Selective activation of quadriceps muscle fibers according to bicycling rate

Abstract

Moving average electromyography (MA) of quadriceps muscle bellies has been recorded during bicycling at different rates (30-70 cycles/min) or forces (1-3 kg). For power increments (50-100%) achieved by increasing force at constant rate, MA during pedal downstroke always increased. For similar power increments achieved by increasing the rate at constant force, MA did not increase (37% of cases), increased less (37%), or increased similarly (26%). Investigations by others on the rat suggest that the lack of increase of MA despite power increment was not compensated by other muscle activity; hence it indicates a shift from slow to fast fibers, which provide greater power per unit stimulus. Smaller increase of MA with increasing rate rather than force at isopower could depend on this shift or on muscle properties, if operating on ascending limb of power-velocity curve. This, however, does not seem the case for slow fibers, which should develop peak power at about 25 cycles/min. Hence, fibers of quadriceps muscle of humans seem selectively activated according to movement speed, as previously found in inspiratory muscles of rabbits.

Early deactivation of slower muscle fibres at high movement frequencies

“…Preferential recruitment of faster muscle fibres during rapid contractions has been shown in various animal studies, including humans (Gillespie et al, 1974;Gollnick et al, 1974;Grimby and Hannerz, 1977;Hoffer et al, 1981;Citterio and Agostoni, 1984;Wakeling et al, 2006;Hodson-Tole and Wakeling, 2008a;Lee et al, 2013). It has been suggested that complete inhibition of slow muscle fibres in a muscle with mixed fibre types would provide resistance to whole muscle shortening and therefore be detrimental to power production for very fast contractions (Josephson and Edman, 1988).…”

“…However, the size principle of motor unit recruitment (Henneman et al, 1965a;Henneman et al, 1965b) dictates that slower fibres, with longer relaxation times, are deactivated last. While the size principle adequately describes many recruitment strategies, alternative strategies may also exist (Gillespie et al, 1974;Gollnick et al, 1974;Grimby and Hannerz, 1977;Hoffer et al, 1981;Citterio and Agostoni, 1984;Wakeling et al, 2006;Hodson-Tole and Wakeling, 2008a) when the demands exceed the abilities of the slowest muscle fibres.…”

Comparison of human gastrocnemius forces predicted by Hill-type muscle models and estimated from ultrasound images

“…This is the first study to test Hill-type models against in vivo estimates of time-varying muscle forces from human subjects at a range of mechanical conditions. Electromyographic (EMG) recordings from the gastrocnemii of cyclists have shown that the activation patterns of different muscle fibre types vary with cadence (Citterio and Agostoni, 1984;Wakeling et al, 2006;Wakeling and Horn, 2009). However, while muscles are composed of a mix of slow and fast fibres with different physiological and biomechanical properties, Hill-type models are typically composed of a single contractile element with force-length and force-velocity properties that have been estimated from in vitro data collected from single fibres under maximally activated conditions.…”

“…Both models were driven by subject-specific measures of fascicle length, velocity and pennation angle derived from ultrasound images and by activation patterns of slow and fast fibres derived from surface EMG recordings and waveletbased time-frequency decomposition techniques (von Tscharner, 2000;Wakeling and Horn, 2009). We hypothesized that the twoelement model would better reproduce the ultrasound-based estimates of gastrocnemius force at the higher cadences, where preferential recruitment of fast fibres has been reported (Citterio and Agostoni, 1984;Wakeling et al, 2006). This is an extension of our previous studies on the gastrocnemius muscles in goats, where we showed that a two-element model, driven by the independent activation of slow and fast muscle fibres, provided better predictions of time-varying muscle forces than traditional one-element models for some in situ (Wakeling et al, 2012) and in vivo conditions.…”

Recruitment of faster motor units is associated with greater rates of fascicle strain and rapid changes in muscle force during locomotion

“…Different motor unit types have muscle fibers with different electrical membrane properties (Luff and Atwood, 1972), and there is an intrinsic speed dependence of the EMG frequency such that faster fibers generate higher-frequency signals (Gerdle et al, 2000;Hodson-Tole and Wakeling, 2008b;Lee et al, 2011). For example, when human subjects cycle at increased cadences, EMG signals from the gastrocnemii shift to higher frequencies (Wakeling et al, 2006), consistent with the recruitment of faster motor units (Citterio and Agostoni, 1984).…”