The force-velocity curve is one of the most badly understood concepts in Strength and Conditioning.
Firstly, in classical mechanics there is no instantaneous force and instantaneous velocity. Many strength and conditioning coach assume there is. There is a universal relationship between instantaneous force and acceleration. This is Newton’s 2nd law: F=ma: force is directly proportional to acceleration. The existence of a relationship between force and velocity is a property of muscle – ie the FV relationship commonly talked about applies just to muscle. It is a large leap to assume that a relationship that is seen in individual muscle fibers will also be seen in the movement of the whole organism. But this is an assumption that is very common in strength and conditioning. This is especially strange as there are a number of bodily structures which can mitigate the FV relationship. For instance, the biarticular muscles can permit large joint excursions without experiencing much change in length. This means tension in them can be high even if the joints are changing angle quite fast. Similarly, muscle tissue is just one component of a muscle-tendon unit. Tendons can stretch even if the muscle doesn’t change length. This can allow both force and velocity to be high when considering the whole muscle-tendon unit. So that’s the theory, but does the body actually exhibit a force velocity relationship in different movements? It’s a really common to see graphs like this, showing where different exercises lie on the FV curve. But there are a few problems with this graph.
For instance, this graph says that force is higher during squatting than weightlifting. Is this true? Peak forces are pretty high during weightlifting. Similarly, peak forces are pretty high in maximal sprinting, and are just for one foot. It’s pretty easy to imagine that an elite sprinter has a peak Ground Force during sprinting that is much greater than half of their peak force in squatting. The graph should really be labeled as the load/velocity curve and then would make more sense. This is a classic mechanical misunderstanding – confusing load and force – they are not the same thing. When people talk about the force velocity relationship, they normally suggest that we should train at all points on the curve. But do we need to invoke this curve to give this training guideline? Can’t we express the same guideline by saying that athletes should train across a range of velocities or a range of external loads? I appreciate is doesn’t sound as pretty. So it is very difficult to answer our original question if we compare different movements. If we look at just one movement though, is there a force velocity – relationship? First, let’s remind ourselves of the force relationship in muscle – a hyperbolic relationship.
In squatting? A linear relationship (note differences in methods and populations account for different locations on lines).
In a power clean? Maybe no relationship.
What is important here is that in whole body movements force can still be substantial at higher velocities. The force – velocity relationship of muscle is mitigated by other factors.
So where are we?
1. A hyperbolic FV relationship is a property of muscle not movement.
2. Often a particular movement will also display a FV relationship but this relationship can be very different to that in isolated muscle fiber.
3. It is hard to establish where different exercises lie relative to each other on a FV curve. If it is possible the relationship is higly unlikely to be a curve.
4. Is peak force in squatting really greater that in weightlifting which is in turn greater than sprinting?
5. Do we really need to classify exercises on a FV curve to establish a training guideline around at a range of velocities or loads?
3. Neural Factors
3.1. Motor Unit Recruitment
Motor Units (MUs) are generally recruited in a orderly fashion: Slow MUs to Fast MUs. This is stated by the Henneman Principle. If a task does not require all MUs to be recruited, low threshold MUs will be called into action. However, if slow/low threshold MUs are not enough to complete a task, fast/high threshold MUs need to be recruited. Obviously, this mechanism takes time to be performed, and the time needed to recruit high threshold MUs appears to be modifiable up to a certain point depending on training strategies.
3.2. Firing Frequency
In order to have a muscle action, electrical impulses need to be sent to muscle fibers. The mechanism behind this process is the ALL OR NOTHING law. It states that a single twitch is always sent at its maximum capacity. What changes the degree or intensity of a muscle action is the frequency of those twitches are sent. When firing frequency is high, a state of tetanus occur, resulting in maximal activation of MUs. Firing Frequency can be increased through strength/power training, resulting is increases in Rate of Force Development (RFD).
3.3. Motor Unit Synchronization
As we saw before, within the same muscle there are different fiber types. Additionally, there are also dozens to thousands of different MUs within each individual muscle (intramuscular). As we saw with Hennrman Principle, MUs are recruited according to the task demands and MUs threshold. The synchtonization of MUs enables in more coordinated MUs activation for a certain muscle, resulting in more efficient muscle action with greater potential to produce force.
3.4. Intermuscular Coordination
3.4.1 .Activation of Synergists
As we saw with intramuscular coordination and MUs synchronization, the ability to synchronize synergistic muscles is crucial for performance, and it is a result of motor learning.
3.4.2. Co-Activation of Antagonists
Activation of antagonist muscles have a negative effect over concentric ballistic performance. Untrained subjects show a higher co-activation of antagonists, and one of the most pronounced intermuscular adaptations after ballistic training is its decrease, resulting in performance enhancement.
4. Muscle Environment
Fatigue, Muscle Temperature and Hormonal Concentrations can negatively affect muscle force production
2. Morphological factors
2.1 Muscle Fiber Type
There are differences in muscle fiber distribution, both intra and inter individual. Different athletes show different % of slow vs fast fibers. Additionally, different muscles within the same athlete have different fiber type distribution. Even further, within a muscle there are different fiber types. This distribution is dependent both on genetic factors and training history.
Depending on the fiber type dominance, force production potential can be affected.
2.2 Muscle Architecture
2,2,1 Cross – Sectional Area
—Physiological CSA —Anatomical CSA
Generally, the greater the PCSA the greater the force production potential. In other words, if a muscle has more sarcomeres in parallel. it has the ability to produce more force. That can be achieved both through an increase of number of sarcomeres and/or an increase in pennation angle, as we will see next.
2.2.2 Pennation Angle
An increase in pennation angle is associated with greater potential to produce force. This is due to an increase in sarcomeres in parallel, increasing therefore PSCA, resulting in greater ability to produce force. An increase in pennation angle in a typical adaptation after training with heavy loads. On the other hand, a decrease in pennation angle is associated with greater potential to higher contraction velocities, and as we saw before this reduces the potential to produce force. This is due to more sarcomeres in series and greater fascicle length, as we will see next.
2.2.3 Fascicle Length
An increase in fascicle length is a result of more sacromeres in series (longitudinal). Since a decrease in pennation angle results in a decrease of PSCA, it is associated with reduced force production potential. However, it allows for greater contraction velocities, which is a common adaptation after ballistic and speed training.
2.3 Tendon Properties
Tendon or muscel tendon unit (MTU) has two distinct properties: stiffness and compliance. Particularly, tendon stiffness has important implications for performance since it allows for a more efficient storage and utilization of elastic energy, which drives greater force production as we saw before.