When a muscle cannot shorten as it tries to, or is lengthened instead, and the stiffness that remains
Even with the athlete’s usual care, stiffness or difficulty moving may remain in a particular muscle. In Zeroka Seitai, we place importance on “muscle lock” as one way of thinking about the background to this.
We consider that lock arises from situations in which a muscle is trying to shorten but cannot because of a load, or is lengthened instead.
Here we explain our interpretation of how muscle lock arises, focusing on the size of the load at that moment and on speed on both sides: the muscle side and the load side.
The muscle trying to shorten, and the load acting against it
A muscle tries to shorten and actually becomes shorter. We first use this state as the reference for comparison. When a load is applied, however, the muscle may be unable to become shorter even though it is trying to, or may instead be pulled longer.
The figure sets these three states side by side. The teal arrows represent the force with which the muscle tries to shorten, and the ochre arrows represent an external force lengthening it.
Three states of a muscle trying to shorten
An interpretation based on Zeroka Seitai’s practice
- ① Shortens
The muscle tries to shorten and actually becomes shorter
(the reference state) - ② Cannot shorten
The muscle tries to shorten, but the load stops it becoming shorter - ③ Lengthened instead
The muscle tries to shorten, but the load pulls it longer
Teal arrows: the force with which the muscle tries to shorten. Ochre arrows: an external force lengthening the muscle.
What we see as triggers are ② and ③. In those situations we consider three things together:
- Size of the load
How large a load acts on the muscle trying to shorten - Muscle side: speed of intended shortening
How quickly the muscle is trying to shorten - Load side: speed at which the load is applied
Including the lengthening speed when the muscle is lengthened
The two speeds are considered separately. They are not treated as the same quantity, and they are not added together.
This figure does not mean that every situation like ② or ③ is muscle lock. Muscle lock is a Zeroka Seitai concept, not an established diagnosis. Force, speed and the degree of lock shown here were not measured.
Figure R-1-1 | A schematic showing Zeroka Seitai’s interpretation of how muscle lock arises. In the Japanese edition this figure is an AI-generated illustration; this English version presents the same elements as an HTML diagram. The shape and length of the muscle and the arrows are for explanation only; the figure is not a measurement of actual force or speed, or of the degree of lock.
What we see as the triggers for onset are ② and ③ in the figure. The muscle is working to shorten but cannot become shorter in response. We consider that, from this situation, a state remains in which the muscle does not readily relax by itself.
“Shorten” here means the muscle becoming shorter. We do not judge every exercise in which the muscle’s length does not change, or in which it produces force while being lengthened, to be muscle lock.
Considering the size of the load together with speed on both sides
In situations where a muscle’s attempt to shorten is blocked by a load, we consider that the greater the load and the higher the speed, the greater the degree of lock.
“Speed” here is not limited to the speed at which the muscle is pulled longer. We consider it from both sides: the muscle trying to shorten, and the load being applied.
Scroll sideways to read the full table.
| What we focus on | What we consider in our interpretation |
|---|---|
| Size of the load | How large a load acts on the muscle that is trying to shorten. |
| Speed on the muscle side | How quickly the muscle is trying to shorten (the speed of intended shortening). We consider this separately from the speed at which it actually shortened. |
| Speed on the load side | How quickly the load is applied (the speed at which load is applied). Where the muscle is lengthened instead, we also include the speed at which it is pulled longer (the lengthening speed). |
Rather than judging from a single speed, we look at the relationship between the muscle trying to shorten and the load acting against it. This explanation does not treat the speeds on the two sides as the same quantity, nor add them together.
This is Zeroka Seitai’s interpretation, based on experience in hands-on practice. Force, speed and nerve signals at the moment of onset have not been directly measured, and neither a causal relationship with lock nor the conditions for its onset have been established. We use this interpretation while comparing the situation in which the problem occurred, the condition of the muscle, and the changes before and after our response.
Linking the feel of “stiffness” to movement
In muscle lock, we consider that the muscle does not relax sufficiently from a shortened state, and that its ability to lengthen and shorten as needed is impaired.
For this reason, we check not only the stiffness felt by touch but also the response when the muscle is engaged and relaxed, and the actual movement.
Scroll sideways to read the full table.
| What we check | What we consider from the perspective of muscle lock |
|---|---|
| The state when the muscle is relaxed | Whether stiffness remains that does not readily relax by itself |
| The response when trying to engage the muscle | Whether it can do the work needed for that movement |
| Situations in which it needs to lengthen | Whether the stiffness is restricting movement or range of motion |
| The work of other areas | Whether other muscles are compensating for the part that cannot move |
We do not settle on a single cause from these responses alone; we form our interpretation together with the quality and distribution of stiffness and how it has developed over time.
Linking our interpretation of onset to checks of movement
We consider that when a muscle cannot lengthen and shorten sufficiently, the necessary movement becomes difficult and other parts that can move need to compensate. A figure showing the effect on movement and how the load is shared is in A-1, and why we also check the areas that were compensating is explained in A-4.
In actual support, we compare the situation in which the problem occurred with the stiffness and difficulty moving that remain afterwards.
When a muscle stiffens suddenly, and when stiffness persists
When we interpret that muscle lock is involved in stiffness or difficulty moving that has arisen suddenly, we describe this as “acute lock” — a term we use in Zeroka Seitai, not an established diagnosis. We also consider the possibility that muscle lock is involved when stiffness has persisted for a long time.
Scroll sideways to read the full table.
| What we are seeing | What we check |
|---|---|
| A particular movement triggered sudden stiffness and difficulty moving | In what situation it happened. Where the stiffness is, and which movements have become difficult. |
| Stiffness has remained since a previous injury or load | Whether problems remain not only at the original area but also in the areas that were compensating. |
| Stiffness returns with the load of training or competition | Whether it is the same state as before, or whether new strain or another factor is involved. |
These are Zeroka Seitai’s ways of organising how a problem has developed. They do not equate acute lock with tissue damage from a sudden injury.
The mix of stiff and mobile parts, the concentration of load, and the relationship with injury are covered in more detail in B-5.
How do we respond when a muscle does not readily relax?
Zeroka Seitai understands a muscle that does not readily relax by itself as being in “a state in which the muscle’s safety mechanism keeps over-reacting”. Rather than stretching the muscle, we aim to release the loop of the response that keeps it shortening to protect the body, using hands-on techniques that slacken the muscle.
What changes do we check when responding to lock?
In what Zeroka Seitai calls “responding to lock”, we check how the stiffness and the reluctance to relax seen before hands-on work have changed.
We then have the athlete perform movements involving that muscle, and check whether the movement they want to achieve has changed.
Quality and distribution of stiffness, response when engaging / relaxing, the movement that is difficult
Change in muscle condition + the athlete’s own sense + change in actual movement
Changes after hands-on work are cues for examining our interpretation. However, we do not treat a change in movement alone as proof of the whole mechanism we describe.
There are also cases where responding to lock alone does not soften the muscle sufficiently, or where the change does not last. In those cases, we review the situation, including the several factors explained in R-4.
Using the perspective of muscle lock to understand the stiffness and difficulty moving that remain, and checking the changes before and after our response. Through these repeated checks, we put together the response each athlete needs.