Journal of Biomechanics 32 (1999) 71—79 Acute eﬀects of intramuscular aponeurotomy on rat gastrocnemius medialis: Force transmission, muscle force and sarcomere length R.T. Jaspers, R. Brunner, J.J.M. Pel, P.A. Huijing  *  Instituut voor Fundamentele en Klinische Bewegingswetenschappen, Faculteit Bewegingswetenschappen, Vrije Universiteit, Amsterdam, The Netherlands  Peadiatric Orthopaedic Department, Children’s Hospital, University of Basel, Switzerland  Integrated Biomedical Engineering for Restoration of Human Function, Biomedisch Technologisch Instituut, Universiteit Twente, Enschede, The Netherlands Received in ﬁnal form 14 October 1998 Abstract Acute eﬀects of intramuscular aponeurotomy on muscle force and geometry as a function to muscle length were studied in rat m. gastrocnemius medialis (GM). Acutely after aponeurotomy, activation of the muscle at increasing lengths (acute trajectory) showed a spontaneous and progressive but partial tearing of the connective tissue interface between the ﬁbres inserting directly proximally and distally to the location of the section. After this the muscle consisted morphologically of a stable proximal and a distal part (post-aponeurotomy). Post-aponeurotomy mean active sarcomere length within ﬁbres of the proximal part was shown to be unaﬀected. In contrast, mean sarcomere length within the distal part was reduced substantially after aponeurotomy. However active sarcomeres in the distal part were still attaining higher lengths with increasing muscle lengths (p(0.005), indicating myofascial force transmission through the intact part of the connective tissue interface of the muscle parts. Post-aponeurotomy optimum muscle force was reduced substantially to less than 45% of pre-aponeurotomy values. During the acute trajectory the muscle yielded approxim- ately 20% higher forces than post-aponeurotomy, indicating that myofascial force transmission was related to the area of connective tissue interface. It is concluded that after aponeurotomy of the proximal aponeurosis of rat GM, ﬁbres without direct myotendinous connection to the origin of the muscle are still able to contribute to muscle force. As the magnitude of reduction in muscle force can only be explained partially by the spontaneous rupture of the connective tissue interface between proximal and distal muscle part, other factors causing a decrease of muscle force are present. Clinical implication of acute eﬀects of intramuscular aponeurotomy are discussed.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Intramuscular aponeurotomy; Myofascial force transmission; Sarcomere length; m. Gastrocnemius medialis; Rat 1. Introduction In orthopaedics several techniques are used to lengthen overly short muscles. One such surgical inter- vention is intramuscular aponeurotomy (Baumann and Koch, 1989; Majestro et al., 1971; Scherb, 1935). Depend- ing on the type of muscle and the clinical diagnosis the aponeurosis is transsected perpendicular to its length at one or more locations. Even though such operations are clinically successful in restoring more normal patterns of gait in spastic children (Barnes and Herring, 1991; * Correspondence address: Peter A. Huijing. Faculteit Bewegin- gswetenschappen, Vrije Universiteit, van de Boechorststraat 9, 1081 BT Amsterdam, Netherlands. Tel.: # 31 20 4448476; fax: # 31 20 4448529; e-mail: p — a — j — b — m — huijing @fbw.vu.nl Nather et al., 1984; Nene et al., 1993; Reimers, 1990; Ruda and Frost, 1971), relatively little is known about the mechanisms active and their eﬀects on muscle mechanics. Due to transverse sectioning of an intramuscular apo- neurosis, part of the muscle ﬁbres loose their direct myotendinous connection to either muscular origin or insertion. However, the intramuscular network of con- nective tissue, consisting of endomysium and per- imysium, stays intact partially. This means that the ﬁbres without direct myotendinous connection still have paral- lel connections via the intramuscular connective tissue to the tendon. On the basis of experiments using small fascicles it is known that ﬁbres are able to transmit force laterally onto the network of endomysium which is shared with adjacent ﬁbres (Street, 1983). Furthermore, it has been shown that after tenotomy of heads of the in situ m. extensor digitorum longus (EDL), muscle force was 0021-9290/99/$ — see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 1 - 9 2 9 0 ( 9 8 ) 0 0 1 5 9 - 6