Definitive Guide To Flexibility Series - Part 2
Stretching and subsequent performance. Part 2: Evidence based guidelines for stretching protocols in an integrated warm-up.
In our last article I alluded to the complexity of stretching effects as ultimately it is dependent on the configuration of several variables. The reason for going into this in more detail is to dissect the discrepancies in opinions about its use in sports performance. I will explore these factors in more detail and provide some basic guidelines for designing a stretching protocol within an integrated warm-up to acutely enhancing certain biomotor abilities to facilitate performance.
Stretch type: dynamic vs static
The first component that needs to be considered is stretch type. Typically stretching is divided into dynamic and static. Although we could further sub-divide these categories for simplicity we will avoid this. Generally speaking dynamic stretching is often associated with continual movement at a pre-determined speed whereas static stretching requires a specific position that is held for a set duration. There is a general consensus that dynamic stretching is more appropriate for physical preparation which is also confirmed by athlete perceptions (1). This is warranted as there is a large and growing body of evidence that repeatedly demonstrates an improvement in speed-power performance such as jumping activities or sprinting (2). Dynamic stretching requires active participation of the individual. This may impact what components of the musculotendinous complex lengthen as the active portions will need to contract to control the movement thereby causing viscoelastic changes to the connective tissue elements such as the tendon (3). This can influence the fluid characteristics of the tissue that lead to an eventual increase in the stiffness properties whilst improving range of movement (2, 4). Furthermore, being more dynamic will stimulate haemodynamic activity and increase peripheral temperature which appear to be major components for the benefits of a warm-up prior to performance (5).
Static stretching as a part of a warm-up sparks more controversy when it comes to its effect for athletic preparation. As previously mentioned, there is a large variation of outcomes on performance measures (2). However, the discrepancies in these results can be better understood with closer observation of the intervention protocols (I will explain these in more detail below). Static stretching by itself is not a problem and in fact there certainly is clear indications that it can enhance performance measures such as running time (6). Furthermore, static stretching can provide larger acute changes in range of movement without decreasing subsequent performance (7). This is particularly relevant for enhancing flexibility capacity for an individual that may have a specific impairment following injury, surgery or simply from misuse that allows them to achieve certain movements more effectively and therefore improve their biomechanics.
Active vs passive
One other important element that needs to be considered is the concept of active vs passive stretch. Although the concept of purely ‘passive’ is something that is not physically possible under conscious conditions, it is possible to appreciate that there are stretching manoeuvres that would require less activity from the agonists and/or antagonists than others based on the technical set-up. I think this is important to allude to as many of the stretching protocols utilised in research settings often set arbitrary subjective measures such as ‘just before the point of discomfort’ or until you feel a ‘pulling sensation’ which generally coincide with very gentle intensities. Such measures generally lead to less contraction through the active elements of the muscle-tendon complex which sustained for a certain period of time may elicit unwanted effects of the autonomic nervous system (generally upregulating parasympathetic activity). Although it may be useful for upregulating recovery mechanisms (via parasympathetic stimulation), in the context of a warm-up I believe can be problematic (as seen in the literature) as it often creates a physical stimulus that assists in relaxing the individual which given the very basis of a warm-up is counter-intuitive. This is one of the clear benefits of dynamic stretching in a warm-up as this phenomenon is less likely to occur based on the nature of the movement. Admittedly this is based more on empirical evidence. Nevertheless, this is an area that I feel is generally absent from the research world but warrants further investigation as I will discuss this further below (see ‘Intensity’ section).
The other main stretch variable that has been extensively studied is duration. Interestingly, the benefits of dynamic stretching appear to be more obvious with prolonged duration (i.e. > 90 seconds). As previously mentioned, it may simply be that the longer time spent performing dynamic stretching causes a greater change in temperature leading to a greater effect on physical measures (5). The other element is that with dynamic stretching there is the requirement to use active contraction of musculature whilst moving through range for the given task, thus mitigating the potential for a relaxation response and adverse impacts on the autonomic nervous system that impede physical output. This is in contrast to static stretching whereby performance decrements are generally seen with longer protocols (1). As I have already highlighted an explanation for these different responses may be to do with prolonged stretching creating a more ‘passive’ effect. That is, as you hold a certain position in a non-dynamic way (i.e. low intensity parameters) the body relaxes. This may down-regulate the nervous system negatively impacting neural drive (8) and cause more of the length changes to occur within the muscular potion of the musculotendinous unit (9). This may be an unintended consequence for prolonged static stretching that unless other related variables (i.e. technique, intensity) are considered will not be overcome.
Whilst on the topic of duration, I think a more important point of discussion needs to be related to the duration of stretching used practically versus the stretch duration used in clinical settings. Generally speaking, the negative impacts associated with static stretching are duration dependent with usually > 60 seconds often (but not always depending on other factors) associated with a negative response to physical output (1). However, if we consider how long static stretching is used in most settings, this has little relevance. Outside of gymnasts, most of the time static stretching durations would very unlikely be more than 10-20 seconds which as discussed may potentially enhance performance (10) and at least not impede it (1).
Rest vs no rest intervals
A factor intimately related to duration is the use or configuration of sets (i.e. rest intervals) versus no sets (i.e. no rest interval). Of the available literature there are mixed outcomes that may in part be explained by the different characteristics of the participants mainly in regards to their stretch training history (11, 12). However, it does appear to have a large impact on the acute response to stretching whereby the use of intervals (i.e. shorter duration but more sets) may not only negate the unwanted impact of prolonged static stretching but further enhance power performance (11). This alludes to the factor that it is not simply the absolute stretch duration that has a negative impact but the configuration of sets / reps (i.e. rest intervals) that impacts subsequent performance perhaps due to alterations in the magnitude of intensity (i.e. the longer the duration, the more likely a point of ‘relaxation’ is reached).
As I discussed in the previous post this appears to be an area of research that is severely lacking which given the practical and theoretical understanding of applying a novel stimulus to a person does not make sense why this hasn’t been considered further (as I discussed previously consider the PAP effect). It’s worth mentioning that the previous study cited (11) did use a group characterised by an extensive stretch history which certainly will impact the acute neurophysiological response. Of the research available there is evidence to suggest chronic stretching can alter the acute neurophysiological response that negates any potential unwanted effect (13, 14). This in my opinion has to be one of the largest limitations to studies assessing static stretching and subsequent performance as it simply dismisses common sense – providing a novel stimulus to an individual is more likely to evoke side effects. This applies to any training modality.
The final factor to consider is intensity. Unfortunately, there is little research observing the relationship between stretching and subsequent performance with most studies using arbitrary subjective measures (i.e. pain onset, maximum stretch, or maximum pain tolerated) to determine the intensity. However, there is some research that demonstrates different neurophysiological responses based on more objective intensities parameters for static stretching. Although there was a reduction in peak passive torque immediately following the stretching protocol, the higher intensity stretching created a subsequent increase from 30 – 60 minutes that was not seen in the lower intensity group (15). This is in contrast to typical static protocols whereby peak passive torque is decreased and is often considered a factor that contributes to the performance decrement seen in prolonged static stretching protocols (16). The enhancement in passive torque from higher intensity stretching occurs in conjunction with greater increases in ROM which is a general reason for using stretching in a warm-up in the first place. This finding coincides with my previous discussion on the timing of performance measures following stretching as well as there clearly seem to be several factors that not only determine if an increase will be seen, but when. There is still work to be done exploring this variable and its impact on performance measures, however, from the neurophysiological mechanisms known higher intensity stretching may be beneficial for force producing capacity.
To summarise the main points:
1. Static stretching itself is not a problem, it is the lack of understanding of related variables
2. Both dynamic and static stretching can be beneficial in an integrated warm-up whether that be to acutely enhance range of movement capacity and performance measures
3. The discrepancies in opinion about static stretching can be explained by the configuration of related variables, namely duration and intensity. It is best to use static stretching for shorter durations (i.e. < 30 seconds for each pose/movement with higher intensity potentially being more effective). Start with doing < 5 minutes total stretching.
a. Avoid ‘gentle’ stretching movements that stimulate a relaxation response.
4. Familiarise yourself with stretching. The more stretching training history, the more likely you are to realise the positive effects associated with it. Additionally, you may be able to integrate more stretching into your warm-up the more exposure you get (if indicated). As you become more accustomed to the stimulus, you may be able to increase the total amount of time under tension stretching by increasing the duration (best to do multiple sets/reps i.e. rest intervals so the intensity can stay high) or amount of movements used.
a. Increasing the total time of stretching is not necessary, but may be indicated in certain circumstances, particularly in rehabilitation settings whereby there are severe deficiencies in range of movement of myofascial compliance