Voluntary force production

Under normal conditions, the intent to voluntarily generate force involves the sequence of tightly regulated events originating in the brain and ending with the activation of skeletal muscle and, usually, movement around a joint. This sequence of events will, depending on the external load or task, result in the muscle remaining the same length (isometric), shortening (concentric) or lengthening (eccentric) under tension.

2.2.1.1 Neural control of force

Briefly, voluntary force production originates in the cerebral cortex, basal ganglia and cerebellum. Activity in these regions may be altered by input from a number of sensory fibres including group I and II afferents, which sense changes in muscle length and tension, and group III and IV afferents which sense pain. Convergent input from the pre - motor cortex, basal ganglia and cerebellum leads to excitation, or inhibition, of neurons in the primary motor cortex. Output from the primary motor cortex brings about depolarisation of motor neurons in the pyramidal tract, located in the spinal cord, resulting in excitation of anterior (α) - motor neurons. Each muscle may be innervated by a motor neuron pool made up of a large number of α - motor neurons; for example the gastrocnemius is controlled by approximately 580 α- motor neurons. Depolarisation of the axonal membrane, through opening of voltage gated Na+ channels, allows for action potential propagation along the axon to the terminal branches. Branching of the axon may result in the innervation of up to 1000, or more, muscle fibres depending on the particular muscle. Together the α- motor neuron and the fibres it innervates make up a motor unit (MU) (Grabowski and Tortora 2003; Komi 2003; McArdle et al. 2009).

The magnitude of force generated during voluntary muscular contraction is dependent on the number of MUs recruited and the firing frequency (rate coding) of these active MUs. Recruitment occurs in an orderly manner with small, low-threshold MUs recruited first and as greater force, or faster muscular action, is required more MUs are recruited until the large, high-threshold MUs are active (Henneman et al. 1965; Milner-Brown et al. 1973b). Additional force is produced by an increase in the firing frequency of active MUs (Milner-Brown et al. 1973a) however the contribution of MU recruitment and firing frequency as strategies to increase force development differs between muscles (De Luca et al. 1982; Kukulka and Clamann 1981).

2.2.1.2 EC coupling

The arrival of an action potential at the presynaptic membrane opens voltage – gated Ca2+ channels resulting in an increase in [Ca2+]i. This in turn leads to exocytosis of

synaptic vesicles which deposit ACh into the synaptic cleft. ACh diffuses across the synaptic cleft and binds with ACh receptors which open allowing for Na+ influx and

K+ efflux, thus depolarising the end plate. Any remaining ACh is either taken up by the presynaptic membrane or broken down to acetate and choline by acetylcholinesterase. The end plate potential propagates across the sarcolemma into the T-tubule system. Here, the activation of dihydropyridine receptors leads to activation and opening of ryanodine receptors which release Ca2+ from the SR into the cytoplasm (Latash 2008; McArdle et al. 2009).

2.2.1.3 Cross-bridge cycle

Beginning at the end of a “power stroke”, binding between actin and myosin

(actomyosin) is broken when adenosine triphosphate (ATP) binds to the nucleotide

“pocket” of the myosin subdomain (S1). The increase in [Ca2+]i, brought about by

depolarisation of the end-plate, activates the troponin complex of the actin filament. The binding of Ca2+ to troponin C brings about a conformational change allowing S1 access, albeit weak, to its binding site on the actin molecule. Further conformational changes on the actin filament result in movement of the tropomyosin coil so that S1 has direct access to the binding site. The hydrolysis of ATP to adenosine diphospahte (ADP) and inorganic phosphate (Pi) by the myosin ATPase brings about a swing in

the lever arm (cocking) of approximately 60ᵒ which allows S1 to come into close proximity with the binding sites on the actin filament. Binding at this stage remains weak until the release of Pi results in tight binding and the formation of a force -

generating state. The release of ADP returns S1 back to its original position, however because it is strongly bound to the actin filament, this action results in movement of the actin filament towards the centre of the sarcomere. This step is typically referred

to as the “power stroke”. Binding of ATP to the now empty S1 “pocket” releases the binding between myosin and actin so that the cycle may continue. Cross-bridge formation and cycling will continue unabated as long as [Ca2+]i is high enough to

inhibit the troponin-tropomyosin system that would otherwise block the actin - myosin binding sites. Withdrawal of CNS stimulation leads to a decrease in [Ca2+]i as

Ca2+ release from the SR is halted and the action of SR Ca2+ ATPase returns Ca2+ back into the SR from the cytosol (Billeter and Hoppeler 2003; Cooke 1997; McArdle et al. 2009).

The rate at which ATP is utilised in type IIB fibres is approximately four times that of type I fibres (Stienen et al. 1996). This is due to the fibre type specific characteristics

of myosin and SR Ca2+ ATPases which have greater rates of ATP hydrolysis in type IIX and IIB fibres compared to those of type I and IIA fibres (Stienen et al. 1996; Szentesi et al. 2001). These specific characteristics, along with increased rates of Ca2+ release from the SR in type II fibres (Li et al. 2002), are responsible for the greater rates of shortening, and therefore power production (Figure 2), and specific tension (kN m-2) found in the fast fibre types (Bárány 1967; Bottinelli et al. 1996).

Figure 2 Velocity – power curve for isolated human type I (slow), IIA and IIB fibres

at 12˚ C (Bottinelli et al. 1996).