Isokinetic Dynamometers: Getting Voltage from Muscle Force

Examples of Isokinetic Dynamometers

One of the functions of an isokinetic dynamometer is to measure the amount of force the user of the machine is outputting. They do this in different ways, but one design goes as such:

  1. User applies force to the machine at it’s load cell (such as extending their leg like in the image above)
    Dynamometer Load Cell
  2. The strain gauge in the load cell deforms from the force

    Strain Guage Deformation

  3. The deformation of the strain gauge causes a change in it’s electrical resistance.
  4. Using a Wheatstone bridge and voltmeter, the change in resistance can be measured.

Wheatstone Bridge

So in the end, the computer program reads a voltage and from that determines the muscles force. It needs some sort of equation relate the two. We’ll walk through the steps in coming up with that equation.

  1.  How does force deform the load cell?
    When the user kicks his/her leg out against the machine, the load cell wants to stay in place, and the user isn’t able to accelerate the load cell no matter how hard they push. Since that force isn’t causing motion, it’s causing the load cell (and it’s strain gauge on the inside) to stretch a little bit, causing a CHANGE IN LENGTH. The load cell will follow the following response due to the stress it feels during the exercise:
    The materials chosen for the load cell will be such that the maximum force expected to be experienced by the machine would keep the plot in the linear, elastic region. In this region the load cell can elastically return to it’s original length when the force is removed and doesn’t permanently deform.

    1. We can determine the STRESS the load cell experiences if we control the LOAD CELL AREA. The MUSCLE FORCE is variable and depends on the user (see equation in plot)
    2. We can solve for STRAIN by using young’s modulus to relate STRESS to STRAIN
    3. We can get CHANGE IN LENGTH if we know STRAIN and the ORIGINAL LENGTH Of the strain guage.
    4. All together, the CHANGE IN LENGTH is given by this equation:
  2. How does deformation affect resistance of the strain gauge?
    The load cell deforming causes the strain gauge to deform the same amount. Let’s assume the strain guage is a cylindrical wire.

    Resistance of A Wire

    As long as the cross sectional area and resistivity of the wire are controlled, we can solve for the change in length to get the strain gauge’s electrical resistance.

    2. Bringing it all together
      With resistivity, original length of wire be constants and cross-sectional area changed being assumed miniscule, so also constant for simplicity
  3.  How is change in resistance measured?
    For this, a wheatstone bridge is used.
    Using the voltage divider rule we are able to know what the voltage at both sides of the path in the wheatstone bride above are. Using a voltmeter, we can read the difference in voltage across the two paths. The resistors R1,2,3 are all constants chosen by the designer. Only the strain gauge resistance is variable which scales based on the muscle force applied.

    1.  Bringing it all together:

Everything in this equation is constant besides muscle force. The material properties and sizes are all determined by the materials and dimensioning of the designer. Remember that we assumed that the cross sectional area of the wire remains constant. In actuality it thins a little as the wire is elongated, which would cause a a greater increase in the change in resistance than if it was constant. This greater increase in resistance would give us a smaller voltage readout.

Also, as the machine is used over time it’s material properties will degrade and some of the constants will shift, so calibration of this equation over time will be necessary for accurate results. But this gives us the fundamental equation to measure force using a dynamometer.


I hope that was easy to follow along. Of course, feel free to reach out in the comments below if you have any questions.

Still yet further another Exercise Machine again

In stark contrast to the common forms of weight training is isometric training, in which instead of moving weights the goal is to try as hard as you can and FAIL to move it.[1] This form of training has been shown to elicit increased strength development in some ways, and is often used in rehabilitation settings and is being used increasingly in recreation gym use. Unfortunately, these gains are also seen specifically in the joint angles that are trained. [1] This means that if you’re doing an isometric bicep curl with your elbow at 90 degrees, you’ll only develop strength at that 90 degree angle, not angles far from it.

To apply this training to the full range of joint motion, Isokinetic exercises can be used. Isokinetic exercises are exercises that are performed at constant speeds, regardless of how strongly one pushes or pulls. This allows for an isometric-like training across all joint angles. Studies have shown in some capacities isokinetic training leads to rapid growth, such as in jump athletes in which high speed isokinetic training had tenfold faster improvements when compared to other exercise groups[2].

In order to do isometric exercise, however, special exercise machines that move only at fixed speeds must be used. For elite training and physical training aspects, being able to regulate the speed as well as read out the force the user is applying to the machine to log their efforts. BIODEX is a company that designs isokinetic dynamometer exercise machines that do just that.

The Invention

  • Patent Title:
    1. Muscle exercise and/or rehabilitation apparatus using linear motion
  • Patent Number:
    1. US490777A
  • Patent filing date:
    1. May 25, 1988
  • Patent issue date:
    1. March 13, 1990
  • How long it took:
    1. 22 months
  • Inventors:
    1. Walter Gezari, Daniel Y. Gezari
  • Assignee
    1. Biodex Corporation
  • US Classification
    1. A63B21/154: Using flexible elements for reciprocating movements (ropes, chains, special pulley assemblies
    2. A63B21/0058 Exercising apparatus for developing or strengthening the muscles or joints of the body by working against a counterforce, with or without measuring devices using electromagnetic or electric force-resisters using motors
    3. Y10S482/90 Ergometer with feedback to load or with feedback comparison
    4. Y10S482/901 Exercise devices having computer circuitry
  • How many Claims:
    1. 24

This invention is an isokinetic dynamometer exercise machine used to simulate a whole body lift, as often done in manual tasks at a workplace. For example lifting up a box and placing it on an overhead shelf. This is a common function and one that needs to be rehabbed in injured manual labor populations in which goals include returned to work, and having a machine that works specifically on that function can be useful. This more accurately replicates the stresses on the spine as well as supporting muscles, ligaments, and tendons than a surrogate exercise would.

The claims on this patent describe all parts of the physical apparatus. As can be seen in the image above, this includes the base on lockable wheels upon which the user stands on. The user then holds onto the handles and attempts to slide the bar up along two vertical guide poles. Ropes are rigidly fixed to the bottom and top of the bar and are held vertical by passing through pulleys directly above and below the bar. These pulleys redirect the rope towards a wheel, upon which it tightly winds around.

This effectively converts the linear motion of the bar and rope to the rotational motion of the wheel. There is a shaft that rotates along with this wheel which connects to a dynamometer and a servo motor to fulfill it’s claims of being able to measure the speed and force at which the user moves the bar as well as control the speed at which the user can move the bar. This dynamometer filed under a different patent (US4691694) and as such will not be covered in the context of this post.

But this dynamometer doesn’t actually read the force or the linear speed at which the user is moving the bar. It’s reading rotational measures: torque and angular velocity. This information needs to go through some postprocessing in order to get the linear values. As the radius of the wheel is known, the conversion is done by the following equations:

Force  = Torque / Radius

Linear Velocity = angular velocity * radius

When the device is used for isokinetic motions in which the speed is controlled, the servo motor is engaged, which controls how the shaft and wheel rotate, therefore controlling the rope and the bar that the user is trying to lift. A servo motor is a motor that can rotate in both directions a certain amount at a certain angular velocity. This angular velocity is also calculated based the equations above and the desired linear speed set by the users.

Prior art has simulated the exercise of lifting as done in the workplace but they haven’t had any control systems for speed prior to this one. There have also been prior works that use servo motors to control rotational motions but not linear ones as lifting workplace items tends to be. The linear systems in the past have used other techniques that use force readouts of the person during exercise to determine the speed at which the exercise should be done, but these require complex systems with real time computation in order to function, and the motion may not be smooth as a result.

In comparison to these prior arts, this patent is novel in its ability to simulate linear loads in isokinetic ways, in being simple in doing so, and having a smooth motion in doing so as well.

[1]Smith, M. J., & Melton, P. (1981). Isokinetic versus isotonic variable-resistance training. The American Journal of Sports Medicine, 9(4), 275–279. doi: 10.1177/036354658100900420
[2]Folland, J. P., Hawker, K., Leach, B., Little, T., & Jones, D. A. (2005). Strength training: Isometric training at a range of joint angles versus dynamic training. Journal of Sports Sciences, 23(8), 817–824. doi: 10.1080/02640410400021783

Myofascia-nating, does myofacial release work?

Many people envision an evening under the hands of a masseuse as the perfect example of a relaxing experience, even as those hands dig deep into the “hurts so good” territory. This is often justified with the long term effects promised as the gain to the pain, such as relief of chronic pain, reduced soreness, and feeling looser. These claims are primarily backed by anecdotal evidence rooted in experiences of clients ranging from casual massage goers to professional athletes, as well as from those of practitioners.

Ubiquitously coined by masseuses, physical therapists, and athletic trainers, myofascial release is one of those deep soft tissue massage techniques that gets people excited to let them above their personal pain thresholds. But does myofascial release have any scientific evidence behind it’s effectiveness?

But what really is myofascial release?

The theory is that a primary factor in muscle tightness and pain is the condition of the sticky, gooey web that holds our muscle fibers together and to other parts of our body.

Fig 1: Fascia around and between layers of muscle

Originally posed by physician Stephen Typaldos came the idea that virtually all musculoskeletal injuries were due to distortions of connective tissue, particularly when sticky masses of fascia clump in between muscular fibers. Myofascial release is the practice of, in theory, releasing these fascial clumps to relieve tension on the musculoskeletal system. Dr. Typaldos found a lot of success in his career by using rubbing, friction, sliding, and pulling on acute pain- success that has inspired practitioners to adopt his strategies and aim for the best results.  However, while clumps of fascia melting away is easy to visualize, there is no scientific proof that distortions actually exists, nor that they can be removed by manual work. Some argue that fascia doesn’t even matter while others swear by it and pose theories to back their claims, but currently the mechanism, if any, behind myofascial release is unknown.

There is also no common consensus over how myofascial release is really used. For example, some professionals apply myofascial release to trigger points, which don’t seem to be related to fascia at all as per the video below:

Of course, for anyone looking for relief from pain and soreness myofascial release sounds like a good idea, but the costs of repetitively visiting therapists and masseuses is a deterrent. Thus enters the market of self myofascial release including products such as rollers and massage tools, where one can supposedly achieve myofascial release at home without the need of a practitioner. However, things get a little murky here as well – some argue that these techniques can’t actually achieve myofascial release, and there is no proof in either direction:

Ultimately, these grievances are rooted in the lack of understanding over the mechanism behind myofascial release. Even as these techniques aren’t tightly defined, we’re still left wondering the question: “But does it work?”

Does the massage work?

This one is tricky to answer, using scientific evidence, as there is a lack of high quality, highly controlled studies I myofascial release massage. A 2013 review on the effect of myofascial release on adults with orthopedic conditions found only 10 peer reviewed articles on the topic. Of these, 6 were case studies of which 5 had degrees of improvement ranging from slight to full recovery, and one on which the treatment failed. Of course, there is no way of knowing if massage really had any effect on those results. Of the studies, it was found that in treating plantar fasciitis, hamstring tightness, and misaligned pelvis myofascial release was useful (especially in plantar fasciitis, with on average 60% better pain reduction than the group without three months down the line!. However, the study with the largest subject was on low back pain, in which it was found that myofascial release did improve back pain, but no better than other manipulation techniques. There was no control in this study, however, so again it’s hard to tell if time alone healed all the subjects. With only one randomized control study (plantar fasciitis), I concurred with the authors here that there was a greater need for stronger studies on the subject.

A 2017 review looked only at randomized controlled trials where individuals and personnel were blinded to which treatment group they belonged in. Only 8 were found, all of which indicated myofascial release was beneficial. The conditions studied were tennis elbow and low back pain. Among these, two of them found that myofascial release on top of physical therapy was more useful than just physical therapy.

As we can see here, there really aren’t many conclusive studies on the matter, without a big enough sample of studies to draw a consensus from. I’ll agree it looks like from what we’ve seen myofascial release therapy seems to help, but only two of the above studies actually compare it to fake massage or other massage techniques. While those two studies found myofascial release was better than faking a massage, two are hardly enough to conclude that myofascial release is responsible for reduced pain and not just any massage. Looking at a third study, we see that there is no significant difference between myofascial release and Swedish massage in pain symptoms.

Okay…but what about foam rollers?

An example of a foam roller

Many people use foam rollers as a cheaper alternative to hands on massage to achieve myofascial release. It doesn’t look like we’re sure if myofascial release is even a real thing, but let’s not just go throwing our rollers out feeling dejected and lied to all along. Even if we aren’t sure how foam rollers work, they may still help.

One study looked at 20 gym-going males and how foam rolling affected them while doing a resistance squat and jump height protocol. They were evenly and randomly split into a foam rolling group and non foam rolling group. The study found that throughout their training which consistent of five consecutive days of exercise, the group that foam rolled had consistently lowered muscle soreness and improved range of motion at each time point. The participants that foam rolled did not have better gains in their squat one rep max, but did have better jump height improvement in comparison to the control. One limitation of this study is that the control group had no replacement to foam rolling, such as just laying down on foam, after their workouts, so there could have been another factor involved in the difference between the two groups.

Another study looked at the effect of foam rolling in delayed onset muscle soreness (DOMS), in which a squat regiment was used to induce the pain in both the no foam rolling and foam rolling group, who foam rolled immediately afterward the workout and then 24 hours later and 48 hours later for 20 minutes each time. The foam roller group had significantly reduced muscle soreness and increased tenderness of the quadriceps. The athletes had recorded performance measures such as sprint time and squat reps before the DOMS protocol, and the group the foam rolled had less reduction of performance 24, 48, and 72 hours after. Again, we can say the lack of a more robust control condition applies here, but again the results are promising.

So lets roll it all together

Even though it looks like overall we’re really not too sure what myofascial release massage is, how it works, or if its effective, we can still draw some conclusions from the research. The first is that myofascial release isn’t harmful. With neither the foam rollers or the manual massage did pain increase for subjects or performance decrease. Its true that myofascial release could be no different than any other massage in it’s effects, but they trend to show that whether or not a release of clumps of fascia occurs, the massage does help with pain for certain cases. The same thing goes for foam rolling the legs. Maybe no form of release is occurring at all, but spending the time to foam roll is showing to increase flexibility and reduce soreness at least over the span of time that DOMS is a factor. Importantly, there is no case here to say that if you feel like myofascial release helps you that there is any reason to give it up.

Questions to Consider:

Is it important to know how myofascial release works or just that it does work? If you had limited resources and to support one of those two types of studies, which would it be?

Are randomized controlled trials important to understanding how effective myofascial release is? Or is that being too strict, and looking at case studies and less controlled studies is sufficient enough? Why?


Meltzer, K. R., Cao, T. V., Schad, J. F., King, H., Stoll, S. T., & Standley, P. R. (2010). In vitro modeling of repetitive motion injury and myofascial release. Journal of Bodywork and Movement Therapies, 14(2), 162–171. doi: 10.1016/j.jbmt.2010.01.002

Whitehead, M., Jeffrey, E., khurana, A., Gail, Oster, D., Wilson, S., … Miller, C. (2018, March 8). Self Myofascial Release- What is MFR and how does it work? Retrieved from

Ingraham, P. (n.d.). Fascia Science Review. Retrieved from

Problems MFR Helps. (n.d.). Retrieved from

American Fascial Distortion Model Association. (n.d.). Retrieved from

Mckenney, K., Elder, A. S., Elder, C., & Hutchins, A. (2013). Myofascial Release as a Treatment for Orthopaedic Conditions: A Systematic Review. Journal of Athletic Training, 48(4), 522–527. doi: 10.4085/1062-6050-48.3.17

Laimi, K., Mäkilä, A., Bärlund, E., Katajapuu, N., Oksanen, A., Seikkula, V., … Saltychev, M. (2017). Effectiveness of myofascial release in treatment of chronic musculoskeletal pain: a systematic review. Clinical Rehabilitation, 32(4), 440–450. doi: 10.1177/0269215517732820

Liptan, G., Mist, S., Wright, C., Arzt, A., & Jones, K. D. (2013). A pilot study of myofascial release therapy compared to Swedish massage in Fibromyalgia. Journal of Bodywork and Movement Therapies, 17(3), 365–370. doi: 10.1016/j.jbmt.2012.11.010

Macdonald, G. Z., Button, D. C., Drinkwater, E. J., & Behm, D. G. (2014). Foam Rolling as a Recovery Tool after an Intense Bout of Physical Activity. Medicine & Science in Sports & Exercise, 46(1), 131–142. doi: 10.1249/mss.0b013e3182a123db

Pearcey, G. E. P., Bradbury-Squires, D. J., Kawamoto, J.-E., Drinkwater, E. J., Behm, D. G., & Button, D. C. (2015). Foam Rolling for Delayed-Onset Muscle Soreness and Recovery of Dynamic Performance Measures. Journal of Athletic Training, 50(1), 5–13. doi: 10.4085/1062-6050-50.1.01