Needle Motion in a PMU Machine: What Don’t Hz and Stroke Tell You?

Two PMU machines can operate at 60 Hz, have a 3.5 mm stroke and use the same cartridge, yet still leave a different trace. One creates clearly defined, consistent pixels with light contact. The other requires a slower hand movement, more passes or begins to behave differently once the needle meets tissue resistance.

There is no contradiction here. Frequency and stroke describe important parameters, but they do not describe the needle’s entire motion cycle. They do not tell us how quickly the needle accelerates, how the drive responds to cartridge resistance or how long the needle tip remains in actual contact with deforming tissue.

This is why the setting displayed on the screen is not a complete description of the impulse. It is only one part of a larger system: machine–cartridge–pigment–hand–tissue.

What does the “impulse” of a PMU machine mean?

In this article, the term impulse refers to the practical characteristics of a single needle cycle: its forward movement, contact with the surface, reversal of direction and return. It is not a single standardised unit published by every manufacturer.

The characteristics of the impulse are influenced by factors including:

• the needle’s motion profile over time,

• its speed and acceleration during different phases of the cycle,

• stroke length,

• the mass of the components moving together with the push rod,

• motor torque and how the motor is controlled,

• play, friction and the geometry of the mechanism,

• resistance from the cartridge membrane and guiding system,

• the system’s response under load.

This is why two machines operating at the same frequency do not necessarily deliver identical needle motion. Hz tells us how often the cycle repeats. It does not reveal the complete shape of that cycle.

Hz describes the number of cycles, not the time spent in contact with the skin

One hertz means one cycle per second. A machine operating at 60 Hz is intended to complete 60 cycles every second. The average duration of one complete cycle is therefore:

1 ÷ 60 s = 0.0167 s, or approximately 16.7 ms.

This distinction is important: 16.7 ms is the duration of the entire cycle, not the amount of time the needle spends in the skin.

During this interval, the needle must extend, reach its most forward position, reverse direction and retract. It remains inside the cartridge tip during part of the cycle and extends beyond it during another part. It only comes into contact with the tissue when the needle protrusion, hand position and surface deformation allow it.

Parameter	What it actually describes	What it does not determine on its own
Hz	Number of cycles per second	Shape of the cycle, tissue contact, depth or amount of pigment delivered
Stroke	Total travel of the needle system during one cycle	Pigmentation depth or actual contact time
Needle protrusion	Position of the needle tip relative to the cartridge tip	Actual penetration into deforming tissue
Motion profile	How the drive and control system execute the cycle	The final effect without considering the cartridge, hand and tissue

Contact time and “dwell time” are not simple consequences of stroke

In the tattoo and PMU industries, the term dwell time is sometimes used to describe the time the needle spends in the most forward part of its movement or the time it remains in contact with the surface. These definitions are not identical.

It is possible to measure how long the mechanism keeps the needle within a specific position range. This does not mean that the needle remains inside the tissue for that entire period.

Actual contact also depends on:

• needle protrusion,

• the angle of the machine,

• pressure and hand stability,

• tissue deformation before penetration,

• swelling that develops during the procedure,

• hand movement relative to the surface.

A longer stroke changes the range of movement and may affect needle dynamics, but it does not automatically mean that the needle remains in the skin for longer.

Determining the contact time in milliseconds for a specific machine under particular working conditions would require a position measurement or a sufficiently high-frame-rate recording performed with a specific cartridge and load. This value cannot be calculated reliably from stroke and Hz alone.

Why can two machines operating at 60 Hz create different pixels?

1. The same cycle duration does not mean identical speed during every phase

Two periodic movements can repeat 60 times per second while following different position, velocity and acceleration profiles. One system may move more dynamically through the working phase, while another may decelerate and reverse direction more gradually.

In PMU, this may be perceived as more direct or softer contact. However, this sensation alone is not a measurement of force or proof of better pigment implantation. It is an observation that should be assessed alongside pixel consistency, tissue response and healed results.

2. The cartridge changes the load placed on the drive system

The membrane, guide friction, plunger mass and needle configuration create resistance that the machine must overcome during every cycle.

Two cartridges labelled 1RL 0.30 may place different loads on the drive system if they differ in design or manufacturing tolerances.

In a system without effective compensation, increased resistance may cause the actual speed to drop. Closed-loop control compares the measured value with the target value and adjusts the drive response accordingly.

This is a general principle of control engineering. It does not mean that every PMU machine manufacturer implements it in the same way, nor does the word “brushless” alone guarantee identical behaviour under load.

3. Tissue is not a rigid surface

Before penetration occurs, the needle first deforms the tissue. Research into needle insertion mechanics—although not conducted specifically on PMU procedures—shows that insertion speed, tip geometry and material properties affect both the force required and the degree of deformation. These relationships were observed, among other studies, in 2021 research into needle insertion mechanics in tongue tissue published in Medical Engineering & Physics.

This helps explain why the same impulse may behave differently on stretched brow skin, soft lip vermilion and practice latex.

It does not mean that there is one correct machine or one correct frequency for every type of tissue. It means that tissue is an active part of the mechanical system, not merely a passive recipient of pigment.

What does the motion profile change in an artist’s work?

The motion profile does not replace technique. It does, however, change the starting point to which the artist adjusts their hand speed, movement length, cartridge and method of stabilising the treatment area.

Frequency and hand speed determine the potential density of contacts along the path of movement. If a machine operates at 50 Hz while the hand moves at 25 mm/s, there are theoretically two cycles per millimetre of movement.

This does not mean that two identical pigment deposits will be created. One cycle may not result in effective tissue contact, the tissue may deform, and the amount of pigment present on the needle tip may vary.

We can therefore say that:

• Hz and hand speed influence the distribution of potential contacts,

• motion profile and behaviour under load influence the characteristics of each cycle,

• the cartridge and pigment influence resistance and product delivery,

• tissue and technique determine how that movement is actually used.

What is established fact, and what is an engineering conclusion for PMU?

It is a fact that frequency determines the number of cycles per second and that feedback control can correct speed when the load changes.

Laboratory studies also show that needle insertion mechanics depend on speed, needle geometry, tissue properties and fluid characteristics.

In a 2022 study using a tattoo machine, a five-needle configuration and a model gel, the volume of delivered liquid was only slightly dependent on frequency but increased when lower-viscosity fluid was used.

This was not a study of permanent makeup performed on human skin, so its numerical results must not be applied directly to a PMU procedure. It does, however, demonstrate that frequency is not the only variable controlling the amount of product delivered.

The engineering conclusion for PMU is that evaluating a machine solely on the basis of Hz or stroke overlooks some of the variables that affect contact between the needle and tissue.

How should you compare the behaviour of two PMU machines?

A comparison is meaningful only when the number of variables is limited. Instead of performing two different brow treatments using two different sets of tools:

1. Use the same type of practice latex and mark equal-length sections.

2. Use the same cartridge model from the same production batch.

3. Maintain the same needle protrusion, pigment, angle and hand movement.

4. Compare the actual frequency in Hz only if both machines display it.

5. Record several repetitions and evaluate the entire series rather than the best individual trace.

6. Test performance at several frequencies, changing only one parameter at a time.

Do not assess darkness alone. Pay attention to:

• consistency at the beginning and end of each movement,

• spacing between pixels,

• width of the trace,

• tendency to create scratches or lines,

• changes in sound after fitting the cartridge,

• number of passes required to achieve a comparable result.

A latex test cannot predict a biological response. It can, however, reveal whether the machine and cartridge form a repeatable system before introducing the most variable element: living tissue.

FAQ

Does a higher frequency mean a stronger impulse?

No. Higher Hz means more cycles per second. The contact sensation also depends on stroke, motion dynamics, mechanism design, cartridge, motor control and technique.

Can the needle’s contact time with the skin be calculated from Hz?

No. Hz can be used to calculate the duration of the entire cycle. Determining actual tissue contact requires knowledge of the motion profile, needle protrusion, machine position and tissue deformation.

Does a longer stroke mean the needle remains in the skin for longer?

Not automatically. A longer stroke changes the distance travelled and may affect speed and acceleration, but contact time depends on the cycle profile and working conditions. Stroke should not be treated as equivalent to penetration depth or penetration time.

Why does a machine operate correctly without a cartridge but slow down after one is fitted?

The cartridge introduces membrane resistance, guide friction and the mass of moving components. If the symptom occurs repeatedly with several verified cartridges, the ability of the drive and control system to maintain operation under load should be assessed.

Can a better motion profile correct poor technique?

No. It may provide a more stable and predictable starting point, but it cannot replace proper tissue tension, angle, hand speed, control over the number of passes or appropriate cartridge selection.

Conclusions

Hz answers the question: How often does the cycle repeat?

Stroke answers the question: How far does the needle system travel?

Neither parameter, on its own, answers the question: How exactly does the needle behave when it comes into contact with tissue?

Answering that question requires looking at the entire system: motion profile, motor control, behaviour under load, cartridge, pigment, hand and the biomechanics of the treatment area.

The point is not to disregard Hz or stroke. The point is to use these parameters according to what they actually describe—without attributing properties to them that they do not measure.