Brief Summary
Nicola Maffiuletti, Per Aagaard, Anthony Blazevich, Jonathan Folland, Neale Tillin, Jacques Duchateau
https://link.springer.com/content/pdf/10.1007/s00421-016-3346-6.pdf
In this month's research summary, we are taking a closer look at the Rate of Force Development (RFD) and the physiological factors that impact how quickly an athlete can develop neuromuscular tension or force. Maffiuletti et al provide a narrative review that outlines key physiological and methodological considerations for accurately assessing and interpreting RFD in research and clinical settings. Part 1 of this review will explore the physiological considerations outlined by the authors.
In the clinical setting, explosive strength is generally seen as the rapid increase of force or torque during a quick voluntary contraction from a low or resting level. Rate of force development (RFD) data are derived from force or torque-time curves during brisk or rapid voluntary contractions and commonly used to assess an athletes' explosive strength. The authors state that compared to maximal voluntary contraction (MVC) strength, RFD is better related to most sport-specific and functional tasks and more sensitive to acute and chronic neuromuscular changes.
In short, both isometric MVIC and RFD testing are useful. During RFD testing, the athlete's intention, patterns of neuromuscular recruitment and physiological mechanisms are different and can potentially provide useful information as to athletic profile, recovery, tissue function and injury risk. However, accurate and time efficient quantification, understanding and interpretation of RFD is crucial for practitioners in elite sport operating in a Multi Disciplinary Team (MDT).
Physiological Considerations
Motor Unit Recruitment and Discharge Rate:
In this section, the authors review previously published literature to suggest that the magnitude of muscle activation and force production is dependent on the number of motor units (MUs) recruited and the discharge rate of motor neuron action potentials. Contraction speed has been found to influence both MU recruitment and discharge rates. Slow contractions involve a progressive activation of MUs up to approximately 80–90% of maximum force. In contrast, during rapid actions, MUs are recruited at much lower force levels; for example, the majority of MUs in the tibialis anterior are recruited at merely one-third of maximum force during ballistic contractions. Unlike slow contractions, where the discharge rate of MUs increases progressively, rapid contractions are characterized by an initially high discharge rate at the onset of activation, which declines progressively with successive discharges. These findings highlight the differences in MU activation patterns related to contraction speed.
Association between muscle activation and RFD
Studies have examined the influence of muscle activation on RFD using EMG. The findings indicate that the ability to produce force rapidly is more dependent on the increase in muscle activation at the onset of contraction rather than on the muscle's speed-related properties. The authors further elaborate that agonist EMG activity significantly contributes to the explained variance in force throughout the entire 150 ms period of rising muscle force, particularly during the initial phase (25–75 ms). In contrast, the evoked RFD, assessed from octet (tetanic) contractions—representing intrinsic muscle contractile properties—was the primary determinant of the steeper phase of voluntary RFD (50–100 ms).
Muscle fibre type composition
In this section, the authors discuss the common conception that fibre type is often considered a major factor influencing muscular RFD based on the observation that the rate of tension development is faster in type II than in type I muscle fibres. Although this may be the case, there is a large variability in fibre-type composition between various skeletal muscles. For example, vastus lateralis, vastus medialis and vastus intermedius contain ~50 % type I fibres, whilst soleus contains 75 % and gastrocnemius 45–75 % type I fibres. Furthermore, very large inter-individual differences in human skeletal muscle fibre type composition also exist. Both factors have been found to correlate with RFD. In a study by Harridge et al. (1996), RFD elicited by 50-Hz electrical stimulation increased in the order plantar flexors < knee extensors < elbow extensors, which was consistent with the increase in type II myosin heavy chain percentages of the soleus (30 %) < vastus lateralis (53 %) < triceps brachii (67 %). Hvid et al. (2010) reported a significant correlation between vastus lateralis type II fibre area and knee extensor RFD measured to 50 ms in young men. These studies indicate that fibre type composition may be an important discriminating factor for interindividual and inter-muscular differences in RFD.
Muscle size and architecture
Maximal strength capacity has been found to correlate significantly with RFD. In a paper identified by the authors Andersen and Aagaard (2006), MVC strength explained 18, 29, 57 and 78 % of the variance in voluntary RFD recorded over the first 10, 50, 100 and 200 ms, respectively, of a rapid voluntary contraction. Therefore, it is reasonable to suggest that factors influencing MVC strength (the main determinants of which are neural drive and muscle cross-sectional area) may also influence RFD. The effect of muscle architecture on RFD is currently poorly understood, but increases in pennation (fascicle) angle allow for a greater muscle physiological cross-sectional area for a given muscle size (volume) and, thus, for a greater absolute rate of force rise (particularly, later in the rise of force).
Musculotendinous stiffness
The velocity of force transmission through a material is influenced by its stiffness, described by the equation 𝑣 = 𝑘𝑥/𝜇, where 𝑣 is the transmission speed, 𝑘 is stiffness is the mass-to-length ratio, 𝑥 is length relative to the resting length, and 𝜇 is the mass-to-length ratio. As tissue stiffness is inversely related to length, longer tissues (e.g., muscles and tendons) are more compliant, leading to slower force transmission. This was demonstrated by Wilkie (1949) and could explain differences in force transmission across muscles; for example, the plantar flexors, which rely on the Achilles tendon, transmit force slower than the quadriceps, which use the shorter patellar tendon. Inter-individual differences in the rate of force development (RFD) might also be influenced by tendon stiffness, as shown by correlations with RFD in studies onpatellar and Achilles tendons. While the role of muscle stiffness in RFD has been studied less, it is hypothesized that given the muscle's greater mass than that of the tendon, muscle stiffness may have a more significant effect on RFD. However, current data on the relationship between musculo-tendinous stiffness and RFD are inconsistent, and further research is needed to clarify these effects.
Contribution of neural factors
The rapid generation of contractile force within the first 300 ms of a voluntary contraction, indicated by a high rate of force development (RFD), is crucial for elite athletes. Strength training methods can enhance both RFD and muscle activation (e.g., EMG amplitude) during this initial contraction phase. These improvements are primarily due to neural adaptations, though increases in muscle size, type II fibre proportion, and tendon stiffness also contribute. Both heavy-resistance and explosive-type strength training significantly increase RFD, with EMG analysis showing consistent parallel gains in muscle activation after training. For example, six weeks of lower-limb strength training increased knee extensor RFD by 33% alongside an 80-100% rise in EMG activity, while endurance training did not affect RFD or muscle activation. The strong correlation between RFD and EMG activity suggests neural adaptations play a key role. These adaptations likely involve changes in spinal circuitry and motor unit (MU) discharge rates, particularly during explosive-type exercises, which are most effective for maximizing RFD. Training-induced increases in MU discharge rates, including ultra-high frequency doublet discharges, have been linked to significant RFD gains. Explosive-type strength training, which emphasizes maximal RFD, is effective across various sporting codes. While heavy, non-explosive training can also boost RFD, explosive exercises generally yield greater and faster improvements in rapid force production.
Contribution of musculotendinous factors
Beyond the neural adaptations associated with strength training, several changes may occur in the musculoskeletal system that influence RFD. Duchateau and Hainaut (1984) first demonstrated that strength training can enhance RFD through muscle adaptations independent of neural changes. They observed significant increases in peak RFD following three months of strength training, suggesting muscle adaptations contribute to RFD gains. One key factor is muscle hypertrophy, particularly in type II fibres, which are known for higher RFD compared to type I fibres. Strength training, especially heavy resistance-based exercises, promotes type II fibre hypertrophy, leading to gains in both MVC force and RFD. Studies have shown positive correlations between changes in muscle fibre composition and improvements in RFD, further supporting this muscle-driven adaptation. Additionally, increases in tendon stiffness due to training may alsocontribute to RFD improvements, though more research is needed to clarify this relationship. The authors suggest future research should focus on the interplay between neural and muscular adaptations to develop a more comprehensive model of how RFD adapts to training.
Read part two here.