Dorsiflexion, Plantar-Flexion, and Neutral Ankle Positions During Passive Resistance Assessments of the Posterior Hip and Thigh Muscles

Passive musculotendinous resistive properties are commonly assessed via the application of a dynamic stretch.^1–4 For the hamstrings specifically, the use of a straight-legged–raise (SLR) movement to assess passive resistive properties, such as passive torque and range-of-motion (ROM) measurements, may be important for determining athletic^5,6 and health⁷ status and predicting lower body injuries.⁸ Tafazzoli and Lamontagne⁷ showed that passive torque values of the hamstrings measured during an SLR effectively discriminated between individuals with and without low back pain. Furthermore, Witvrouw et al⁸ reported that the maximal ROM of the hamstrings achieved during an SLR could be used to identify athletes who were at risk for developing hamstrings muscle injuries.

Passive musculotendinous properties contributing to the resistance to stretch traditionally have been attributed to several structural factors, including the stretching of the stable cross-links between the actin and myosin filaments, direct resistance from the actin and myosin filaments, stretching of the noncontractile proteins of the endosarcomeric and exosarcomeric cytoskeletons, and deformation of the noncontractile connective tissues located within and surrounding the muscle belly.² Eliciting the stretch reflex may also be a factor that contributes to increases in torque measured during passive stretch.^9–11 Specifically, Lamontagne et al¹⁰ indicated that stretch-reflex excitation depends on velocity; therefore, passive SLR assessments performed at higher stretch velocities may elicit greater stretch-reflex excitation, causing increased activation of the stretched muscles and possible contamination of the passive torque measurements with both active force production and passive tension. The need for a device to control for stretch velocity has been addressed by several authors^3,11–14 who have used isokinetic dynamometers when performing SLR testing. Whereas researchers have attempted to control for the potential confounding effects of stretch velocity, few investigators have examined the importance of controlling for ankle position during the SLR. For example, SLR assessments have been performed with the ankle fixed in dorsiflexion (DF),¹⁵ plantar flexion (PF),^16–18 or neutral (NTRL).^3,4,7,11,12 However, it is unclear whether ankle position contributes to differences in the passive resistance to stretch. If ankle position influences the passive resistance to stretch as determined by the SLR, then ankle position may need to be standardized when performing SLR testing to reduce discrepancies (and possibly help explain contrasting findings) across studies and provide more consistency to enhance physiologic comparative analyses.

The incorporation of sensitizing maneuvers, such as ankle DF or cervical flexion, into a passive SLR assessment has been shown to increase tension on the neural structures of the proximal thigh without increasing hamstrings stretch.^15,19,20 Given these findings, authors^21,22 have suggested that greater neural tension created by sensitizing maneuvers may play a role in the passive resistance produced during hamstrings stretch. McHugh et al²³ recently provided evidence to support this hypothesis by demonstrating in vivo that adding cervical flexion to a seated passive knee-extension assessment increased resistance to hamstrings stretch. In addition, Hall et al²⁴ reported that passive resistance during manually applied passive SLR movements was greater with the ankle positioned in DF than NTRL. However, they did not examine the effect of PF on resistance to stretch, nor did they perform a statistical analysis. Moreover, manually applied passive movements do not yield consistent acceleration and deceleration from trial to trial¹⁰; therefore, potential differences in stretch velocity may have been a confounding factor in the previous study.²⁴ The use of an isokinetic dynamometer to perform SLR testing at a constant movement velocity provides reliable measurements of passive torque and ROM that are not confounded by velocity-related differences.¹¹ To our knowledge, no investigators have made direct statistical comparisons of the differences in ROM and passive torque measurements involving SLRs performed at a slow, constant movement velocity using an isokinetic dynamometer with the ankle positioned in DF, PF, and NTRL. Given the importance and prevalence of the SLR as a tool to assess passive musculotendinous resistive properties along with the relationships among these passive properties, health status, and sport-related injuries, further research is warranted to examine the potential effects of placing the ankle in several positions across ankle-joint ROM on passive torque and ROM measurements during stretching of the posterior muscles of the hip and thigh. Therefore, the purpose of our study was to examine the influence of ankle positions (DF, PF, and NTRL) on the passive torque, ROM, and hamstrings electromyographic (EMG) characteristics measured during passive isokinetic SLR assessments of the posterior hip and thigh muscles.

METHODS

Assessment of Passive Torque and Range of Motion

Passive torque and ROM of the posterior hip and thigh muscles were examined with an iSLR technique, which consisted of using a calibrated Biodex System 3 isokinetic dynamometer (Biodex Medical Systems Inc, Shirley, NY) programmed in passive mode to move the limb toward the head at 5°/s (Figure 1). For each iSLR assessment, participants lay supine with the knee braced in full extension and the ankle immobilized in 10° of DF, 10° of PF, or NTRL (0°) with an adjustable, custom-made cast that was fixed around the foot and held with straps placed above the ankle and over the toes and metatarsals. During the iSLRs, the input axis of the dynamometer was aligned slightly superior and anterior to the greater trochanter of the femur to account for movement of the greater trochanter during the iSLR, and restraining straps were placed over the left (unstretched) thigh and ankle. All iSLR assessments were performed on the right limb to the point of discomfort but not pain as indicated by the participant; this measurement was regarded as the maximal ROM, and the limb was returned immediately to the baseline position.

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Signal Processing

During each iSLR assessment, torque (Nm), joint-angle position (°), and EMG (μV) signals were sampled simultaneously at 1 kHz (model MP100WSW; BIOPAC Systems, Inc), stored on a personal computer (model Inspiron 8200; Dell Inc, Round Rock, TX), and processed offline using custom-written LabVIEW software (version 11.0; National Instruments, Austin, TX). Torque and position signals were low-pass filtered with a zero-phase lag, fourth-order Butterworth filter that had a cutoff of 10 Hz. The EMG signal was scaled and bandpass filtered with a zero-phase lag, fourth-order Butterworth filter from 20 to 400 Hz. All subsequent analyses were conducted on the scaled and filtered signals.

For passive torque, gravity correction was performed during each iSLR using a cosine function in which the limb mass was subtracted from the torque signal across the ROM. The scaled and gravity-corrected torque and joint-angle signals were plotted as passive angle-torque curves and fitted with a fourth-order polynomial regression model based on the equation reported by Nordez et al.²⁹ Passive torque and electromyography were quantified at 4 common joint angles (θ) separated by 5° during the final common 15° of ROM (at 0°, 5°, 10°, and 15°) for each participant. Consequently, the same absolute joint angles could be used for each participant to calculate passive torque and electromyography for each iSLR assessment (Figure 2).

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RESULTS

Maximal ROM was lower for the DF condition than the NTRL (P = .003) and PF (P < .001) conditions (Figure 3). For passive torque, we found a condition × joint-angle interaction (F_6,72 = 4.149, P = .04; Figure 4). Passive torque was greater for the DF condition than the NTRL (P = .008) and PF (P = .03) conditions at θ₃ and was greater for the DF than the NTRL condition at θ₄ (P = .02). However, no differences in passive torque were observed between the DF and PF conditions at θ₄ (P = .07) or among the DF, PF, and NTRL conditions at θ₁ (P range, .27–.99) and θ₂ (P range, .12–.99). Passive torque also increased with joint angle (θ₁ < θ₂ < θ₃ < θ₄) for the DF (F_3,36 = 37.404, P < .001), PF (F_3,36 = 35.665, P < .001), and NTRL (F_3,36 = 33.208, P < .001) conditions. For EMG amplitude, we found no condition × joint-angle interaction (F_6,72 = 0.903, P = .44), no main effect for condition (F_2,24 = 0.157, P = .86), and no main effect for joint angle (F_3,36 = 0.096, P = .87). The EMG amplitude values were 0.70%, 0.64%, and 0.74% of MVIC for the DF, PF, and NTRL conditions, respectively.

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DISCUSSION

Passive torque was greater at θ₃ and θ₄ and maximal ROM was lower for the DF condition than the NTRL and PF conditions despite a lack of differences among conditions for EMG amplitude (Figures 3 and 4). Researchers have reported similar findings regarding the influence of ankle position on ROM^21,25,30,31 and passive torque²⁴ during SLR testing. For example, Hall et al²⁴ noted that passive torque during SLR testing was greater with the ankle positioned in DF than in NTRL. However, in contrast to our findings, they found that the increases in resistance to stretch were accompanied by increases in EMG activity of the hamstrings. The authors hypothesized that because EMG activity was higher for the DF than the NTRL condition, the greater passive torque induced by ankle DF was due to an elicitation of the stretch reflex.²⁴ Alternatively, the EMG-amplitude values in our study were not different among the DF, PF, and NTRL conditions and remained relatively low (ie, <1% of MVIC) across the ROM, suggesting that an elicitation of the stretch reflex likely did not contribute to the condition-related differences observed in resistance to stretch. The discrepancies in EMG activity between our findings and those reported by Hall et al²⁴ may be due to differences in the techniques used to assess passive resistance. We assessed passive resistance using an isokinetic iSLR technique, whereas Hall et al²⁴ used a manual technique, which consisted of the primary investigator applying resistance against a load cell positioned immediately posterior to the heel while the limb was moved toward the head. Consequently, differences in stretch velocities may contribute to these discrepancies,¹¹ as the SLRs in our study were performed at a slow, constant velocity (ie, 5°/s), which may not have been the case in the previous study²⁴ as stretch velocities were not reported. Investigators have provided support for velocity discrepancies during manual application, indicating that manually applied passive movements are often performed at high stretch velocities (ie, >5°/s),^9–11 and given that higher stretch velocities may evoke greater stretch-reflex excitation,¹¹ manual techniques possibly have a greater potential for eliciting the stretch reflex and, consequently, higher EMG activity than iSLR techniques.

The lack of an observable contractile response to stretch for all conditions, which was supported by the EMG inactivity, suggests the absence of a detectible stretch reflex. Therefore, other factors may be responsible for the differences observed in passive torque and ROM among the DF, PF, and NTRL conditions. In support of this hypothesis, Gajdosik et al²¹ demonstrated smaller hamstrings ROM values during SLR assessments with the ankle positioned in DF than in PF; however, because EMG amplitude remained low for both the DF and PF conditions, they suggested that the lower ROM in the DF condition was not due to an elicitation of the stretch reflex but rather to an increase in passive tension on the sciatic nerve. Researchers^{19,23–25,32,33} have indicated that the sciatic nerve and other peripheral nerves are parts of a large, continuous neural tissue tract in which increases in tension and movement in 1 part created by maneuvers, such as ankle DF or cervical flexion, may create tension and movement of the neural structures in other parts. Using animal models, Boyd et al¹⁹ showed that adding ankle DF to SLR testing increased tension and movement in the sciatic nerve of the proximal thigh. Furthermore, McHugh et al²³ recently demonstrated with in vivo human musculotendinous models that adding cervical flexion to a seated passive knee-extension assessment caused increased passive resistance to hamstrings stretch without any changes in EMG activity. They hypothesized that, in the absence of EMG activity, the greater resistance to stretch for the cervical-flexion condition was due to an increase in passive tension in the neural tissues.²³ Thus, the possibility of increases in neural tension may explain why we observed greater passive torque and lower ROM values for the DF condition. Moreover, our finding of greater passive torque for the DF condition within the final common 5° of ROM (ie, at θ₃ and θ₄) further supports the notion that the increased passive resistance to stretch with the ankle in DF may be a consequence of tension within the neural tissues, which would be subjected to increased tension at greater joint angles as previous authors^34–36 have suggested. Alternatively, however, variations among fascicle connections of the lower body musculature may also help to explain the passive torque and ROM differences that we observed. For example, investigators^21,25,31,37 have suggested that fascicle connections at the popliteal region between the hamstrings and gastrocnemius muscles may allow DF to increase the amount of passive tension on the hamstrings and, thereby, influence the resistance to stretch measured during passive SLR assessments. However, given the scope of our study and the limited data available regarding these findings, it was not feasible to ascertain the underlying mechanisms resulting in the higher passive torque and lower ROM values for the DF condition than the PF and NTRL conditions. Thus, future research involving more invasive measures (ie, ultrasound imaging) is necessary to elucidate more specifically the mechanisms that may be responsible for influencing the passive torque and ROM differences displayed among iSLRs with various ankle positions.

CONCLUSIONS

Our findings align with those of other authors indicating greater passive torque²⁴ and lower ROM^21,25,30,31 values during SLRs with the ankle positioned in DF than in PF and NTRL. Some authors^24,25 have hypothesized that the increase in passive resistance to stretch for the DF condition may be due to an elicitation of the stretch reflex; however, given that the EMG amplitude values in our study remained low for the DF, PF, and NTRL conditions, we conclude that other factors may be contributing to the condition-related differences observed in passive torque and ROM. One factor may be an increase in the amount of neural tension on the sciatic nerve due to the DF condition²¹; however, we are aware of no studies in which the authors have shown this to definitively contribute to the greater resistance to stretch measured during an SLR assessment. Nevertheless, our findings further support the influence of ankle position on resistance to stretch and highlight the potential importance of ankle position during SLR testing while elucidating the possibility that tension in the neural tissues may contribute to the passive torque measured during posterior hip- and thigh-muscle stretching. Researchers^3,11–14 have indicated that the use of an isokinetic dynamometer during an SLR assessment provides reliable and quantitative measurements of passive torque and ROM of the posterior muscles of the hip and thigh by controlling for the velocity of stretch. However, considering the potential influence of ankle DF on these variables, our findings highlight the importance of standardizing ankle position by fixing the ankle in either PF or NTRL when conducting these types of passive assessments to avoid the confounding effects on resistance to stretch observed in the DF ankle position. Furthermore, athletic trainers, physical therapists, and other practitioners may use these findings and perhaps exercise caution when interpreting passive-stiffness data from SLRs because these types of tests may be influenced by ankle position, which could adversely affect the capacity of the SLR as a diagnostic tool to identify and assess individuals with low back pain and other sport-related injuries.