Handel Et Al., 1997

Eur J Appl Physiol (1997) 76: 400 ± 408

Ó Springer-Verlag 1997

ORIGINAL ARTICLE

M. Handel á T. Horstmann á H.-H. Dickhuth R.W. GuÈlch

Effects of contract-relax stretching training on muscle performance in athletes

Accepted: 1 March 1997

Abstract The e€ects of an 8-week unilateral contractrelax (CR) stretching training program (passive stretch after isometric contraction) on muscular performance were investigated in a group of 16 athletes. The ¯exibility, maximum torque and angular position as well as contraction work in movements of the knee joint were determined before training and after 4 and 8 weeks of training. The torque measurements were performed under isokinetic conditions, eccentrically at angular velocities of 60° á s)1 and 120° á s)1, isometrically at ®ve di€erent joint positions, and concentrically at angular velocities of 60, 120, 180 and 240° á s)1 using an isokinetic dynamometer. A surface electromyogram (EMG) of the thigh muscles (quadriceps and hamstrings) was recorded simultaneously. As compared to untrained control limbs, signi®cant improvements in active and passive ¯exibility (up to 6.3° in range of motion), maximum torque (up to 21.6%) and work (up to 12.9%) were observed, and these were especially pronounced under eccentric load conditions. A comparison between integrated EMG recordings during eccentric and concentric loads, as well as the interpretation of the training-induced changes in the EMG, suggest that muscular activity under eccentric loads may be impaired by mental processes. Key words Skeletal muscle á Contract-relax stretching training á Flexibility á Isokinetic dynamometry á Force-velocity relationship

M. Handel á R.W. GuÈlch (&) Institute of Physiology II, University of TuÈbingen, Gmelinstrasse 5, D-72076 TuÈbingen, Germany T. Horstmann á H.-H. Dickhuth Medical Clinic V, Department of Sports Medicine, University of TuÈbingen, HoÈlderlinstrasse 11, D-72074 TuÈbingen, Germany

Introduction In recent years, muscle stretching exercises have begun to play an important role in rehabilitation, public sports, and especially in high-performance sports. In rehabilitation gymnastics, stretching exercises are frequently applied to alleviate chronic muscle shortening (Spring 1985). In sports, stretching exercises ± chie¯y recommended during warm-up (Anderson and Burke 1991; Williford et al. 1986) ± exhibit the following e€ects: improved ¯exibility in the sense of an enlarged range of motion (ROM) in the trained joint (Anderson and Burke 1991; Lucas and Koslow 1984; Sady et al. 1982); prevention of strain injuries (Bixler and Jones 1992; Stanish and Hubley-Kozey 1984); and shortening of the muscular contraction and relaxation time as a basis for the facilitation of faster movements (HortobaÂgyi et al. 1985). In addition, the sti€ness of muscle series elasticity has been observed to be reduced, with the consequence that the storage of elastic energy is more ecient in certain ``stretch-shorten cycle'' movements (Wilson et al. 1992). Stretching as a protection against ``delayed onset muscle soreness'' (DOMS; High et al. 1989) is still a controversial theme. The e€ects of stretching on ¯exibility, i.e. on ROM, have been investigated by numerous authors. Here, a distinction has been made between alterations due to stretching that is performed repeatedly over longer periods of time (investigated by, among others, Holt et al. 1970; Lucas and Koslow 1984; Sady et al. 1982; Wiemann 1991) and short-term e€ects as measured immediately after stretching exercises (Cornelius et al. 1992; Etnyre and Abraham 1986a; Moore and Hutton 1980). In these investigations, ¯exibility was not measured in a uniform way. Cornelius et al. (1992) and Starring et al. (1988) determined ROM under passive load conditions; the subjects were asked to voluntarily relax their muscles while the joint was moved for measurement by the experimentalist. In contrast, other authors performed the measurement of ¯exibility actively, that is the subjects

401

were asked to move the joint of interest to its end position by supreme e€ort using the muscle antagonistic to the one being stretched (Etnyre and Abraham 1986a; Holt et al. 1970; Lucas and Koslow 1984; Sady et al. 1982). Generally, stretching is classi®ed according to ®ve di€erent techniques (SoÈlveborn 1983; Wydra 1993): dynamic stretching (DS), static stretching (SS), stretching immediately after a short and almost maximal isometric contraction of the muscle (CR), stretching by antagonist contraction (AC), and stretching by antagonist contraction after agonist contraction (CR-AC) as a combination of CR and AC. In evaluating the individual techniques, increases in ¯exibility were measured in most cases. In this respect, di€erent authors reported controversial results: whereas some authors stated a higher e€ectiveness of techniques including contraction of agonist or antagonist (Cornelius et al. 1992; Etnyre and Abraham 1986a; Holt et al. 1970; Moore and Hutton 1980; Osternig et al. 1990; Sady et al. 1982), others found no di€erences between the individual stretching techniques (de Vries 1962, Lucas and Koslow 1984). In contrast to this, Schober et al. (1990) found positive e€ects of DS, but negative e€ects of SS. Wydra et al. (1991) found a highly signi®cant superiority of DS after 2 weeks. Starring et al. (1988) preferred cyclic passive stretching rather than a single sustained stretch. As a reason for the higher e€ectiveness of the CR and CR-AC stretching exercises, Etnyre and Abraham (1986b) as well as Guissard et al. (1988) described an attenuation of the H-re¯ex that occurred immediately after muscle contraction, which should improve distensibility of the muscle. This e€ect was demonstrated by Etnyre and Abraham (1986b) only during the ®rst seconds after the beginning of the stretch. Guissard et al. (1988) found a 20% attenuation of the H-re¯ex lasting at least 30 s that was independent of the duration of the preceding muscle contraction (between 1 and 30 s). The possible e€ects of regular stretching training on various muscle parameters have rarely been subject to systematic investigations. Wiemann (1991) obtained partly contradictory results: whereas the isometric maximum force increased signi®cantly in female volunteers, a non-signi®cant decrease in ``explosive force'' was observed in male subjects. Worrell et al. (1994) found a signi®cant increase in torque of the ¯exors in the knee joint under eccentric load conditions at velocities of 60 and 120° á s)1 and under concentric load conditions at 120° á s)1, but not under concentric load conditions at 60° á s)1. Wiktorsson-MoÈller et al. (1983) described a non-signi®cant decrease in maximum torque during ¯exion and extension of the knee joint under isometric and concentric load conditions at velocities of 30 and 180° á s)1 immediately after CR stretching. The question as to whether the observed positive changes in muscle parameters are to be considered as consequences of structural alterations of the muscle, in the sense of length increases or hypertrophy, rather than as consequences of changes in re¯ex activity or other electrophysiological adaptations, is also controversial.

The e€ects of regular stretching training on muscle performance have not yet been systematically investigated on a wider scale. Thus, the aim of the present study was to determine the changes in various relevant parameters of muscle performance due to CR stretching training lasting several weeks. In particular, the following questions were broached: 1. Can certain contractile properties of muscles be affected by stretching training in such a way that the force-velocity relationship is changed? 2. Does stretching change the force development or the torque pro®le in the respective joint movement? 3. Can possible alterations in muscle properties be explained by a process of morphological reorganization, or can they be considered to be functional changes in muscle electrical activity, in the sense of a neurophysiological adaptation process?

Methods Subjects Sixteen male athletes (eight league swimmers, six amateur footballers, two long-distance runners) collaborated in this study [age: 23.6 (3.9) years; body mass: 82.0 (10.3) kg; height: 182.7 (8.6) cm; mean (SD)]. The swimmers belonged to a ®rst-division swimming club and trained at least three times a week; the amateur footballers were members of a district league club, training twice a week with matches each weekend. The long-distance runners trained approximately three times a week. The volunteers were fully informed about the training and experimental procedures and gave their consent to participate. Stretching training A 10-min CR stretching training program (contract-relax stretching), according to Anderson and Burke (1991) and SoÈlveborn (1983), was performed regularly three days per week over a period of eight weeks. In part, the exercises had been modi®ed so as to prevent training of the contralateral side and to adapt the protocol to the subjects' sports-speci®c training conditions. The stretching exercises were performed after a short warm-up period of at least 2 min of, for example, running and jumping prior to the athletes' speci®c training program. The stretching exercises were restricted to the extensors and ¯exors of the knee joint and the adjacent muscles of one side of the body; the contralateral muscle groups served as controls. Within the stretching program, each muscle group was subjected to eight CR stretching cycles comprising a 10-s strong contraction (the athletes were told to contract their muscles with at least 70% of their maximum force), 1±2 s of relaxation and 10±15 s of passive stretching. After two cycles of CR stretching, the muscles were brie¯y limbered up by shaking. The extensors and ¯exors were stretched alternately with two di€erent exercises each and one complete repetition of the program. Measurement of ¯exibility Flexibility was measured by reading the extreme knee joint angles (using an elongated orthopaedic goniometer above the bone points: trochanter major, epicondylus lateralis and malleolus lateralis) during active as well as passive movement. Flexibility in ¯exion was determined at a hip joint angle of 0° (standing position, leaning against a wall); ¯exibility in extension was determined at a 90° hip joint angle (with the subject in a sitting position, ®xed to a measuring chair).

402 Experimental set-up The e€ects of stretching training on the performance of the muscles acting on the knee joint were investigated using an isokinetic dynamometer (LIDO-Active 2.1 dynamometer, Loredan, USA). Muscle performance was assessed on the basis of eccentric, isometric and concentric torque measurements under preset isokinetic velocities to obtain torque-angular velocity relationships. Torque curves and joint angles were recorded during de®ned isokinetic movements, thereby allowing the work done during such movements and the angular position of maximum torque to be monitored. In addition, the electromyographic (EMG) activity was taken into account by recording the surface EMG of the thigh muscles. The evaluation of the stretching training was based on the ¯exibility as determined by the active and passive ROM. In recording the force-velocity relationship, it was necessary to analyse a wide range of velocities. Therefore, in preliminary experiments the technical limits of the dynamometry were investigated, especially for high velocities (Handel et al. 1996). In order to record torques under the highest possible velocities with acceptable accuracy, a muscle group that was able to produce high torques had to be used. In addition, the muscles under consideration had to be accessible to stretching training. For our investigations, we chose the extensors and ¯exors of the knee joint, since they meet both of the above requirements. Since we wanted to investigate the e€ects of long-term stretching training, the in¯uence of the regular training speci®c to the athletes' primary sport as well as possible ¯uctuations in their physical conditions had to be kept under control. For this reason, the stretching training was applied to one extremity only (as, for example, by Grady and Saxena 1991). The contralateral side served as the individual's control. In this way it was possible to ®lter out in¯uences that uniformly a€ect the state of both legs (e.g. the general improvement in ®tness in the course of the training season; ``adaptation'' to the measuring procedure) by examining the differences in muscle performance between the stretch-trained and the control leg. Torque measurement The torque curves of the knee joint during muscle contraction were recorded using a LIDO-Active 2.1 dynamometer. In preliminary experiments it was frequently found that in the concentric mode high velocities (adjustable up to 400° á s)1) could not be attained since the torque produced by the subjects could not overcome the relatively large resistance of the dynamometer during the acceleration phase. Therefore, in a ®rst series of measurements the reliability of the isokinetic diagnostics in general and of the dynamometer in particular were investigated (Handel et al. 1996). From these ®ndings, a restriction to velocities up to 240° á s)1 had to be observed with respect to the expected strength of the subjects, thus guaranteeing isokinetic conditions over the angular range of movement under consideration. Torque measurement in the subjects The isokinetic measurements were performed by encouraging the subjects while in a sitting position, to develop maximum force covering the whole angular range of movement between knee joint angles of 8° (extension) and 108° (¯exion) (all angles cited according to the neutral zero method commonly used in orthopaedics). The tests were performed under eccentric conditions ( passive mode) at angular velocities of 60 and 120° á s)1, concentrically at 60, 120, 180 and 240° á s)1, and isometrically at ®ve di€erent joint positions (108°, 83°, 58°, 33°, 8°). Torque maxima, as well as their angular positions, were read from the individual curves using the cursor function of the LIDO software. The values representative for the eccentric and concentric tests are means of the three highest values out of a series of ®ve experiments at each angular velocity. In the isometric measurements we used the maximum torque reached during a 4-s muscle contraction. In ad-

dition, the work performed in the eccentric and concentric tests during the movement between joint angles of 30° and 90° was calculated using the LIDO software. EMG recording For monitoring the EMG activity, in each case a surface EMG was measured between two recording points (bipolar) at the extensors and the ¯exors of the knee joint using the Myosystem 2000 (Noraxon, Finland). The recording points were located proximally and distally, adjacent to a line half-way between the epicondylus lateralis and trochanter major in the middle of the muscle belly at the front and back of the thigh (rectus femoris of the quadriceps femoris, or caput longum of the biceps femoris, respectively). We used circular adhesive electrodes (``blue sensor P-00-S''; Medicotest, Denmark) that had an active diameter of 1 cm. The electrodes were attached at a distance of 3 cm between their centres and an indi€erent electrode was axed to the left olecranon. All of the recording points on the skin were shaved, degreased with alcohol and permanently marked. The EMG was sampled at a rate of 1000 Hz per channel. Simultaneous recording of joint angle and torque using the dynamometer permitted assignment to the corresponding phase of movement. The EMG data were integrated over time using the Myosoft 2000 software (Noraxon) and averaged over the isokinetic phase of the movement between joint angles of 30° and 90° . Experimental protocol The three test series were taken immediately in the pre-training period, and after 4 and 8 weeks of training. To make sure that the results represented training-induced long-term rather than shortterm e€ects, no stretching exercises were performed for at least 24 h before the tests. Prior to each series, the subjects passed a 5-min warm-up exercise on a bicycle ergometer at 80 W and 90 rpm. In addition to the morphometric data (body mass and height), thigh circumference was measured half-way between the trochanter major and the epicondylus lateralis at hip joint and knee joint angles of 90° using a measuring tape. The EMG recording points were marked at the same positions. After attachment of the adhesive electrodes, the subjects were ®xed to the dynamometer chair with two belts at the shoulder and one at the hip. The thigh was ®xed at a joint angle of 90° with a padded clamp to avoid any movement of the hip. Each measurement started with ®ve extensions and ¯exions of the trained leg under concentric conditions at angular velocities of 240, 180, 120 and 60° á s)1. The subjects were encouraged to exert maximum force against the lever arm over the whole range of movement. After each measurement at a particular velocity a 1 min pause was enforced to minimize any fatigue e€ect. The complete concentric procedure was repeated in the contralateral extremity. Thereafter, measurements were performed under isometric load conditions. Starting with a joint angle of 108° (¯exion), the subjects were asked to push against the lever arm of the dynamometer with maximum force at ®ve di€erent joint positions, ®rst in the direction of extension for 4 s and then, after a break of 2 s, in the direction of ¯exion. Each isometric test was followed by a break of 10 s. The eccentric investigations were performed subsequently with ®ve movements of the knee joint at a velocity of 120° á s)1. Thereby, the subjects were encouraged to decelerate the lever arm of the dynamometer with maximum force. After a 1-min pause the measurement was repeated at the slower velocity of 60° á s)1 and, thereafter, on the contralateral leg. Statistical methods Averages and standard deviations were calculated from individual measurements for di€erent groups of measurements. To judge the signi®cance of changes occurring between the time before and after 4 and 8 weeks of training, the di€erences in the measured values were tested for signi®cance (P < 0.05) using a ``one-dimensional

403 repeated measurement analysis of variance'', the Kolmogoro€Smirnov test for normal distribution having yielded no signi®cant deviations. For the assessment of the di€erence between two dependent groups of measurements, Student's ``paired t-test'' was applied. Di€erences were termed signi®cant if the corresponding tests yielded an error probability of P < 0.05, and highly signi®cant if P < 0.01. In order to compensate for ®tness ¯uctuations and improvements induced by basic club training during the test period, measurements on the stretched leg were additionally normalized with respect to the contralateral extremity. The normalized alterations were related to the results of the pre-training test series.

Results The increase in ¯exibility due to the stretching training proved signi®cant or highly signi®cant (see Table 1) if evaluated by the index ROM whereby the passive ROM increased without exception more markedly than that under the active condition. In the athletes, a slight but signi®cant increase in thigh circumference was observed, amounting to an average of 0.8 cm (1.1) after 8 weeks [0.3 cm (0.9) after 4 weeks, non-signi®cant]. Changes in body weight [)0.3 kg (1.8) after 4 weeks, +0.5 kg (1.8) after 8 weeks], however, were not signi®cant. The di€erences detected in the mean torque-angular velocity curve of the stretched extremities before and after 4 and 8 weeks of training (Fig. 1a) and their statistical evaluation are given in Table 2. The torque increments under eccentric load conditions were signi®cant in both the extensor (up to +56 Nm, or 23.0% as related to the initial value) and the ¯exor (up to +29 Nm, or 18.2%). The increase in maximum isometric torque was signi®cant in the ¯exor only (+19 Nm, or +11.3% after 8 weeks). Under concentric load conditions, a signi®cant increase in torque was found in the ¯exor at velocities of 60, 180 and 240° á s)1 (up to +13 Nm, or +9.4%). In the control leg, in all cases no signi®cant di€erences could be found between the pre-training period and after 8 weeks of training. As can be seen from the integrated EMG (iEMG) shown in Fig. 1b, much smaller values were measured in the extensor under eccentric load as compared to concentric load conditions at the same velocity. These differences were highly signi®cant and the values became smaller after several weeks of stretching training, resulting in a signi®cant increase in the iEMG values at eccentric loads. The corresponding values for the control Table 1 Increase in the range of active and passive motion in the knee joint [mean degrees (SD)] Type of motion

after 4 weeks

after 8 weeks

Active extension Passive extension Active ¯exion Passive ¯exion

+0.4 +3.1* +3.5** +4.6**

+1.1 +5.6** +2.6* +6.3**

* Signi®cant change ** highly signi®cant change

(4.0) (5.2) (4.6) (6.3)

(2.9) (6.6) (4.9) (7.5)

Fig. 1 Torque-velocity relation of the stretch trained leg (a) and averaged integrated electromyograms (iEMG) obtained during the torque measurements (b). (Open symbols ¯exor, solid symbols extensor, triangles measurements prior to training, squares after 4 weeks of training, circles after 8 weeks of training. (ecc Eccentric load, iso isometric load, con concentric load)

leg did not change signi®cantly (data not shown). The changes in iEMG under isometric and concentric loads were also non-signi®cant. In addition, the absolute changes in the ¯exor iEMG were non-signi®cant. Moreover, in the latter case no signi®cant di€erences could be found between the eccentric and concentric load conditions. Figure 2 shows the changes related to the control leg. By evaluating the training e€ects with regard to di€erences between the two legs, errors should be eliminated due to in¯uences equally a€ecting both legs, irrespective of the CR training of only one leg. However, in the eccentric measurements no substantial qualitative deviations from the examination of the trained extremity alone ± mentioned above in the context of the forcevelocity relationship ± were found. With both the extensor and the ¯exor, the increase in torque under the

404 Table 2 Changes in torque under di€erent loading conditions in the stretched leg [mean value (SD); the percentile value is given below in brackets] as related to the ®rst measurement in the considered time period, e.g. D21 represents the di€erence in torque of Type of loading

ecc 120° á s

)1

ecc 60° á s)1 isom max )1

conc 60° á s

conc 120° á s)1 conc 180° á s)1 conc 240° á s)1

the second measurement related to the torque of the ®rst one. (ecc Eccentric torque, isom max isometric maximum torque, conc concentric torque)

Changes in torque [Nm] under extensor loading

Changes in torque [Nm] under ¯exor loading

D21

D32

D31

D21

D32

D31

+22.4** (21.1) (+9.5%) +24.2 (35.2) (+9.0%) +26.9 (50.7) (+10.4%) +4.1 (8.2) (+1.8%) )0.5 (10.3) (0%) +2.9 (4.9) (+1.8%) +2.1 (11.5) (+1.4%)

+33.5* (45.7) (+12.4%) +25.5* (30.9) (+9.0%) )4.8 (44.1) ()1.4%) +11.4 (22.0) (+5.0%) +1.4 (16.2) (+0.5%) +3.1 (12.4) (+1.8%) +0.5 (13.9) (0%)

+55.9** (+23.0%) +49.7** (+18.9%) +22.1 (+8.8%) +15.5 (+6.9%) +0.9 (+0.5%) +6.0 (+3.7%) +2.6 (+1.4%)

+11.5 (22.9) (+7.6%) +20.4** (15.0) (+12.6%) +16.6* (4.2) (+10.1%) +5.1 (12.1) (+3.6%) +4.4 (6.5) (+3.2%) +6.3** (5.8) (+5.3%) +11.6** (7.6) (+11.4%)

+14.3 (24.2) (+8.2%) +9.0 (18.7) (+5.0%) +2.6 (15.8) (+1.0%) +8.4* (10.6) (+5.6%) +1.0 (20.4) (+0.7%) +2.7 (11.2) (+2.5%) )1.3 (9.7) ()0.9%)

+25.8** (+16.5%) +29.4** (+18.2%) +19.1** (+11.3%) +13.5** (+9.4%) +5.4 (4.0%) +9.0* (+8.0%) +10.3** (+10.4%)

(43.7) (44.8) (40.9) (3.0) (17.6) (9.6) (12.9)

(22.7) (26.7) (19.7) (10.0) (18.1) (6) (10.6)

* Signi®cant change; ** highly signi®cant change

eccentric load was particularly striking (up to 21.6%). The increase under isometric load conditions at di€erent joint positions was more pronounced in the ¯exor than in the extensor. In the concentric tests slight changes were observed, of which only a few were signi®cant. Figure 3 shows the relative changes in work performed under di€erent loads (as related to the control leg). Under eccentric load conditions, greater increases were found when compared to the concentric load (up to 12.9%). The relative increase in concentric work, in most cases not signi®cant, was slightly higher than that in concentric maximum torque (see Fig. 2a and b). Analysis of the joint angle at the peak torque of the movement yielded signi®cant changes only for an eccentric load at 120° á s)1 (see Table 3). Under eccentric load conditions the torque maximum was shifted towards joint angles corresponding to increased muscle length. Table 4 shows the angular position prior to the training period where the torque maximum was attained under the di€erent loads. It is striking that the torque maximum under ¯exor loading occurred at a relatively straight position of the knee joint. At higher velocities, the ¯exor torque maximum was shifted towards joint angles that are reached later in the course of the movement (towards greater joint angles under concentric loads, and towards smaller angles under eccentric loads).

Discussion On the basis of the signi®cant (P < 0.05) and, in some cases, highly signi®cant (P < 0.01) improvement in ¯exibility by the CR stretching training performed in our experiments this form of training can be considered to be an e€ective method for improving ¯exibility in man. The increase in ROM was similar to that observed by Wallin

et al. (1985), who found increases of 5±10° in the same joint after 30 days of CR stretching training. In addition, in the sense of a side-e€ect, the stretching training caused an improvement in muscle performance as characterized by the torque-angular velocity data of the concentric, isometric or eccentric mode. When compared to the increases in torque recently reported by Worrell et al. (1994), the e€ects observed in our investigations were twice as large in the eccentric range, and were similar in the concentric case. The di€erences between our ®ndings and those of Worrell et al. may be due to the type of stretching training applied, as well as to the individually loaded and pre-trained muscle groups of the subjects. This would also account for the di€erent e€ects of training on the extensors and ¯exors. The control leg showed no signi®cant changes in the torque-velocity relationship between the ®rst and the third measurement. A contralateral transfer, as observed by Smith (1970) for example, after ``myotatic strength training'' (a type of re¯ex training induced by abrupt muscular stretch), and by Kannus et al. (1992) after combined isokinetic and isometric training, appears rather improbable in our case. If such a transfer had occurred in our training, it would have been very small so as to remain lower than the limit of detection. The insigni®cant alterations in the control leg argued against any cross-over e€ects. Therefore, it is justi®able to relate the changes that occurred during the stretching training to the control muscles in order to eliminate any change a€ecting equally both legs. For the same reason any alteration in isokinetic muscle performance a€ecting both legs in a similar way when repeating the test series after a certain time can also be eliminated. According to the classical physiological force-velocity concept of Hill, the eccentric torque should de®nitively, exceed the isometric level (Edman et al. 1978). This is

405

not valid for the torque-velocity relationships seen in Fig. 1a, however, particularly in the pre-training state. These ®ndings can be explained by the fact that the iEMG values, at least for the extensor, under eccentric load are highly signi®cantly lower than those under the corresponding concentric load at the same angular velocity (Fig. 1b). This may be interpreted as mental inhibition, since many subjects only developed submaximal forces, probably because of the rather disagreeable or even painful eccentric intervention they described after the tests. Such ``inhibitory neural in¯u-

Fig. 2a±d Relative changes in maximum torque and corresponding iEMG values as compared to the control leg. a Torque during extensor load, b torque during ¯exor load, c averaged iEMG during extensor load, d averaged iEMG during ¯exor load. The bars indicate standard deviation. The open/solid columns symbolize the state after 4/ 8 weeks of training, (e Eccentric load, i isometric load, c concentric load). * Signi®cant change P < 0.05

ences'' in man have already been postulated by GuÈlch (1994) as a possible cause for the reduction in torque at high eccentric velocities. Since these in¯uences have been found to be weakened by stretching training, this may be considered to be an indication that the neuromuscular activity is positively in¯uenced by stretching. It is conceivable that after stretching training the often painful extending of the muscle during eccentric tests can be better tolerated, resulting in improved eccentric performance as shown in Fig. 1 and Table 2. Komi et al. (1978) found a 38% increase in iEMG after 12 weeks of isometric strength training, with an increase in the isometric maximum force of 20%. It should be noted that, strictly speaking, CR stretching also involves a kind of isometric strength training during the active phase, and that the changes in the torque, muscle size and iEMG observed in our investigations may thus likewise be due to a kind of isometric training. Findings supporting a more ecient muscle activation have been reported by Moritani and de Vries (1980).

406

Fig. 3a, b Relative changes in contraction work compared to the control leg. a Extensor load, b ¯exor load. The bars indicate standard deviation. The open/solid columns symbolize the state after 4/8 weeks of training. (e Eccentric load, c concentric load) Table 3 Changes in the angular position of the torque maximum [in deg. (SD)] as related to the value of the control leg (positive values=shift towards greater joint angles, negative values=shift towards smaller angles

Type of loading

eccentric 120° á s)1 eccentric 60° á s)1 concentric 60° á s)1 concentric 120° á s)1 concentric 180° á s)1 concentric 240° á s)1

Extensor

Flexor

after 4 weeks

after 8 weeks

after 4 weeks

after 8 weeks

+7.2* +2.3 ) 1.3 +0.9 +0.9 +1.9

+5.7 +2.0 ) 0.2 ) 0.1 +0.6 ) 2.6

+1.4 ) 6.8 +1.4 ) 1.4 +0.1 ) 2.3

) 2.7 ) 2.5 +3.5 +0.2 +2.3 +0.3

(12.6) (7.5) (6.5) (7.0) (7.3) (9.8)

(9.1) (9.1) (3.6) (8.4) (5.8) (6.1)

(20.6) (26.6) (8.1) (6.9) (7.7) (9.6)

(13.2) (26.2) (8.4) (8.6) (5.3) (9.8)

* Signi®cant change Table 4 Angular position at torque maximum measured prior training period

Type of loading

)1

eccentric 120° á s eccentric 60° á s)1 concentric 60° á s)1 concentric 120° á s)1 concentric 180° á s)1 concentric 240° á s)1

Extensor

Flexor

control leg

stretched leg

control leg

stretched leg

65.5 68.1 63.7 59.0 58.5 61.8

63.6 67.1 64.0 58.2 57.1 59.9

38.0 42.8 29.9 34.3 38.0 46.5

35.1 40.1 29.2 37.1 39.1 50.6

After an 8-week isotonic strength training, these authors observed a force increase of 30%, but an improvement in the iEMG of only 12%, corresponding to an actual improvement in the force-iEMG ratio. Structural adaptations of the muscle, however, are considered to be responsible for the improved resistance to eccentric loads. This can explain the e€ects of stretching training on the reduction of muscle strain injuries which, according to Glick (1980) and Zarins and Ciullo (1983), should occur predominantly under eccentric load conditions. Such a reorganization may, for instance, occur in the form of an increase in the length of the muscle,

(7.2) (4.9) (4.3) (4.3) (4.7) (5.3)

(8.2) (6.4) (7.0) (4.8) (5.9) (4.7)

(11.7) (15.8) (6.7) (6.2) (5.6) (8.0)

(8.4) (9.0) (4.1) (8.2) (6.7) (6.7)

with the consequence that longer muscles are not so easily overstretched. An increase in muscle length would also concur well with the observed improvement in ¯exibility (see above), since its measurement was performed in such a way that the muscle distensibility could be considered to be the essentially limiting factor of the ROM. In animal experiments, various authors (e.g. Williams et al. 1986) described muscle hypertrophy as a growth in thickness, indicated by an increase in muscle circumference, as well as a lengthening of muscle after sustained stretches that were sometimes combined with electrical stimulation. The slight increase in thigh cir-

407

cumference we observed would therefore speak in favour of an increase in muscle thickness due to hypertrophy. Even when underestimating the increase in muscle crosssection on the basis of an increase in thigh circumference (Young et al. 1983), this may not be sucient to explain the enhanced torque observed, since the measured circumferential changes were too small. The results obtained by other authors also support the idea of a functional rather than a structural induction of strength improvement during strength training. Stretching training may comprise various stimuli that lead to muscle hypertrophy. The contraction phase of CR stretching may have the same e€ect as isometric muscle training. Furthermore, it is possible that during the second CR stretching cycle the isometric contraction of the pre-stretched muscle acts as a particularly strong trigger in a similar way as in animal experiments where muscles were stimulated under stretched conditions (Williams et al. 1986). In the so-called AC stretching technique, the muscle is stretched by the antagonist. In most of these stretching programs agonist and antagonist are stretched alternately through the application of active tension. This may also act as isometric muscle training. To single out those di€erent components of a stretching program responsible for the hypertrophic effects, comparative studies would be necessary on the e€ects of a purely isometric training program without any stretching, and purely passive stretching without agonistic or antagonistic activation. In our training protocol, a total contraction time (TCT) of the extensor and ¯exor muscles of 1920 s each was attained (eight times 10 s on 24 days of training). Comparing that with the protocol of Komi et al. (1978), where a 48-day isometric training of the quadriceps with a TCT of 1200 s yielded a maximum voluntary contraction increase of 20%, the torque increases under isometric load found in our investigations could be explained suciently by the isometric component of the CR stretching performed. Structural alterations of the muscle may also provide an explanation for the changes in the torque pro®le during motion. The basic evidence is derived from the fact that a muscle develops maximum force at an optimal degree of ®lament overlapping (Gordon et al. 1966). If we assume, for simplicity, that the lever arm conditions of the investigated joint do not change during muscle shortening, then the angular position where the maximum torque is attained characterizes the optimum degree of actin-myosin overlapping. If a stretching training program were to lengthen the muscle tendons alone without a€ecting the muscle itself, then the torque maximum would be shifted towards greater lengths, with no e€ect on the shape of the torque curve. On the other hand, an exclusive increase in muscle length due to an increase in the number of linearly arranged sarcomeres would have to lead to a widening of the torque curve (torque vs joint angle) during contraction, and would therefore essentially e€ect an increase in work without a€ecting the magnitude of the

torque maximum. An isolated increase in thickness with an increase in the number of ®brils would lead to an improved torque maximum and a proportional increase in work. A combined increase in both length and thickness with parallel and serial addition of sarcomeres, however, e€ects the work more than the torque. This was observed in the extensor as well as in the ¯exor under concentric load conditions (see Figs. 2a, b and 3a, b). A signi®cant change in the angular position of the torque maximum has not been found (except for eccentric movement at 120° á s)1) (see Table 3), although this would be dicult to detect due to the rather low reproducibility of this parameter and the comparably small di€erences due to changes in ¯exibility. A tendency in the shift of the torque maximum towards muscle lengthening was observed under eccentric loads. Altered biomechanical conditions in the joints due to stretching training must also be considered. If there was a lengthening of joint-stabilizing structures it would then be possible that, despite the same angle in the joint, di€erent lever conditions may occur not only due to a shift of the rotational axis, but also to di€erent angles of the tendon. Consequently, the torque curve may be changed during movement in such a way that the joint angle and the magnitude of peak torque are a€ected without any change in muscle properties. A change in the angular position of the peak torque may also be caused by electromechanical alterations. Since under high shortening velocities of the ¯exor its torque maximum is shifted towards later phases of the movement (see Table 4), the time to maximum activation of the muscle may be of importance, at least at higher velocities. Therefore, training e€ects in¯uencing this time interval should also a€ect the angular or time course of the torque. Principally of course, it cannot be ruled out that the subjects were highly motivated and thus produced higher torques after the stretching training. Concluding remarks Our results show that CR stretching training may favourably in¯uence the force-velocity relationship of the trained muscle as well as shape of the torque curve during movements at a given velocity. This appears to be a positive side-e€ect of stretching which in the ®rst place is considered to be and is applied as a method for improving ¯exibility. The extent to which the observed improvements in muscle performance can be applied successfully to competitive sports or within the framework of rehabilitation programs, as well as establishing which persons would bene®t most from it, could not be clari®ed conclusively in this study. More distinct results might be expected by increasing the number of volunteers from the same sport in additional studies. Further comparative investigations are necessary to determine the proportion of the improvement in muscle performance due either to the stretching stimulus or to the pure isometric contraction manoeuvre without stretching.

408

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