, it is possible that the brain is involved in force loss after stretching. Sensory receptors that respond to muscle stretch are connected with both sensory and motor areas within the brain (Gandevia 2011). However, our review did not find any experiments that explicitly tested this hypothesis. Thus, it is currently unknown what areas of the brain might be involved in force loss after stretching.
SIGNIFICANCE AND IMPLICATIONS
Our review paper puts forth a neural mechanism (i.e., the altered amplification system) which can partly explain the reductions in muscle force and muscle activation that occur after static stretching. Because this amplification system is hindered for 5 minutes after stretching, clinicians should consider waiting 10 minutes after stretching, before letting their patients perform exercises that require high levels of muscle force (e.g., resistance training).
Trajano GS, Nosaka K, Blazevich AJ. Neurophysiological mechanisms underpinning stretch-induced force loss. Sports Med (January 24, 2017) doi: 10.1007/s40279-017-0682-6.
Gandevia SC. Kinesthesia: roles of afferent signals and motor commands. Compr Physiol, 128-172, 2011.
Heckman CJ, Enoka RM. Motor unit. Compr Physiol, 4: 2629-2682, 2012.
Kay AD, Blazevich AJ. Effect of acute static stretch on maximal muscle performance: a systematic review. Med Sci Sports Exerc, 44: 154-164, 2012.
Kokkonen J, Nelson AG, Eldredge C, Winchester JB. Chronic stretching improves exercise performance. Med Sci Sports Exerc, 39: 1825-1831.
Trajano GS, Seitz LB, Nosaka K, Blazevich AJ. Can passive stretch inhibit motoneuron facilitation in the human plantar flexors? J App Physiol, 117: 1486-1492, 2014.
Gabriel Trajano is a Lecturer at Queensland University of Technology. His research investigates neuro-mechanical adaptations in response to exercise training. You can follow Gabriel on Twitter @gabrielstrajano and ResearchGate.