Why are strength gains velocity-specific after fast movement training?
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We can calculate a force-velocity profile for an athlete in a lift by measuring the bar speed and the force produced at a range of loads. Measuring this force-velocity profile at varying times during a training block can be used to monitor the effects of a training program. Very often, the gradient of the force-velocity profile will change after a block of a certain type of training. This indicates that the program block has unique effects on force produced at different speeds (some types of training have larger effects on force at slow speeds, while other types of training have larger effects of force at fast speeds). However, we can also consider how the underlying physiological adaptations that occur after fast movement training might alter the force-velocity profile.
#1. Adaptations that increase maximum velocity without changing maximum force
Many of the adaptations that occur after fast movement training actually increase maximum velocity without changing maximum force. This causes a shift in the gradient of the force-velocity profile that looks like this:
The adaptations that produce this effect are both neural and peripheral in nature, and include:
- an improvement in the coordination of the tested exercise (since such coordination improvements are velocity-specific, which means that fast movement training is unlikely to affect the coordination of the same lift at slow velocities). One coordination element that differs between fast and slow movements is the sequence of muscle activation. In fast movements (and in short-duration, explosive isometric contractions), there is a three-phase sequence of muscle activation (agonist-antagonist-agonist). In contrast, in slow movements (and in sustained isometric contractions), there is only a single phase of muscle activation, involving the agonist muscle acting with a certain level of antagonist coactivation. Additionally, some researchers have suggested that fast movements (and explosive, short-duration isometric contractions) involve a pre-programmed motor pattern that cannot be interrupted once it has been triggered. Evidently, this differs from movements that last for longer periods of time (which can clearly be interrupted), such as lifting heavy loads with maximal efforts;
- a reduction in antagonist coactivation levels (since such alterations are well-known to be velocity-specific);
- an increase in motor unit firing frequency. Motor unit firing rates are lower during slow movements than in fast movements, and while heavy strength training does not alter motor unit firing rate, fast movement training does. It is likely that there is no benefit to increasing motor unit firing rate for force production in slow movements (since a low level of firing rate already achieves near-maximal crossbridge formation). In contrast, there is great benefit to increasing motor unit firing rate for force production in fast movements, since crossbridge formation is much harder to maintain due to the very high detachment rates that result from the fast movement speeds. Consequently, it is logical that fast movement training is better at increasing motor unit firing rates than heavy strength training, and that this produces a much larger increase in maximum velocity than in maximum force; and
- an increase in single fiber contractile velocity. This adaptation is not very well-known in the fitness industry. Single muscle fibers (of any type) can increase their maximum shortening velocity after fast movement training, but this does not happen after heavy strength training. This change is not the result of fiber type shifts, since the maximum shortening velocity changes even when controlling for fiber type. The exact mechanism that causes this to happen is still unknown.
Practically speaking, improvements in coordination are most likely to be generated by a focus on improving movement skills, using (either visual or auditory) feedback, implementing an external focus of attention, and avoiding fatigue during exercise (since fatigue impairs motor learning).
The practical methods for improving reductions in antagonist coactivation are unclear. Yet, reducing antagonist coactivation very likely occurs through similar mechanisms to coordination. Indeed, stability-specific strength gains are often the result of reductions in antagonist coactivation. Therefore, we can probably accelerate reductions in antagonist coactivation by using the same methods as we use to improve coordination.
Little is known about what stimulates increases in motor unit firing frequency. Yet, given the relatively close relationship between motor unit recruitment thresholds and motor unit firing frequencies at a given velocity, it seems likely that the same methods that can be used to increase motor unit recruitment might similarly be valid for increasing motor unit firing frequency. Such methods work by enhancing motivation and therefore increasing the level of effort, which is closely linked to motor unit recruitment)
Similarly, little is known about what factors might enhance improvements in single fiber contraction velocity. Yet, since it is clear that only fast movement training produces this effect (while heavy strength training does not), it is highly likely that a fast movement speed is necessary to create the adaptation, and that faster movement speeds might lead to superior improvements.
#2. Adaptations that increase maximum velocity and also increase maximum force
Some adaptations that occur after fast movement training increase maximum velocity but also increase maximum force. This means that the gradient of the force-velocity profile does not change, and the whole line is shifted vertically upwards, like this:
The key adaptation that produces this effect is an increase in voluntary activation, which is the ability to recruit high-threshold motor units. Untrained individuals do not have the ability to access all of their high-threshold motor units, so they cannot activate the muscle fibers that they control. After strength training workouts that involve maximal efforts (including high-velocity movement training), this ability often improves. As a result, lifters are able to access more high-threshold motor units in future workouts. Since each of these additional motor units controls a group of fast twitch muscle fibers, this enhances force production at both slow and fast speeds to a similar extent.
Mechanistically, it is probably the recruitment of a large number of motor units in a workout that increases the number of high-threshold motor units that can be recruited in future workouts, rather than the use of a high level of effort. Effort is very closely related to the size of the signal from the central nervous system (CNS) that leads to motor unit recruitment. However, as a set progresses, CNS fatigue occurs and this reduces the size of the signal from the CNS that corresponds to a given level of perceived effort. Indeed, when lifters use light loads and train to failure, a high level of effort is required at the end of the set, but this does not cause an increase in the ability to recruit high-threshold motor units.
Practically speaking, the exact training variables that might enhance gains in voluntary activation are unclear. Yet, strength gains are enhanced by many psychological techniques (including giving autonomy, providing a challenging target to hit, or issuing feedback) that produce temporary and long-term improvements in performance by enhancing motivation and therefore effort. Since effort is closely linked to motor unit recruitment, it seems likely that any technique that increases effort in a workout (while avoiding CNS fatigue) might lead to increased changes in voluntary activation over time.
What is the takeaway?
Fast movement training often changes the gradient of the force-velocity profile. Usually, it produces an overall increase in maximum velocity alongside a reduction in maximum force (although the reduction in maximum force is probably caused by a lack of mechanical tension stimulus, rather than by a negative adaptation resulting from fast movement training). Overall, the effect is the net result of multiple adaptations, each of which produce different changes to the force-velocity profile.
Some adaptations produce increases in maximum velocity without altering maximum force (coordination, antagonist coactivation, motor unit firing frequency, and muscle fiber shortening velocity). In contrast, other adaptations produce increases in maximum velocity and also increase maximum force (voluntary activation ).
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