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Original Research

A Dynamic Warm-up Model Increases Quadriceps Strength and Hamstring Flexibility

Aguilar, Alain J.; DiStefano, Lindsay J.; Brown, Cathleen N.; Herman, Daniel C.; Guskiewicz, Kevin M.; Padua, Darin A.

Author Information
Journal of Strength and Conditioning Research: April 2012 - Volume 26 - Issue 4 - p 1130-1141
doi: 10.1519/JSC.0b013e31822e58b6
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Abstract

Introduction

The majority of the athletic population performs a warm-up before activity to enhance performance and reduce the risk of injury (2,40,44). Acutely enhancing performance ability immediately before activity may maximize early parts of training and competitions. Also, acutely enhancing performance is essential for competitions of short duration (i.e., shot put, 100-m dash). Furthermore, a warm-up may mitigate injury risk factors by acutely changing them immediately before activity. Several modifiable risk factors for specific injury, sport, and population are indicated in the literature. For example, 2 risk factors for hamstring muscle strain injury are flexibility deficits and a decreased hamstring to quadriceps (H:Q) ratio (28,48), which may be influenced by a warm-up. Therefore, warm-up methods should focus on causing acute daily changes that will reduce an athlete's injury risk during a training session, eventually causing chronic changes that will reduce an athlete's injury risk over time and ideally improve performance.

A common warm-up includes a brief period of a submaximal aerobic warm-up activity and a bout of static stretching. The submaximal aerobic warm-up activity may elevate muscle and core temperature, causing decreased stiffness of the musculoskeletal tissue and increased anaerobic metabolism, circulation, and nerve conduction (7). As a result, the submaximal aerobic warm-up activity has been shown to improve several short-term performance measures (6). The purpose of static stretching is to improve flexibility, which has been indicated as an injury risk factor for athletes with specific muscle inflexibility (28,48). Although long-term flexibility training is important to maintain health muscle tissue length and reduce injury risk for specific individuals, there is no evidence that static stretching, immediately before activity, will significantly reduce muscle injury rates (38,45). Furthermore, static stretching for the hamstrings has been shown to reduce conventional and functional hamstring to quadriceps ratio (H:Q) via decreased concentric and eccentric hamstring peak torque (14). Although debated and not validated, the H:Q ratio has been used as a screening tool to help prevent lower extremity injury (13,18). A reduction in the H:Q ratio as a result of static stretching could possibly increase risk of lower extremity injury (13). Moreover, static stretching has been shown to acutely impair the following performance measures: strength (15); power (49); balance, reaction, and movement time (4); vertical jump height (11); and sprint performance (24). Therefore, because evidence shows that static stretching before acitvity may not reduce injury risk and may cause performane deficits, the use of static stretching before activity has been questioned (33) and new methods for preparticipation activities have been proposed.

An alternative to the traditional warm-up mentioned above is called a “dynamic warm-up” (DWU). Although the DWU often varies in protocol, it typically includes dynamic stretching (49), agility and plyometric activities (37), and specific motor pattern movements (1). In essence, a combination of these techniques may prepare the body for performance by improving core and muscle temperature(25), enhancing nervous system function (i.e., postactivation potentiation) (29,49) and using similar movements that occur during subsequent exercise (47). Recently, dynamic stretching has been shown to improve several performance factors such as shuttle run time, medicine ball throw distance, 5-step jump distance (33), electromyography activity (29), lower extremity power (49,50), and vertical jump (29).

Therefore, because a DWU may positively influence performance and static stretching may negatively impair performance, researchers have compared the acute effects of the 2 types of warm-ups. These studies indicate that a DWU can significantly improve power and agility (33), sprinting performance (24), vertical jump (22,23,46), and long jump (46) when compared with static stretching alone. Although these findings are promising, there are limitations to these studies. For example, none of the above mentioned articles used a preintervention assessment, which limits the ability to understand any true acute improvements observed. Also, the previous DWU studies did not investigate the inclusion of a short distance “acceleration run,” which may assist in preparing the body for activity by further elevating heart rate and body temperature. Moreover, the previous studies did not compare static stretching warm-up (SWU) and DWU on acute changes in variables that are related to both muscle injury risk factors and performance measures, such as flexibility and strength.

Therefore, the purpose of this study was to use a randomized controlled study design to investigate the acute effects of a control program (CON), SWU, and a DWU protocol with a short distance “acceleration run” on acute changes in the injury risk factors of proximal leg flexibility and hamstrings to quadriceps peak torque ratio and acute performance changes in quadriceps and hamstrings peak torque and vertical jump performance. Based on the previous literature, we hypothesize that the SWU will improve flexibility, reduce H:Q ratio, and negatively affect peak torque and vertical jump, whereas the DWU will cause acute increases in flexibility, H:Q ratio, peak torque, and vertical jump.

Methods

Experimental Approach to the Problem

This study was designed to observe how 2 warm-ups (SWU and DWU) acutely affect specific muscle injury risk factors and performance factors. We used a randomized controlled trial study design to compare the acute effects of 2 warm-up protocols (SWU or DWU) with a control group (CON). All subjects were evaluated immediately before and after a 15-minute warm-up intervention. We also included a control (CON) group to specifically observe the difference between the static stretching protocol and the DWU protocol. The dependent variables included muscle injury risk factors hamstrings, quadriceps, hip flexor, and rectus femoris flexibility and conventional and functional H:Q ratio and performance measures of hamstrings and quadriceps concentric and eccentric peak torque and predicted vertical jump height and power.

Subjects

Forty-five healthy male (n = 23) and female (n = 22) recreational soccer players from club and intramural teams volunteered to participate. Subject demographics are provided in Table 1. Soccer players were recruited because the DWU protocol was designed specifically for that sport. Subjects were included in the study if they exercised at least 3 days per week for 30 minutes and possessed an adequate level of previous soccer experience, which was defined as at least 1 year of varsity soccer in high school. Exclusion criteria included any muscle strain injury sustained in the past 6 months or any current lower extremity injury, head injury, or illness that would prevent them from playing soccer at the time of testing. Before testing, all subjects read and signed an informed consent form approved by the institutional review board and completed a healthy subject questionnaire. The questionnaire contained general questions to ensure inclusion and exclusion criteria.

Table 1
Table 1:
Subject demographics, mean (SD).*†

Procedures

Before testing, the subjects were stratified for sex and randomly assigned without replacement into 1 of 3 groups: CON, SWU, or DWU (n = 15 per group) (Table 1). Subjects reported to a research laboratory for a single testing session. They were required to wear athletic clothing, including shorts, a T-shirt, and athletic shoes. The testing session lasted approximately 90 minutes and involved a pretest, an intervention period, and a posttest. Flexibility (hamstrings, quadriceps, and hip flexor), peak torque (hamstrings and quadriceps concentric and eccentric), and vertical jump height and power measurements were recorded during both pretest and posttest assessments. The dominant limb, which was defined as the limb used to kick a ball for maximal distance, was used for flexibility and peak torque measurements. Flexibility was the first measurement obtained during the pretest and posttest to ensure that the other testing procedures would not confound the flexibility results. After flexibility measurements, the order of testing peak torque and vertical jump was counterbalanced between subjects to control for a potential order effect. During the pretest and posttest, subjects rested for 2 minutes between each assessment and for 15 minutes before beginning their warm-up intervention, so that a possible increase in body temperature resulting from the pretest would not add to the effects of the warm-up treatments (7). To establish reliability of the individual measurements, 6 subjects were pilot tested using identical testing procedures between 2 different testing sessions separated by 15 minutes. The reliability results are presented for each dependent variable in Table 2.

Table 2
Table 2:
Reliability results.

Warm-up Interventions

Each intervention (CON, SWU, and DWU) began with a 5-minute warm-up on a stationary bicycle, and subjects were asked to adjust their effort so that they would score a “light” rate of perceived exertion (RPE) (32) by the end of the 5 minutes. After cycling, the CON group sat quietly for 10 minutes, whereas the SWU and DWU groups performed the protocols outlined in Tables 3 and 4, respectively.

Table 3
Table 3:
Static warm-up protocol.
Table 4-a
Table 4-a:
Dynamic warm-up protocol.
Table 4-b
Table 4-b:
Dynamic warm-up protocol.

Including the cycling, the SWU protocol lasted 15 minutes and consisted of a 10-minute static stretching protocol for the lower extremity (Table 3). Each muscle group was stretched for 2 sets of 20 seconds to the point of discomfort. The stretching sequence is listed in Table 3, and each muscle group was stretched bilaterally, starting with the right side, before moving to the next muscle group in the stretching sequence. Subjects rested 2–5 seconds between each stretch. The stretch duration is within the American College of Sports Medicine's guidelines for static stretching and also kept the SWU protocol duration equivalent to the DWU protocol.

Including the cycling, the DWU (Table 4) lasted 15 minutes and consisted of a 10-minute DWU. The DWU was divided into 3 phases: dynamic exercises (dynamic stretching, agility, and plyometric exercises), an acceleration run, and a short recovery jog. Two consecutive 10 yd distances were marked by cones in an area where subjects could run 20 yd and decelerate safely. Subjects performed a dynamic exercise in the first 10 yd distance, an acceleration run in the second 10 yd distance, and finished with a recovery jog back to the starting cone for the next dynamic exercise in the sequence. This process was repeated until all dynamic exercises in the protocol were completed. The sequence was the same for each participant and was only performed once. Within the first 10 yd distance, all dynamic stretches were alternated 5 times bilaterally and subjects were asked to bring the limb to the point of mild stretch and then quickly release, without “holding” the stretch. Between stretching the alternate leg, subjects were asked to “bounce” for 3 steps as if they were jogging in place. In the second 10 yd distance, the subjects transitioned to a 10 yd acceleration at a certain percentage of their perceived maximum sprint effort (Table 4). The intensity and difficulty of the dynamic exercises and the acceleration run increased as the protocol progressed.

To ensure proper technique and efficiency during the DWU, subjects reported to 2 familiarization sessions before testing. All familiarization sessions lasted 20 minutes and were conducted between 1 and 7 days of each other. During both familiarization sessions, subjects watched a video of the DWU protocol and then practiced the DWU protocol. The SWU group did not perform familiarization sessions because the static stretches in the protocol are commonly used in the physically active population.

Assessments

Hamstrings, quadriceps, hip flexor, and rectus femoris flexibility were assessed in a random order using a digital inclinometer (Model # 670099; Saunders Group Inc, Chaska MN, USA). The inclinometer was zeroed on a flat and level surface before pre- and posttesting. Inclinometer placement for the hamstrings, quadriceps, and rectus femoris assessment was on the medial tibial shaft distal to the tibial tuberosity. Inclinometer placement for the hip flexor assessment was on the posterior leg, along the medial hamstrings group and proximal to the popliteal crease. During pretest, marks around the inclinometer were made directly on the subject's body using a permanent marker so that the placement was consistent during the posttest. Each muscle group was assessed 3 times, and the average was used for analysis. Between the 3 consecutive trials, the subjects were repositioned and the inclinometer was removed and replaced. Specific positioning and instructions for each flexibility assessment are presented in Table 5 (Figures 1–5). Only the dominant limb, which is defined as the one used to kick a ball for maximum distance, was measured.

Table 5
Table 5:
Flexibility assessment.
Figure 1
Figure 1:
Inclinometer placement on posterior leg.
Figure 2
Figure 2:
Hamstring flexibility ending position measurement.
Figure 3
Figure 3:
Quadriceps flexibility ending position measurement.
Figure 4
Figure 4:
Hip flexor flexibility ending position measurement.
Figure 5
Figure 5:
Rectus femoris ending position measurement.

Quadriceps and hamstrings concentric and eccentric peak torque were measured by an isokinetic dynamometer (Biodex 3 Isokinetic Dynamometer; Biodex Medical System, Inc., Shirley, NY, USA). Instructions and verbal encouragement were similar for each participant. Only the dominant leg, which was considered the leg they would use to kick a ball for maximum distance, was measured. During pretesting, the subjects were familiarized to the equipment by performing 3 submaximal attempts (50% capacity), followed by 1 minute of rest and then 3 maximal attempts for both quadriceps and hamstring testing (30).

Velocity for the isokinetic tests was set to 60° per second, and motion was set from 0° (extension) to 90° (flexion). Subjects were positioned sitting upright with their hips flexed 90° and were secured using torso, pelvic, thigh, and shin stabilization straps. The axis of rotation of the dynamometer was aligned with the axis of rotation of the subject's knee. A shin pad was placed 1–2 cm proximal to the subject's lateral malleolus. All testing began with the subject's knee at 90° of knee flexion. For the quadriceps test, the subjects concentrically extended their knee to full extension (0°), then eccentrically resisted as the dynamometer moved the knee back to 90° of flexion. For the hamstring test, the subject eccentrically resisted the dynamometer as it moved the knee to full extension (0°) and then concentrically flexed the knee back to 90°. Subjects performed 1 set of 5 continuous maximum effort repetitions for the quadriceps test and 1 set of 5 continuous maximum effort repetitions for the hamstring test. Subjects rested for 2 minutes between the quadriceps and hamstring tests.

To eliminate the possibility of an order effect, subjects randomly selected the order of muscle testing by drawing numbers. After each test, subjects rated their perceived effort on a 19-point RPE scale (32). A successful trial was considered a score of 17 or higher, which equated to a “very hard” perceived exertion (20). If the subjects' score was less than 17, then they repeated the trial. If the subject reported lower than 17 for 2 consecutive tests, then the subject was excused from the study because fatigue may have confounded the data.

Predicted vertical jump height and power were evaluated using a double leg countermovement vertical jump on a strain gauge force plate (Bertec model number 4060-08A; Bertec Corp., Columbus, OH, USA). An overhead goal in the form of a suspended soccer ball was used to encourage maximal performance (26). Subjects were instructed to keep their hands on their hips, jump straight up and down (vertically), and jump as high as they could to try to “head” the soccer ball. During the pretest, subjects were allowed 3 maximal jump practice trials. The overhead goal was placed slightly higher than the subject's highest practice jump. Three trials were performed with 30 seconds of rest between each trial. Force plate calibration occurred before each trial.

Data Reduction

Peak torque data from the second, third, and fourth repetitions of each isokinetic test were averaged to determine the peak torque for each muscle group and the hamstring to quadriceps ratio. The first repetition was eliminated because of a possible learning effect, and the fifth repetition was not considered because of the possible effects of fatigue. The average peak torque for concentric and eccentric hamstrings and quadriceps muscle actions were then used to determine both the conventional H:Q ratio (Hcon:Qcon) and the functional H:Q ratio (Hecc:Qcon). The average peak torque values were divided by body mass.

A customized software program (MatLab v12; The Math Works, Inc., Natick, RI, USA) was used to calculate the impulse and predicted vertical jump height and power from the countermovement vertical jump. The subjects' impulse from the takeoff portion of each jump was recorded at a sampling rate of 1,000 Hz. Impulse was used to calculate velocity (V = impulse created during takeoff/subject mass), and velocity was used to calculate height (height [m] = [V]2/[2*g]; gravity [g] = 9.81 m/s2) (3). Predicted vertical jump height was then used to calculate predicted vertical jump power by using Harman's equation: power (W) = 61.9 × jump height (cm) + 36.0 × mass (kg) − 1822 (9).

Statistical Analyses

We used a mixed model repeated measures design with one between factor (3 levels: DWU, SWU, or CON group) and one within factor (2 levels: pretest, posttest). Separate 2 × 3 mixed model analyses of variance were used to investigate differences between time and warm-up protocol for each dependent variable. Post hoc testing was performed with a Tukey's Honestly Significant Difference (HSD) analysis and an a priori alpha level of ≤0.05 was set.

Results

All 45 original subjects completed the study. No significant differences in subjects' height, weight, or age were observed between groups (p > 0.05). Means and SDs for the flexibility assessments are presented in Table 6. We observed a significant group by time interaction for hamstring flexibility (F2,42 = 12.004, p < 0.0001). The post hoc analysis revealed that hamstring flexibility in the DWU group was significantly greater at posttest than at pretest. We did not observe any significant differences for quadriceps, hip flexor, and rectus femoris flexibility (p > 0.05).

Table 6
Table 6:
Flexibility results.*†

There was a significant group by time interaction for eccentric quadriceps peak torque (F2,42 = 4.930, p = 0.012) (Table 7). The post hoc analysis revealed a significant increase in the DWU group from pretest to posttest. Likewise, concentric quadriceps peak torque demonstrated a significant group by time interaction (F2,42 = 3.671, p = 0.034) (Table 7), and post hoc testing revealed that the DWU was greater than the CON and SWU at posttesting. The DWU appeared to cause an increase in torque from pre- to posttest (12.2 N·m) with a moderate effect size, but the difference was not statistically significant (critical difference = 13.3 N·m). No significant differences were observed for concentric hamstring peak torque, eccentric hamstring peak torque, concentric hamstring to concentric quadriceps ratio, or eccentric hamstring to concentric quadriceps ratio (p > 0.05) (Table 7).

Table 7
Table 7:
Isokinetic results.*†

No significant differences were observed for vertical jump height or vertical jump power (p > 0.05) (Table 8). Thus, it appears that none of the warm-up groups significantly changed vertical jump performance.

Table 8
Table 8:
Vertical jump results.*†

Discussion

The purpose of the study was to investigate the acute effects of a control program (CON), SWU, and a DWU protocol with a short distance “acceleration run” (DWU) on acute changes in proximal leg flexibility and hamstring to quadriceps peak torque ratio and acute performance changes in quadriceps and hamstring peak torque and vertical jump performance. This study found that that the DWU acutely improved hamstring flexibility and eccentric quadriceps peak torque, which may lead to reduced injury risk and improved performance immediately after the warm-up activities.

Our hypothesis that the SWU would cause improvements in flexibility and a decrease in H:Q ratio was not supported by our findings. With regard to flexibility, we may not have seen improvements because of a shortened stretch duration in the SWU protocol. Other studies showed flexibility improvements after a duration of 1.5 to 4.5 minutes (16,17,35), which is much longer than the 40-second duration (2 sets of 20 seconds) used in this study. Another reason why the SWU did not improve flexibility could be because of an absence of flexibility deficit. For example, the subjects in this study possessed greater hip flexor flexibility (27°) than the national normative values for the closest age group (22°) (39). Similarly, the subjects had a quadriceps flexibility value that was within 6° of these normative values (39). Therefore, because of a shortened stretch duration and lack of flexibility deficits, acute increases in flexibility may not have been possible. Likewise, no significant changes to conventional or functional H:Q ratios were found in this study after the SWU. The reason for this finding is also possibly because of the short stretch duration. One study showed a significantly decreased H:Q ratio after 6 minutes of static stretching (14), but another study showed no significant decrease with a 6-minute protocol (13). These findings suggest that H:Q ratio may be impaired with a prolonged stretch duration.

The short stretch duration included in the SWU may have also prevented changes in peak torque and vertical jump performance. Behm and Chaouachi (5) observed in a recent static stretching literature review that when a static stretch is held for longer than 90 seconds, there is strong evidence for strength impairment. However, when a stretch is held for less than 30 seconds, the findings of strength impairment are more variable. In this literature review, Behm and Chaouachi also reported that vertical jump was more likely affected with a longer stretch duration (i.e., >90 seconds). However, even in those studies, the effect sizes were small, so the clinical implication of these findings is questionable. Because the SWU static stretch duration was relatively short, it is possible that there was not enough time to cause physiologic viscoelastic property alterations and mechanical changes (31) and alterations in neurological activity and processes (27) to cause strength and performance impairments.

Our hypothesis that the DWU would improve flexibility was supported for the hamstrings but not for the other muscle groups. Another study showed a similar result for the hamstrings; however, the flexibility increase was only evident when dynamic stretching was preceded by a general aerobic warm-up (35). Interestingly, in the same study, the authors observed a decrease in flexibility when dynamic stretching was not preceded by a general aerobic warm-up. Perhaps this highlights the necessity of a general aerobic warm-up, similar to the one included in the DWU, before dynamic stretching to elevate muscle and body temperature, decrease muscle stiffness, and improve flexibility.

Another possible explanation for the DWU's increase in hamstring flexibility is that the group began with less flexibility than the other groups. Therefore, it is difficult to ascertain if the increase was because of the intervention or because the group had more room for improvement. However, all 3 groups demonstrated limited hamstring flexibility, as defined as lacking 20° from full knee extension (17), which suggests that each group had equal opportunity to improve hamstring flexibility. The DWU may not have improved flexibility in the quadriceps and hip flexor muscle groups because there may not have been much room for change, as all subjects had flexibility ranges that were within normal values. Therefore, a 15-minute DWU consisting of dynamic stretching, agility, plyometric, and acceleration runs may cause improvements in hamstring flexibility without any detriment to quadriceps and hip flexor flexibility.

In contrast to our hypotheses, the DWU only improved quadriceps peak torque. The results showed a significant increase in eccentric quadriceps peak torque and a statistically nonsignificant increase in concentric quadriceps peak torque. Although not statistically significant, the increase observed in concentric peak torque may be clinically meaningful because the magnitude of increase (7%) is similar to the increase seen in another study after dynamic stretching (43) and similar to the magnitude of decrease after static stretching (34). The mechanisms that may have caused our concentric quadriceps peak torque to improve are an increase in body temperature (25,49,50), postactivation potentiation (29,47), which is defined as an increase in muscle efficiency to produce force after a conditioning contractile activity (41), and possibly an increased rate of cross-bridge formation (49). These changes may improve neurologic and metabolic function (6) of the muscles and result in improved strength performance.

The mechanism responsible for improved eccentric quadriceps strength may include heightened muscle spindle activation caused by the quick discontinuous dynamic exercises of the DWU (43). Concentric peak torque of the quadriceps is essential in performance as it controls the forcefulness of knee extension, which is one of the primary components in generating any movement involving triple extension. Concentric force would be important for accelerating the body from a stand still position or after making a change of direction (12). Likewise, eccentric quadriceps function is essential for athletic movements including direction change, deceleration, and lower extremity body control (12). Eccentric strength may also protect against quadriceps injury because of its role in dissipating ground reaction forces during jump landing (19) and deceleration, which is the main mechanism for quadriceps muscle injury (36).

Despite observing improvements in quadriceps torque, we did not observe a reduction in the H:Q ratio. To our knowledge, this is the first study to evaluate the effects of a DWU on H:Q ratios. We believe the H:Q ratio may not have decreased with the concurrent increase in quadriceps torque because the hamstring peak torque did increase, although this finding was not statistically significant.

The literature has conflicting evidence regarding the effect of dynamic activities on vertical jump performance. Some studies show a significant increase (29,46), whereas others show no effect (8,42). One explanation to the difference could be described in the previous studies' dynamic protocol. The dynamic protocols that improved vertical jump typically used dynamic exercises that mimic functional exercises (i.e., side lunges) or a prolonged dynamic stretching protocol (7 minutes). The dynamic protocols that did not change vertical jump typically used a low volume of dynamic stretches only. Although our study used dynamic stretching and dynamic functional exercises, we may not have seen improvements in vertical jump because the time frame where dynamic activities cause improvements in vertical jump (6 minutes) (21) may have elapsed by the time we measured it during the posttest. Furthermore, it is possible that our DWU protocol did not include enough specific jumping activity to sufficiently cause improvements as other studies have used (10).

Finally, it is interesting to note that the 2 variables that significantly changed after the DWU had an antagonistic muscle relationship. We observed improvements in both quadriceps strength and hamstring flexibility. Perhaps the DWU improved this muscle balance relationship where a strength increase of the quadriceps allowed greater range of motion for the hamstrings, resulting in a significant change. Further research should examine this relationship.

Practical Applications

Our findings suggest that a preactivity DWU that includes dynamic stretching, agilities, plyometrics, and acceleration running will improve hamstring flexibility and quadriceps strength. Furthermore, using a static stretching protocol with short durations (40 seconds per muscle group) will not significantly reduce quadriceps and hamstring strength and vertical jump performance. Static stretching is important in health-related benefits; however, a prolonged duration is necessary to achieve this goal. To ensure appropriate duration for all major muscle groups, static stretching may be better used in a setting other than a preactivity warm-up. A DWU may be more appropriate for a preactivity warm-up because it appears to acutely improve strength production of the quadriceps and improve hamstring flexibility, which may reduce injury risk.

Acknowledgments

The authors have no professional relationships with companies or manufacturers who will benefit from the results of the study. The results of the study do not endorse any products of the authors or the National Strength and Conditioning Association.

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Keywords:

static stretching; isokinetic dynamometer; vertical jump

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