Muscle injuries can be debilitating for active sports participants. We have previously shown that sprint runners with a history of hamstring injuries have decreased eccentric hamstring strength (12), which is also confirmed by Croisier et al. (8). Thus, decreased eccentric strength may be a risk factor for muscle injury. In prospective, randomized clinical trials, eccentric exercises have shown good results in the rehabilitation of, for instance, Achilles (9,19-21) and patellar tendinopathy (15), as well as chronic lateral epikondylalgia (22).
Eccentric exercise is prone to give delayed-onset muscle soreness (DOMS) with a peak at 24-72 hours post exercise (1,10). There is a cellular response to eccentric exercise that includes initial tissue breakdown, followed by upregulation of the muscle protein desmin, infiltration of inflammatory cells, and increased permeability of the cell membrane (16). Yu et al. (25) have proposed that the DOMS after eccentric exercise may reflex the myofibril remodeling. The DOMS is also characterized by a significant loss of maximal muscular strength directly after intensive exercise (5,11).
Recently, eccentric hamstring training has been shown effective as a rehabilitation treatment for hamstring injuries (8), and, in another study, hamstring training has been suggested to be effective in the prevention of hamstring injuries (2). However, both these studies were performed using advanced equipment that normally is available only at certain special training centers.
Thus, there is a need for studies of specific eccentric exercises, which do not require advanced equipment. In fact, one training method that athletes, especially sprint runners, commonly use-the so-called “Nordic hamstring”-has recently been demonstrated to increase eccentric hamstring strength more effectively than training with a dynamometer (13).
We believe that the athletic community can help us in our search for new rehabilitation methods. Different kinds of forward lunges represent a training method that is widely used by athletes, especially sprint runners and jumpers. Athletic coaches consider this training to be a very strenuous exercise, and it is believed to act eccentrically both with regard to the quadriceps and the hamstring muscles, when breaking the ground forces. Forward lunges are performed as closed kinetic chains, which have been found to be superior to open kinetic chains when strength and functional parameters are measured (3).
In this study, we investigated the acute and long-term effects of 2 kinds of forward lunges: the walking forward lunge (WFL) and the jumping forward lunge (JFL). The forward lunges are shown in Figure 1. First, the acute effects of training on performance and pain parameters were studied directly after exercise. Second, 2 days after the initial exercise, the same parameters were studied; this time period reflects the impact of DOMS. Third, after a 6-week period of training, the effects on strength and performance of these 2 training regimens were examined.
Experimental Approach to the Problem
The study was performed during the off-season for soccer in Sweden, from January to March. All subjects were randomized into 1 of 3 study groups. The WFL group (n = 11) performed WFLs, doing 4 sets with 12 repetitions twice a week after their ordinary training sessions. The JFL group (n = 10) performed JFLs, doing 4 sets with 12 repetitions twice a week after their ordinary training sessions. Controls (n = 11) completed their ordinary training without any intervention and, thus, served as a control group. The ordinary training was focused on general physical training during this period and included aerobic and strength training as well as speed and technical skills. Training was scheduled for 3-5 times a week. All players, including controls, were instructed to document all their daily training in a training diary.
A test leader supervised the first training session to ensure that the exercises were done correctly. For quality control, the exercises were also filmed during the supervision. After 3 weeks, another supervised training session was held, this time without filming.
Evaluations of strength, functional, and pain parameters were done at 4 occasions: 1) immediately before and 2) after the first exercise, 3) after 2 days, and 4) after 6 weeks. The subjects were instructed not to do any hard strength training 5 days before the first and last testing sessions.
Forty players in a local junior male soccer team were personally invited to participate in the study. Without giving any reason, 6 players chose to decline, and another 2 were excluded, 1 because of fever and the other because of a sprained ankle. Thus, 32 players were included. Four subjects did not perform the last test because of illness or injury unrelated to this study. Thus, 28 subjects performed all the tests and completed the study. The mean (range) age of the subjects was 18 (17-20) years, their average height was 178 (166-190) cm, and their mean body weight was 68 (53-82) kg. They were all well trained and had been in organized training for at least 5 years. All subjects of the study gave their written informed consent. The ethical committee at the Karolinska Institutet approved the study (03-776).
Maximal Strength Testing on a Biodex Dynamometer
Warming up included 600 m of jogging, mild stretching of the hamstrings and quadriceps, and 5 submaximal squat jumps. The subjects were thereafter seated on a Biodex isokinetic dynamometer (Biodex Medical Systems, Shirley, NY) with a hip angle of 100°. The hip and the nondominant leg that was to be examined were anchored to the bench by belts to avoid extra movements. The dominant leg was defined as the leg with which the player preferred to kick the football. During the tests, each subject held his arms folded over his chest. The motion axis of the Biodex was aligned with the bilateral motion axis of the knee joint. The lower leg was placed in the resistance arm, and the angle was calibrated with a plumb line. Concentric peak torque was tested at 180°·s−1 for both the hamstring and quadricep muscle groups. During the test, each subject first acquainted himself with the machine by doing a few submaximal trials, followed by 3 maximal test contractions. The test started with the quadriceps, followed by the hamstrings. The subjects were encouraged verbally and got feedback on their performance by following the torque curve displayed on the screen. The torque between 10 and 90° of knee flexion was measured, and the highest peak torque value of the 3 trials was recorded.
Each subject performed 2 maximal 30-m sprints, after an initial acceleration distance of another 30 m. Electric timing was measured by photocells. The athletes used ordinary training shoes and ran on an indoor rubber track. They rested for about 3 minutes between races. The result of the fastest sprint was recorded.
One-Leg Hop Test
Each subject made 3 maximal 1-leg horizontal long jumps on the dominant leg (17). Ordinary training shoes were used, and the ground surface was an indoor rubber track. They were allowed to move their arms freely, but they were not allowed to lift their heels before takeoff. The best result was recorded, measured from the toes at toe-off to the heels at landing.
Pain Evaluation Using a Visual Analogue Scale
During the hop test, each subject also scored his pain in the jumping leg, on a visual analogue scale (VAS), with marks numbered from 0 to 10. The subjects were instructed that, on this scale, “0” was no pain or discomfort, and 10 indicated the worst pain imaginable.
Pain Evaluation Using an Algometer
A special algometer (A J Tech Commander Algometer, Kom Kare Co., Middletown, Ohio), with a point diameter of 9 mm, was used to estimate pressure pain in 8 defined muscle points of the dominant leg (Figure 2). Initially, the points were marked with a pen. The test leader pressed the algometer perpendicularly on the marked points while slowly increasing the pressure. Two values during a single pressure were recorded for each point. The first value was the pressure of the algometer when the subject started to feel pain-the pain detection threshold. The second value was the pressure of the algometer when the subject could not stand the pain any more-the pain tolerance threshold. For safety reasons, the maximal pressure of the algometer is 25 N. In the majority of the recordings, the subjects scored 25 N at the second value-the pain tolerance threshold. The second value could, thus, not be used for statistical analysis. We therefore calculated the probability to score 25 at the first value-the pain detection threshold.
A linear mixed model was used to evaluate the differences between the randomized groups with regard to the 4 continuous outcome variables (hamstring and quadriceps strength, sprint performance, and 1-leg long jump test). The time effect was also taken into account. Because of differences at baseline, the following ratio was used in place of the original data values:
Because of unequal variances for the time measures, a “heterogeneous compound symmetry” covariance structure was fitted to each model. Different variance components were also estimated for each randomized group. For the outcome variable of hamstring strength, 1 observation was marked as having an extreme value and, hence, excluded from the analysis. For the same reason, another observation was excluded from the jump test. For the ordinal variable of the algometry score, a proportional odds model was set up to evaluate the differences between randomized groups and the time effect. The differences in pain score, between randomized groups at the time points “after” and “2 days,” were analyzed using the Kruskal-Wallis nonparametric test. All pairwise comparisons were adjusted using either the Bonferroni or Tukey criterion. A p value ≤ 0.05 was considered to be statistically significant. Spearman rank was used for correlation between VAS after 2 days and change in functional parameters.
Twenty-eight subjects completed the whole study. The testing equipment worked well, and the dropouts (n = 2) were attributable to injuries or diseases without relation to the study.
Hamstring Strength Testing
Results from maximal concentric hamstring strength testing are shown in Figure 3. The WFL group improved their concentric hamstring strength by 35% after 6 weeks of training, from 92 N·m before the study period to 124 N·m at the end (p < 0.001). The WFL group also showed a significant increase in strength at day 2 compared with baseline; the mean strength at day 2 was 107 N·m (p < 0.01). The JFL group had a mean hamstring strength of 103 N·m at the start and 121 N·m after 6 weeks of training (NS). The control group had a hamstring strength of 102 N·m at baseline and 112 N·m (NS) after the 6-week period. There were no significant differences between the training groups during the 6-week period.
Quadriceps Strength Testing
Immediately after the first training, all groups showed significant decreases in quadriceps strength, even the control group (Figure 3). The WFL group had a decrease from 171 to 162 N·m (p < 0.05), the JFL group had a decrease from 150 to 132 N·m (p < 0.001), and the control group had a decrease from 169 to 157 N·m (p < 0.05). At 2 days, the JFL and control subjects still had significant decreases in quadriceps strength compared with baseline, with mean results of 133 N·m (p < 0.05) and 156 N·m (p < 0.001), respectively. The WFL and JFL training groups had no significant improvements in quadriceps concentric strength after the 6-week training period. The control group, however, was weaker in the quadriceps after 6 weeks; they performed 169 N·m at the beginning and 158 N·m at the end (p < 0.01). No differences were found when comparing the training groups with each other.
Sprint running maximal speed was tested during a 30-m sprint (Figure 4). The WFL group increased its performance by 3% (NS) after 6 weeks of training, the JFL group increased its performance by 2%, and the control group increased its performance by 1% (NS). Only the improvement by the JFL group compared with baseline was significant (p < 0.001); there were no differences between groups. Immediately after the exercises, sprint running performance decreased by 2% in the WFL group and by 4% in the JFL group, but it decreased only by 1% in the control group. The difference between the JFL and control groups was significant (p < 0.01).
All 3 groups improved their jumping performance in the 1-leg hop test after 6 weeks, but there were no differences between groups. The WFL group improved their mean performance at 6 weeks compared with baseline, from 198 to 212 cm (p < 0.001). The JFL group improved from 191 to 205 cm (p < 0.01), and the control group improved from 186 to 198 cm (p < 0.001).
Pain Measured by Visual Analogue Scale
Overall muscle pain measured by VAS was analyzed during the 1-leg jump test (Figure 5). The WFL group increased their mean VAS from 0.2 to 1.6 immediately after the exercise, whereas the mean VAS increased from 0.4 to 3.3 in the JFL group and from 0.3 to 0.6 in the control group. The difference between the JFL group and controls was statistically significant (p < 0.01). After 2 days, the VAS for the WFL group was 2.9, and for the JFL group it was 3.5. The VAS for the control group was 1.3. The number of subjects was too small to show any group differences in VAS between the study groups. There were significant (p < 0.05 and p < 0.01) correlations between increase in VAS after 2 days and loss of function in running (r = 0.49) and jump (r = 0.39). However, no such correlation was found in strength parameters.
Pain Measured by Algometer
We found that the probability to score 25 increased significantly during the testing period (Figure 6). The odds ratio between the first and the fourth test sessions was 3.3 (p < 0.0001). There were also significant differences between the points that were tested. Gluteus medius was least tolerable to pressure pain, and the most tolerable point was 5 cm proximal to the patella. The probability values of scoring 25 at 2 days after exercise, for each group and point, are shown in Table 1.
Junior soccer players underwent a 6-week training study of 2 types of forward lunge. No differences could be found between the study groups. This can be explained by the power, by the fact that the number of subjects was too small to detect differences, or by the fact that that the intervention was done during too short a period. However, compared with baseline, the WFL group improved the hamstring strength, whereas the JFL group improved the performance of sprint running. Different kinds of forward lunges are considered by athletic coaches to be very strenuous. These exercises also are thought to be eccentric when breaking the ground forces at the first part of the stance phase. One typical characteristic of an eccentric exercise is the loss of strength after an intense bout of contractions (5,11). Immediately after exercise, the JFL group exhibited decreased hamstring concentric strength as compared with baseline (Figure 3a), indicating that the work performed by the hamstring muscles during the JFLs was strenuous and/or eccentric. This finding was supported by the significantly decreased sprint performance in the JFL group, compared with the control group (Figure 4), right after exercise. All 3 groups, including the control group, demonstrated significant decreases in quadriceps strength immediately after training (Figure 3b). This was probably caused by exhaustion from the testing procedure itself.
Even though the subjects were randomized by lot to either training group, there were differences in baseline values. There were no differences between the groups in anthropometric data that could explain the minor differences in baseline values for the various tests.
In this study, no eccentric strength testing on the Biodex was performed. We did not want the testing procedure itself to produce extra eccentric training because it is known that 1 bout of eccentric exercise can have a major impact on muscle adaptation (7). We believe that the sprint test represents a good test of eccentric muscle function (2), and thus we included functional testing, which is also recommended by other authors (4,18). Concentric strength testing is, however, less interchangeable than functional testing (14).
Another characteristic of eccentric exercise, especially if it is unaccustomed, is the DOMS that typically occurs after 2 days. After 2 days, the VAS pain scoring for the WFL and JFL groups was 2.9 and 3.5, respectively, whereas this score for the control group was only 1.3. The number of subjects, however, was too small to show any statistically significant difference between groups.
As can be seen in Figure 5b, about half (n = 10) of the subjects from the WFL and JFL groups scored very high (VAS = 5-7) after 2 days and, thus, seemed to experience a distinct DOMS, whereas the other half (n = 11) scored very low (VAS = 0-2). It could be speculated that different preparations explain this phenomena; that is, those who did not have any DOMS may have been used to similar exercises earlier. Interestingly, DOMS had a negative impact on functional parameters such as running and jumping, but it had no impact on strength parameters.
We wanted to find out whether there were differences in pain perceptions from different muscles and different parts of the muscles using the algometer. The algometer has been used in studies of muscle pain (23), and it has been shown to have acceptable reproducibility (6). We found that those young soccer players were able to tolerate pressure pain very well. Interestingly, the probability of scoring 25 N increased significantly during the testing period. Thus, it seems that the subjects were able to tolerate pain better with time. This contradicts, in some aspects, the good reproducibility of the algometer reported above (6). Earlier studies (24), though, have shown similar phenomena when measuring pain thresholds with heat. However, as far as we know, this has not been reported earlier concerning pressure algometers. This finding is important to consider if the algometer is used to evaluate treatment for pain in clinical studies.
After 6 weeks of training, the WFL group had a 35% increase in concentric strength of the hamstrings, an increase that is surprisingly high. In the study of the Nordic hamstring exercise (13), soccer players increased their isometric hamstring strength by 7% and their eccentric strength by 11% after a 10-week training period. This difference may be explained by the age and level of the players; we used junior players, but in the Norwegian study the participants were probably older than ours and, possibly, more trained.
Neither of the groups increased their quadriceps strength significantly, but the control group had a significant decrease (−7%) of their quadriceps strength. This is difficult to explain because one would think that these adolescent athletes would increase their muscle strength by their ordinary soccer training. The players who trained using JFLs had a small but significant increase in sprint performance after the 6-week training period. The exercise also gave cause to a significant acute soreness and decrease in sprint performance directly after the first training session compared with controls. Thus, it seems that the JFL training method might have a significant impact on sprint performance.
Surprisingly, algometer testing itself increases the possibility for tolerating pressure pain. This is important to consider when using algometers in clinical trials.
Forward lunges are easy to perform in the open field without any specific equipment. There are many types of lunges. In conclusion, we have found that a 6-week period of training with WFLs improved hamstring strength, whereas training with JFLs improved sprint running performance. Thus, different kinds of forward lunges are beneficial for different muscle properties and could be included in training programs for young soccer players. For example, junior soccer players may include 2 × 12 WFLs and 2 × 12 JFLs twice a week in their ordinary training during the off-season. These exercises can be performed on the soccer field.
This study was supported by the Swedish Sports Research Council (grant 135-04) and the Karolinska Institute foundations. We wish to thank physiotherapists Jenny Fridh and Linda Waara for skilful technical assistance. Jacob Bergström KI Lime is acknowledged for help with the statistical work. Illustrations are made by Lars Gyldorf and Eva Hall. There are no conflicts of interest.
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