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plyometrics

Plyometrics exercises

two-leg drop jumping exercises help prevent injuries, rather than causing them – and they may improve running speed as well

Athletes who run in their sports are often told to avoid ‘high-impact’ exercises for their legs, including plyometric activities which involve bounding and drop-jumping from benches and platforms, in the belief that such exertions carry with them a high risk of injury. As a result, many running athletes avoid high-impact drills, even though such efforts are believed to increase running power. Are the high-impact-exercise naysayers correct? Should athletes who run really stay clear of such activities?

Focusing on the shin bone

In an attempt to find out, researchers at the Hebrew University Medical School in Jerusalem, Huddinge University Hospital in Sweden, and the Indiana University Medical Center recently took a look at the forces acting on the shin bones (tibias) of athletes during both running and jumping activity (1). The choice of bone as a general tissue to study and the selection of the tibia as a specific bone to monitor were excellent decisions. The tibia is very prone to stress fractures in athletes who run (in fact, about 50% of all stress fractures in athletes occur in the tibia), and it is believed that jumping exercises may increase the risk of such fractures. In addition, bone is a highly mechanical organ which must be strong enough to withstand both high, sudden forces as well as low-level repetitive impacts; it is important to understand which kinds of forces are most likely to induce bone injury. The forces placed on the tibia during running and jumping are the result of both muscular ‘pulling’ on the bone during activity and, of course, gravity. Bone strains are measured in units called ‘microstrains’, and most forces placed on the tibia during activity produce strains which are only 20% to 30% of its ‘yield strength’ (breaking point) of about 10,000 microstrains. This makes it seem as though the tibia would almost never be fractured during normal sporting actions, but one problem is that repetitive activity can create small-scale damage in the tibia which can accumulate and eventually cause the bone to fail at strains below 10,000 microstrains.

The vulnerable square-bashers

Research reveals that military recruits are at a very high risk of experiencing tibial stress fractures, possibly because their leg muscles are weak and fatigue easily (leg muscles can act to absorb impact forces and thus protect underlying bone, but they do this poorly when they are tired); in addition, the recruits’ biomechanics may be sub-optimal (flexion of the knee and ankle during jumping can diminish the amount of ‘hammering’ the tibia receives, while failure to achieve such flexion puts added pressure on the tibia), and they may have low rates of repair of tibial microdamage as a result of previously sedentary lifestyles (it is possible that bone-repair rates are a function of fitness; as training level increases, within reason, bone-synthesis rates may also accelerate). Athletes who increase their volume of training quite suddenly, however, are also at risk of tibial stress fractures, presumably because the increased number of impact forces associated with the new training creates a large amount of microdamage, which can eventually lead to failure in one section of the bone. The average person, of course, would expect that jumping exercises would produce greater tibial strains and therefore a greater risk of tibial stress fracture than mere running (and of course there are a lot of ‘average people’ in the coaching and sports-publishing ranks – that is why runners are often told to avoid plyometrics). After all, jumping activities allow the tibia to fall from greater heights, thus enhancing tibial acceleration towards the ground and the resultant impact forces with terra firma. To see if this is really the case, the Jerusalem-Huddinge-Indiana researchers worked with six subjects (four male and two female), aged 27 to 52, stapling strain gauges to the middle, inside portions of their right tibias (surgical implantation of the strain gauges was performed on an outpatient basis at the Hadassah University Hospital in Jerusalem). Happily, the gauges – and staples – were removed on the same day after completion of the data collection.

What happened next

The gauges were utilised to measure tibial strain as the subjects ran for 100 metres on a cinder track at a velocity of 17 kilometres per hour (about 5:41 per mile), which was set by a pacer. The drop-jump measurements were made sequentially as the athletes jumped from wooden blocks of 26cm, 39cm, and 52cm heights (10, 15, and 20 inches) onto a force plate. Interestingly enough, there was no statistically significant difference in tension, shear strain, or compression forces on the tibias as drop-jump height increased from 26 to 52cm. In addition, there was an actual reduction in shear strain rate (the rapidity with which shearing force is applied to the tibia after impact) as jumping height increased. With the highest wooden block (52 cm), sheer strain rate was actually reduced by 30%, compared with the lowest block! Similarly, compression and tensile strain rates at the greatest height were reduced by 35%, compared with the rates associated with jumping from the lowest box. Since there was no difference in the total magnitude of the compression strain of the tibia associated with the height of the drop, it was clear that the subjects were able to dissipate part of the potential energy of successively higher jumps and not transmit that energy to their tibias. Video recordings of the jumps suggested that this dissipation of force occurred as a result of increased knee flexion and ankle dorsiflexion during landing; to put it another way, changes in the subjects’ mechanics of landing protected their tibias. Video analyses of the athletes revealed that the ankle was particularly important in dissipating force; in fact, ankle dorsiflexion (ankle movement which brings the top of the foot and the shin closer together) increased by roughly 66% at the greater jumping heights. Other studies have also shown that athletes naturally modify their landing techniques according to the height of a jump (2).

The key findings

Now, here are the key findings which will help us resolve our original question about the dangers of high-impact activities for athletes who run. The principal strains and forces associated with drop-jumping from the greatest height of 20 inches were not significantly different from the strains and forces associated with running. In fact, compression and tension strain rates were actually greater during running than they were during drop-jumping! So much for the idea that two-leg drop-jumping produces dramatically higher forces on athletes’ tibias, compared with running. One of the beauties of the present study was that it employed in-vivo measurements of bone strain; previous work had utilised force plates, video-motion analysis, and accelerometry to estimate – rather than directly measure – the strains placed on leg bones during landing. This Jerusalem-Huddinge-Indiana work gives us a much better picture of the forces which the tibia actually experiences during running and jumping.

Stay fresh

One factor to keep in mind, however, is that muscular shock-absorbing mechanisms in the legs are most effective when an athlete is not in a fatigued state. One study found that the strains on the tibia increase by about 30% during walking when leg muscles become tired, for example(3). A basic problem which muscles have when they are fatigued is that they transfer energy from their stretched states to their contracted states more slowly, an effect which can produce a 50% increase in tibial acceleration during normal gait(4). Thus, it is critically important to carry out high-impact exercises when one is fresh and non-fatigued. Naturally, a thorough – but non-fatiguing – warm-up should always precede high-impact activity. The astute reader will also note that an athlete, particularly one who is susceptible to stress fractures, should also avoid running when the leg muscles are particularly fatigued. Completing a second or long workout on a day when one is significantly fatigued can be an invitation for stress fractures to begin developing.

A one-leg activity

Running produces greater accelerations of the legs than does walking, and running consequently induces tibial strains and strain rates which are two to three times greater than those associated with walking. Drop-jumping can produce downward-directed accelerations of the legs which are larger than those produced during running, and thus it is only natural to expect that drop-jumping would create tibial forces higher than those linked with running. However, note that running is a ‘one-leg activity’, with full body weight and impact forces being absorbed by one leg at a time, not two. In this study, drop-jumping was carried out on two legs, which should help to mitigate the forces experienced by each limb; indeed, most athletes begin plyometric programmes with two-leg drop-jumping and build up significant coordination and strength with the technique before moving on to one-leg dropping. In addition, the research shows that athletes naturally modify their mechanics in order to moderate the strains experienced by their tibias when jumping from increasing heights. The two-leg technique and the biomechanical alterations are quite effective, as witnessed by the actual decreases in tension and compression strain rates exhibited by the athletes in this study during drop-jumping, compared with running. The Jerusalem-Huddinge-Indiana researchers were quite justified in concluding that two-leg drop-jumping exercises do not produce higher tibial strains and strain rates than running, and thus would be unlikely to increase the risk of bone fatigue stress fracture in the running athlete who carries out drop-jump training in a reasonable way.

Increasing bone density

Interestingly enough, drop-jumping exercises actually tend to increase bone density in the legs (5), an effect which should help to decrease – not increase – the risk of stress fractures. Drop-jumping should also improve coordination, an effect which can lower injury rates. So the next time you hear someone say that drop-jumping activities and their ilk will hurt you, it’s OK to chuckle a bit. If the purveyor of such ‘wisdom’ is one of your competitors, there is no need to provide a retort. If not, however, you will be helping the poor fellow by indicating that two-leg drop jumping should actually be an anti-injury technique, not an injury promoter, and it may help improve running speed, too.

Jim Bledsoe

References

  1. ‘Do High Impact Exercises Produce Higher Tibial Strains than Running?’. British Journal of Sports Medicine, Vol. 34, pp. 195-199, 2000
  2. ‘The Evaluation and Prediction of Impact Forces During Landings’. Medicine and Science in Sports and Exercise, Vol. 22, pp. 370-377, 1990
  3. ‘The Effects of Muscle Fatigue on Bone Strains’. Journal of Experimental Biology, Vol. 188, pp. 217-233, 1994
  4. ‘What Leads to Age and Gender Differences in Balance Maintenance and Recovery?’. Muscle Nerve Supply, Vol. 5, pp. 60-64, 1997
  5. ‘Pre- and Postmenopausal Women Have Different Bone Mineral Responses to the Same High-Impact Exercise’. Journal of Bone Mineral Research, Vol. 13, pp. 1805-1813, 1998

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