Why IET   The Concept   General Principals   IET Operation   How it Works   IET vs. Other Exercise   Sample Programs

How the Concept Works


What good is a muscle if you can’t turn it on when you need it?

The primary benefit of Inertial Exercise is training the user proper functional timing in any activity

If an individual is unable to perform a task requiring a given level of coordination, you may easily train the learning ability by changing the weight on the sled.  The intent of the process is to train the timing of an activity, not necessarily the intensity of an activity.  Intensity of the actual activity can be trained as the patient progressively improves.   The following motion/force waveform analysis charts may help to demonstrate the process.

accelerationb chart 1


This is the waveform of a professional major league baseball pitcher (a Cy Young Award Winner) after rehabilitation.  Prior to rehabilitation, the patient was diagnosed with a denervated right infraspinatus muscle from suprascapular nerve neuritis with status post surgical nerve decompression.  He suffered from weakness at all elevation movements and required protection of the right arm during most activities of daily living.  This patient was extremely motivated to return to work.


The motion recorded here is external rotation of the right shoulder- shoulder in neutral, elbow at ninety degrees, from forty-five degrees internal rotation to forty-five degrees external rotation.  The large downward spike is the eccentric decelerating activity


Changes in motion are occurring in the 35ms range (deceleration) and the 29ms (acceleration) with a peak force of about 395 pounds.  These times are faster than we can think.   In viewing these accelerometry charts the important observation is the smoothness of the G Force line and the timing of deceleration and acceleration.  This waveform is typical of a high performance activity.  


Training high performance activity is the objective of Inertial Exercise.  Obviously, an injured patient is not able to produce the extremely high forces demonstrated in the high performance example.  What the patient needs is low forces over a long period of time.  In this way, the patient is able to think through the motion while developing a new motor program.  This can be accomplished by increasing the mass (weight) on the sled.  Low forces on a relatively high mass produce low levels of acceleration and deceleration.  In this way, the patient can learn new coordinated skills without the fear of injury.  As the patient progresses with the skills of the motion, the time required to perform the skill can be changed.  This is the process of developing coordination.  So we start with high weights on the sled and pull with extremely low forces.  Isaac Newton explained this concept with the formula: F=ma (force = mass x acceleration).


The above chart demonstrates the timing involved with this concept.  Because the Impulse is a gravity free training device relative to the weight on the machine, the weight is not felt; only the mass of the weight is felt.  Applying a small amount of force over a long period of time produces very slow acceleration rates.  The timing of motion is so slow that the patient is able to consciously feel and control the activity with assuranceDeceleration in this example occurred in about 112ms with acceleration at 240ms.   This time is easily within the realm of conscious thought.  As the patient becomes more confident, a natural increase in performance will be observed.  When confidence in control of this large mass is evident, reduce the weight, i.e. the mass.


The fundamental concept in patient training is to begin with the tonic technique of motion.  This technique does not have a “catch” phase and is therefore more simplistic in its ability to provide a smooth “feel” and control experience.  It stimulates co-activation of motor synergists while providing acceleration and deceleration times, with weights of 17.5 pounds and greater, in the realm of conscious thought control.  The example of timing in the chart on page 10 is an example of the timing changes that occur when using the phasic technique.  If the patient does not understand the concept of the “feel” of control of a catch, then attempting training in this technique could create anxiety.

tonic wave comparisons


These waveforms are complete repetitions of exercise with different weights on the sled recording the forces produced during each cycle.  Note the force is always, regardless of the mass, in the 7 to 8 pound range.  What changes is the timing.  It is not until the weight on the sled is 2.5 pounds and less that the timing to peak force is in the 100 ms range.  If the patient can perform exercise in the tonic technique with 2.5 pounds on the sled, then performing the catch with 25 pounds on the sled in the phasic technique will require approximately the same control.  When the patient learned control in tonic with 2.5 pounds, he was also learning phasic control at 25 pounds.  All control is a matter of timing.



Phasic wave comparison


The waveforms on the right are also complete repetitions of exercise with different weights on the sled.  Note the force is always, regardless of the mass, in the 18 to 22 pound range.  As with the tonic technique, the timing becomes less and less as the weight on the sled is reduced. It is not until the weight on the sled is 7.5 pounds and less that the timing to peak force is less than 100 ms.  This force wave-form chart and the previous tonic chart demonstrate exercise intensities typical of rehabilitation.  In these examples, the operator was instructed to perform as gently as possible while maintaining smooth and absolute control of the sled.  These charts demonstrate the compressive effects of control as the weight of the sled changes. But we are looking at only one repetition at each sled weight.  What is the relationship of repetitions per second as the sled weight changes?



tonic vs phasic comparison

The chart at left offers a comparison of tonic to phasic repetitions from the same data pool the above single repetition examples were extracted.

In general, phasic exercise offers more cycles per second than tonic in intensities of user perceived comfort. The Phasic exercise offers a rest period between each cycle of work activity.  Phasic produces 3+ times the force in each cycle of exercise and phasic produces deceleration times in periods much less than conscious thought is capable of regulating.  The fundamental activities of each exercise cycle are acceleration and deceleration.  The relationship of each activity varies from tonic to phasic and sled weight. The following charts depict these relationships



Fundamentally, as the sled weight is reduced the timing is also reduced regardless of technique.  The frequency of each cycle does not change greatly from one technique to the other respective of sled weight.  However, the deceleration phase of each cycle is generally greater than the acceleration phase with the tonic technique while the relative peak force is low.  The reverse is true of the phasic technique.  The deceleration phase of each cycle is much less than the acceleration phase while the relative peak force is high. Note also, the acceleration time of the 7.5 pound and less sled weight, in phasic, is just on the verge of conscious human thought timing.








In this environment of exercise, the user cannot actually feel the deceleration phase as it is occurring faster than thought is possible.  However, the motor program for controlling activity has been formed and is utilized from activity learned using the same techniques of motion with heavier sled weights.  The user knows what to do and does not need to think about it. At this level, the deceleration phase of activity is just on the verge of conscious control. With practice, the user will gain automatic control of this sled weight and develop confidence in the ability to move in these short time periods.


Confidence is the key in training higher performance levels of human motion.  This is true whether you are training motive ability with a head injury patient or higher performance levels of an Olympic athlete.  Having a belief in performance attainability is the most important aspect of any exercise.  The single most important ingredient in performance attainability is timing.   


With Inertial Exercise, it is not the intensity of the exercise; it is the training of the timing of the activity.  The selection of the technique coupled with the sled weight automatically governs the timing of the activity during the initial training phases of implementation.  An experienced therapist/trainer can transition the average orthopedic type injury to 0 pound sled weight in the first training session developing confidence in motion by creating or reeducating functional motor programs.  Once the user crosses the 7.5-pound sled weight, functional coordination has begun.  Using heavier sled weights builds confidence in control. When confidence in control of a sled weight is evident, reduce the weight; i.e. the mass.


acceration chart 2

This is an example of the timing of deceleration in a therapeutic mode with just the sled weight (3.2pounds). Remember the baseball player chart on page 9? Same person in therapy.  Now the max force is 18.5 pounds.  Note; however, the similarity of the shape of this chart and the chart on page 9.  The deceleration phase of activity is within 36ms and much quicker than the ability of the mind to consciously control.  And, the same may be said of the acceleration phase. With practice this patient gained control of this mass and developed confidence in the ability to move in these functional short time periods.  The development of functional coordination (a motor program) has occurred. 


The activity in the above chart does not require strength.  The activity requires coordinationThe power demonstrated in the chart on page 9 could only be developed if first the co-ordination required for the task is trustworthy.  This trust in one’s ability to move is developed by repetition of task in a coordinated fashion.  With this newly gained level of coordination, your patient will be more apt to utilize the injured area in daily life without fear of injury or pain.

   What good is a muscle if you can’t control it? 


Analysis of Phasic Exercise Motion Related to Sled Weight

Weight on Sled (Pounds)

Travel Distance (inches)

Repetitions per second

Average power per rep

Approximate duration to fatigue

Maximum reps/ session

Deceleration ratio to acceleration

High-Range Degree/Sec

Shoulder rotation

Stopping Time

Starting Time





2 min










10 sec














































Note: the sled weighs 3.2 pounds

The above chart exhibits the relationship of endurance, force, and sled weight when performing high performance phasic exercise utilizing the Impulse.  The subject here is a professional football player active with a prestigious NFL team.  The motion is the same as all the examples in all the charts above.  That is, external rotation, shoulder in neutral at 90 degrees flexion performing 90 degrees external rotation from 45 degrees of internal rotation.  The forces here are similar to those of the professional pitcher displayed earlier (page 9) with 10 pounds on the sled.  Utilizing this specific motion, motor unit substitution to maintain activity was greatly reduced.  In this example, the user was the most powerful in the 10 to 5 pound sled weight.  Coincidentally, he exhibited the least endurance in this sled weight range.  This individual was very familiar with Inertial Exercise and was asked to perform at 100% of his ability. 


This example gives a demonstration of the true functional nature of Inertial Exercise.  It also indicates that, when ingraining high performance activity, utilizing 5 to 7.5 pounds on the sled is appropriate.  In this motion, this seems to be the weight where the human body feels good about generating the maximal torque. 


It is not known but suspected that at the 2.5 and less sled weights everything is happening so fast that the user does not feel comfortable.  Sensory overload may prevent the user from just letting go with everything available.  Research is needed in this area as it has application to patient care as well.