The conversion of physical force into biochemical information is fundamental to development and physiology. It provides a simple means by which cells and organisms can ensure structural stability. With cells responding to both the magnitude and distribution of forces, this strongly indicates that communication through physical events are just as important or if not more than chemical messengers (2). Physical forces regulate a large array of physiological processes, and dysregulation of mechanical responses contributes to major human diseases (i.e., cancer) (3).
With stretching being a mechanical force, my research focused on the intensity (magnitude) of this force, suggesting that this attribute was the most important compared to duration, frequency, and volume. The issue however was, how does one quantify a qualitative response, since the perception of stretch intensity is varied amongst individuals? A solution that presented itself was the measurement of blood, in particular blood biomarkers associated with inflammation. The testing of blood afforded the opportunity to look for distinct biological markers (biomarkers), an objective measurement of something happening inside the body. Biomarkers provide information about the body’s response to a mechanical perturbation (i.e., stretching).
Studies were designed to specifically answer the questions: 1) does aggressive stretching cause inflammation (Study 1); 2) is there a dose dependent response associated with the intensity of the stretch and inflammation (Study 2); 3) Comparing low- versus hi-intensity stretching and their effects on pain soreness, and muscle function (Study 3).
According to our knowledge, these studies at the time, were the first designed to specifically address inflammation with stretching with the use of blood biomarkers and human participants.
In this randomized crossover trial with each participant (n=12) acting as their own control, we assessed whether high-intensity passive static stretching results in acute inflammation. Explicit reference was made to C-reactive protein (CRP), specifically high sensitivity (hs)CRP . CRP is produced by the body as a normal response to inflammation, being released quickly at the start of any inflammatory conditions.
The glutes, hamstrings, and quadriceps of each participant were stretched by a therapist for one minute each starting on the right and progressing to the left completing one cycle. This cycle was repeated two more times for a total of three amounting to approximately 18 minutes of stretching. Blood was taken pre- and post- and 24 hours post-aggressive stretching. On a numerical rating scale (NRS) anchored at 0 (zero) for no stretch and 10 for extreme stretching associated with pain, the participants were stretched at an intensity level of 8 out of 10 (pain with some discomfort).
CONCLUSION: Within the study’s limitations (i.e. timing of blood collection), the present data demonstrated that high-intensity passive static stretching is associated with an increase in systemic hsCRP attributed to inflammation and the inflammatory response. Further research into the probable causes of inflammation will elucidate if this form of activity is detrimental to the health of the muscle tissue and its proper function.
Similar to study 1, study 2 was also a randomized crossover trial, with all 11 participants (n=11) being part of all three interventions: 30, 60, and 90% of maximum range of motion of the right hamstring muscle group. With blood taken pre- and post- and 24 hours post intervention, explicit reference was made to CRP, a protein released by the body as a normal response to inflammation.
With each participant in the supine position on the bench of the isokinetic dynamometer (Kin Com), with their right knee being immobilized with use of knee immobilizer brace, and their pelvis and left knee strapped down to prevent any movement insuring that the proper intensity will be applied to the right hamstring, their right hamstring was placed in one of three (3) ranges of motion as indicated above. Each stretch was held at 60 seconds, repeated five times with a 10 second rest in between each stretch position.
CONCLUSION: The data revealed that increases in percentage maximum range of motion were associated with a progressive increase in hsCRP. Since the production of hsCRP has been linked to inflammation, intensities between 30 and 60% maximum range of motion were less likely to cause inflammation. However, although these results were confirmed in our collective analysis (RMANOVA and effect sizes), these results should be treated with caution, due to some unexpected discrepancies in hsCRP baseline values between the three conditions, suggesting further research.
This randomized controlled trial consisted of 30 participants allocated to one of three groups: low-intensity stretching (LiS) (n=10), high-intensity stretching (HiS) (n=10), and control(n=10). The intent of the study was to determine the influence of stretch intensity on muscle soreness and function (eccentric & isometric peak torque) with duration (60 sec) and frequency (3 times per muscle group, once per-day) remaining the same.
After creating muscle damage using an eccentric protocol consisting of 6 sets of 10 eccentric repetitions through a range of motion from 20 to 100 degrees of knee flexion at a speed of 1.05 rads/s with a 2-min rest between sets using an isokinetic dynamometer, the two stretching intervention groups were instructed to perform similar 3 stretching exercises for the hamstring, hip flexor and quadriceps, respectively. These stretches were to be performed for 4 consecutive nights prior to going to sleep, for a total of 18 minutes (6 minutes per exercise).
CONCLUSION: Compared to HiS and no stretching, LiS results in small-to-moderate effects on perceived muscle soreness and recovery of muscle function post-unaccustomed eccentric exercise. Since soreness associated with DOMS affects athletic performance, a decrease in perceived muscle soreness may aid in the restoration of maximal function and an increased adherence to regular exercise with non-athletes.
Skeletal muscle is a highly vascular tissue with an abundant capillary bed that forms an extensive system of anastomosis (1). In addition, most muscles receive small collateral arterioles from tendons, fascial sheaths, and major nerve trunks (2). Compelling evidence exists, at least in isolated animal preparations, that muscle blood flow decreases proportionally with increases in muscle length (i.e. stretching) (3, 4, 5).
Shear stress, caused by the flow of blood during exercise, is a very important stimulus for the endothelial cells lining blood vessels, promoting a beneficial adaptation. However, if the exercise stimulus is too strenuous or unattainable for both aging and diseased populations, low-intensity forms of exercise, such as passive skeletal muscle stretching, have been suggested as a method(s) for improving vascular function (6). An abstract in the journal Circulation by Hotta, K. et al (2014) observed that daily passive muscle stretching on aged rats enhanced the endothelium dependent vasodilation, contributing to increased skeletal muscle blood flow during exercise (7).
With high-intensity stretching being associated with inflammation and muscle soreness (see above studies), the intent of this pilot study was to compare the effects of both a high- and a gentle low-intensity (microStretching®) stretch on the circulation of the hamstring muscle with use of a near infrared spectroscopy (NIRS) technique. NIRS allows for the assessment of microvascular muscle tissue oxygenation [oxyhemoglobin + myoglobin concentration ([O2Hb]), deoxyhemoglobin + myoglobin concentration ([HHb]), and/or muscle blood volume (total hemoglobin + myoglobin concentration [Hbtot]) in response to skeletal muscle stretch (8, 9, 10). This tool has been recognized for its non-invasive and real-time assessment of tissue oxygenation and metabolism (11).
In this pilot study (n=1), the aim was to investigate if a pronounced difference exists between low- and high-intensity stretching with regards to the oxygenation level of the muscle being examined. With the hypothesis being that high-intensity stretching may elicit a reduction in blood flow and blood and an increase in O2 extraction of muscle causing the tissues to become ischemic, dependent upon the results a further study with more subjected . The subject’s hamstring muscle was exposed to a two minute low- or high-intensity stretch.
CONCLUSION: Low-intensity stretching (microStretching®) allowed for the continued oxygenation of the hamstring muscle suggesting that the muscle tissue was less ischemic as compared to the high-intensity stretch associated with pain and discomfort. Interestingly, after the ischemia of the muscle associated with stretching, during reperfusion (reactive hyperemia) of the muscle, the low-intensity stretch was associated with less of a hyperemia compared to the high-intensity stretch. These results suggest that during low-intensity (microStretching®), the hamstring muscle remained somewhat oxygenated as compared to high-intensity stretching.
