Muscle Architecture PDF

MUSCLE ARCHITECTURE Dr Bart Bolsterlee Benninghoff-Goertler (1964). Lehrbuch der Anatomie des Menschen, 9th edition, Urban & Schwarzenberg, Berlin Outline Part 1  Recap: force generation in sarcomeres  Muscle architecture and muscle function Part 2  Measurement of...

MUSCLE ARCHITECTURE Dr Bart Bolsterlee Benninghoff-Goertler (1964). Lehrbuch der Anatomie des Menschen, 9th edition, Urban & Schwarzenberg, Berlin Outline Part 1  Recap: force generation in sarcomeres  Muscle architecture and muscle function Part 2  Measurement of muscle architecture  The structure of a sarcomere A sarcomere is the basic unit of striated muscle tissue. It is the repeating unit between two Z lines. A longitudinal section of a sarcomere Titin Myofibril Z line Thin filament Thick filament How sarcomere length affects muscle tension Sarcomeres produce tension most efficiently within an optimal range of lengths. When A decrease in the resting resting sarcomere length is sarcomere length reduces within this range, the maximum tension because stimulated number of cross-bridges can sarcomeres cannot shorten form, producing the greatest very much before the thin tension. filaments extend across the An increase in sarcomere length center of the sarcomere and reduces the tension produced by collide with or overlap the thin reducing the size of the zone of overlap filaments of the opposite side. and the number of potential cross-bridge interactions. Tension (percent of maximum) Tension production When the zone of overlap is reduced to falls to zero when zero, thin and thick filaments cannot the thick filaments interact at all. The muscle fiber cannot are jammed against Normal produce any active tension, and a the Z lines and the range contraction cannot occur. Such sarcomere cannot extreme stretching of a muscle fiber is shorten further. normally prevented by titin filaments (which tie the thick filaments to the Z lines) and by the surrounding Decreased length Increased sarcomere length connective tissues.  Definition of muscle architecture Muscle architecture is the physical arrangement of muscle fibres (or fascicles) relative to the muscle’s force-generating axis. In other words, muscle architecture describes how sarcomeres are arranged. De Motu Musculari (1670; The Wellcome Institute for the History of Medicine, London) Hierarchical structure of skeletal muscle Level Order of magnitude Whole muscle centimetre Muscle fascicles millimetre Muscle fibres μm × 100 Myofibrils μm Myofilaments nm Lieber (2010) Lippincott Williams & Wilkins Classification of muscle architecture Muscles are often classified according to their architectural design. ML = muscle length FL = fascicle length Parallel-fibred Pennate Multi-pennate Lieber (2010) Lippincott Williams & Wilkins Classification of muscle architecture Architectural type Fascicle orientation Examples relative to force- generating axis of the muscle Parallel Parallel Biceps brachii, biceps femoris Pennate One angle Gastrocnemius, vastus lateralis Bipennate Two angles Tibialis anterior, rectus femoris Multi-pennate Multiple angles Gluteus medius, deltoid Muscle architecture is typically quantified by:  Fascicle length  Pennation angle  Physiological cross-sectional area (PCSA) Fascicle length Optimal fascicle length determines the range of lengths and velocities the muscle can produce force over. Short fascicles  small range optimal fascicle length = 10 cm sarcomere/muscle fibre optimal fascicle length = 5 cm Long fascicles  large range Fascicle length Optimal fascicle length determines the range of lengths and velocities the muscle can produce force over. optimal fascicle length = 10 cm optimal fascicle length = 5 cm Pennation angle  Pennation angle is the angle that muscle fascicles make with the axis of force generation.  In pennate muscles, muscle fibres attach to aponeuroses (tendinous sheets). Physiological cross-sectional area  PCSA is the sum of the cross-sectional areas of all fibres within a muscle.  PCSA is directly proportional to the maximum force the muscle can produce. Physiological cross-sectional area PCSA determines the maximum force a muscle can produce. Physiological cross-sectional area  PCSA = muscle volume / optimal fascicle length  In parallel-fibres muscles, PCSA is equal to the cross-sectional area (CSA; measured perpendicular to the muscle’s line of action).  In pennate muscles, physiological CSA does not equal CSA. Line of action Haxton HA (1944). J Physiol 103 p267-273. Muscle architecture and muscle function In-series sarcomere arrangement Long fascicles, small PCSA Large range, low force Parallel sarcomere arrangement Short fascicles, large PCSA Small range, high force Lieber (2010) Lippincott Williams & Wilkins Muscle architecture and muscle function  Stacking sarcomeres in series results in muscles with long fibres and a large range of operating lengths and velocities. ─ These muscles are sometimes called the “prime movers”.  Stacking sarcomeres in parallel results in muscles with a large capacity to generate force, but over a relatively small range. ─ These muscles are sometimes called “stabilising” muscles. PCSA and fibre length for muscles that cross the ankle High force, small range Small force, large range Ward et al. Clin Orthop Relat Res 2009 (437) p1074-1082 The vastus lateralis is about a third heavier than the soleus, but can produce about a third less force! Ward et al. Clin Orthop Relat Res 2009 (437) p1074-1082 Summary (part 1)  Muscle architecture is the physical arrangement of muscle fibres relative to the muscle’s force-generating axis.  Muscle architecture determines: ─ how much force a muscle can produce ─ the range of lengths and velocities the muscle can produce force over  Next: how is muscle architecture measured? Measurement of muscle architecture  Cadaver measurements  B-mode ultrasound  Magnetic resonance imaging (incl. diffusion tensor imaging)  Cadaver measurements  Muscle architecture can be measured directly using rulers and goniometers.  More advanced techniques involve dissection and digitisation to make 3D reconstructions. Ward et al. (2009) Clin Orth Rel Res Lee et al. (2015) CMBBE)  Cadaver measurements  Cadaver measurements are often made on muscles of elderly people, who may suffer from sarcopenia.  Cadaver measurements are, therefore, presumably not representative of in vivo muscle architecture. Ward et al. (2009) Clin Orth Rel Res Lee et al. (2015) CMBBE) B-mode ultrasound  Human muscles can be imaged in vivo using ultrasound. B-mode ultrasound Diong et al. (2012) Muscle & Nerve B-mode ultrasound  Ultrasound provides two-dimensional images of muscles. ─ But most human muscles have complex three- dimensional structures.  Human muscles are typically larger than ultrasound images, so ultrasound provides an incomplete picture.  Ultrasound is used extensively to study how muscle architecture changes during dynamic tasks (e.g. walking and running) or over time (e.g. ageing or training). MRI and diffusion tensor imaging  Magnetic resonance imaging can be used to image whole human muscles in 3D.  On conventional MRI scans, muscle fibres cannot be discerned. MRI and diffusion tensor imaging  DTI can be used to make 3D reconstructions of the architecture of human muscles in vivo. Thank you for your attention! Further reading Chapter 3 of: Lieber, Richard L. Skeletal muscle structure, function, and plasticity. Lippincott Williams & Wilkins, 2010.