Hamstring tightness is a common clinical issue that manifests itself as restricted motion at the hip and knee due to the reduced ability of the muscle to elongate and permit sufficient joint motion necessary to perform the desired activities.1 Hamstring tightness is thought to be the result of either a reduced ability of the muscle tissue to elongate, which could possibly be mechanical or neural in nature,1,2 or myofascial adhesions,3,4 which hamper the muscle's ability to glide freely and permit full joint motion.
Muscle tightness is commonly encountered in clinical sports medicine practice. Although the commonly believed link between muscle tightness and injury risk has not been clearly substantiated,2 there is some evidence that muscle tightness may predispose athletes to lower extremity muscle strains5–7 and is related to lower back8,9 and patellofemoral10,11 pain. Therefore, the resulting dysfunction may have negative implications for sport performance12–16 due to pain and/or the inability to achieve the full joint range of motion necessary to optimally perform sport-specific maneuvers. In a previous study,15 restricted knee extension range of motion due to hamstring tightness affected knee biomechanics during running. Accordingly, another study showed that increasing hip and knee range of motion by stretching the hamstrings facilitated a larger dynamic range of motion during soccer kicking, which the authors concluded would translate to greater hip angular velocity and resultant ball speed.13
Because of the widely held belief that hamstring tightness is related to injury risk and movement restriction, stretching routines are a common preparatory activity for physical activity and an important component of the rehabilitation process for restoring normal function after injury. Hamstring tightness is addressed with various techniques that are aimed at increasing range of motion. This responsibility often falls on the athletic trainer, regardless of whether it is for preventative or rehabilitation purposes. Approaches such as static and dynamic stretching or myofascial release are available, but they require varying degrees of need for clinician assistance, thus potentially creating a burdensome patient load. Therefore, it would be advantageous to gain an understanding of the efficacy of self-administered techniques.
One such technique, static stretching, involves applying a sustained force that elongates the muscle for a period of time, which reduces muscle stiffness and increases compliance.2 Although this traditional form of stretching has been shown to be effective for increasing hamstring flexibility when used in prolonged protocols,17 acute benefits after a single bout of stretching may not be apparent unless the muscle is stretched for at least 4 minutes2; shorter durations such as those typically observed in practice do not appear to be effective.18 There is also an abundance of literature indicating that static stretching acutely impairs muscle force production,19 so this technique has fallen out of favor for exercise preparation.
Alternatively, dynamic stretching is commonly performed prior to physical activity and involves actively moving the body part through the full range of motion to mimic movements that are associated with the specific sport. This technique increases blood flow, motor neuron excitability, and tissue temperature,20 therefore increasing tissue compliance and range of motion. Unlike static stretching, there is some evidence of performance-related benefits during sprinting14 and jumping14,21 activities. Consequently, dynamic stretching is used more frequently in preparation for high-level sport activities, despite the inconsistent findings as to its superiority over static stretching for increasing range of motion.13,14,21–24
Another option is proprioceptive neuromuscular facilitation (PNF), which is a technique that involves cyclic contraction and stretching of the targeted muscle,25 resulting in autogenic inhibition (ie, relaxation)26 and increased compliance of the muscle. Various PNF techniques exist and are implemented based on clinician and patient goals. Previous investigations have consistently demonstrated improvements in range of motion following PNF, regardless of the number of sessions or duration of treatment.27–30 Additionally, when comparing the outcomes to static stretching, several studies have reported that PNF results in larger increases in range of motion compared to static stetching.27,28,30 However, when looking specifically at the hamstrings, a recent study31 identified three studies that evaluated the efficacy of static stretching versus PNF and concluded that, although both techniques can elicit statistically significant increases in range of motion, no conclusive evidence exists suggesting that one technique is superior to the other when examining short- or long-term effects.
Another technique that is common to the athletic population is the use of a foam roller, which is thought to heat the muscle and fascia via friction that is created by the rolling action, with concurrent tissue deformation (ie, stretching) resulting from the pressure exerted by the patient's body weight.3 Also commonly considered a “myofascial release technique” despite our lack of understanding of the exact mechanism by which it may work, it is theorized that foam rolling breaks up fibrous adhesions in the fascia, which enables the muscle to glide more freely, thus improving range of motion.4 The evidence for the efficacy of this modality to immediately increase range of motion is equivocal.3,32 However, one study showed that a 4-week (three times per week) foam rolling protocol induced a small (3 cm) improvement in stand-and-reach test performance. Despite not having a clear understanding of the effects, foam rolling is replacing stretching techniques in many settings without knowledge of whether fascial restrictions underlie the muscle's ability to lengthen, hence restricting full joint motion.
Although each technique appears to be useful in some way, the technique used depends on the clinician, setting, and patient/athlete's preference. These factors have resulted in inconsistency in the methodology of previous studies, such as time (eg, acute vs prolonged effects) and outcome measured (eg, knee extension angle vs sit- or stand-and-reach). Additionally, it is still unclear which technique is superior for increasing flexibility. Most studies have only compared one technique (PNF, dynamic warm-up, or foam rolling) to static stretching because the latter is the most traditional stretching technique. Therefore, we do not have an understanding of the efficacy of these techniques compared to each other. A search of the literature has failed to identify any study that compares multiple techniques. This knowledge is important because it could offer direction in developing a more efficacious plan for patient care by helping to determine which technique will best fit the patient's abilities and rehabilitation or performance goals. Additionally, in settings where clinicians may have a large patient load, understanding which technique is most effective and that a patient can self-administer will save clinician time while also achieving the desired outcome.
Therefore, the purpose of this study was to (1) determine the acute effects of three self-administered techniques (dynamic warm-up, PNF, and foam rolling) commonly used in the clinical sports medicine setting on hamstring tightness as measured by the peak knee extension angle; and (2) compare the relative effectiveness of the interventions. We hypothesized that the peak knee extension angle would increase after the dynamic warm-up and PNF interventions, but not after the foam rolling intervention.
College students who participated in physical activity at least 3 days per week for at least 30 minutes per session and had hamstring tightness/inflexibility were recruited for this study. Because we were investigating the effect of an intervention for hamstring tightness, we included participants who had at least a 20° deficit in peak knee extension angle when the hip was flexed to 90°. Participants were excluded if they had a neuromuscular disorder affecting the lower extremity, hamstring or any other pain that would contraindicate stretching, or had undergone lower extremity or back surgery in the previous 12 months. After determining qualification for participation, informed consent was obtained from all participants according to California State University, Northridge, Institutional Review Board protocol. Participants were then familiarized to all study procedures. Following familiarization, the study included four visits separated by at least 3 days. During each visit, each participant completed one of the stretching protocols (dynamic warm-up, PNF, foam rolling, or no intervention [control condition]) in a randomly assigned counterbalanced order. Participants were instructed to refrain from any stretching, weight-training, or arduous physical activity the day before and during testing days.
At the beginning of each session, we assessed the knee extension angle at baseline. The participant laid supine on a standard treatment table (Figure 1). A standard goniometer was used to position the participant's dominant (ie, kicking) limb in 90° of hip flexion33,34 and then a vertical wooden board was positioned against the anterior thigh and clamped to the table to indicate proper positioning and serve as the 0° vertical reference.
Experimental set-up for measuring passive knee extension angle with a digital inclinometer.
A digital inclinometer (Acumar; Lafayette Instrument Co., Lafayette, IN) was placed on the mid-point of the anterior tibial crest, which was determined by using measuring tape to measure the participant's tibia length from the medial malleolus to the medial joint line of the knee. With the hip positioned in 90° of flexion, the researcher passively extended the knee until the participant reported discomfort in his or her hamstring (Figure 1). Peak knee extension position (in reference to the position of the anterior thigh) was determined with the inclinometer. During pilot testing, excellent day-to-day reliability (intraclass correlation coefficient2,1 [standard error of measurement] = 0.94 [3.4°]) was established.
After recording the knee extension angle at baseline, all participants completed a 5-minute jog at a self-selected pace, followed by their assigned treatment for that day (dynamic warm-up, PNF, foam rolling, or control).
Participants performed a series of dynamic exercises35 (Table 1) that first required hamstring contraction and then stretching. Sixteen repetitions of each exercise were performed over an 18-meter distance, followed by a return jog to the starting position before the next exercise in the series. The entire protocol took approximately 8 minutes.
8-Minute Dynamic Flexibility Warm-up Protocol
Proprioceptive Neuromuscular Facilitation
While sitting on a treatment table with the knee of the treated leg extended and the uninvolved foot on the ground, participants performed 8 minutes of the hold–relax PNF technique.26 We chose an 8-minute period to be consistent with the dynamic warm-up intervention and reduce the potential effect of duration and repetition on knee extension angle. While keeping the knee and foot pointed toward the ceiling to avoid hip rotation, participants first leaned their trunk down toward the table (trunk + hip flexion) until their hamstrings were stretched to the point of discomfort. They sustained the stretching position for 5 seconds. They were then instructed to push their heel into the table (to contract the hamstrings) for 5 seconds using maximum effort before leaning their trunk further toward the table and holding the new position for 5 seconds. A 10-second break was permitted every three repetitions. We devised these procedures in the interest of investigating a protocol that could be easily self-administered (ie, easy to track time). With the 8-minute protocol, we theorized that the shorter hold time with this protocol could be balanced with more repetitions, which is in accordance with a previous study36 that evaluated the interchangeability of the number of repetitions with stretching time.
Participants performed 8 minutes of foam rolling over the entire length of the hamstring, from the ischial tuberosity to the posterior knee. Once again, we chose an 8-minute period for consistency in the duration of the intervention. The participants rolled out their hamstrings at a rate of three to four times per minute,3 with 10 seconds of rest after each minute. The researcher ensured that the participants were rolling at the appropriate pace by observing a stopwatch and also provided feedback if participants were off pace.
Participants completed a 5-minute jog at a self-selected pace, then sat in a chair for 8 minutes. The 8-minute period was chosen to ensure a time interval between the measurements at baseline and following intervention that was equivalent to the other interventions.
Measurements After Intervention
Immediately following each intervention, three consecutive measurements of the participants' knee extension angle were acquired to determine the effect of the intervention. A single investigator (LD) completed all measurements. To reduce potential bias, the investigator recorded the knee extension angle measurements at baseline and following intervention on separate data collection sheets. The investigator did not review the participants' knee extension angle measurements from the previous days on which they completed the other interventions until they completed the study.
Data Reduction and Analysis
The peak knee extension angle achieved of the three trials was used for analysis. The change score was calculated and used as the dependent variable for analysis. To determine whether the interventions resulted in a significant increase in knee extension angle, a 2 (time) × 4 (intervention) repeated measures analysis of variance (ANOVA) test was used. In the case of a significant omnibus test, post hoc one-way ANOVA and paired t tests with Bonferroni corrections were analyzed. Finally, to determine whether any of the interventions elicited larger increases in the knee extension angle compared to the others, a one-way ANOVA compared the change scores in knee extension angle between interventions.
Thirty-five participants (female: 12; male: 23; height: 1.74 ± 0.9 m, mass: 72.1 ± 13.8 kg, age: 24.6 ± 2.5 years) completed each of the interventions. The right leg was dominant for all participants (ie, participants indicated that they preferred to kick a ball with their right foot). Descriptive statistics for the knee extension angle at baseline and following each intervention and the knee extension angle change and effect size are displayed in Table 2.
Descriptive Values for Passive Knee Extension Angle at Baseline and Following Each Intervention
When comparing knee extension angle measurements at baseline and following the different interventions, the overall ANOVA test revealed significant main effects for time. The knee extension angle following intervention was greater than at baseline (F1,136 = 420.6; P < .001) for all interventions: dynamic warm-up (t = 12.3; P < .001), PNF (t = 13.3; P < .001), and foam rolling (t = 8.9; P < .001) (Table 2). We also noted a significant increase in knee extension angle during the control condition (t = 5.2; P < .001). There was no main effect for intervention (F1,136 = 2.44; P = .07). There was a significant time × intervention interaction (F3,136 = 26.2; P < .001). Post hoc testing revealed that there were no differences in knee extension angle measurements at baseline between interventions (F1,136 = 0.36; P = .78). However, the absolute knee extension angle was larger following the dynamic warm-up and PNF interventions compared to the control condition (P = .02 and P < .001, respectively). Additionally, the knee extension angle following the PNF intervention was larger than that during the foam rolling intervention (P = .04).
To determine whether any of the interventions resulted in a larger increase in knee extension angle compared to the others, a one-way ANOVA compared the change in knee extension angle between interventions. The overall test was significant (F3,136 = 26.1; P < .001), indicating a difference in the increase in range of motion between one or more of the interventions. The post hoc pairwise comparisons (Figure 2) revealed that each of the interventions resulted in a larger increase in knee extension angle compared to the control condition (all P < .001). The PNF intervention resulted in a greater improvement in knee extension angle when compared to the foam rolling intervention (11.1° vs 7.1°, respectively; P = .001) than when compared to the dynamic warm-up intervention (11.1° vs 9.1°, respectively; P = .31). There was no difference in improvement between the dynamic warm-up and foam rolling interventions (9.1° vs 7.1°, respectively; P = .31).
Changes in the peak knee extension angle following each intervention. All interventions (dynamic warm-up [DYN], proprioceptive neuromuscular facilitation [PNF], and foam rolling [FR]) resulted in a significant increase in knee extension angle, revealing a significant increase compared to the control group (no intervention; CON). The PNF intervention resulted in a significant increase compared to the foam rolling intervention. †Increase > control (P < .001); ‡Increase > foam rolling (P = .001)
Previous studies have demonstrated the efficacy of individual modalities aimed at acutely increasing range of motion; however, we were unable to find any that compared across interventions to determine which self-administered method was most effective. Our primary finding was that each of the self-administered techniques elicited significant increases in knee extension angle, ranging from 7° to 11°, on average. Additionally, the PNF intervention resulted in a larger increase in knee extension angle compared to the foam rolling intervention. However, because no minimal clinically important difference has been identified for knee range of motion, we are not able to conclude whether our statistically significant findings translate to practical increases in function.
We observed a 9.1° increase in knee extension angle following our dynamic flexibility warm-up, which is similar to another study that used a similar warm-up protocol24 and larger than those that used shorter protocols that focused on the entire lower extremity.14,21 The current protocol included a 5-minute general warm-up jog, followed by an 8-minute dynamic stretching intervention focused on the hamstrings (rather than distributing the treatment time among many muscle groups). Previous studies have indicated that focused stretching of a muscle group should be applied for at least 4 minutes to see a significant effect2,37; the fact that our dynamic protocol included 8 minutes of focused warm-up and stretching exercises on the hamstrings could explain the larger effect we observed in the current study.
Dynamic warm-ups are commonly used in preparation for high-demand athletic activities. The increases in range of motion are thought to be achieved by creating an exercise-induced increase in blood flow and local tissue temperature, which results in greater muscle and tendon extensibility. However, similar to PNF, dynamic warm-ups also include a neural aspect because some of the exercises may incorporate the reciprocal inhibition mechanism. For example, the participants performed maximal dynamic straight leg raises (hip flexion) that required quadriceps contraction and concomitant hamstring relaxation and lengthening to achieve maximum hip flexion. Therefore, dynamic warm-ups may be the most “functional” because they also require coordinated movement between limbs and muscle groups while weight-bearing. Hence, it is likely that the neuromuscular adaptations over time may result in larger increases in range of motion as the coordination between agonist and antagonist improves, allowing for greater joint motion.
Proprioceptive Neuromuscular Facilitation
Previous studies that have investigated PNF consistently report improvements in range of motion regardless of the number of sessions or duration of treatment.27–30 In the current study, the PNF hold–relax technique yielded the largest increase in range of motion, although it was only statistically superior to the foam rolling intervention and not to the dynamic warm-up intervention. PNF is thought to effectively increase the knee extension angle via two underlying mechanisms: mechanical and neural. In the hold–relax technique, hamstring relaxation is elicited via autogenic inhibition by actively extending the knee against resistance (in this case, from the treatment table). The ham-strings are then mechanically stretched. In the current study, we observed an 11.1° increase in the peak knee extension angle, which is nearly identical to a previous study.16 However, it is important to note that this was the result of just one treatment. One study reported a 19.2° increase after 6 weeks of 4-minute treatments,29 indicating that there is potential for greater improvements in range of motion with additional treatments.
On average, we observed a 7° increase in range of motion following the foam rolling intervention, which is consistent with a previous study that reported a 6.5° increase in knee extension angle after rolling the hamstrings with a foam-covered hollow plastic roller two times for a duration of 1 minute each.38 Accordingly, another study reported a 6.9° increase in hip flexion range of motion after foam rolling the hamstrings three times for a duration of 1 minute each.39 Because our protocol consisted of foam rolling the hamstrings eight times for a duration of 1 minute each, it appears that there may be a ceiling effect of time, after which there is no improvement. Therefore, our 8-minute protocol may have been excessively long; however, we thought it important to ensure that differences in the effects between interventions were not simply due to differences in treatment duration. Although the foam rolling intervention in the current study may have been longer than what is traditionally seen in some clinical settings, there is evidence that treatments lasting less than 2 minutes may be ineffective.32 Therefore, more studies are needed to determine the optimal treatment duration.
Foam rolling is thought to release myofascial adhesions that may restrict the muscle from lengthening and allowing full joint motion.4 Thus, immediate improvements in range of motion following foam rolling may be attributed to a release of myofascial restrictions. However, in the current study and a previous study,39 participants were not evaluated for myofascial restrictions because a clinical technique that can quantify myofascial restrictions does not currently exist. Instead, a diagnosis via treatment approach is typically used whereby increases in range of motion following supposed myofascial techniques are thought to confirm the presence of myofascial restrictions.
Although we cannot evaluate the presence of myofascial restrictions via the musculoskeletal evaluation process, patients with myofascial adhesions should theoretically show an increase in range of motion after foam rolling, whereas those without myofascial adhesions would not. This supposition may be supported by the large increases in range of motion reported during a combination of foam rolling and static stretching protocols.39 As an exploratory analysis, we inspected the current dataset to determine whether we could identify an independent effect of the foam rolling intervention above that observed from the other two interventions (dynamic warm-up and PNF) that arguably work via similar mechanisms. Eleven participants appeared to respond better with the foam rolling intervention compared to either the dynamic warm-up or PNF interventions. However, only 3 participants responded better to both the dynamic warm-up and PNF interventions. This could imply that foam rolling works by a different mechanism and that, although dynamic warm-ups and PNF work most of the time, there may be an additional benefit to be gained from foam rolling, although it is not clear exactly through which mechanism. It has been proposed that the pressure from an individual's body mass that is exerted on the hamstrings is sufficient to stimulate the Golgi tendon unit and decrease muscle tension.4 If that is the case, foam rolling has the potential to work via a neural mechanism as well, although it is typically thought of as a mechanical treatment.
We observed a statistically significant increase in knee extension angle with the control condition. However, it is doubtful that this 2.4° increase in range of motion would translate into a clinically relevant increase in function. The increase was most likely the result of the 5-minute warm-up jog22 performed after the baseline measurements during each intervention and is indicative of the increased tissue extensibility that resulted from tissue warming. It is interesting to note that we measured the increase in range of motion after the participant had sat for 8 minutes. It is likely that the increase may have been even larger had we measured range of motion immediately after the 5 minutes of jogging. In that case, it is plausible that the increases in range of motion were largely due to tissue warming, not the actual interventions. This finding gives further credence to the common understanding that tissue warming should precede techniques aimed at elongating tissue.
Comparing the Interventions
When addressing a clinical issue, practitioners typically have several tools from which to choose. Thus, determining the relative efficacy of comparable tools makes up a large proportion of our clinical questions. In the current study, we observed statistically significant increases in the peak knee extension angle after each of the interventions (range: 7.1° to 11.1°). We also observed a statistically significant effect of the PNF intervention compared to the foam rolling intervention. However, when comparing the magnitude of difference between those two interventions, it is difficult to conclude whether the 4° difference would translate into a clinically meaningful difference. One previous study40 hypothesized that 5° was the minimal clinically important difference, meaning that a 5° improvement in range of motion would be detectable by patients during activities of daily living. However, they were unable to directly relate a 5° change in knee extension range of motion to a difference in function.40 Future studies are needed to determine how degrees of motion translate into function in activities of daily living and athletic performance.
The current study had several limitations. We chose common interventions for hamstring tightness, indicated by restricted knee extension angle. Although we typically assess this as “hamstring tightness,” we were unable to determine the underlying cause for the hamstring tightness. Therefore, it is likely that the efficacy of the interventions was different across our participants, depending on the root source of their range of motion restrictions. We also acknowledge that there was the potential for bias in the researcher's measurements because she was not blinded to the treatment that the participants completed. However, we attempted to mitigate that by blinding her to the knee extension angle measurements at baseline, as well as those following each of the previous interventions.
Implications for Clinical Practice
We found each of the interventions investigated to be useful for increasing the passive peak knee extension angle following a 5-minute general warm-up jog. We chose each of these interventions because they could be self-administered, thus increasing athlete/patient self-sufficiency and clinician time, which is a known barrier in the athletic training practice. Because the interventions ranged from being fairly passive (eg, foam rolling) to very active (eg, dynamic warm-up) with the desired goal of increasing range of motion, our results indicate that the clinician has reasonable choices for approaching the issue in consideration of other limitations that the athlete/patient may have (eg, no or limited weight-bearing ability during the early phases of postoperative rehabilitation). Also, if the clinician was able to determine the origin of the patient's range of motion dysfunction (eg, myofascial restriction vs muscle extensibility) and choose the intervention that best addresses the issue, it is likely that the clinician may see a larger effect than we did in the current study. Future studies should investigate evaluation techniques to determine the specific pathology underlying a range of motion restriction and then examine the effect of the intervention that most specifically addresses that pathology. Additionally, studies are needed to examine the relationship between range of motion, patient-reported function, and biomechanics to help clinicians determine whether the small differences in the expected effect of each intervention will likely translate to an appreciable improvement in a patient's function.
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- MacDonald GZ, Penney MD, Mullaley ME, et al. An acute bout of self-myofascial release increases range of motion without a subsequent decrease in muscle activation or force. J Strength Cond Res. 2013;27:812–821. doi:10.1519/JSC.0b013e31825c2bc1 [CrossRef]
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8-Minute Dynamic Flexibility Warm-up Protocola
|Butt kicks||Bend knee and drive heel to buttock while keeping knee pointed toward the ground. Alternate between legs.|
|Lunges||With chest in upright position, take a step forward, lower body by flexing knee and hip of lead leg to 90°, using the lead foot to support body weight and the back leg for balance. Lead foot remains flat on ground, and when the 90° hip and knee flexion is attained, push off of lead foot to bring body into standing position. Alternate between legs.|
|Walking high knee pull||With chest in upright position, step forward into a single-leg balance. Bring knee toward chest with foot dorsiflexed, grab knee with both hands, and pull knee as close to chest as possible. Alternate between legs.|
|Walking hamstrings||Keeping back flat, step forward and extend knee fully while dorsiflexing foot maximally. Bend waist forward and reach for toes. Alternate between legs.|
|Walking toe-touch||Step forward, place foot flat on the ground, extend knee fully, bend forward at waist, and touch ground while simultaneously lifting opposite leg behind body and extending hip. Alternate between legs.|
|High knees||With chest in upright position, maximally flex knee and hip to bring knee as high as possible, quickly alternating between legs and moving in a forward direction. Opposite arms coordinate movement of legs.|
|Russian walk||With chest in upright position, maximally flex hip by bringing knee to chest, then fully extend knee with foot dorsiflexed. Alternate between legs.|
|Flexed leg drive||With chest in upright position, drive knee and leg maximally upward into hip flexion with knee slightly flexed, and swing opposite arm to coordinate the movement. Alternate between legs.|
|Extended leg drive||With chest in upright position, drive foot upward while keeping knee extended, and reach hands forward and upward toward foot. Alternate between legs.|
|Graduated sprints||Sprint at 50% of maximum speed for 10 meters, at 75% of maximum speed for 15 meters, and at 100% of maximum speed for 20 meters.|
Descriptive Values for Passive Knee Extension Angle at Baseline and Following Each Interventiona
|Intervention||N||Passive Knee Extension Angle at Baseline||Passive Knee Extension Angle Following Intervention||Change||Effect Size|
|Dynamic warm-up||35||59.8° ± 7.8°||68.9° ± 8.7° b,c||9.1° ± 4.4°||1.05|
|Proprioceptive neuromuscular facilitation||35||60.7° ± 8.2°||71.7° ± 9.1° b,c,d||11.1° ± 4.9°||1.21|
|Foam rolling||35||58.6° ± 8.6°||65.7° ± 8.9° b||7.1° ± 4.7°||0.80|
|No intervention (control)||35||60.0° ± 8.8°||62.4° ± 9.8° b||2.4° ± 2.8°||0.24|