The kiio Sensor; A New Innovation For Assessment of Muscle Function

Reliability, Responsiveness, and Criterion Validity of the Kiio Sensor, a New Tool for Assessment of Muscle Function

Patrick Grabowski, MPT, PhD; Michelle Narveson, DPT; Stephen Siegle, DPT


Musculoskeletal injuries are a leading cause of disability in the general population and U.S. military, resulting in enormous direct and indirect costs. Prerequisite to any program of injury management, from prevention to rehabilitation, is the ability to effectively quantify the many facets of musculoskeletal function. The World Health Organization defines function at three different levels:

  • the body part function and structure
  • the whole person
  • the whole person in a social context

Disability results from dysfunction at one or more of these levels – in the form of impairments, activity limitations, and participation restrictions, respectively. To effectively assess any individual, all three levels must be tested.

Although tests of the activity and participation levels are important for estimating daily function, they lack sensitivity to underlying impairments of muscle function allowing potential compensatory movements, which may predispose to future injury. For example, strength impairment of the ankle plantar flexors is a risk factor for development of Achilles tendinopathy. However, plantar flexor strength has very low correlation with tests of activity and participation. Thus, assessment at the level of body part must also be considered.

Additionally, activity and participation tests provide only limited information to guide the progress of individuals who fail to meet required standards. For instance, an individual who fails to achieve the required proficiency in a push-up test would benefit from knowing if this was related to weakness in the pectoralis musculature, the dynamic stabilizing muscles of the glenohumeral joint, or the scapulothoracic stabilizers. This is especially critical after injury, as medical and rehabilitative treatment primarily aim to resolve impairments in body part function and structure.

Therefore, to comprehensively guide decisions for musculoskeletal training – either to prevent or rehabilitate injury – specific objective measures at the level of key individual body parts is a necessity, and should be used in conjunction with other functional tests. Objective performance standards serve a number of important purposes, from identifying and reducing injury risk factors, to easing clinical decision-making with legally defensible criteria, to improving efficiency of fitness training and rehabilitation. To develop and administer specific, comprehensive, and validated performance standards, effective methods for measurement of muscle function must be identified.

Despite recent technology advancement, measurement of regional musculoskeletal function remains a complex entity rooted in long-standing paradigms. The most popular methods, which have been in use by clinicians and fitness professionals for decades, all have positive aspects but significant limitations.

Manual muscle testing (MMT) is the most widely used method. An examiner applies hand pressure to a body segment as the individual exerts countereffort. The examiner then subjectively rates the maximal exertion of the tested muscle on a 0 to 5 Likert-type scale. This technique is efficient and cost-effective, but unreliable and negatively impacted by human factors such as size and strength of the examiner. Additionally, this only provides a single metric of maximal isometric strength. Recent studies show that more complex metrics requiring a force/time curve (such as rate of force development) provide useful information for return to activity decisions. For these reasons, MMT fails to provide adequate data needed to develop performance standards.

On the other end of the spectrum are isokinetic dynamometers (ID), which provide highly complex and dynamic measures (such as indices of power and work) with excellent reliability. These are generally considered the gold standard for muscle assessment. The major limitations for IDs include high cost (up to $50,000 per instrument), nonportability with a large footprint (up to 64 square feet), and complex setup and operation. The ID instruments are computerized, recording a number of useful metrics based on a torque/time curve, but their high cost and labor-intensive methodology have limited their applicability on a large scale.

The most commonly used compromise between MMT and ID devices is handheld dynamometry (HHD). The HHD adds a quantitative force sensor, either spring-loaded or digital, to an MMT. This removes the subjectivity of MMT, and provides a quantifiable metric of force output. The devices are cost-effective, generally in the range of $1,000 to $2,000, and efficient to operate. Some have wireless technology to expedite data collection, and some have the ability to generate a force/time curve for more complex performance metrics. The major limitation for these devices is that numerous authors find them subject to the same human factors as MMT, such that if the muscle to be tested is stronger than the examiner, the data are no longer reliable.

A solution to this is to provide external stabilization to the device, which can be cumbersome and time consuming, and adds to the equipment necessary to conduct tests. Recently, tension myometers have become more appealing as strength testing devices (sometimes referred to as pull-type HHD). These strain gauges eliminate the examiner from the testing procedure, resulting in reliable strength data. Unfortunately, very little data exist on the measurement properties of myometers.

A new device, the kiio Sensor (from Kiio of Madison, Wisconsin), is a tension myometer that has been validated and shown to be reliable in mechanical laboratory testing, but has not been studied with human subjects (personal communication with Kiio). Therefore, the purpose of this study is to investigate the reliability, responsiveness, and criterion validity of the kiio Sensor in comparison to a gold standard ID.

FIGURE 1. Participant position for data collection with the (A) isokinetic dynamometer and (B) the kiio Sensor. The sensor is attached inline between the handle and a steel cable anchored to the locked dynamometer seat.



Forty-four (24 male, 20 female) civilian adults (age, mean [SD] = 21.2 [1.5] years) with no history of upper extremity injury in the last year and no current complaints of pain, weakness, or functional limitation participated. Males averaged 179 cm tall (range 168–199 cm), with mean weight of 84.1 kg (range 64.4–161.0 kg).


Individuals participated in two separate sessions 1 week apart, at the same time of day to maximize consistency of recent activity levels. Both sessions began with a 3-minute warm-up on an upper extremity ergometer, followed by submaximal isometric external rotation (ER) contractions at approximately 25, 50, and 75% of full effort.

FIGURE 2. Method comparison with difference calculated by subtracting the maximum force value obtained with the ID from that obtained by the kiio Sensor. The solid line represents mean difference between devices with the dotted lines showing the 95% limits of agreement.


Current methods of muscle assessment are limited in their ability to objectively quantify impairments of localized muscle function, rendering them impractical for widespread augmentation of comprehensive functional testing. The kiio Sensor demonstrates excellent reliability, responsiveness, and validity compared with a gold standard ID in a group of healthy participants. Because of several key attributes, this technology may be an excellent tool for muscle assessment in widespread settings, and with additional study could assist in the establishment, validation, and administration of objective performance standards. Although this study is limited to the measurement of isometric shoulder ER, the device appears readily applicable to the measurement of more complex metrics for many muscle groups, efficiently increasing the quantity of useful data for the prevention and rehabilitation of musculoskeletal injuries.

kiio Sensor


The kiio Sensor accurately measures, stores, and wirelessly communicates force over time during any exercise. On-board sensors, CPU, and special electronics calculate actual force, power, exercise pacing, and more. The kiio Sensor is integrated with kiio FLEX software to offer a comprehensive solution for assessing, training, monitoring, and improving the overall performance of patients and athletes. With 50 samples per second and over 99% accuracy, results are continuously and instantly delivered wirelessly throughout an exercise, making quantitative metrics available in real time for each and every repetition. Objective data is automatically recorded and displayed against target effort and timing, assisting with proper effort and frequency for each exercise. Audio and visual alerts indicate when limits are exceeded, helping to ensure proper performance and safety.


The kiio Sensor is approximately the size and weight of an average smartphone, and includes a rechargeable battery with greater than 6 hours of life per charge. In addition to allowing real-time transmittal of data to kiio FLEX software, the kiio Sensor can also store hundreds of hours of data, ready to be downloaded at a later time. The kiio Sensor thus enables objective data collection virtually anywhere, anytime.


The kiio Sensor is a highly adaptable in-line device. The patented quick interchange system allows the kiio Sensor to interconnect with almost any equipment. A short list of some products the kiio Sensor can connect with includes:


6.1″ x 2.7″ x 1.4″

4 oz

Battery Life:
8 hrs

Charge Time:
90 min

Charging Port:

WiFi Range:
up to 300 ft

Sample Rate:
50 ms

Built-in Storage:
microSD Card

Force Range:
0-250 lbs

>99% accuracy full scale

Units of Measure:
Pounds (lbs) Kilograms (kg) Newtons (N)


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