Posted: June 17, 2005

Science of Sport: How Does Explosive Training Change Your Leg Muscles?

By Owen Anderson, Ph. D. (Copyright © 2004-2005)

Our skeletal muscles are highly plastic tissues which respond rapidly to the things we do during training. That being the case, what specific changes occur in our muscle cells when we engage in explosive training? Does an understanding of explosive-training-related muscle alterations help us plan more-effective explosive workouts?

To find out, Dr. Heikki Kyrolainen and his colleagues at the University of Jyvaskyla in Finland, Appalachian State University in North Carolina, and the Copenhagen Muscle Research Center in Denmark recently studied the leg muscles of 13 athletes as they carried out explosive training, along with 10 individuals who served as controls (1). The subjects in both groups were young (24-25 years old) and relatively lean (9- to 11-percent body fat), and had not participated in any systematic power training prior to the study.

After a two-week, preparatory phase of training which included the performance of squats, dead-lifts, abdominal exercises, and calf exertions, the 13 experimental subjects carried out two explosive workouts weekly for a period of 15 weeks. The explosive-session exercises included jumping drills (with a special sledge apparatus to provide resistance), drop jumps with minimal displacements of the knee joints from heights of 20 to 70 centimeters (8.5 to 27.6 inches), jump squats (squats which incorporated a maximal vertical jump once the full squatting position was reached) with a resistance of 30 to 60 percent of the one-repetition maximum, two-leg hopping, one-leg hopping, and hurdle jumps. Note that these drills correspond closely with the kinds of routines which many runners utilize during their explosive phases of training. For each exercise in the explosive workouts, there were five to 10 reps per set, and each rep was completed with maximal effort in an attempt to enhance explosive force production in the leg muscles. The overall number of reps gradually increased from 80 per workout to 180 reps per session across the 15-week period. In addition to these power workouts, the experimental athletes engaged in about six hours of supplemental physical activity per week (cycling, walking, and ball games). Meanwhile, control subjects exercised for about four and one-half hours per week - but carried out no power training, of course.

Why did Kyrolainen and his colleagues ask the experimental athletes to carry out their drop jumps with minimal changes in knee angle after impact? Performing drop jumps in this way loads the plantar-flexor muscles (i. e., the sinews in the calf area) more effectively, compared with allowing the knees to flex once the feet strike the ground. The utilization of significant knee flexion takes the pressure off the calf muscles and allows the quads to produce much more of the force required to jump upward following impact. The use of major knee flexion also takes more time, compared with straight-knee drop-jumping, and is therefore a less powerful movement (remember that power is force expressed per unit time; as the denominator - time - increases, power diminishes). Utilizing expansive knee flexion is also less specific to running, compared with employing rather minimal knee flexion and deriving power from the foot and ankle.

As you might expect, maximal force production by the calf muscles increased significantly during the study for the power trainees (control subjects achieved no improvement in force output). Interestingly enough, calf-muscle force production expanded at a high rate over the first five weeks of the training, slowed its rate of increase over the subsequent five weeks, and then remained essentially stable over the last five weeks of the research project. One might conclude that this means that 10 weeks is a maximum duration of time to spend on power training, since no gains in calf-muscle strength were apparent after that time. However, note that the use of greater resistance or higher jump heights for the drop jumps might have produced additional strength benefits for the calves; further work needs to be carried out in this area.

Similar to the time course of the changes in the magnitude of calf-muscle force production, the maximal rate of muscular-force development during knee extension (extension is essentially the straightening of the leg at the knee by the quads) increased (by about 35 percent) over the first 10 weeks of the investigation but remained unchanged after that. This increase in the rate of force development means that if you focus on any relevant force produced during knee extension - say, for example, 2500 Newtons, this amount of force was attained more quickly during the act of extending the knee as a result of explosive training. Of course, this is exactly the kind of change you want to occur in response to power training, which is supposed to speed up muscle reactivity and get your feet on and off the ground more quickly as you run. Control subjects were not able to augment the rate of knee-extension force production during the investigation.

At the beginning of the Finnish-Danish-Appalachian study, the centers of gravity of the experimental subjects rose by an average of 30 centimeters per drop jump (this occurred with an actual drop-jump height of 50 centimeters, or 20 inches, which simply means that the bench used for the drops was 50 centimeters above the floor of the laboratory); by the end of the investigation those centers were rebounding upward by 37 centimeters after impact with the ground, a 23-percent upswing. Importantly, contact time with the ground did not change significantly; it was .23 seconds at the beginning of the 15-week period and .23 seconds at the end. This means that the explosive training enhanced muscular force production following impact with the ground (after all, you have to produce more force to jump higher); since the expanded force was produced in the same amount of time as before (foot-contact time was unchanged), the athletes were truly more powerful (remember that power is force per unit time).

In theory, this should translate into faster maximal running velocities for the athletes who carried out the explosive training. The only "fly in the ointment" here is that the muscular activities which produce the vertical body movement which follows ground impact during a drop jump is not exactly the same as the sinew actions which produce horizontal motion during running. It is simply assumed that there is a carry-over effect and that once the nerves controlling the leg muscles have learned to recruit the muscles more quickly, this rapid recruitment process will also occur during running. However, the specific effects of drop-jump training per se on running ability have not been carefully examined in a carefully controlled investigation, to our knowledge. Drop jumps have been featured as components of power workouts which have upgraded raw running speed, and it has simply been assumed that drop jumping was an essential factor in the speed-up process.

There has been some interesting research which has linked drop-jump ability with maximal running velocity, however. In one study carried out in the Fitness Department of the Irish Rugby Football Union in Dublin, 17 trained, competitive runners completed three tests: (A) Countermovement Jumps for vertical distance (such "countermovement" jumps involve extensive flexion at the knees prior to the upward leap), (B) Drop Jumps for best-possible height with the minimum-possible ground-contact times (a great test of explosiveness), and (C) Five-Step Bound Tests, which called for the athletes to cover as much ground as possible with five quick bounds. Total ground-contact time during the drop jumps was also assessed by the Irish investigators, body fatness was estimated, and the 17 athletes also ran 30, 100, and 300 meters as fast as possible (2).

The Irish researchers analyzed a very interesting variable called the BDJ (Bounce-Drop-Jump) index, utilizing units of centimeters per second. In effect, the BDJ index was a measure of power during drop jumping, since it indicated how far an athlete's body could rise (in centimeters) for each second of ground-contact time (determined in seconds). The group average for the 17 runners for the BDJ index was 167 centimeters per second. They were probably moving their centers of gravity upward by about 33 centimeters or so after ground contact during their drop jumps, which meant that actual ground-contact time was about .2 seconds (33 cm/.2 seconds is approximately the same as 165 cm/1 second). As it turned out, the BDJ index was strongly correlated with running performances over 30 and 100 meters: The higher the BDJ (and thus the greater the power during drop jumping), the faster the 30- and 100-meter times. It is not too much of a stretch to suggest that drop-jump training, with resulting improvements in drop-jump performances, might lead to faster maximal running velocities.

Speaking of stretch, it is important to note that even though drop jumping and fast running are not identical from a kinematic standpoint, each depends on the optimization of something called the "stretch-shortening cycle" to maximize performance. The word "stretch" in the phrase "stretch-shortening cycle" refers to the stretch which the calf muscles, quads, and hip extenders instantly undergo when the foot/feet hit the ground (during both running and drop jumping). "Shortening" refers to the subsequent rebound of the muscles, to a large part a function of elastic recoil, which occurs after the muscles have been stretch. As this process is optimized during drop-jump training (by adjusting the stiffness of the muscles so that the amount of stretch is just right - not too much, not too little, and by adjusting the shortening so that it takes place at just the right time and with just the correct and well-coordinated force), the optimization process may well carry over to running, which also involves stretches and shortenings of the calves, quads, and ham-glutes with each ground impact.

In the Irish study, there was no connection between body fatness and running performance over 30, 100, or 300 meters (the runners were fairly homogenous with regard to body composition). However, there was a decent correlation between counter-movement jump height and 30-, 100-, and 300-meter speeds (there's that stretch-shortening cycle in action again). The strongest correlations, however, were between the BDJ index and running performance. Note that drop jumps are more similar to running, compared with counter-movement jumps (the latter employ considerably more knee flexion, for example, than is the case with running, and since there is greater knee flexion there is also a fatter ground-contact time during counter-movement jumps, compared with either drop jumps or running).

Although drop jumping seems to be a pretty good explosive exercise for runners who want to improve their speed, one important drop-jump issue concerns foot placement at impact. Most runners carry out their drop jumps with forefoot landings, and yet many of these runners are "heel-toe strikers," hitting the ground with their heels first when they are actually running. Should heel-toe strikers land on their heels first during drop jumping, not on their forefeet? Is there a "best" foot-placement strategy for performing drop jumps?

To check on these possibilities, researchers in the Department of Biomechanics at the Hungarian University of Physical Education in Budapest recently studied 10 healthy male student-athletes as they performed two types of drop jump from a 40-centimeter-high (16-inch) box (3). In one case, the students were instructed to land first on the balls of their feet, without the heels touching the ground at all during the subsequent vertical jump (this was the forefoot-landing drop jump). In another trial, the athletes landed on their heels and then rocked forward onto their metatarsals and toes before jumping (this was the heel-toe-landing drop jump). Each type of drop jump involved landing on a force plate so that power outputs could be measured.

As it turned out, impact forces tended to be greater when the heel-toe-landing strategy was employed; in fact, they were up to 3.4-times larger. In addition, heel-toe landing tended to aggrandize muscular activity in the hip and knee joints, whereas forefoot landing put the onus for force production on the knees and ankles. For example, during the flexion phase of heel-toe landing (when first impact with the ground is made and the hips and knees flex simultaneously while the ankles are going through dorsiflexion), the hip and knee joints contributed 40 and 45 percent of the total required torque (85 percent in all), leaving a lowly 15 percent for the ankles. In contrast, forefoot landing called the ankles into action during the flexion phase of the exercise, with the ankles chipping in 37 percent of the total muscular torque; the knees were still in the game, at 37 percent too, while the poor hips dropped their contribution down to 26 percent. It is clear that forefoot landing during drop jumping activates the ankles to a greater extent (during the milliseconds which follow first impact with the ground), compared with heel-toe landing. This would appear to be a good thing, since ample ankle responsiveness should improve explosiveness and efficiency during running. Many runners have rather low-power, unresponsive ankles.

During the extension phase of drop jumping (when the legs straighten and the athlete leaves the ground, jumping upward), the ankles also were more active when forefoot landings were utilized, compared with heel-toe landings. With heel-toe landings, the knees chipped in 41 percent of the torque and the ankles offered 45 percent, but with forefoot landings the ankles' contribution shot up to 55 percent of the total (the knees fell to 34 percent). Note that the hips were rather quiescent during extension with both techniques, contributing just 14 percent of the torque with heel-toe and 11 percent with forefoot landings.

Overall, total power production was 40-percent greater during the extension phase of drop jumping when forefoot landings were utilized, compared with heel-toe landings. As you might expect, EMG (electromyogram) analyses of muscular activity revealed that the key calf muscle - the gastrocnemius - was significantly more active during forefoot landings, compared with heel-toe impacts. It would appear that forefoot landings during drop jumping are associated with unique changes in muscle recruitment and activity, and that these changes are desirable for runners who want to optimize their "stretch-shortening cycles," ankle power, and overall explosiveness after impact with the ground. For these reasons, RRN recommends the use of forefoot landings during drop jumping (in a later article, we'll address the question of whether heel-toe-striking runners should attempt to shift over to the forefoot-landing strategy during actual running).

Three final points about drop-jumping are in order: (1) For your first attempts at drop-jumping, drop onto a very "forgiving" surface, such as soft grass, relatively soft earth, moderately packed sand, or a basketball or gym floor with some "give" to it. Keep the number of reps to a minimum during your first few drop-jump sessions, too; a seemingly paltry six to eight reps should be about right. Finally, employ low box or step heights until you have built up a modicum of drop-jump strength and coordination; four to six inches are actually good starting heights. (2) When you hit the ground after your drop, jump forward instead of vertically, while of course minimizing ground-contact time. This will make the action more similar to running. (3) After you have been drop-jumping for a considerable period of time without trouble (four to six months or so), shift over to one-foot drop jumps, landing on one foot instead of two before springing forward. When you do this, reduce the height of your box or step at first, and fall back to just six or eight reps per leg. You can then gradually make a progression to more reps and greater height.

Going back to the Finnish-Danish-Appalachian research for a moment, it is important to note that there were no changes in the athletes' muscle-fiber compositions or muscle-fiber sizes (for either slow-twitch or fast-twitch cells) as a result of the power training, nor were there any changes in the make-ups of the key proteins involved in contraction and elasticity within the muscles. These observations forced the researchers to make a logical conclusion: The improvements in strength, power, the rate of force development, and drop-jump ability were all the result of adjustments in how the nervous system controlled the muscles, not a response to changes in muscle structure or organization. True, muscle cells are highly plastic, in terms of their ability to change size and composition in many situations, but they are also flexible with respect to their ability to respond to variations in "input" from the nervous system. With power training, it appears to be the latter plasticity which is most important, especially during the early stages of strength gain.

Of course, with increased power comes increased speed while running. We know that maximal running speed is a great predictor of performance for both sprinters and endurance runners, and we also know that there are three proven ways to bolster max running velocity:

(1) By running faster during training,
(2) By carrying out explosive training, with an emphasis on jumping, hopping, and bounding movements and maximal muscular contractions completed in the shortest-possible amount of time, and
(3) By performing high-resistance strength training for the leg muscles, with a reliance on resistances which can not be utilized for more than six reps prior to failure.

However, what we don't know is how these types of training should be sequenced within an overall program. Is it best to perform all three types of max-velocity enhancing training simultaneously? Should # 3 take place before # 2, which in turn should precede # 1? We simply don't have the answers right now.

What is also unclear is how this form of training, which we may call power or explosive training (although admittedly # 3 does not fit precisely into this category), should be coordinated with the rest of one's running program. By convention, runners and other endurance athletes have tended to place explosive training near the ends of their overall training cycles. One of the reasons for this is that historically we have believed that aerobic development, not raw speed, is the key to endurance success, making speed training of relatively minor importance. It has been believed, for example, that speed training is primarily carried out so that one can surge or "kick" dramatically in the latter stages of a race - not so that higher speed can be utilized throughout an entire competition. Another common belief has been that speed training somehow harms aerobic development (making the identities of muscle cells more "anaerobic") and therefore should not be continued for long periods. For a variety of reasons, true, systematic explosive training (which is something more than going to the track on Tuesdays with a group of friends or club members) has been relegated to the workout dustbin or treated almost as an afterthought.

There is a certain, attractive logic which goes along with this, of course. After all, does it not make sense to develop running-specific strength first - and then learn to apply that strength more quickly (by carrying out explosive training)? The result would be longer, faster strides - and a higher maximal running velocity.

Of course, that sort of scheme does make sense, but it does not necessarily make more sense than the employment of a strategy which first makes muscles quicker and more reactive - and then burgeons their strength. Of course, a key "knock" against the use of explosive training early in a training cycle has always been that such training would significantly increase the risk of injury. There is no scientific support for this whatsoever, but the "knock" appears to be reasonable to many of us who have strained a muscle or tendon during a high-speed workout - or who have had to endure a red-hot knee the day after a high-intensity interval workout, while enjoying cool knees on days following moderate runs.

Note, though, that there is no necessary connection between explosive training and injury. If one embarks on explosive training in a reasonable manner and very gradually adds reps, difficulty, height (for the drop jumps), and speed over time, there is nothing about explosive training per se which should make it risky or tendon-tearing. In fact, one could argue that slow, prolonged training, with more total impacts with the ground, would be more conducive to injury.

What if we argued the question from a fitness-gain standpoint? Here, the potential advantages of putting explosive training first in the overall training cycle quickly become apparent. Let's face it: A runner will probably be faster after a six-week period of explosive training, compared with six weeks of general-strength (circuit) training (the latter is a common starting point for training cycles). That improved speed could then spike the quality of subsequent training, heightening vVO2max, lactate-threshold speed, and running economy to a greater extent, compared with opening up the training cycle with circuits or some other kind of base period. The result would then be higher fitness at the end of the second, running-specific strength phase of work - and greater readiness for the hill training which would follow. One might then utilize circuit training to fine-tune whole-body strength and coordination, inching overall fitness up even further as the true race season commences.

Even if you do not accept this kind of reasoning, our understanding of the importance of max running velocity and the kind of training can make us run faster forces us to reevaluate the way in which we construct our circuit workouts. Should circuits really revolve almost entirely around exercises which use little more than body weight for resistance, for example? Would it not make more sense (from the standpoint of improving speed) to include some heavy-duty squatting exercises (using six-rep-max resistance) within our circuits, for example, as well as some high-resistance lunges? Traditionally, the running segments of circuit workouts are progressed by expanding the lengths of the running intervals, but might not it make more sense to vary the nature of the running within the circuits - focusing on short, very high-speed intervals for some of the circuit sessions (to improve neural coordination of the muscles at advanced velocities) and longer-distance intervals (to work on the aerobic side of things) for others? At this point in time, we can not look to science for the answers to these questions, but the logic is certainly appealing.

Here is an example of a speed-developing circuit workout which incorporates all three principles of speed enhancement and which can be used during almost any phase of the overall training cycle:

(1) Run 400 meters at a fast - but not maximal - pace (be certain that you have warmed up thoroughly prior to starting this workout).
(2) Perform 15 regular push-ups, activating your core muscles throughout the overall movement to keep your body linear.
(3) Carry out 20 low-back extensions from a prone position, using your glutes and hamstrings to lift your legs off the ground each time you extend your back.
(4) Do 20 two-leg squats at moderate speed with a barbell poised on your shoulders (very light resistance).
(5) Run 400 meters at close-to-maximal pace, staying relaxed and rhythmic throughout your effort.
(6) Perform 20 sit-ups.
(7) Carry out 20 squat thrusts with jumps (burpees).
(8) Do 6 two-leg squats, using the most barbell weight you can handle for six reps (early in the season this will be a modest amount of weight; if you can perform eight or more reps, increase the weight).
(9) Run 400 meters at close-to-maximal pace, staying relaxed.
(10) "Parade-walk" with a barbell held on your shoulders. To do this, walk on the balls of your feet with springy strides for 20 meters, emphasizing exaggerated dorsiflexion and plantar flexion of your ankles as you do so. Very important: As your left foot moves forward and lands on the ground, rotate your upper body in a counter-clockwise manner; as your right foot moves forward and lands on the ground, rotate your upper body in a clockwise direction. Use your abs and low-back muscles to control these motions - and to keep your torso upright. Also very important: Keep your feet pointed straight ahead at all times as you move along. For many runners, this is an exercise which takes some time to learn, so don't be frustrated if you don't "get it" right away - you will gradually increase your proficiency (while you are learning, you may temporarily omit the exaggerated dorsiflexion and plantar flexion of your ankles). Initially, utilize very light weight on the bar, gradually increasing resistance over time.
(11) Perform 12 feet-elevated push-ups, activating your core muscles throughout the overall movement to keep your body linear.
(12) Do 6 two-leg squats, using the most barbell weight you can handle for six reps (see further explanation above in step # 8).
(13) Run 400 meters at close-to-maximal pace, without straining or tightening up your muscles.
(14) Carry out 25 bench dips.
(15) Complete 20 low-back extensions from a prone position, using your glutes and hamstrings to lift your legs off the ground each time you extend your back.
(16) Do 6 two-leg squats, using the most barbell weight you can handle for six reps (see further explanation above in step # 8).
(17) Run 400 meters at close-to-maximal pace, staying relaxed at all times.
(18) Hit 20 burpees.
(19) "Parade-walk" with barbell on shoulders for 20 meters (please see step # 10).
(20) Perform 15 two-leg squats, utilizing moderate resistance on the barbell (approximately your 15-rep max) and moving as explosively as possible (while still maintaining good form).
(21) Run 400 meters at a strong, relaxed pace.
(22) Do 20 sit-ups.
(23) Perform 15 lunges on each leg, while holding barbell on shoulders (moderate resistance at first).
(24) Run 400 meters strongly - while relaxed.
(25) Cool down with 1600 meters of easy jogging.

Some cautionary notes are in order concerning this workout: (A) The first few times you try this circuit, stop after step # 13 to minimize muscle soreness. In addition, take it easy with the 400s at first, focusing simply on coordination and quick foot contacts rather than all-out running. (B) Be very careful with the barbell squatting. Until you have a firm grasp of your six-rep max, be conservative with the amount of weight you are piling on the bar. It is better to start with loads which are too light, rather than too heavy. Gradually (over a period of several workouts) develop an appreciation of what resistance is required to limit you to just six reps. As you perform the squats, be sure to keep your back linear; it is OK to lean forward slightly as you squat, but do not curve your back.

If about two miles of running are included in the warm-up for the above circuit, please note that the session includes about 4.75 total miles of running, with two miles (42 percent) at good velocity (this assumes that there are about four 100-meter "strides" included in the two-mile warm-up). Note that each 400-meter bout of fast running brings you back to the "station" at which your barbell is positioned.

Of course, you may be thinking: "That's way too much fast running for an early-season workout!" Bear in mind that this workout will be carried out according to your capacity at the time of the session. In the early part of your season or training cycle, the 400s may not be very fast; it would certainly not be a good thing if they were just as fast at the beginning of your macrocycle, compared with the end. The important thing to do is to simply relax and run the 400s as well as you can, focusing on quick, rhythmic, coordinated strides. You will naturally progress over the course of the season to faster and faster splits. Similarly, you will employ greater weight on the bar as your strength and power improve. Toward the end of your major cycle, you will chuckle about the small amount of weight which comprised your six-rep max at the beginning of the training period.

Here are explanations of some of the exercises in the circuit (please accept my apologies if you already know them):

Feet-elevated push-ups are normal push-ups, except that your feet are elevated (on a bench, chair, or wall).

To perform bench dips, seat yourself on a bench or chair, with your hands at your sides. Your hands should be gripping the front edge of the bench or seat. While keeping your hands in the same position, slide forward off the chair and put your feet as far forward as possible, so that all of your body weight is supported only by your hands and the heels of your extended feet. Then, simply lower your buttocks to the floor (or almost to the floor), and bring yourself back up again to complete one rep.

To do low-back extensions, lie on your stomach, with your arms by your sides and your hands extended toward your feet, with palms touching the floor. To achieve the basic starting position, contract the muscles at the back of your neck, so that you are gazing forward and upward. A rep is simply a contraction of your low-back muscles which lifts your trunk well off the ground, followed by a slow easing of your torso back to the floor. For the modified low-back extensions included in the above circuit, please use your glutes and hamstrings to lift your legs off the ground each time you elevate your trunk (so that the lifting of trunk and legs is simultaneous).

Lunges are simply exaggerated steps. Begin with erect posture and your feet directly under your shoulders. You should be relaxed, as though you were about to run. Then, step forward with one foot, taking a slightly longer step than usual. After this forward foot makes contact with the ground, go into a squatting position, so that the thigh of the forward leg becomes almost parallel with the ground (it's OK to let your upper body incline forward a little bit as you do so). Return to the starting position (feet back under your shoulders, erect posture), and you have completed one rep. ©

References

(1) "Effects of Power Training on Muscle Structure and Neuromuscular Performance," Scandinavian Journal of Medicine & Science in Sports, In Press, Accepted for Publication December 17, 2003
(2) "Relationship of the Stretch-Shortening Cycle to Sprint Performance in Trained Female Athletes," Journal of Strength and Conditioning Research, Vol. 15(3), pp. 326-331, 2001
(3) "Foot Placement Modifies Kinematics and Kinetics during Drop Jumping," Medicine & Science in Sports & Exercise, Vol. 31(5), pp. 708-716, 1999

To learn about Owen's running camp in Malibu, California this summer, please visit www.RRNews.com, scroll to the bottom of the page, and click on the running-camp "splash."

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