Rate of Force Development: Physiological and Methodological Considerations
Abstract
The evaluation of rate of force development (RFD) during rapid contractions has gained popularity for characterizing explosive strength in athlete. RFD is determined by the capacity for maximal voluntary activation in the early phase of an explosive contraction, improved by strength training, and difficult to evaluate reliably.
Introduction
Explosive strength, the ability to increase force quickly during a rapid contraction, is crucial in sports. RFD is increasingly used to characterize this strength because it relates better to performance and is sensitive to changes in neuromuscular function. The maximum level of force a muscle can produce is called the maximum voluntary contraction.
The rate of force development is a function of the time in which high levels of force are produced. MVC and RFD are related but distinct parameters. Each of them are influenced by motor unit (MU) recruitment and action potential discharge rate.
Physiological Considerations: The Underlying Mechanisms
Neural Determinants
Rapid contractions involve high MU discharge rates at the onset of contraction. which decline with successive discharges. Neural factors, such as MU discharge rate and recruitment, play a significant role in RFD, especially in the early phase of contraction. The recruitment threshold is lower for rapid contractions. It can be as low as 1/3 of maximal force.
Slow and fast muscle fibers respond differently to stimulus. Slower fibers display a ramp-like increase in force production, progressively activating until reaching an upper limit, whereas fast fibers exhibit a highly synchronized burst at the onset of activity.
Increased force production above the limit of MU recruitment is due to an increased discharge rate. Trained individuals can exhibit, during rapid contractions, a discharge rate 4-6 times faster than untrained.
While fiber type plays a role, it appears that RFD is initially influenced by neural factors more than intrinsic properties of the muscle, such as contraction speed. Neural factors such as discharge rate are also more impactful on fast-twitch fibers.
Motor Learning also appears to impact RFD. Training which was focused on motor learning improved RFD substantially. In other words, the neural determinants may include adaptations in the brain as well as to the spinal circuitry.
Muscular Determinants
Muscular factors like muscle fiber type composition, myofibrillar mechanisms, muscle size, and architecture also influence RFD. Type II fibers, which have faster contraction speeds and greater Ca2+ release per action potential, are critical for high RFD. Muscle architecture and musculotendinous stiffness also affect RFD.
One experiment showed that 5 weeks of sprint training increased the rate and magnitude of CA2+ release per action potential, leading to greater force production. This can also lead to faster propagation of the action potential throughout the cell.
The level of force produced (MVC) seems to be more associated with slightly delayed force production.
Adaptive Changes with Training
Strength training, particularly explosive-type training, enhances RFD by increasing muscle activation and altering neural and muscular properties. Training-induced increases in MU discharge rates and muscle size contribute significantly to improvements in RFD.
Strength training increases rate of force development. Endurance training does not. However, explosive or ballistic-style training (acceleration only) seemed to be more beneficial for RFD than traditional resistance training, mostly through increased discharge rates.
Importantly, training performed with maximal RFD intention, regardless of load, increased RFD and muscle activation significantly more than traditional strength training.
KEY TERMINOLOGY
Rate of Force Development (RFD): A measure of how quickly force or torque can be developed during a rapid contraction
Motor Unit (MU): A motor neuron and the muscle fibers it innervates, responsible for generating force.
Maximal Voluntary Contraction (MVC): The maximum force that a muscle or group of muscles can exert voluntarily.
Electromyogram (EMG): A technique for recording the electrical activity produced by skeletal muscles.
Myofibrillar Mechanisms: Processes involving the contractile proteins within muscle fibers that contribute to force production.
Musculotendinous Stiffness: The resistance of muscles and tendons to stretching, affecting the speed of force transmission.
Sarcoplasmic Reticulum: A network within muscle cells that stores and releases calcium ions to trigger muscle contraction.
Motor Evoked Potential: An electrical response in a muscle following direct stimulation of the motor cortex.
H-Reflex: A reflexive electrical activity in a muscle following stimulation of the sensory fibers.
Dihydropyridine Receptor: A voltage-sensitive receptor in the muscle cell membrane involved in the release of calcium from the sarcoplasmic reticulum.
Ryanodine Receptor: A calcium channel in the sarcoplasmic reticulum membrane, responsible for releasing calcium into the cytosol.
Post-Activation Potentiation: The increase in muscle contractile response following a preceding contraction.