It is important to accurately define and prescribe the types of stretching modalities that are most effective for achieving the common goals of increased flexibility and decreased musculotendinous stiffness (MTU) 1. However, the most recent literature questions the effectiveness of pre-activity stretching on performance 2. Numerous studies show that static stretching will improve the joint range of motion (ROM) but will reduce force produced by the muscle 3 and very few others show that performance is not impaired 4. On the other hand dynamic stretching has shown to improve both joint ROM and force produced 3. No previous studies could be found looking at the effects of dynamic stretching vs. static stretching measuring the variables of this study, more specifically muscle stiffness.
Muscle stiffness and decreased flexibility are generally considered etiological factors in musculoskeletal injuries and hence, stretching exercises are usually recommended before activity to prevent injury 5. Most studies just look at joint ROM 5 before and after stretching and not at the actual change in elasticity of the muscle, causing results to differ considerably 6. The increase in ROM can be due to an increase in stretch tolerance and not necessarily elastic changes to the muscle 6. There are studies that look at passive muscle stiffness but there is no stretching intervention 7, whilst there are others that do use stretching but only look at muscle stiffness and not joint ROM 6.
Joint range of motion and stiffness - review of methods
Whilst there is evidence that compares static and dynamic stretching and their effect of joint ROM and performance 8,9, there are no papers that could be found that look at the elastic properties of the muscle in question with regards to these types of stretching. Raymond Lee 7 studied these elastic properties by measuring forces applied to the leg during a straight leg raise (SLR), minimizing error by using a knee and ankle brace to keep the SLR as pure as possible. However, a combination of subject feedback and the testers feel for end of range was used to determine limit of the SLR. Y.S Yoon et al 10 carried out a similar study but instead used feedback from an electromyography (EMG) machine to determine the end of passive range of motion. This would have made Lee's study stronger as he was looking at passive stiffness and therefore muscle activity should be minimal.
Brad S. Curry et al 9 used the hip extensor group of muscles to test joint ROM and muscle performance (Peak force). In addition to static and dynamic stretching groups, there was also a group involving a light warm up. However, all groups did a 5 min cycle as a warm up before pre-intervention testing and before the stretching/warm up intervention. Although the time frame of cycling was constant throughout all groups, the involvement of a warm up for the stretching groups would have brought into play a number of variables that could influence joint ROM as opposed to pure static and dynamic stretches. Also the stretching interventions targeted a number of lower limb muscles, yet only the quadriceps were tested for peak force and joint ROM. Again this renders conclusions non-specific to hip extensors as a number of other muscles were stretched. The right leg of all participants was used which does not account for leg dominance, hence expanding room for inaccurate results.
Limb dominance
The dominant limb can be defined on the basis of muscle strength, functional use and personal preference and these parameters may interfere with balance. Limb dominance should be determined according to which leg the individual chooses and relies on to carry out a variety of functional activities, including maintaining balance.11However, there is a lack of consensus regarding the definition and determinants of lower-limb dominance. The methods most used have been evaluations of kicking and hopping on a single leg.12
Hoffman et al.11 carried out a series of functional tests that they called "functional determination of the dominant limb" and established that the dominant limb was the one that performed the movement with more precision and skill. They confirmed that the dominant leg was the one used for kicking a ball.
Stretch tolerance
Stretch tolerance is often described as the primary mechanism for increased muscle extensibility following training, although there is often no dependent variable measured to represent this mechanism.13-15
Halbertsma and Goeken13 and Magnusson et al16 also reported altered stretch tolerance following stretch interventions. Both groups of authors investigated the hamstring muscles with relatively intensive regimens of 10 minutes of stretch twice a day for 4 weeks13 and 225 seconds of stretch twice a day for 20 days.16 Bjorklund et al17 similarly showed sensory adaptations in the rectus femoris muscle after a 2-week stretch protocol. Interestingly, although Bjorklund et al17 used a relatively mild stretch regimen (80 seconds of
stretch, 4 times a week for 2 weeks) compared with the 2 previous studies,13,16 the intervention was still sufficient to alter stretch tolerance. Recent evidence from studies by Folpp et al18 and Ben and Harvey19 add further support for this proposition. These investigators reported increases in stretch tolerance but not extensibility following fairly intensive stretch protocols (20 minutes a day, 5 days a week for 4 weeks,18 and
30 minutes a day, 5 days a week for 6 weeks19). Roberta Y.W. Law et al20 suggests that even a shorter and less-intensive regimen (1 minute daily over 3 weeks) can increase stretch tolerance and improve apparent muscle extensibility.These findings have important implications for clinical practice if the aim of stretch is to achieve a greater joint ROM regardless of the underlying mechanism.
Paul W.M. Marshal et al21 also included stretch tolerance in their study of the effects of passive stretching on hamstring extensibility, passive stiffness and strength. However, the measurement for stretch tolerance used was pain feedback from the patient using a VAS scale. It would have strengthened the study to take measurements of joint ROM alongside passive stiffness measurements to conclude whether there was an increase range of extensibility owing to stretch tolerance.
Stretch duration
Literature is unanimous in its support for stretching resulting in increased range of motion (ROM). However, there is a lack of consistency with regard to how long stretches should be held to obtain optimum benefits 22. Recommendations for duration of stretching in flexibility training programmes range from 5 to 60 seconds,22 yet justiï¬cations for these selections have largely been absent.23 Madding et al24 compared the effects of 15, 45 and 120 seconds of stretching on hip abduction, and reported that sustaining a stretch for 15 seconds was as effective as 120 seconds. These results, however, are based on one stretching session rather than an extended training programme.
Borms et al 25 compared the effects of 10, 20 and 30 seconds of active static stretching on active coxo-femoral flexibility. The programme lasted for 10 weeks and consisted of two sessions a week. No signiï¬cant differences were observed between the three groups, implying that a duration of 10 seconds static stretching is sufficient to elicit improvements in ROM. Bandy and Irion5 compared the effects of stretching durations of 15, 30, and 60 seconds on hamstring flexibility. The training programme lasted six weeks and involved subjects stretching passively ï¬ve times a week. Measures of passive knee extension before and after the training programme showed signiï¬cantly greater improvements in ROM in the 30 and 60 second groups than the 15 second group, but no differences existed between the improvements shown by the 30 and 60 second groups.
It is clear that there are limited and conflicting ï¬ndings in this area. Comparison and subsequent conclusions are difficult because of the lack of consistency in the selection of stretching methods employed and whether active or passive movement is assessed. In the literature reviewed, no study has indicated attempts to control for total amount of time spent stretching while manipulating stretch duration.
Some studies have revealed no change in passive moment following three 45-s stretches26 or a single 90-s stretch,27 indicating that the increased range of motion (ROM) that succeeded the stretch was attributable to an increased "stretch tolerance" rather than any change in the mechanical properties of the tissues. Together, these studies are also suggestive that longer duration stretches (<5 min) might impact mechanical properties of the MTC, whereas shorter duration stretches might not. Furthermore, some recent studies28-33) have indicated a clear dose-response effect where reductions in maximal force become more significant with longer stretch durations. Although a number of studies have reported a negative effect of acute stretching on force production28-30,32-38, the extensive stretch durations (10-60 min) used in many studies do not reflect the durations of stretch commonly employed by athletes before performance. Limited data exist describing the effects of stretch on changes in muscle force production, neuromuscular activity, and mechanical properties of the MTC when more commonly used stretch durations (<3 min) are utilized.39
Roberts et al 22 studied the effect of stretching duration on active and passive range of motion in the lower extremity. 24 university sports team/club members were recruited (19 men and 5 women) and were randomly assigned to either a control group or one of two treatment groups. No justification for the number of participants was made and using equal numbers of each sex would have been beneficial in minimizing confounding variables. One treatment group was required to hold a stretch for 15 seconds while the other was to hold it for 5 seconds. There were 3 repetitions for each stretch with a 15 second rest between, however the 5 second group was to carry out 3 sets, resulting in a total of 45 seconds of stretching. Whilst this does look at the duration of the individual stretch, the total time spent stretching is still the same and therefore does not essentially look at stretch duration.
The study should also have been a repeated measures design as this would eliminate a number of variables that can occur across different subjects. Subjects were also told to maintain their normal exercise activity levels for the duration of the study which again may lead to very individual differences depending on what type of sport/training the individual takes part in. A 10 minute aerobic warm up was performed prior to stretching which would again allow room for error as other factors are being involved in the joint ROM as opposed to a pure static stretch. It may be the case that the warm up purely had a significant impact on joint ROM as shown in other studies40. Active and passive joint ROM was assessed using hip flexion and knee extension and flexion. The end of passive range of motion was determined once the tester felt resistance or the subject vocalized discomfort. This method is very inaccurate and it may have been more effective to use an EMG machine to determine the end of passive ROM as in the study by Y.S Yoon et al 10.
Review of research equipment
Passive stiffness and joint ROM
Brad S. Curry et al 9 used a Thomas test was used to quantify any improvements in joint ROM using a goniometer. The test was not standardized using any knee or ankle braces to keep the movement pure and the participant was told to hold one knee while the other was expended, leading to subjective measures. Lee and Yoon 7,10 both used braces as discussed above during their procedures. This is too minimize any other directional movements such knee and ankle rotation during testing. The complication with using braces is that there is an added weight on the participant's leg. Lee 7 used a lightweight, thermoplastic orthosis to keep the knee straight and the ankle in plantigrade at all times, but using even lighter bracing equipment would be beneficial in reducing error. Yoon 10 did not document what kind of brace was used, nor did he make any reference to the weight of the brace. Again, taking into account the weight of the brace and minimizing it as much as possible will reduce variability.
A limitation of goniometry is that it requires the clinician to use both hands, making stabilization of the extremity more difficult, and thus increasing the risk of error in reading the instrument.41 Inclinometry is another practical alternative that incorporates the use of constant gravity as a reference point to assess joint mobility.42-44 Digital inclinometers are portable, lightweight, and require training similar to that of goniometry.45 The inclinometer uses a fixed vertical reference point realized by gravity, thus is stable provided the zero point is accurately calibrated and established.45 Traditional goniometry requires visualization of the vertical reference point, which may compromise measurement reproducibility.45
A study by Herda TJ et al 8 looked at the effect of static vs. dynamic stretching on isometric peak torque, electromyography (EMG) and mechanomyography (MMG) of the biceps femoris muscle. The purpose of using MMG is that it has been suggested that MMG amplitude is inversely proportional to the active stiffness of a muscle therefore; stretching-induced decreases in muscle stiffness might be detected by increases MMG amplitude 8. It may have been more accurate and appropriate to use similar equipment and methods as Lee and Yoon 7,10, as these take into account various forces acting on the muscle to reflect and actual changes in muscle stiffness.
Use of Cybex dynamometer for performance testing
Isokinetic assessment of muscle performance is widely used for epidemiological studies, for performance prediction, and for rehabilitation purposes.46-50
Li RC et al51 concluded that the Cybex 6000 isokinetic dynamometer shows high reliability in measuring isokinetic concentric and eccentric peak torque, total work, and average power of knee extensor and flexor muscle groups when using the continuous concentric and eccentric cycle. Isokinetic concentric strength measurements were more reliable than the measurements of eccentric strength.
Curry 9 used a KinCom isokinetic dynamometer to measure the time to peak force. However, the speed of contraction (calibrated to manufactures recommendations), measured in degrees per second, was kept constant. Using a variety of speeds e.g. 60 and 240 degrees per second would account for the action of different muscles types and give a broader spectrum of information. In agreement with previous studies,47,52-54 peak torques of both knee extensor and extensor muscle groups obtained from continuous concentric-eccentric cycles at the angular velocity of 60°s-1 and 120°s-l were highly reliable (ICCs 0-82 to 0 92). Total work and average power reliability during knee extension and flexion was also high.47,52 In most instances, the ICCs of peak torque at 120°s-1 were greater than those at 60°s-1 2 in contrast with the report of Steiner et al.46 A possible reason for this observation is that subjects found it more difficult to maintain maximum effort throughout the entire range of motion at the slower speed. This would also explain why the reliability of total work and average power were greater at the faster speed.
Several issues must be considered when using strength data collected from isokinetic dynamometers for research and diagnostic purposes. Torque "overshoot" and "oscillation"
may occur before the constant angular velocity is attained and deceleration occurs toward the end of the contraction.55,56 The duration of constant angular velocity decreases as the present angular velocity increases.55,57,58 This phenomenon is primarily due to the fact that, under the same experimental condition, longer time (or angular distance) is needed to accelerate a body
segment to a higher angular velocity. Errors in torque measurements occur when the gravitational and inertial effects are not considered.57,58,59 However, the feature of gravity-correction is available in most modern isokinetic dynamometers. Mixed results on the within-day, interday, inter-machine reliability (or reproducibility) of strength data have been reported for different exercises.60-62
Summary
In general, static stretching has been shown to increase ROM but decrease muscle performance and dynamic stretching has shown to increase joint ROM and improve, or have no effect on muscle performance. No studies have assessed these types of stretching on passive muscle stiffness. It is clear that minimizing variables is crucial to yielding good results. However this can also be very difficult in the case of measuring passive muscle stiffness and joint ROM. The stretching exercises carried out should also be specific to the tests that are being carried out in order to make the study more relevant. Futures studies in similar areas should thoroughly review their methodology and use of appropriate equipment for implementation purposes, clinical significance and relevance.
