Reliability of isokinetic tests of velocity- and contraction intensity-dependent plantar flexor mechanical properties

Abstract

"Flexibility" tests are traditionally performed voluntarily relaxed by rotating a joint slowly; however, functional activities are performed rapidly with voluntary/reflexive muscle activity. Here, we describe the reliabilities and differences in maximum ankle range of motion (ROM_max ) and plantar flexor mechanical properties at several velocities and levels of voluntary force from a new test protocol on a commercially available dynamometer. Fifteen participants had their ankle joint dorsiflexed at 5, 30, and 60° s^-1 in two conditions: voluntarily relaxed and while producing 40% and 60% of maximal eccentric torque. Commonly reported variables describing ROM_max and resistance to stretch were subsequently calculated from torque and angle data. Absolute (coefficient of variation (CV%) and typical error) and relative (ICC_2,1 ) reliabilities were determined across two testing days (≥72 h). ROM_max relative reliability was good in voluntarily relaxed tests at 30 and 60° s^-1 and moderate at 5° s^-1 , despite CVs ≤ 10% for all velocities. Tests performed with voluntary muscle activity were only reliable when performed at 5° s^-1 , and ROM_max reliability was moderate and CV ≤ 8%. For most variables, the rank order of participants differed between the slow-velocity, relaxed test, and those performed at faster speeds or with voluntary activation, indicating different information. A person's flexibility status during voluntarily relaxed fast or active stretches tended to differ from their status in the traditional voluntarily relaxed, slow-velocity test. Thus, "flexibility" tests should be completed under conditions of different stretch velocity and levels of muscle force production, and clinicians and researchers should consider the slightly larger between-day variability from slow-velocity voluntarily relaxed tests.

Keywords: flexibility; muscle stretching; passive and active stretching; range of motion; stiffness.

Figure 1

Experimental design. (A) Participants attended the laboratory on four occasions separated by ≥ 72 h. The first and second visits were devoted to familiarization of the test procedures, and the experimental protocols were performed on the third and fourth visits for reliability purposes. In the familiarization sessions, participants were fully familiarized with the maximum range of motion (ROM_max) passive tests at all joint rotation velocities (for Experiment 1) and with the active ROM_max tests (for Experiment 2). Before practicing tests for Experiment 2, participants performed two to three maximal voluntary eccentric plantar flexion contractions (Ecc‐MVC) to 90% of their maximum dorsiflexion joint angle achieved in the passive tests to determine the contraction intensity to be used during the active ROM_max tests in Experiment 2. (B) Experimental design, showing the timeline for the performance of maximal voluntary isometric contractions (MVICs) and passive (Experiment 1), performed at 5, 30 and 60° s⁻¹, and active (Experiment 2) ROM_max stretching tests that were performed at 5 and 30° s⁻¹. 5 min of passive rest separated Experiments 1 and 2. MAC condition: plantar flexor isometric contractions at increasing intensities to condition the musculo‐articular complex; ROM_80%: passive dorsiflexion at 5° s⁻¹ to 80% of ROM_max achieved in the low‐velocity stretching test; 60% MVIC: 5‐s sub‐maximal conditioning contraction at 60% of MVIC

Figure 2

Schematic representation of the procedures used to determine the target joint moment levels during active stretching tests, and the visual feedback provided, using data from one subject. The joint angle (from neutral) corresponding to 90% of the maximal range of motion (ROM_max) during the passive, 5° s⁻¹ joint rotation was calculated and then used during the maximal plantar flexion eccentric contractions at 5 and 30° s⁻¹ (Ecc‐MVC; Panel A). A second‐ or third‐order polynomial regression line was fitted to the best Ecc‐MVC moment‐angle data from contraction start to 95% of the final joint angle of Ecc‐MVC (blue line; Panel B). The resulting regression equation enabled estimation of joint moments at angles greater than those used in the Ecc‐MVC test (shaded area in Panel B) and calculation 40% and 60% of the maximal moment at each joint angle in order to provide target lines for subsequent eccentric contractions, that is, “active” stretches at 40% (Ecc‐40) and 60% (Ecc‐60) of Ecc‐MVC. The difference between the active moment and target contraction intensity should equal to zero in order to maintain the contraction intensity required, and this difference was visually provided in real time so that the participants were able to maintain and adjust the target plantar flexion moment within ± 5 Nm during the active stretch protocol (shaded area in Panel C1). Note that, for this participant, ±5 Nm was approximately 1%‐9% of the 60‐Ecc joint moment developed during the test, depending on the joint angle calculated. Panels C1 and C2 show the visual feedback (C1) provided to participants during the active maximum range of motion stretching trial as well as the actual joint moment‐angle curve (C2) during the tests (5° s⁻¹; 60% Ecc‐MVC). The vertical black dashed line represents the start of stretch, whereas the vertical red dashed lines represent joint positions at end of stretch (maximum range of motion except for panel B that final ROM was pre‐set)

Figure 3

Left Panel: Relationships between maximum joint range of motion (ROM_max) during 5° s⁻¹ passive and active stretches performed at 40‐ and 60‐Ecc from Sessions 1 and 2 (top and bottom, respectively). Overall, moderate‐to‐strong significant relationships were found between ROM_max in passive and active tests except for the 60‐Ecc active stretching tests performed in Session 2. Right Panel: The individual relative (to the cohort mean, Z scores) change in scores for ROM_max during passive and active stretches. The nonstandard changes in rank scores obtained at each stretching test suggest that different results can be drawn from these tests