Posted: June 12, 2005

Science of Sport: Are Norwegian Hams As Well-Preserved As Swedish Ones?

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

In a recent issue of Running Research News, I described some special Swedish exercises for curing hamstring ills ("Swedish-Cured Hams Are Right for Runners," Volume 19-8 (October), pp. 1-4, 2003; http://www.rrnews.com). Now, Norwegian researchers are getting into the hamstring act with a unique and extremely effective exercise which expands hamstring strength greatly and - unlike the Swedish effort - requires no special equipment to perform.

In the new Norwegian research, Roald Bahr and four colleagues from the Oslo Sports Trauma Research Center at the Norwegian University of Sport and Physical Education and the Stabaek Clinic in Bekkestua, Norway worked with 22 competitive soccer players who were from the first-division national club Stabaek Fotball (10 athletes) and also from some second- through fourth-division teams (12 players). At the beginning of the study, all athletes underwent basic tests of hamstring flexibility and strength, as well as quadriceps-muscle forcefulness (1). None of the 22 players were suffering from prior hamstring strains which had not been fully resolved by the start of the study.

After the pre-tests, the 22 athletes were divided into two equal groups - a "traditional-hamstring-curl" (HC) collection of players and a "Nordic-hamstring" (NH) group; members of each group then began a unique 10-week training program. The HC athletes performed their hamstring curls on a traditional hamstring-curl machine. As each athlete did this, he lay flat on the curl bench, stomach down, keeping his hips in a fixed position and his ankles hooked under a padded bar attached to a weight rack by a cable pulley. Each player rested his forehead on the bench, grabbed the handgrips, and brought his heels up towards his rump as quickly and as forcefully as possible, maximizing effort during this concentric phase of the action. During the eccentric phase (the lowering of the weight), the athletes used as little effort as possible, providing minimal resistance to the dropping of the weight.

During the first week of the 10-week study, the HC athletes trained just once, completing a series of exercises on the curl machine which determined 10-repetition max (the amount of weight which could be successfully lifted 10 - but not more than 10 - times). During the second week, they trained twice, hitting two sets of six reps at 10-rep-max intensity in each workout. For week three, they moved up to three workouts, with work load set at three sets of six to eight reps at the 10-rep max. During week four, there was a progression to three workouts with three sets of eight to 12 reps at the original 10-rep-max intensity (the athletes were stronger, so they could now do 12 reps at the initial 10-rep max). For weeks five through 10, the HC players continued training their strings three times a week, with each workout consisting of three sets of eight to 12 reps. Progressive loading was utilized during this final six-week period, with the resistance increasing by 2.5 kilograms (approximately 5.5 pounds) whenever an athlete could complete 3 X 12 reps with the previous weight.

The Nordic-hamstring protocol proceeded quite differently. As it turns out, the Nordic-hamstring exercise is a partner-assisted exertion (although it may also be performed solo with the ankles under a low bar) in which an athlete attempts to resist a forward-falling motion by using his/her hamstrings. As he does so, the hamstrings are loaded nearly maximally in their eccentric phase of action (i. e., while they are lengthening). This creates a great contrast from the hamstring-curl exertion, in which the mega-loading occurs during the concentric (shortening) phase of hamstring activity.

To carry out their Nordic-hamstring exercises, each athlete kept his hips fixed in a slightly flexed position throughout the whole range of motion and braked the forward fall of the body for as long as possible using his hamstrings, trying to keep tension in the hamstrings even after they have to "let go". Each player used his arms and hands to buffer the fall (please take a look at Figure 1), letting his chest touch the surface of the floor or mat at the end of the plummet. Once the chest touched the floor, the athlete immediately went back to the starting position by forcefully pushing with his hands, thus minimizing hamstring work in the concentric phase of action.

The Nordic-Hamstring athletes used a progression similar to the one employed by the HC subjects, beginning with just one training session during the first week (two sets of five reps) and moving up to dual workouts during the second week (two sets of six reps each time). From the third through 10th weeks, the NH participants trained three times a week, using first three sets of six to eight reps (third week), three sets of eight to 10 reps (fourth week), and then three sets with 12-10-eight reps (weeks five through 10). As the athletes became stronger while performing the exercise, the load on the hamstrings increased (the subjects could sustain the forward fall longer without falling onto their chests). Whenever the players became quite adept at handling three sets of 12 reps, speed was added to the starting phase of the overall motion, making the exercise more difficult and increasing the strain on the strings. In some cases, training partners heightened the hamstring work load by pushing on the backs of the shoulders of the athletes doing the NH exertions (making it more difficult for the hamstrings to prevent forward collapse).

Each group completed about 23 hamstring workouts during the 10-week training period, and there was no difference in the total amount of other training carried out, including soccer training, strength training, and endurance work. Post-workout soreness remained fairly minimal, with no real difference between groups (a "plus" for the NH athletes, since eccentric exercise is often linked with the invocation of muscle pain), and there were no changes in hamstring flexibility in either the NH or HC subjects. No hamstring injuries occurred during the training (another "plus").

Over the course of the 10 weeks, the HC athletes did achieve significant gains in strength while performing concentric hamstring-curl exercise, boosting their 10-rep-max resistance up to 45 kilograms (99 pounds). However, when the two groups were compared for maximal torque during isometric and eccentric actions (the latter being the ones believed to cause most hamstring injuries), the NH athletes were absolutely dominant. In fact, there were no improvements at all in either isometric or eccentric strength for the HC players, while the Nordic-hamstring subjects exhibited strength upswings in all of the tests.

Specifically, NH athletes boosted maximal eccentric hamstring torque by 11 percent and max isometric hamstring strength at 90, 60, and 30 degrees by 7 percent; HC participants failed to get better on any of these four tests.

This is of course an example of mode specificity, which basically means that if athletes train their muscles using eccentric activities, their eccentric strength will improve, but they should not expect gains in concentric strength. Likewise, focusing on concentric actions tends not to lead to much improvement in eccentric strength. Overall, the gain in strength is specific to the mode of muscle activity.

That's why a separate, well-regarded study carried out with college students produced results similar to those found by Bahr and his colleagues. In the college research, 12 workouts which revolved around eccentric hamstring strengthening (two training sessions per week for six weeks) produced significantly greater gains in peak eccentric hamstring torque, compared with the same number of workouts stressing concentric work for the poor hams (2).

But, if mode specificity is so important, why did the NH athletes get stronger eccentrically and isometrically, when the basic action of the NH exercise appeared to be eccentric? Bear in mind that the hamstrings performed a lot of braking activity during the forward-fall portion of the NH exercise, bringing the body to temporary halts on the way down and in effect inducing short-duration isometric phases of activity (when the 'strings were highly active but were neither elongating or shortening). This being true, it is not surprising that isometric strength swelled in the NH group.

Why didn't the HC players enjoy some surges in isometric strength, too, transferring some of their concentric strength from curling over to isometric actions, just as NH subjects moved strength from eccentric to isometric? The problem for the HC athletes was that they in effect performed almost no isometric activity; they kept the weights moving steadily as they carried out their curling activities. Thus, there was little chance for their 'strings to work isometrically, and so there was no change in isometric strength. Don't forget that gains in muscular strength are almost always tightly linked with the form of muscular activity utilized during training (eccentric, isometric, or concentric), as well as with the specific movement involved and the speed with which the movement is performed.

Sports-medicine physicians, trainers, coaches, and athletes are often interested in something called the H:Q ratio, which is simply the ratio between eccentric hamstring torque (H) and concentric quadriceps torque (Q). Ratios which are too low are often believed to be linked with a higher risk of hamstring injury during sporting activity, since a low ratio would imply an imbalance in strength between these two key muscle groups controlling the knee joint, with the quads having the upper hand. The basic idea is that with a low ratio the quads could exert powerful forces during knee extension which would outstrip the hamstrings' abilities to control the action, leading to potential hamstring damage. Over the course of this 10-week Norwegian study, concentric quadriceps strength did not change in either group of athletes, which meant that the H:Q ratio rose for the NH players but remained stable in the HC group. For the NH athletes, H:Q ascended from .89 to .98 during the 10-week training period; HC players kept their H:Q right at .88 for the whole 10 weeks.

Low hamstring strength (and thus - in many cases - an impoverished H:Q) has been linked with an increased risk of hamstring strain in a variety of different scientific studies (3, 4, & 5). Runners have notoriously low H:Q ratios and also are quite prone to hamstring injury. For example, evidence suggests that hamstring problems account for about 11 percent of all running injuries (6), and it is likely that a lack of eccentric (rather than concentric) hamstring strength is the source of most of the maladies (2). Concentric hamstring curls do not heighten eccentric hamstring strength, but the Nordic-hamstring exercise represents a viable means of improving such strength. As Bahr and his colleagues note, the Nordic-hamstring exertion is also a very practical exercise, in the sense that it can be performed anywhere; the only requirement is that an athlete's knees be at least slightly cushioned. Another practical aspect of the exercise is that - in contrast to exertions such as hamstring curls - an entire team of runners can carry out Nordic-hamstring drills at the same time, taking turns between actually doing the exercise and holding a partner's legs.

The body posture utilized with the Nordic-hamstring maneuver is not running-specific, however, and thus the unique exercise should be utilized in a workout "partnership" with more-running-specific, eccentric hamstring exertions such as bicycle leg swings and explosive thigh raises. As you incorporate Nordic-hamstring work into your overall program, be certain to utilize a slow increase in the number of repetitions of the exercise; this will minimize your risks of running-thwarting soreness and hamstring strain.

As always, you should also work on the speed with which you perform the exercise. Bear in mind that the knee angular velocity during very fast running (as the knee swings forward and the leg extends at the knee) may be as high as 600 to 700 degrees per second (7). Note, too, that the greatest eccentric strain on the hams occurs as the forward movement of the foot and leg is decelerated during the late forward swing phase of gait - until the leg is stopped about 30 degrees from full extension at the knee (8); that is when electromyographic activity of the hamstrings reaches its apex (9). As Bahr and his co-workers point out, from an injury-prevention (or rehabilitation) point of view it would appear that the optimal exercise would be an eccentric one in which a very high angular knee velocity was most aggressively controlled (with maximum hamstring force production) at around 30 degrees of knee flexion.

The Nordic-hamstring exercise comes close to doing this. Toward the end of the 10-week training period, many of the Norwegian athletes were able to stop the downward motion of their bodies completely before touching the ground - at about 30 degrees of knee flexion, even when they had been pushed by their partners at a significant speed! When this stage is reached, the NH exercise is probably very close to the scenario which takes place when injury occurs, with eccentric hamstring action, strong forces, and close-to-full extension at the knee. Thus, one would expect NH to be a significant injury-preventer. ©

References

(1) "A 10-Week Randomized Trial Comparing Eccentric vs. Concentric Hamstring Strength Training in Well-Trained Soccer Players," Scandinavian Journal of Medicine & Science in Sports, Vol. 14, pp. 1-7, 2004
(2) "Concentric Versus Enhanced Eccentric Hamstring Strength Training: Clinical Implications," Journal of Athletic Training, Vol. 33, pp. 216-221, 1998
(3) "Relationship between Hamstring Strains and Leg Muscle Strength. A Follow-Up Study of Collegiate Track and Field Athletes," Journal of Sports Medicine and Physical Fitness, Vol. 33, pp. 194-199, 1993
(4) "Hamstring Injuries in Sprinters," American Journal of Sports Medicine, Vol. 22, pp. 262-266, 1994
(5) "Preseason Hamstring Muscle Weakness Associated with Hamstring Muscle Injury in Australian Footballers," American Journal of Sports Medicine, Vol. 25, pp. 81-85, 1997
(6) "Injuries in Runners," American Journal of Sports Medicine, Vol. 15, pp. 168-171, 1987
(7) "Testing Strength and Power." In: MacDougall, J. D., Wenger, H. A., & Green, H. J., eds. Physiological Testing of the High-Performance Athlete. Champaign, IL: Human Kinetics, pp. 21-106, 1990
(8) "Muscle Strain Injuries: Clinical and Basic Aspects," Medicine and Science in Sports and Exercise, Vol. 22, pp. 436-443, 1990
(9) "Amplitude and Timing of Electromyographic Activity during Sprinting," Scandinavian Journal of Medicine & Science in Sports, Vol. 6, pp. 15-21, 1996

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