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It is well established that a high rate of oxygen usage (VO2max) is an important quality in elite runners. However, for those already possessing this capacity, VO2max is a poor predictor of performance, and other factors become decisive in determining success. One such factor is the athlete’s lactate threshold and thus the ability to run at high speeds without a muscle-inhibiting increase in lactic acid concentrations. Another, often overlooked, is the submaximal energy cost of running at a given speed – otherwise known as running economy.
Classic studies have shown that, among elite runners, running economy is a powerful predictor of successful performance, which can vary between individual runners by as much as 30%. Reduced submaximal energy expenditure gives economical runners an edge over non-economical runners of equal VO2max and lactate threshold, since they can run at faster speeds at any given percentage of VO2max, and run at a lower percentage of VO2max (with lower heart rate and lactate production) for any given speed. Factors known to affect running economy include age, training, stride length and frequency. It is also widely believed that suppleness and flexibility can enhance economy – a belief based on the observation that both running economy and flexibility decline with age.
Advocates of flexibility training argue that decreased flexibility could result in shorter, less economical stride lengths, or in increased effort to move the limbs against tight muscular resistance, particularly at extreme ranges of motion. There is, however, little evidence to support claims that non-pathological muscle tightness reduces running economy, so impairing performance. Indeed, there is a growing body of evidence to suggest the opposite – that a lack of flexibility in certain areas of the body may be linked with increased running economy. And it is interesting to note that studies of competitive distance runners have shown them to be less flexible than non-runners.
Recent research studies have suggested that trunk and lower limb flexibility are inversely related to running economy. For example, Gleim and associates tested 100 male and female subjects across a range of treadmill running speeds (0.9-3.13m per second), taking oxygen intake as a measure of running economy and using a battery of 11 tests to assess trunk and lower limb flexibility. Analysis of the data revealed that subjects who exhibited tightness in the trunk (limiting turnout of the leg from the hip and trunk rotation) were the most economical at every test speed.
In another study, the researchers followed a similar procedure for 19 male runners with performance times of less than 40 minutes for 10k. A total of nine measures of trunk and lower limb flexibility were taken and treadmill running economy was assessed at a speed of 4.13m/sec. As with the previous study, the researchers also found that decreased flexibility (in this case of the hip, limiting external rotation of the leg) was significantly linked with decreased energy cost of running – i.e. increased running economy.
The inverse relationship between flexibility and running economy
In addition, the results of this study showed a strong relationship between improved economy and limited dorsiflexion of the ankle (tightness in the calf and soleus muscles, limiting raising of the foot towards the shin). More recently, this same issue has been investigated in elite, international distance runners (12). Again, the findings revealed an inverse relationship between flexibility and running economy: this time it was decreased flexibility of hamstring/lower back muscles that appeared to boost economy.
How might tightness in the musculoskeletal system decrease submaximal energy expenditure in running? Craib and colleagues suggested that inflexibility of the ankle in dorsiflexion could reduce energy expenditure by enhancing elastic energy storage and return in the Achilles tendon and calf muscles. A similar explanation was given by Jones with regard to inflexible hamstrings. A mechanism involving the return of elastic energy would, in theory, reduce the amount of active muscle contraction required to push the runner forwards and so reduce energy expenditure.
Previous work has suggested that elastic recoil of muscle and tendons can contribute 25-40% of the energy necessary for subsequent movements in a maximally-stretched muscle. It is reasonable to suggest that inflexibility around the ankle joint would result in a greater relative stretch of the tight muscles and tendons, storing more elastic energy for subsequent recoil and reducing the active work of the muscles.
Musculoskeletal tightness can also explain the beneficial effects of limited hip/trunk flexibility demonstrated in the studies mentioned above. Limited external hip rotation could enhance running economy by stabilising the pelvic region at the time of foot impact. Since running occurs primarily in a forward direction, rotational motion is potentially energy-wasting as it does not contribute to forward movement.
Presumably, rotary movements in flexible runners must be neutralised by active muscle contraction, making them less economical than runners with more rigid trunk and hip muscles. Similarly, an increased relative stretch of tight hamstrings in the forward swing of the leg may store elastic energy that can be used to pull the body over the limb and propel the runner forwards, thus saving on active muscle contractions. The research provides good support for these suggestions, although the extent of trunk rotation and hip flexion during the running trials were not actually measured in any of the studies.
The results of the studies outlined above suggest that inflexibility in the hip and calf regions is associated with improved running economy in recreational, moderately-trained and elite runners. On the basis of previous work, it is possible to speculate that inflexibility reduces the submaximal energy cost of running by minimising the need for active stabilising of the trunk and increasing the storage and return of elastic energy in the muscles and tendons of the lower leg.
However, these findings must be applied with caution: while it seems reasonable to suggest that a little ‘tightness’ in the hip and lower leg may be beneficial, we need to define what is meant by tightness. There is a cut-off point where inflexibility ceases to be tightness within a normal range of motion and becomes excessive to the point of increasing injury risk. Clinically, excessive muscle tightness is believed to be an important cause of such injuries as muscle strains and inflammation of tendons. There is also evidence that regular running leads to inflexibility in the calf and hamstrings anyway, due to the hypertrophy of these muscle groups.
With these points in mind, runners should beware of concluding that general inflexibility is desirable for distance running performance. Rather, we suggest that runners with normal levels of flexibility should avoid flexibility training designed specifically to increase the range of motion around a joint, particularly when targeted at the muscles limiting external hip rotation, the calf/soleus complex and hamstrings.
In conclusion, while general stretching, designed to maintain existing levels of flexibility and muscle function, should remain an important aspect of every runner’s warm-up and cool-down routines, improving flexibility beyond levels normal for runners is likely to impair rather than improve performance.
Mick Wilkinson and Alun Williams
