Effects of repeated ankle stretching on calf muscle-tendon and ankle biomechanical properties in stroke survivors

Abstract

Background: The objective of this study was to investigate changes in active and passive biomechanical properties of the calf muscle-tendon unit induced by controlled ankle stretching in stroke survivors.

Methods: Ten stroke survivors with ankle spasticity/contracture and ten healthy control subjects received intervention of 60-min ankle stretching. Joint biomechanical properties including resistance torque, stiffness and index of hysteresis were evaluated pre- and post-intervention. Achilles tendon length was measured using ultrasonography. The force output of the triceps surae muscles was characterized via the torque-angle relationship, by stimulating the calf muscles at a controlled intensity across different ankle positions.

Findings: Compared to healthy controls, the ankle position corresponding to the peak torque of the stroke survivors was shifted towards plantar flexion (P<0.001). Stroke survivors showed significantly higher resistance torques and joint stiffness (P<0.05), and these higher resistances were reduced significantly after the stretching intervention, especially in dorsiflexion (P=0.013). Stretching significantly improved the force output of the impaired calf muscles in stroke survivors under matched stimulations (P<0.05). Ankle range of motion was also increased by stretching (P<0.001).

Interpretation: At the joint level, repeated stretching loosened the ankle joint with increased passive joint range of motion and decreased joint stiffness. At the muscle-tendon level, repeated stretching improved calf muscle force output, which might be associated with decreased muscle fascicle stiffness, increased fascicle length and shortening of the Achilles tendon. The study provided evidence of improvement in muscle tendon properties through stretching intervention.

Copyright © 2010 Elsevier Ltd. All rights reserved.

Figures

Figure 1

Experimental setup.

Figure 2

Measurement of the Achilles tendon length using ultrasonography.

Figure 3

A. Typical data from a stretching trial (only the first 100s are shown). B. Torque-angle relation (hysteresis loop) of two short stretching sessions (3rd [in blue] and 12th session [in red] for the same subject). The passive ROM, slope of the loading phase of the hysteresis loop curve (quasi-static stiffness), joint viscosity (the area enclosed by the hysteresis loop) were derived.

Figure 4

Torque-angle relation of the Control and Stroke groups with the torque values normalized to the corresponding peak values.

Figure 5

Normalized torque-angle relationship of stroke survivors (pre-intervention v.s. postintervention). Significant differences were highlighted by the shaded area (p

Figure 6

Ankle dorsiflexion position at specified resistance torques (A) and joint passive stiffness (B) at the specified ankle positions and index of hysteresis pre- and post-intervention (C) as well as Achilles tendon length (D) for stroke survivors pre- and post-intervention (group average with standard deviation; * P