Isokinetics and Exercise Science 14 (2006) 25–31 25 IOS Press Passive stiffness of the plantar flexion in relation to age and functional performance Iris I. Caroway a , Katharina S. Sunnerhagen b , Christine E. Kasper c and Ulla Svantesson d,∗ a Winston Salem State University, School of Health Sciences, Winston-Salem, NC 11111, USA b G¨ oteborg University, Institute of Clinical Neurosciences, Rehabilitation Medicine, G ¨ oteborg, Sweden c Uniformed Services University of the Health Sciences, Graduate School of Nursing, Bethesda, MD 20814, USA d G¨ oteborg University, Department of Occupational Therapy and Physiotherapy, Department of Orthopaedics, G¨ oteborg, Sweden Abstract. Passive stiffness (Nm/ ◦ ) of the plantar flexors was measured during a passive stretch into dorsiflexion (DF) in 18 healthy female subjects (range 18–60 years old) using an isokinetic dynamometer. Passive stiffness of the ankle joint was measured at a short range of motion (from 10 ◦ of plantar flexion (PF) to 10 ◦ of DF), and at a full range of motion (from 10 ◦ PF to maximal DF for each individual) with a neutral EMG from a subset of four legs. The purpose of the study was to assess the variances in the measurements of passive stiffness of the calf muscle and to correlate passive stiffness to age and jumping ability. There was a significant correlation (r = 0.76) between the FRS and FRS test-retest. The methodological error for duplicate determinations was 7.7% for the SRS and 5.5% for the FRS. There was a significant correlation between age and the (FRS) full-range stiffness (r = 0.83). There was a significant negative correlation between age and jumping (r = -0.54). We conclude that the test model of passive stiffness used in the present study is a useful method with good reliability. The passive elastic properties of muscles and tendons might explain part of the muscle performance in functional tests with ageing. However, other qualities may influence performance as well. Keywords: Passive stiffness, muscle and tendon stiffness, plantar flexor muscle, stretch-shortening cycle, torque output 1. Introduction Passive stiffness may be defined as the length-tension relationship of muscle and tendon when it is passively stretched [15,17]. However, the maximal range of mo- tion of a limb has previously been used as an index of musculoskeletal flexibility [25,39,40]. Previous re- searcher suggest that passive tension in muscle was at- tributed to the extensibility of the connective tissue ele- ments in parallel with the muscle fibres, i.e. the parallel elastic component [30,28,35,36,43]. It has also been ∗ Address for correspondence: Ulla Svantesson, RPT, PhD, Faculty of Health Caring Sciences, Department of Occupational Therapy and Physiotherapy, G¨ oteborg University, Box 455, SE 40530 G¨ oteborg, Sweden. Tel.: +46 31 7735742; E-mail: ulla. svantesson@fhs.gu.se. suggested that passive stiffness could be related to the extensibility of the tendon and aponeurosis [34,44]. The combination of eccentric and concentric mus- cle actions forms a natural type of muscle function called the stretch-shortening cycle or SSC [6,7,29,31, 45]. The ability to accumulate and use elastic energy is a capacity of the compliance or stiffness of the tendons and muscles [22]. It was found that SSC performance was notably affected according to stiffness in tendon structures [32]. It has also been demonstrated that the elasticity of tendinous structures associated with the knee extensors influenced sprinting and jumping [32, 33]. Furthermore, past studies have shown that running economy was affected by passive flexibility measure- ments [8,21,23,34]. Therefore, muscle performance during the stretch-shortening cycle exercise could be influenced by passive stiffness as well as by the elas- ticity of tendinous structures. ISSN 0959-3020/06/$17.00 © 2006 – IOS Press and the authors. All rights reserved