Thought this was extremely relevant after the poll in the training forum.
Strength and Conditioning Journal: Vol. 28, No. 6, pp. 66–74.
Stretching: Acute and Chronic? The Potential Consequences
Mike Stone, PhD, Michael W. Ramsey, PhD, and Ann M. Kinser
Stretching is commonly used by many athletes in different sports. Although acute stretching, as part of a warm-up, can enhance range of motion, it may also reduce performance. Acute stretching can reduce peak force, rate of force production, and power output. Chronic stretching may enhance performance, although the mechanism is unclear. Acute stretching has little effect on injury. However,chronic stretching (not part of warm-up) may have some injury reduction potential.
Key Words: acute stretching, chronic stretching, range of motion
Stretching can be defined as the actof applying tensile force to lengthen muscle and connective tissue. Often stretching is performed as part of a warm-up prior to physical exertion. Typically, stretching is used to enhance the range of motion (ROM) about a joint (flexibility). The resulting enhancement may be viewed as acute (temporary) or chronic.
There are many different types of stretching that can be performed. A quick look at the internet (under “stretching”) offers a variety of stretching types and methods, including:
Passive (or relaxed) stretching
Proprioceptive neuromuscular facilitation stretching
Although in some cases the nature of these methods is essentially the same, it gives the coach/athlete a wide variety of methods from which to choose when acutely or chronically stretching.
Although, the exact timing and degree of stretching varies somewhat from sport to sport, there are basically 2 forms of stretching taking place on a regular basis among athletes: first is acute stretching (as part of a warm-up process), and second is chronic stretching that is often quite extensive and usually occurs after a training session. Athletes and coaches commonly hold 2 beliefs concerning these 2 forms of stretching: (a) acute stretching (part of warm-up) may increase performance and will reduce the injury potential of exercise; (b) chronic stretching will increase performance, reduce aches and pains, and reduce the injury potential of exercise and sports performance.
However, data exist indicating that these beliefs may not be completely true. The purpose of this paper is to answer several basic questions concerning stretching and its relationship to sports performance, with a particular focus on gymnastics.
Will Warm-Up (Acute) Stretching Produce a Better Performance?
Table 1 shows the results of studies dealing with the relationship of various activities and various performance characteristics that would have effects on sport. Although not all studies show a decrease in performance, the large majority do indicate that acute stretching can decrease subsequent performance, particularly for maximum strength– and explosive strength–related movements. So, for a sport such as gymnastics, in which explosive strength is quite important, such a loss of explosive capability may reduce the ability to perform.
The underlying mechanisms that can reduce performance subsequent to acute stretching are not necessarily apparent or easily understood. To begin to understand why acute stretching may reduce performance, a brief discussion of how stretching affects ROM is in order. There are basically 2 mechanistic possibilities that may have an effect individually or in combination: (a) stretching alters ROM by altering the structure and properties of soft tissue (muscle and connective tissue); (b) there is an increase in pain tolerance.
Tissue stiffness is the ability of a tissue to resist change in length and is represented by a change in force per change in length (F/L). A decreased or increased stiffness may alter the stress-strain curve (changes in force when muscle or connective tissue is lengthened or shortened by stretching). Figure 1 (36) shows a passive stress-strain curve in which a tissue is being stretched until failure. Note that to a point, the greater the lengthening of the tissue the greater the force produced. The amount of energy that is absorbed by the tissue before failure is a function of its tensile strength. Therefore, the more energy absorbed, the stronger and the more stretch resistant the tissue. The stiffer the tissue, the more it resists the stretch, and there are 2 possible results: (a) the rate at which force rises is faster; (b) the failure point of the tissue may be reached faster.
Muscle can also be activated to resist a stretching load (e.g., eccentric contractions). Thus, muscle tissue has active stiffness properties. Contraction during stretching can take up the slack in the series elastic elements faster and result in a faster rate of force production and an increased amount of force before failure (36).
A very stiff tissue would require more force to stretch it to a given length. So tissue stiffness could (theoretically) inhibit flexibility. Therefore, an acute exercise reducing tissue stiffness could enhance flexibility. However, in the normal intact human, changes in the length of a muscle (or muscles) also alter the feedback to the nervous system. For example, a less stiff muscle would produce less force at a given length, and the nervous system senses this difference. Thus, alterations in muscle stiffness (active or passive) could change how the nervous system reacts to a given muscle length. Therefore, a change in active or passive muscle stiffness could also effect the stretch reflex characteristics and tissue elastic properties (less energy stored for elastic recoil) such that force transmission is disrupted/muted, decreasing force magnitude, rate of force development, and power output.
Some evidence indicates that an increased ROM as a result of stretching is related to reduced tissue stiffness (20). However, the majority of studies indicate that although tissue viscosity may be altered, muscle stiffness and elasticity are largely unaffected by acute stretching as part of a warm-up (11) or chronic stretching over a 3- to 4-week period (21, 34, 37) and that alterations in ROM after stretching are more related to increased pain tolerance (21, 37). On the other hand, repeated and prolonged stretching for 1 hour (7) adversely affected active and passive muscle stiffness, and 30 sessions of static stretching produced a decrease in passive muscle stiffness (20). The decrease in active tissue stiffness as a result of prolonged stretching could be a fatigue-induced phenomenon rather than simply a stretch result (5, 24). Thus, increased ROMs as a result of stretching may result from decreased muscle stiffness but this appears to be more likely caused by altered tissue viscosity and pain tolerance.
Interestingly, maximum strength and strength training effects appear to be associated with increased active and passive muscle stiffness that is independent of ROM alterations (17,34,37,55). An increase in muscle stiffness appears to be associated with enhanced strength (66) and various types of performances, including the vertical jump and improved running (i.e., enhance running economy; Figure 2 ). Thus, a loss of performance associated with acute stretching could be associated with a decrease in muscle stiffness.
Stretching has also been associated with muscle damage. In mice, Black and Stevens (9) found that acutely stretching muscle fibers 5 % beyond resting resulted in a 5% loss of isometric force production. Strains (stretching), as low as 20% beyond resting length, have been related to muscle damage and decreased force in humans (38). So vigorous stretching could induce enough muscle damage to reduce maximum strength and explosive strength. However, in the authors' opinion, it is unlikely that chronic stretching in well-trained athletes would continue to induce tissue damage. Otherwise, one would expect chronic muscle soreness among advanced and elite athletes, and this clearly is not the case.
A finding noted in most of the performance studies indicates that acute stretching as a part of warm-up reduces maximum strength (force magnitude) and several associated variables, such as rate of force development and power output (8, 46, 53). Additionally, a decreased H-reflex has been noted (6, 7, 20). The H-reflex is a monosynaptic reflex elicited by stimulating a nerve, particularly the tibial nerve, with an electric shock. Thus, it appears that stretching acutely as part of a warm-up can negatively alter force production, power output, and stretch-shortening cycle characteristics such that strength and performance, including such explosive performances as gymnastics, can be compromised. This compromise may be associated with alterations in muscle stiffness (Figure 2) .
Will Chronic Stretching (Non–Warm-Up) Improve Performance?
Many athletes stretch after a training session. The belief is that over the long term, this practice may reduce injury and perhaps enhance performance. Table 2 shows studies that have investigated long-term stretching and performance. These studies generally show that performance, particularly maximum strength and explosive strength performances, were enhanced. When the studies are taken as a whole, the degree of enhancement appears to be small, perhaps 3 to 4 %. However, it should be remembered that in high-level sports, a small percentage of improvement can actually be a lot. For example, in the last 2 Olympics, the difference between first and fourth place (for most sports) was less than 1.5 %. The mechanisms underlying enhanced performance, as a result of chronic stretching, are unclear at best.
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