Relationship between force and size in human single muscle fibres

Abstract

When the contractile properties of single muscle fibres are studied, force is typically normalized by fibre cross-sectional area and expressed as specific force. We studied a set of 2725 chemically skinned human single muscle fibres from 119 healthy adults to determine whether specific force is the optimal way to express the relationship between single-fibre force and size. A linear mixed effects model was used to estimate the slope and slope variability among individuals of log-log plots of force and diameter. For type I fibres, the slope estimate was 0.99 (95% confidence interval 0.36-1.62), and for type IIa fibres it was 0.94 (95% confidence interval 0.77-1.11), indicating that force is proportional to fibre diameter, rather than to cross-sectional area. If force were proportional to cross-sectional area, the slope estimate would be 2.0. In future studies using the chemically skinned single fibre preparation, force may be normalized to fibre diameter rather than cross-sectional area. We propose that a new term, 'normalized force', be used for this variable, with units of newtons per metre. We demonstrate using our data set that when populations of single fibres are compared with one another, the determination of whether the size and force relationship is the same or different is dependent upon the method used to account for fibre size (i.e. specific force versus 'normalized force').

Lion (Panthera leo) and caracal (Caracal caracal) type IIx single muscle fibre force and power exceed that of trained humans

“…It is reasonable to presume that the great metabolic capacity for ATP supply of these athletic mammals is driven by a greater capacity to consume ATP as a result of a greater skeletal muscle fibre contractile capacity. The amount of absolute force a fibre can produce is directly proportional to its CSA (Woledge et al, 1985;Bottinelli, 2001;Krivickas et al, 2011). Thus, as the majority of morphological studies on the skeletal muscle of these wild animals have reported significantly smaller CSA of the fibres compared with that of humans, our hypothesis was that their fibres would produce less absolute force, but greater specific force.…”

“…The latter requires equipment that can additionally calculate the depth of a fibre, or measuring diameters in two planes, resulting in two values for diameter that could yield a more accurate measure of CSA. Recently, Krivickas and colleagues (Krivickas et al, 2011) proposed normalising force to fibre diameter, as a better relationship was found using this parameter compared with CSA, suggesting a new normalisation method that would allow for values obtained from different laboratories to be more easily compared. However, it requires a separate investigation to determine whether this would be true across species.…”

“…It is well known that the CSA plays an important role in the maximum capacity of single fibres to produce force; thus, the amount of force generated by a fibre is related to the CSA of the fibre, and not its length (Woledge et al, 1985;Bottinelli, 2001;Krivickas et al, 2011). The highest specific force in human muscle was achieved by the type IIax fibres (155±8kNm −2 ), whereas the lion and caracal type IIx fibres reached values of 191±8 and 211±22kNm −2 , respectively (Fig.4C).…”

Skeletal muscle myofilament adaptations to aging, disease, and disuse and their effects on whole muscle performance in older adult humans

“…Parenthetically, an additional embryonic isoform of myosin may be expressed in some physiological/pathophysiological conditions (D'Antona et al, 2003, 2006). A simple shift in the type of MHC isoform expressed in the fiber will produce different tissue level velocity and tension (force per CSA) characteristics, as faster contracting MHCs (I < IIA < IIX) produce more tension (D'Antona et al, 2003, 2006; Pansarasa et al, 2009; Krivickas et al, 2011). Thus, this myofilament variation can alter whole muscle performance (Thorstensson et al, 1977; Ryushi and Fukunaga, 1986; Harridge et al, 1996).…”

High Growth Dummerstorf Mice Have Reduced Specific Force of Slow and Fast Twitch Skeletal Muscle

“…It is often assumed that muscle mass reflects muscle force generating capacity (Jones, Bishop, Woods, & Green, 2008), but neurological factors may interfere with force production in humans as well (Degens, Erskine, & Morse, 2009). Moreover, specific force or muscle force per physiological cross sectional area (PCSA) tends to be higher in type 2 than type 1 muscle fibres (Bottinelli, Schiaffino, & Reggiani, 1991;Krivickas, Dorer, Ochala, & Frontera, 2011;Stienen, Kiers, Bottinelli, & Reggiani, 1996;Young, 1984). Little effort has been spent in search for other factors modulating specific force.…”