Full Length Article| Volume 35, ISSUE 3, P488-500, July 2022

Reliability and validity of a novel instrument for the quantification of hand forces during a jar opening task

Published:April 15, 2021DOI:


      • The intrarater reliability and ecological validity of a novel and true-to-form jar instrument was tested in women with hand osteoarthritis
      • Intrarater reliability of the jar instrument was good to excellent and participants reported the experience of using it to be highly representative of jar-opening at home.
      • The jar tool may be used in future study of the required hand kinetics for successful opening of a sealed jar and for testing of joint protection strategies in this population.


      Study Design

      Clinical Measurement


      Jar opening is a task that relies heavily on the ability to generate adequate hand forces and is often reported in the literature as being difficult for women with hand arthritis. Many have studied relationships between diminished grip/pinch strength and occupational performance but few have investigated how much hand force is necessary to successfully engage in manual tasks. Those who studied this have relied on approaches and tools which lack ecology. Additionally, few have instrumented daily objects to understand if joint protection techniques do, in fact, reduce the hand force generation when performing manual tasks.

      Purpose of the Study

      The objectives of this study were to one) determine the within session repeatability of a device used to quantify these forces and two) probe into the ecological validity of a novel device to be used in the future study of women with hand osteoarthritis to measure hand force requirements and study the impact of joint protection interventions on hand force profiles.


      A plastic jar was instrumented with a torque limiter, 6-axis load cell, and six force sensing resistors so as to capture the grip and compressive hand forces which act on a jar lid when opening a sealed jar. To assess intra-rater reliability of the tool and its testing procedures, 31 adult women with hand osteoarthritis were asked untwist the jar's lid so as to break its seal with each hand twice while stabilizing the base with the opposing hand. The agreement between trials of peak forces and torques from each hand was assessed through statistical approaches including Intraclass Correlation Coefficient, Standard Error of the Measurement, and Minimal Detectable Change. The jar's ecological validity was then assessed via survey.


      This instrument and methods yielded good to excellent repeatability across all force outputs. The majority of our subjects (87%) reported the jar to be similar to those used at home, 87% reported to use a similar jar 2-3 times/month or greater, and rated the importance of opening such a jar as being, on average, 8.7/10.


      The jar instrument appears to have high reliability and ecologic validity. It has the potential to reveal hand force requirements for a population known to have difficulties opening jars and understanding these force thresholds could help to inform therapy goal-setting. Beyond this, it has the potential to support the study of the hand forces used across various joint protection approaches so as to inform best “jar-opening” practices.


      This tool was designed to represent the qualities of a standard, large sealed jar while still housing sophisticated kinetic measurement capacities. Our findings support that we have presented a tool which can be used in future study within this population to better understand the hand kinetics associated with the highly problematic task of jar-opening and joint protection strategies intended to reduce hand loads.


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      JHT Read for Credit

      Quiz: # 875

      Record your answers on the Return Answer Form found on the tear-out coupon at the back of this issue or to complete online and use a credit card, go to There is only one best answer for each question.
      • # 1.
        One stated purpose of the study was to determine the ___________ of the jar instrument
        • a.
        • b.
        • c.
        • d.
      • # 2.
        Participating subjects were
        • a.
          patients with either OA, or RA of the hand
        • b.
          all normal student volunteers
        • c.
          post op CMC arthroplasty patients
        • d.
          either physician-diagnosed, or exhibiting symptoms of hand OA
      • # 3.
        The most significant expression of the required force was
        • a.
          measured in ft lbs
        • b.
          rotational force
        • c.
          opening torque
        • d.
          opening power
      • # 4.
        Participants felt the jar device was
        • a.
          too difficult to maneuver repeatedly
        • b.
          reasonably similar to the activity they experience at home
        • c.
          too easy to open
        • d.
          uncomfortable in actual use
      • # 5.
        The authors conclude that the device is suitable for clinical application
        • a.
        • b.
      When submitting to the HTCC for re-certification, please batch your JHT RFC certificates in groups of 3 or more to get full credit.