Strength is the ability to produce force.
Our ability to produce force differs, dependent on many factors, including (but not limited to) the muscle length, contraction speed, and contraction type (shortening or lengthening) used in a test of strength.
Strength training is the process by which we *improve* our ability to produce force.
Strength training programs improve our ability to produce force at most muscle lengths, contraction speeds, and and contraction types (shortening or lengthening). However, they do *not* improve them all to the exact same extent.
If we do strength training at one particular muscle length, contraction speed, or contraction type, we will record a large increase in our strength if we test it at that same muscle length, speed, or contraction type.
In contrast, we will only observe a small increase in strength if we test our ability to produce force with a different muscle length, speed, or contraction type.
In other words, strength training produces different effects, depending on the type of training we do.
Why is our “strength” different when we test it in different ways?
There are *literally* dozens of factors that can affect our ability to produce force, many of which have nothing to do with our muscles.
Indeed, our environment, our psychological state, and our central nervous system all play important roles in force production, in ways that we do not yet fully understand.
But there are also three basic biological factors inside muscle that determine how much force we can exert. These are produced by the internal workings of the muscle fibers themselves, as described by three phenomena as follows:
- the length-tension relationship
- the force-velocity relationship
- force enhancement during lengthening
These three factors determine why we are stronger at some muscle lengths than at others (the length-tension relationship), at slow contraction speeds compared to at high speeds (the force-velocity relationship), and in lengthening contractions than in shortening contractions (force enhancement during lengthening).
After strength training, adaptations occur that affect each of these relationships. These adaptations improve our ability to produce force *specifically* under different conditions.
Naturally, other adaptations occur as well (such as an increase in muscle size) which produce increases in force under a range of conditions, but that is a topic for another day!
Here are some examples of how strength training produces adaptations in these three basic biological mechanisms inside muscle, depending on the type of training we do.
#1. The Length-Tension relationship
According to the length-tension relationship, muscle fibers have an optimal length for producing force, mainly because the force that a fiber produces is determined by the amount of overlap between the strands inside its contractile units (sarcomeres).
Perfect overlap occurs only when the two filaments are totally lined up. A reduced level of overlap can occur either when the muscle fiber is very elongated, or is very shortened. Consequently, whole muscles have a length at which they are strongest, and there is a joint angle at which our ability to produce force is greatest (usually in the middle).
After strength training at long muscle lengths, the joint angle at which we are strongest shifts to a more extended angle, corresponding to a longer muscle length.
This seems to happen *partly* because individual muscle fibers elongate by adding sarcomeres. Since the starting and ending points of the muscle itself are not altered, a greater number of sarcomeres is packed into the length of a fiber. Thus, the length of each sarcomere reduces. This decreases the initial overlap between strands inside the sarcomere, and shifts the optimal length for producing force to a longer muscle length.
By shifting the joint angle at which we are strongest, we increase strength *preferentially* in that part of the joint range of motion, and less in other parts.
#2. The Force-Velocity relationship
The force-velocity relationship tells us that individual muscle fibers produce less force whenever they contract more quickly. Thus, we are not able to express the same amount of strength when we move fast.
We observe the force-velocity relationship because the force produced by a muscle fiber is dependent upon the number of attached crossbridges at any one time. And the number of attached crossbridge is dependent upon the contraction velocity of the muscle fiber.
The main factor that determines the number of attached crossbridges at any one time is the detachment rate at the end of the working stroke. And this increases linearly with increasing contraction velocity. Therefore, as the fiber attempts to contract faster, any attached crossbridges detach more quickly, and this reduces force.
If we can either (1) reduce the rate of detatchment of attached crossbridges, or (2) increase the rate of reattachment of detached crossbridges, we might be able to increase strength specifically at high velocities (this can also be done in other ways, but that is another matter).
One of the really interesting things about high-velocity exercises like ballistic strength training or plyometrics is that they display extraordinarily high levels of rate coding (discharge rates), especially at the start of a contraction. The discharge rate is the rate at which the central nervous system sends its signal to the muscle fibers. The rate tells us how many times per second the signal is sent, and each signal leads to a contraction of the muscle fiber.
Normal strength training exercises typically involve rates of 30–50 times per second, while rate coding in plyometrics reaches 60–120 times per second.
Ballistic strength training or plyometrics are well-known to produce strength gains that greater when tested at high speeds (force exerted against light loads), and smaller when tested at low speeds (force exerted against heavy loads). They are also known to increase discharge rates to as much as 200 times per second, at the start of high velocity contractions.
By sending more signals per second, the increase in the discharge rate during high velocity contractions *more than compensates* for the increase in the rate of detatchment of attached crossbridges, and it probably does this by increasing the rate of reattachment of detached crossbridges.
And this allows the athlete to improve strength *preferentially* at high velocities.
#3. Force enhancement during lengthening
When lengthening, muscle fibers can produce up to 150% of the force that they are able to exert while shortening.
And we are approximately 125–130% stronger when we lower a weight under control (over 3 seconds), compared to when we lift a weight in the same exercise. The reason for the smaller difference in strength between lengthening and shortening at the whole muscle level has been attributed to a protective neural mechanism that selectively reduces activation of the muscle in the lengthening (eccentric) phase.
This greater force production while lengthening has largely been attributed to the behavior of the giant molecule called “titin,” which gradually unravels and resists movement as we stretch a muscle fiber that it is actively producing force.
After strength training with lengthening (eccentric) contractions, we increase maximum force during lengthening (eccentric) contractions more than maximum force during concentric (shortening) contractions.
Although no human studies have yet observed any changes in titin after strength training of any type, studies in rats have identified that the amount of titin inside a skeletal muscle fiber can increase after exercise. If this happens more after training involving lengthening contractions (eccentric training), then it could explain why maximum force in lengthening (eccentric) contractions is increased more after strength training with lengthening (eccentric) contractions.
Additionally, strength training with lengthening (eccentric) contractions seems to reduce the impact of the protective neural mechanism, as it removes the selective reduction in activation of the muscle during the lengthening (eccentric) phase.
Either of these mechanisms might therefore contribute to how athletes are able to improve strength *preferentially* during lengthening (eccentric) contractions.
What is the takeaway?
Strength is the ability to produce force, and strength training is simply the process by which we improve our ability to produce force.
Our ability to produce force differs depending on the muscle length, contraction speed, and contraction type (shortening or lengthening) used in a test of strength. Additionally, the mechanisms by which strength improves after training also differ according to the muscle length, contraction speed, and contraction type (shortening or lengthening) used in training.
And this is (a big part of) the reason why strength training produces different effects, depending on the type of training we do.