Plantarflexor muscle function in healthy and chronic Achilles tendon pain subjects evaluated by the use of EMG and PET imaging Tahir Masood a, ⁎, Kari Kalliokoski b , Jens Bojsen-Møller c,d,e , S. Peter Magnusson d,e , Taija Finni a a Neuromuscular Research Center, Department of Biology of Physical Activity, University of Jyväskylä, Finland b Turku PET Centre, University of Turku, Finland c Department of Physical Performance, Norwegian School of Sport Sciences, Oslo, Norway d Institute of Sports Medicine Copenhagen, Dept. Ortopaedic Surgery M, Bispebjerg Hospital, Denmark e Center for Healthy Aging, Faculty of Health and Medical Sciences, University of Copenhagen, Denmark abstract article info Article history: Received 14 October 2013 Accepted 11 March 2014 Keywords: Biomechanics Triceps surae Achilles tendon Glucose uptake Tendinopathy [ 18 F]-Fluorodeoxyglucose Background: Achilles tendon pathologies may alter the coordinative strategies of synergistic calf muscles. We hypothesized that both surface electromyography and positron emission tomography would reveal differences between symptomatic and asymptomatic legs in Achilles tendinopathy patients and between healthy controls. Methods: Eleven subjects with unilateral chronic Achilles tendon pain (28 years) and eleven matched controls (28 years) were studied for triceps surae and flexor hallucis longus muscle activity in response to repetitive iso- metric plantarflexion tasks performed at 30% of maximal voluntary contraction using surface electromyography and glucose uptake using positron emission tomography. Additionally, Achilles tendon glucose uptake was quantified. Findings: Normalized myoelectric activity of soleus was higher (P b 0.05) in the symptomatic leg versus the con- tralateral and control legs despite lower absolute force level maintained (P b 0.005). Electromyography ampli- tude of flexor hallucis longus was also greater on the symptomatic side compared to the healthy leg (P b 0.05). Both the symptomatic and asymptomatic legs tended to have higher glucose uptake compared to the control legs (overall effect size: 0.9 and 1.3, respectively). Achilles tendon glucose uptake was greater in both legs of the patient group (P b 0.05) compared to controls. Maximal plantarflexion force was ~14% greater in the healthier leg compared to the injured leg in the patient group. Interpretations: While the electromyography showed greater relative amplitude in the symptomatic leg, the results based on muscle glucose uptake suggested relatively similar behavior of both legs in the patient group. Higher glucose uptake in the symptomatic Achilles tendon suggests a higher metabolic demand. © 2014 Elsevier Ltd. All rights reserved. 1. Introduction The Achilles tendon (AT) is among the strongest tendons in the human body (Józsa and Kannus, 1997; Kvist, 1994). Despite its strength, the AT is susceptible to overuse injuries which, accompanied by tendon pain, impair the function of the calf muscle–tendon unit (Silbernagel et al., 2006). Furthermore, it may affect the muscle activation strategies of the individual compartments of the triceps surae (TS) muscle group, and other ankle plantarflexors (Mafi et al., 2001; Roos et al., 2004). Previous studies have established that the relative contribution within different compartments of the TS muscle group is inhomogeneous. Specifically, significant differences in the mediolateral forces within the Achilles tendon have been reported depending on how TS components were loaded (Arndt et al., 1999). The force contribution of various TS components also depends on muscle length since small changes in gastrocnemius length result in major changes in soleus and gastrocne- mius electromyography (EMG), torque, and force (Arndt et al., 1998; Cresswell et al., 1995). Past in-vivo studies have demonstrated considerable individual var- iation in the use of different plantarflexors in healthy individuals, displaying either low or high flexor hallucis longus (FHL) activation with corresponding tissue movement (Bojsen-Møller et al., 2010; Finni et al., 2003). Finni et al. (2006) showed that patients recovering from complete Achilles tendon rupture increased the contribution of FHL to motor tasks in their healthy and affected legs during rehabilita- tion. These observations lead the authors to hypothesize that this coor- dination strategy may contribute to Achilles tendon injuries since the FHL is an important deep plantarflexor muscle (Klein et al., 1996). For this reason, and because the FHL is suitable for both PET and SEMG, Clinical Biomechanics xxx (2014) xxx–xxx ⁎ Corresponding author at: Neuromuscular Research Center (NMRC), Department of Biology of Physical Activity, Faculty of Sport and Health Sciences, University of Jyväskylä, P.O. Box 35 (VIV), 40014, Finland. E-mail addresses: tahir.masood@jyu.fi (T. Masood), Kari.Kalliokoski@tyks.fi (K. Kalliokoski), jens.bojsen-moller@nih.no (J. Bojsen-Møller), spmagnusson@gmail.com (S. Peter Magnusson), taija.finni@jyu.fi (T. Finni). JCLB-03761; No of Pages 7 http://dx.doi.org/10.1016/j.clinbiomech.2014.03.003 0268-0033/© 2014 Elsevier Ltd. All rights reserved. Contents lists available at ScienceDirect Clinical Biomechanics journal homepage: www.elsevier.com/locate/clinbiomech Please cite this article as: Masood, T., et al., Plantarflexor muscle function in healthy and chronic Achilles tendon pain subjects evaluated by the use of EMG and PET imaging, Clin. Biomech. (2014), http://dx.doi.org/10.1016/j.clinbiomech.2014.03.003