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Stretching and altitude training: an update

[This article originally appeared in the July 2001 issue of Northwest Runner magazine.]

Remember last summer? We listened to U.S. presidential candidates repeatedly tell us who to vote for and why.... We watched the world's best athletes convene and compete in Sydney.... And, here in the pages of Northwest Runner, we discussed stretching (July) and altitude training (September).

This month, I'd like to revisit those last two topics, starting with stretching. As you may recall, last July's examination of the scientific literature revealed very little empirical evidence that stretching improves performance or prevents injuries in healthy individuals. Given this lack of evidence -- which has been noted by, among others, Liebesman & Cafarelli (Critical Reviews in Physical and Rehabilitation Medicine 6: 131-60, 1994), Gleim & McHugh (Sports Medicine 24: 289-99, 1997), Neely (Sports Medicine 26: 395-413, 1998), Knudson (Journal of Physical Education, Recreation, & Dance 70: 24-7, 1999), Shrier (Clinical Journal of Sports Medicine 9: 221-7, 1999), and Pope et al. (Medicine and Science in Sports and Exercise 32: 271-7, 2000) -- it seems reasonable to speculate that stretching may not be as beneficial or necessary as most coaches assume it to be.

Where on the U are you?

The "U-shape" hypothesis proposed by Knapik et al. (Sports Medicine 14: 277-88, 1992) may offer a partial explanation as to why the presumed benefits of stretching have been difficult to demonstrate experimentally. According to this hypothesis, both people who are very inflexible and those who are very flexible are more susceptible to injury than people of average flexibility; thus a graph of injury risk (on the vertical axis) versus flexibility (on the horizontal axis) would look like a widened U, with people of normal flexibility represented by the bottom of the U. According to Knudson (Journal of Physical Education, Recreation, & Dance 70: 24-7, 1999), the idea that high flexibility could be a risk factor for injury is plausible because "increased joint mobility may come at a cost of decreased joint stability."

Of the available data on injury incidence and flexibility, some but not all conform to a U-shaped pattern Knapik et al., Sports Medicine 14: 277-88, 1992; Jones et al., Medicine and Science in Sports and Exercise 25: 197-203, 1993; Neely, Sports Medicine 26: 395-413, 1998), implying that one may only see a U for certain muscles in certain populations. In any case, the U-shape hypothesis suggests the possibility that stretching might be good for some individuals (i.e., those with very limited flexibility) but not others (those with average or above-average flexibility).

Stretching for success

Those who do feel the need to stretch -- due to chronic muscle tightness, for example, or as part of an injury rehabilitation program -- may wonder how to get the most out of their stretching routine. How, when, how long, and how often should one stretch to improve one's flexibility in a safe manner? First of all, there are three major styles of stretching: ballistic stretching, static stretching, and proprioceptive neuromuscular facilitation (PNF). Ballistic stretching consists of "bouncing" motions in which the muscles are rapidly stretched and then immediately allowed to relax. Static stretching is essentially the opposite; the muscles are slowly stretched beyond their resting length, and then the stretched position is held for several seconds or longer. In PNF, static stretching is preceded by intense isometric contractions of the muscle about to be stretched ("isometric" meaning that there is no movement of the bones connected by the muscle). All three techniques have been shown to increase range of motion; however, ballistic stretching is considered inferior to the other two methods because of the injury-promoting stress it puts on muscles (Liebesman & Cafarelli, Critical Reviews in Physical and Rehabilitation Medicine 6: 131-60, 1994; Knudson, Journal of Physical Education, Recreation, & Dance 70: 24-7, 1999).

Another stretching-related question is whether it is better to stretch before a workout or afterwards. Some researchers have argued that, since warm muscles should be more stretchable than cold muscles (Noonan et al., American Journal of Sports Medicine 21: 517-22, 1993; Draper & Ricard, Journal of Athletic Training 30: 304-7, 1995), stretching should always follow a period of jogging or other warmup exercises. This logic, while appealing, has been difficult to validate experimentally (Williford et al., American Journal of Sports Medicine 14: 316-9, 1986; Cornelius et al., Journal of Sports Medicine 28: 234-6, 1988; Magnusson et al., Journal of Applied Physiology 88: 1215-20, 2000). Others have opined that, since stretching a muscle can decrease its strength (Rosenbaum & Hennig, Journal of Sports Science 13: 481-90, 1995; Kokkonen et al., Research Quarterly for Exercise and Sport 69: 411-5, 1998; Avela et al., Journal of Applied Physiology 86: 1283-91, 1999; Fowles et al., Journal of Applied Physiology 89: 1179-88, 2000; Nelson et al., Research Quarterly for Exercise and Sport 72: 68-70, 2001), stretching should only be done after workouts have been completed. However, these decreases in strength are usually observed after the muscle in question has been stretched for 20 to 60 minutes -- far longer than most of us would stretch a single muscle group prior to a workout. As far as I'm concerned, the before/after question has yet to be resolved.

How long must a stretch be held for it to affect flexibility? Even stretches of 5 to 10 seconds can improve one's range of motion if they are performed frequently enough (Borms et al., Journal of Sports Sciences 5: 39-47, 1987; Roberts & Wilson, British Journal of Sports Medicine 33: 259-63, 1999). Regarding the optimal duration to hold a stretch, three groups of researchers have found differences in the range-of-motion gains achieved by different stretching regimens. Bandy and colleagues (Physical Therapy 74: 845-50, 1994; Physical Therapy 77: 1090-6, 1997) found that stretching the hamstrings for 30 or 60 seconds per day improved flexibility more than stretching for 15 seconds per day, while increasing the number of 30- or 60-second stretches from one per session to three per session did not lead to additional gains. Roberts & Wilson (British Journal of Sports Medicine 33: 259-63, 1999) reported that, for hip flexion and knee flexion and extension, a thrice-per-week routine of 3 stretches of 15 seconds apiece increased subjects' range of motion more than a routine of 9 stretches of 5 seconds each. Most recently, Feland et al. (Physical Therapy 81: 1110-7, 2001) found that, for elderly subjects, sessions of 4 stretches of 60 seconds each improved hamstring flexibility more than those of 4 x 30 seconds, which in turn led to bigger gains than those of 4 x 15 seconds. Considered collectively, these studies suggest that static stretching of a muscle group for 15 to 30 seconds daily will improve its flexibility and that, for truly patient individuals, doing more and/or longer stretches may lead to additional improvements.

Your own personal mountaintop

On to altitude training. As discussed last September, current evidence favors a "live high, train low" approach to altitude training in which athletes live at high altitude (to boost red blood cell production) but train at low altitude (to enable themselves to complete intense, fast-paced workouts which they could not do at high altitude). In light of this evidence, several companies are now selling hypoxic (low-oxygen) chambers to athletes with the promise that sleeping in a chamber is equivalent to living at high altitude. Here's the problem: the benefits of chamber use have yet to be unequivocally demonstrated in a peer-reviewed publication.

Several papers (Savourey et al., European Journal of Applied Physiology 73: 529-35, 1996; Garcia et al., Respiration Physiology 123: 39-49, 2000; Ricart et al., Wilderness and Environmental Medicine 11: 84-8, 2000; Koistinen et al., Medicine and Science in Sports and Exercise 32: 800-4, 2000; Hahn et al., Comparative Biochemistry and Physiology 128A: 777-89, 2001) report no statistically significant increase in red blood cells following chamber exposure. In many of these studies, however, the duration of time spent in the chambers was rather limited, meaning that more prolonged exposure to hypoxia might yield legitimate increases. On the other hand, work from a group in Barcelona (Rodriguez et al., Medicine and Science in Sports and Exercise 31: 264-8, 1999; Casas et al., Aviation, Space, and Environmental Medicine 71: 125-30, 2000; Rodriguez et al., European Journal of Applied Physiology 82: 170-77, 2000) has revealed increases in red cells following intermittent exposure to hypobaric (low-pressure) hypoxia, yet these studies lacked control groups and didn't demonstrate significant increases in maximal oxygen consumption (VO2max) -- a key issue, since, for athletes, the whole point of making extra red blood cells is to increase VO2max. (For more on VO2max, please see my February 2001 column.)

Don't get me wrong; I think the hypoxic chamber people may be onto something exciting. Several research groups around the world are studying how these chambers can be used to improve athletic performance, and it's probably only a matter of time before we know how many hours per day one must spend at what simulated altitude for how many weeks in order to reap the benefits of "living high and training low." Until these details have been sorted out in careful scientific studies, however, the claims of the chamber merchants must be taken with a grain of sodium chloride.

Thanks to Dr. Ben Levine, director of the Institute for Exercise and Environmental Medicine in Dallas, for his help with this article.


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