Effects of Static and Dynamic Stretching With Strengthening Exercises in Patients With Patellofemoral Pain Who Have Inflexible Hamstrings: A Randomized Controlled Trial

Abstract

Background:

Patellofemoral pain (PFP) syndrome is closely associated with muscle tightness. However, studies regarding the effects of stretching exercises on PFP patients with inflexible hamstrings are scarce. The aim of the study was to compare the effects between static and dynamic hamstring stretching in patients with PFP who have inflexible hamstrings.

Hypothesis:

Compared with static hamstring stretching, dynamic hamstring stretching will improve the parameters of hamstring flexibility, knee muscle strength, muscle activation time, and clinical outcomes in this patient population.

Study Design:

Prospective randomized controlled trial.

Level of Evidence:

Level 2.

Methods:

A total of 46 patients (25, static stretching; 21, dynamic stretching) participated. Hamstring flexibility was assessed according to the popliteal angle during active knee extension. Muscle strength and muscle activation time were measured using an isokinetic device. Clinical outcomes were evaluated using the visual analog scale (VAS) for pain and the anterior knee pain scale (AKPS).

Results:

There were no differences in hamstring flexibility and knee muscle strength of the affected knees between the groups (P > 0.05). Significantly improved muscle activation time and clinical outcomes of the affected knees were observed in the dynamic stretching group compared with the static stretching group (all Ps < 0.01 for hamstring, quadriceps, VAS, and AKPS).

Conclusion:

In patients with PFP who have inflexible hamstrings, dynamic hamstring stretching with strengthening exercises was superior for improving muscle activation time and clinical outcomes compared with static hamstring stretching with strengthening exercises.

Clinical Relevance:

Clinicians and therapists could implement dynamic hamstring stretching to improve function and reduce pain in patients with PFP who have inflexible hamstrings.

Patellofemoral pain (PFP) syndrome is the most common reason for anterior knee pain,⁵² which may be caused by factors such as knee muscle weakness^9,29 and imbalance,²¹ malalignment²¹ and inflexibility^45,56 of the lower extremities, hip muscle weakness,¹³ and altered neuromuscular control.³ A systematic review⁵⁴ and previous studies^39,45 have reported that PFP syndrome could be closely associated with inflexible hamstrings, as the knees cannot generate sufficient extensor torque³² owing to increased pressure and stress at the patellofemoral joint.¹⁶ Therefore, some therapists focus on restoration of hamstring flexibility.

Two systematic reviews have reported that stretching exercises are recommended to improve the function of patients with PFP.^2,54 In addition, recent studies have reported that stretching exercises as well as strengthening exercises for the quadriceps and hip muscles are effective for improving pain and function in patients with PFP.^38,48 However, stretching exercises comprise 3 interventions, namely, static, dynamic, and ballistic stretching,⁴⁴ and both static and dynamic stretching help to increase range of motion and flexibility.^44,46,54 To date, no studies have investigated outcomes after static and dynamic hamstring stretching in patients with PFP who have inflexible hamstrings.

This study aimed to compare hamstring flexibility, knee muscle strength, muscle activation time, and clinical outcomes after static and dynamic hamstring stretching with strengthening exercises in patients with PFP who have inflexible hamstrings. We hypothesized that compared with static hamstring stretching, dynamic hamstring stretching will improve all the parameters.

Methods

This study complied with the Declaration of Helsinki and was approved by the institutional review board of our institute (2017AN0830). All study participants provided written informed consent, and the rights of the participants were protected.

Sample Size Estimation and Test-Retest Reliability

According to a previous study on the strength of quadriceps muscles in patients with PFP,²² a greater than 10% difference in quadriceps strength was regarded as clinically significant. An a priori power analysis was used to determine sample size to achieve an alpha level of 0.05 and a power of 0.8. A pilot study of 5 knees in each group (static vs dynamic stretching) was performed to estimate sample size, and we calculated that 38 patients were required to detect a significant difference in the strength of quadriceps muscles after hamstring stretching in the affected knees between the 2 groups. In the present study, 25 patients were subjected to static stretching and 21 to dynamic stretching. The power of this study was 0.937 for detecting a significant difference between the 2 groups.

To quantify test-retest reliability, intraclass correlation coefficients (ICCs) were calculated for 2 trials of flexibility of the hamstring muscles. ICCs were also calculated for strength and muscle activation time of the quadriceps and the hamstring muscles. According to a previous study by Shrout and Fleiss,⁵¹ ICCs >0.75 and <0.4 indicated good and poor reliability, respectively. In the present study, the overall test-retest reliability for flexibility was acceptable for hamstring muscles (ICC = 0.87), and that for strength and muscle activation time was good for quadriceps (ICC = 0.83 and 0.79, respectively) and hamstring (ICC = 0.77 and 0.81, respectively) muscles.

Patient Enrollment

This prospective randomized study included patients with anterior knee pain from December 2017 to October 2018. All patients were consecutively recruited. Two orthopaedic knee surgeons independently performed the physical examination and evaluated all images,⁵⁴ such as those for patellofemoral osteoarthritis, Insall-Salvati ratio (the ratio of the patellar tendon length to the length of the patella), and abnormal trochlear groove. Any disagreements were resolved by consensus. Only patients with PFP who had <141° of knee extension angle during the hamstring flexibility test were included.⁵⁹ Excluded patients are presented in Figure 1. In total, 60 patients volunteered to participate, and each was assigned using a random-number table to either the static stretching or the dynamic stretching group for a 12-week rehabilitation protocol. Allocation and treatment were performed by a physical therapist who was blinded to the details of the assessments and data analyses. Data were assessed and analyzed by an independent physical therapist who was blinded to the rehabilitation protocol. The final confirmation of the evaluation was made by two orthopaedic knee surgeons, who were blinded to the rehabilitation protocol of the enrolled patients.

Figure 1.

Study flowchart.

Data obtained from 46 of the 60 initial volunteers were used for the final evaluation (25 for static stretching vs 21 for dynamic stretching) (Figure 1). Of these patients, 17 in the static stretching group and 15 in the dynamic stretching group displayed high sports and activity levels, defined as participating in contact sports such as football and basketball.⁴² There were no significant between-group differences in characteristics, including age, sex, body mass index, sports and activity levels, or Insall-Salvati ratio (Table 1).

Table 1.

Demographic data of participants in the static and dynamic hamstring stretching groups

	Static Stretching Group (n = 25)	Dynamic Stretching Group (n = 21)	P
Sex (male/female)	11/14	8/13
Age (years) a	27.2 ± 7.0	25.1 ± 9.2	0.71
Height (cm) a	170.1 ± 5	173.3 ± 2.9	0.57
Weight (kg) a	65.8 ± 10.4	70.7 ± 7	0.89
Body mass index (kg/m2) a	26.4 ± 3.1	28.5 ± 3.4	0.78
Injured side (right/left)	20/5	18/3
Sports and activity, n (low:high)	8:17	6:15	0.21
Insall-Salvati ratio	1.01	0.99	0.75

^aValues are expressed as mean ± SD.

Hamstring Flexibility Test

Hamstring flexibility was assessed based on the popliteal angle (90-90 hamstring flexibility test).⁵⁹ The 90-90 hamstring flexibility test was performed using an increased knee extension angle (180°) during active knee extension in the supine position (Figure 2a).

Figure 2.

(a) 90-90 hamstring flexibility test, (b) static hamstring stretching, (c) dynamic hamstring stretching.

Assessment of Isokinetic Muscle Performance Test

Knee muscle strength and muscle activation time were evaluated by 5 maximal contractions at 180 deg/s using the isokinetic Biodex Multi-Joint System 4 (Biodex Medical Systems Inc). Flexor and extensor strength were regarded as hamstring and quadriceps muscle strength, respectively. Knee muscle strength was evaluated by maximum torque normalized to body weight (peak torque/body weight, N·m/kg × 100; knee muscle strength was recorded at 40° for hamstring muscles and 50° for quadriceps muscles in the present study).^33-35 Muscle activation time was assessed by isokinetic testing–acceleration time (AT)^{5,33,36,37,53} and defined as muscle recruitment time for attaining a preset angular velocity (180 deg/s in our study) during maximal muscle contraction. AT was recorded in milliseconds, and a lower AT indicated delayed muscle activation.

Clinical Outcomes

Anterior knee pain was assessed using a visual analog scale (VAS; 0 points indicated no pain, 10 points indicated the worst possible pain)¹⁴ and the anterior knee pain scale (AKPS),³¹ also known as the Kujala scale, which consists of 13 questions for a total score ranging from 0 to 100, with lower scores indicating greater disability. As reported by a previous study,¹⁴ we defined the minimal clinically important difference for VAS and AKPS to be 1.5 to 2 points and 8 to 10 points, respectively.

Hamstring Stretching Interventions

Static Hamstring Stretching

In the sitting or standing position, the affected leg was maintained in a straightened position with ankle dorsiflexion. The foot was grasped using the ipsilateral hand or a towel, with the contralateral hand keeping the affected knee straight, with a slight trunk forward flexion (Figure 2b).⁴¹ Static stretching was repeated for 3 sets, with a holding time of 15 seconds.³⁰

Dynamic Hamstring Stretching

In the supine position, the affected leg was maintained at 90° of flexion of both the hip and knee. The distal thigh was grasped using a towel; then, active knee extension was performed with contraction of the quadriceps themselves (Figure 2c).¹⁸ In the standing position, this exercise was performed by repeated hip flexion with the knee extended.¹⁰ For dynamic stretching, 3 sets were performed, with 15 repetitions with a 1-second holding time.³⁰

In both groups, the sets and holding time were similar. The final test was performed at our institution at the 12-week follow-up, with a 4-minute recovery period after each stretching exercise.

Conservative Rehabilitation Protocol

All participants followed the same rehabilitation protocol for 12 weeks for both knees, except for the stretching exercises, at our rehabilitation center. The goals of the rehabilitation program were to improve strength, proprioception, and neuromuscular control and normalize the biomechanics. Open and closed kinetic chain exercises, such as multidirectional straight-leg raises, wall squat exercises along with knee adductions, and single-leg squats, were performed to improve the strength of the quadriceps, hamstrings, and hip muscles. Balance exercises with both open and closed eyes were performed to improve proprioception and neuromuscular control. A core and hip muscle strengthening program was implemented with all exercises. The recommended home exercise program consisted of 3 sets twice a day for each stretching exercise.

Statistical Analysis

The Student t test was used to compare differences in hamstring flexibility, knee muscle strength, muscle activation time, and clinical outcomes between the 2 groups. The paired t test was used to compare all variables in each group before and after hamstring stretching. To determine whether a continuous variable followed a normal distribution, the Shapiro-Wilk test was used. Data were analyzed using SPSS (Version 21.0; IBM Corp), at a confidence level of P = 0.05.

Results

After the interventions, there were no significant differences in the hamstring flexibility or knee muscle strength of the affected knees between the 2 groups (flexibility, P = 0.27; hamstring strength, P = 0.54; quadriceps strength, P = 0.44) (Table 2). However, isokinetic testing-AT and clinical outcomes were significantly improved for the affected knees of the dynamic stretching group compared with those of the static stretching group (all Ps < 0.01 for hamstring, quadriceps, VAS, and AKPS) (Table 3).

Table 2.

Hamstring flexibility and knee muscle strength for patients in the static and dynamic hamstring stretching groups ^a

	Preintervention			Postintervention
	Static Stretching Group	Dynamic Stretching Group	P	Static Stretching Group	Dynamic Stretching Group	P
Hamstring flexibility	132 ± 12	138 ± 10.9	0.11	146 ± 10.7	150 ± 10.7	0.27
Mean difference (95% CI)	−5.7 (−12.7 to 1.3)			−3.6 (−10.1 to 2.9)
Hamstring strength	69 ± 24.6	82 ± 40	0.11	100 ± 17	103 ± 11.5	0.54
Mean difference (95% CI)	−13.2 (−32.8 to 6.3)			−2.8 (−11.8 to 6.2)
Quadriceps strength	105 ± 38.7	102 ± 41.8	0.78	170 ± 52.6	182 ± 54.2	0.44
Mean difference (95% CI)	3.4 (−20.9 to 27.7)			−12.6 (−44.9 to 19.37)

^aValues are expressed as mean ± SD. Hamstring flexibility was measured in degrees, and muscle strength was measured as maximum torque normalized to body weight (N·m/kg × 100). All data were recorded and described by a physical therapist.

Table 3.

Isokinetic testing–acceleration time (AT) and clinical outcomes for patients in the static and dynamic hamstring stretching groups ^a

	Preintervention			Postintervention
	Static Stretching Group	Dynamic Stretching Group	P	Static Stretching Group	Dynamic Stretching Group	P
Hamstring AT	66 ± 18.2	75 ± 16.3	0.09	64 ± 19.1	45 ± 15.7	<0.01
Mean difference (95% CI)	−9.1 (−19.6 to 1.4)			19 (8.3 to 29.7)
Quadriceps AT	64 ± 17.3	68 ± 16.4	0.53	63 ± 17.2	41 ± 13.8	<0.01
Mean difference (95% CI)	−3.2 (−13.5 to 7.0)			21.7 (12.1 to 31.3)
VAS	4 ± 1	5 ± 1.2	0.52	3 ± 0.9	1 ± 0.7	<0.01
Mean difference (95% CI)	−0.2 (−0.9 to 0.4)			1.0 (0.6 to 1.6)
AKPS	58 ± 9.1	57 ± 8.2	0.57	68 ± 7.7	76 ± 6.5	<0.01
Mean difference (95% CI)	1.5 (−3.8 to 6.8)			−7.4 (−11.7 to −3.0)

^aValues are expressed as mean ± SD. The measurement unit of isokinetic testing-AT was milliseconds. Boldfaced P values are statistically significant. All data were recorded and described by a physical therapist. AKPS, anterior knee pain scale; VAS, visual analog scale.

Participants who performed static stretching showed significantly improved hamstring flexibility, knee muscle strength, and clinical outcomes in the affected knees (allPs < 0.01) for hamstring flexibility, hamstring and quadriceps strength, VAS, and AKPS) (Figure 3a), but showed no significant improvement in isokinetic testing-AT (hamstring, P = 0.53; quadriceps, P = 0.69) (Figure 3a).

Figure 3.

Hamstring flexibility, knee muscle strength, isokinetic testing-AT, and clinical outcomes of the affected knees before and after hamstring stretching in the (a) static and (b) dynamic stretching groups. AKPS, anterior knee pain scale; AT, acceleration time; VAS, visual analog scale.

Participants who performed dynamic stretching showed significantly improved hamstring flexibility, knee muscle strength, isokinetic testing-AT, and clinical outcomes in the affected knees (all Ps < 0.01) for hamstring flexibility, hamstring and quadriceps strength, isokinetic testing-AT, VAS, and AKPS) (Figure 3b).

Discussion

The most important result of the present study was that compared with the static stretching group, the dynamic stretching group had significantly improved muscle activation time and clinical outcomes but not significantly improved hamstring flexibility or knee muscle strength.

Several previous studies have reported that both static and dynamic stretching are effective in increasing muscle flexibility.^6,12,57 However, other studies have reported that static stretching may be more effective in increasing hamstring flexibility than dynamic stretching.^7,41 The results of the present study are inconsistent with these findings. Here, hamstring flexibility was not significantly different between the static and dynamic stretching groups. This may be attributed to the holding time of hamstring stretching. In our study, the holding time for each stretching exercise was 15 seconds; however, it was 30 seconds in the aforementioned studies. Several previous studies have reported that the holding time of stretching is an important factor affecting changes in muscle stiffness and collagen deposition.^6,19 Bandy and Irion⁶ and Bandy et al⁷ have demonstrated that longer stretching holding times, such as 30 to 60 seconds, were more effective than 15 seconds of holding time for improving muscle flexibility, particularly in static stretching versus dynamic stretching. However, there was no significant difference between static and dynamic stretching at a holding time of 15 seconds.⁷ Therefore, the holding time of 15 seconds during stretching in our study may have been insufficient to identify differences in hamstring flexibility between the 2 groups.

Previous studies have reported that patients with PFP have weak hamstring and quadriceps muscles^15,29,55; thus, restoration of knee muscle strength is important for these patients. We found that muscle strength of the affected knees was not significantly different between the 2 groups after each hamstring stretching exercise. Although the reasons for this result are unclear, it may be attributable to the angle required to produce muscle strength during isokinetic testing. Kannus and Beynnon²⁷ and Kannus and Jarvinen²⁸ have demonstrated that the peak torque for hamstring and quadriceps muscles on both affected and unaffected knees is produced at 30° and 50° of knee flexion angle during isokinetic testing, respectively. In our study, the peak torque angles produced for hamstring and quadriceps muscles were similar to those reported in a previous study (40° for hamstring muscles and 50° for quadriceps muscles), resulting in no difference in muscle strength between the 2 groups, because these angles are smaller in the contact stress of the patellofemoral joint in patients with PFP.¹⁷ Previous studies have reported that the contact stress of the patellofemoral joint in patients with PFP syndrome was increased at knee flexion angles greater than 60°^17,25; this could cause increased knee pain,^17,39 resulting in muscle weakness.^11,50 Therefore, muscle strength testing at knee flexion angles greater than 60° is recommended for patients with PFP.

Several previous studies reported altered muscle activation time in patients with PFP.^3,40,57 In our study, muscle activation time were significantly improved in the affected knees of the dynamic stretching group compared with those of the static stretching group, which could be attributed to a neural inhibitory mechanism. Inhibition of neural factors, such as altered motor control and reflex sensitivity, may affect muscle activation.^4,8,23 Behm et al⁸ and Hough et al²⁴ reported that muscle activation decreased after static stretching, whereas dynamic stretching may have enhanced muscle activation^20,43 owing to a postactivation potentiation mechanism, defined as a transient increase in the contractile performance of muscles after a previous conditioning activity.^43,49 In our study, dynamic stretching was performed during active knee extension to increase the flexibility of the hamstrings. Herda et al²³ reported that dynamic stretching may increase muscle activation, and Aagaard et al¹ reported that active knee extension can optimize the proportion of motor neuron activation in both hamstring and quadriceps muscles. Additionally, a previous study reported that static stretching is not effective in improving motor neuron activation.⁵⁸ The results of previous studies point to significant differences in muscle activation between static and dynamic stretching. Furthermore, improved muscle activation can significantly affect clinical outcomes in patients with PFP.⁴⁷ Especially in the present study, dynamic stretching resulted in greater improvement in clinical outcomes compared with static stretching, based on the minimal clinically important difference proposed in a previous study.¹⁴ Therefore, our results indicate that dynamic stretching may be more beneficial than static stretching for improving muscle activation time and clinical outcomes in patients with PFP with inflexible hamstrings.

Limitations

This study has some limitations. First, there was no healthy control group; thus, a further prospective study is necessary to confirm differences in hamstring flexibility, knee muscle strength, and muscle activation time between patients with PFP and controls. Second, hip muscle strength was not evaluated in this study. Previous studies have reported weaknesses in hip abductors in patients with PFP,^13,26 which may increase contact stresses in the patellofemoral joint owing to abnormal biomechanics of the lower extremities. However, hip muscle strengthening was included in the rehabilitation protocol of both groups in our study. Third, the enrolled patients performed muscle strengthening exercise as well as stretching exercise. The additional strengthening interventions could be a substantial confounding variable to the outcome of the current study. However, strengthening exercises were performed in the same manner for both groups. Finally, in the dynamic stretching group, bias related to the use of either the medial or lateral hamstrings during stretching exercises may have existed. Therefore, further studies on dynamic stretching may be needed.

Conclusion

Dynamic hamstring stretching with strengthening exercises resulted in improved muscle activation time and clinical outcomes compared with static hamstring stretching with strengthening exercises in patients with PFP who have inflexible hamstrings.