The age related slow and fast contributions to the overall changes in tibialis anterior contractile features disclosed by maximal single twitch scan Claudio Orizio a, *, Marta Cogliati a , Luciano Bissolotti b , Bertrand Diemont a , Massimiliano Gobbo a , Jan Celichowski c a Department of Clinical and Experimental Sciences, University of Brescia, Viale Europa 11; 25123 Brescia, Italy b Rehabilitation Service, Fondazione Teresa Camplani-Casa di Cura Domus Salutis, Via Lazzaretto, 3, 25123 Brescia, Italy c Department of Neurobiology, University School of Physical Education, 27/39 Królowej Jadwigi St., 61-871 Poznan, Poland A R T I C L E I N F O Article history: Received 21 December 2015 Received in revised form 29 April 2016 Accepted 2 May 2016 Available online 3 May 2016 Keywords: Neuromuscular electrical stimulation Single twitch modelling Skeletal muscle Single twitch Ageing A B S T R A C T This work aimed to verify if maximal electrically evoked single twitch (STmax) scan discloses the relative functional weight of fast and slow small bundles of fibres (SBF) in determining the contractile features of tibialis anterior (TA) with ageing. SBFs were recruited by TA main motor point stimulation through 60 increasing levels of stimulation (LS): 20 stimuli at 2 Hz for each LS. The lowest and highest LS provided the least ST and STmax, respectively. The scanned STmax was decomposed into individual SBF STs. They were identified when twitches from adjacent LS were significantly different and then subtracted from each other. Nine young (Y) and eleven old (O) subjects were investigated. Contraction time (CT) and STarea/STpeak (A/PT) were calculated per each SBF ST. 143 and 155 SBF STs were obtained in Y and O, respectively. Y: CT and A/PT range: 45–105 ms and 67–183 mN s/mN, respectively. Literature data set TA fast fibres at 34% so, from the arrays of CT and A/PT, 65 ms and 100 mN s/mN were identified as the upper limit for SBF fast ST classification. O: no SBF ST could be classified as fast. Conclusions: STmax scan reveals age-related changes in the relative contribution of fast and slow SBFs to the overall muscle mechanics. ã 2016 Elsevier Ireland Ltd. All rights reserved. 1. Introduction The anatomical changes that take place during the ageing process in the skeletal muscle are well described by using muscular biopsy. Indeed, when contractile elements are considered, both changes in the relative proportion of slow and fast fibres (Ryall, Schertzer, & Lynch, 2008) and changes in the myosin heavy chain isoforms (D’Antona et al., 2003) have been described and may contribute to explain the reduction of the twitch amplitude and velocity (Ryall et al., 2008; Lochy nski, Kaczmarek, Krutki, & Celichowski, 2010). More generally the data obtained by muscular biopsy provide the ratio between the area occupied by the type II and type I fibres without identification of their functional contribution to the tension at the tendon. This goal could be achieved using the invasive method suggested by Buchthal and Schmalbruch (1970). It was designed to characterise the single twitch (ST) of the small bundles of fibres (SBF) in intact human muscle. According to the authors SBF can be considered as the muscle quantum that can be activated by stimulation and should be attributed to “more than one motor unit”. This is in line with the experimental evidence that the very and stable stimulation of the single motor unit in human intact muscles presents several difficulties even when the current injection is made at “a peripheral nerve at an accessible position” (McComas, 1998). The Buchthal and Schmalbruch (1970) invasive stimulation procedure was based on both intramuscular and intraneural needle electrical stimulation (at different sites of the muscle innervation zone and at different levels of stimulation amplitudes). The authors demonstrated that the relative number of slow or fast SBF contraction time (CT) range mirrored the bioptic histochemical results for the relative slow/fast twitch fibre area. Intramuscular or intraneural electrical stimulation are not the only stimulation procedures that can be used to evoke muscle Abbreviations: A/PT, ratio between the area and the peak of the evoked twitch; CT, contraction time; LS, level of stimulation; MU, motor unit; NMES, neuromuscu- lar electrical stimulation; PT, peak twitch; SBF, small bundle of fibres; ST, single twitch; STmax, single twitch obtained at Vmax; TA, muscle tibialis anterior; Vmin/ Vmax, stimulation amplitude eliciting the least/maximal detectable twitch. * Corresponding author. E-mail address: claudio.orizio@unibs.it (C. Orizio). http://dx.doi.org/10.1016/j.archger.2016.05.003 0167-4943/ã 2016 Elsevier Ireland Ltd. All rights reserved. Archives of Gerontology and Geriatrics 66 (2016) 1–6 Contents lists available at ScienceDirect Archives of Gerontology and Geriatrics journal home page: www.elsevier.com/locat e/archger