in fig1 a manual muscle tester 10 according to this invention includes a case 11 with a digital display 12 representing a force f applied to an actuator 13 parallel to the axis of movement 14 of the actuator . on the case of the instrument 10 can be seen , in fig1 a switch 16 . this is the unit &# 39 ; s on - off switch . a switch 18 labeled &# 34 ; reset &# 34 ; sets the meter back to zero as described more fully below . an adjusting screw 20 labeled &# 34 ; zero &# 34 ; sets the display 12 at zero when no force is applied , and an adjustment screw 22 labeled &# 34 ; cal &# 34 ; operates in cooperation with a switch 23 labeled &# 34 ; cal &# 34 ; to calibrate the meter whose display is shown at 12 . the actuator 13 has a stem 24 ( fig3 ) extending into an opening 25 in the case 11 . the stem terminates in a threaded end 26 of reduced diameter as shown in fig3 . in this figure , one sees a cantilever beam or flexure 28 bored and tapped at 29 to receive the threaded end 26 of the actuator stem . the cantilever beam 28 has a t shape with down - turned arms 31 and 32 . these have tapped and threaded openings 33 and 34 aligning with openings 35 and 36 in a mounting block 38 . a pair of bolts 41 and 42 extend upward through the openings 35 and 36 and thread into the openings 33 and 34 to secure the down - turned arms of the t - shaped beam to the block 38 , thus supporting the block in cantilever fashion . a recess 39 in the block 38 aligns with the longer central stem 30 of the t - shaped cantilever beam 28 and permits the beam &# 39 ; s central stem 30 to flex downward between its arms 31 and 32 without interference by the mounting block . in addition to the mechanical components of the instrument , in fig3 one sees a pair of nine volt batteries 44 and 45 that power the unit . the printed circuit card 47 that is the digital volt meter whose display is shown at 12 in fig1 appears in this figure , and a printed circuit card 48 that cooperates with a strain gage bridge to translate input force to the analog signal read by the digital volt meter 47 appears , as well . in fig3 two of the strain gages 50 and 51 that make up the bridge can be seen . two further gages are similarly positioned on the under surface of central stem 30 of the beam 28 . the diagram of fig4 illustrates the manner in which the beam 28 , the actuator 13 , and the strain gage bridge translate force parallel the axis 14 ( or perpendicular to the face of the beam ) into an electrical signal indicative of that force only . that parallel force is designated f in fig4 and a force component perpendicular to the axis 14 is designated s . the two additional strain gages 52 and 53 are seen located spaced apart lengthwise on the stem 30 of the beam 28 directly below the gages 50 and 51 . the four gages are connected together in a bridge circuit 55 , illustrated in fig6 . a reference or input voltage is applied across the bridge from a junction 56 of the gages 51 and 50 to a junction 57 of the gages 52 and 53 . a force indicative output signal v is taken from the remaining junctions 58 and 59 . the strain measurement m 1 in the beam 28 at the location of the gages 50 and 52 is : a is the distance between the two gage locations lengthwise of the beam ; l is the distance between the gages 51 and 53 and the stem 24 ; s is the force parallel the beam ( normal to the axis 14 ); and the strain measurement m 2 at the gages 51 and 53 is : as connected , the bridge subtracts the two measurements to arrive at m 1 - m 2 . substituting : therefore , the bridge output is directly proportional to f , the force normal to the beam . in fig5 a typical test procedure is shown . the patient or test subject sits , elevates his leg and resists the downward force f applied by the examiner . the peak force f which will appear and remain on the display of the meter of the unit 10 will be the force needed to overcome the patient &# 39 ; s resistance and move the knee downward . the forces s transmitted substantially parallel the upper leg from knee to hip will not contribute to the force measured by the instrument . this force , communicated back along the femur to the hip , is only the resistance of the patient to movement backward on the table and is no part of the force f that the muscles exert in resistance to the examiner &# 39 ; s downward force f . fig6 illustrates the circuitry of the manual muscle tester . the two small nine volt batteries 44 and 45 supply the circuit through the on - off switch 16 . a commercial voltage regulator 65 ( national semiconductor lm330 , for example ) provides a positive five volts across a capacitor 66 connected between a positive input or supply line 67 and a zero volt reference or supply line 68 . a voltage convertor 70 serves as a power supply . connected in its conventional manner between the lines 67 and 68 a commercially available integrated circuit 72 ( such as the icl 7660 of intersil ) provides a negative five volts to a line 73 . the ten volts between line 73 and the line 67 are supplied as the input to the strain gauge bridge 55 . the bridge output , at junctions 58 and 59 , is applied , via lines 81 and 82 to input terminals of an instrumentation amplifier 85 ( e . g . analog devices ad521 ). the amplifier 85 is connected to provide a gain of 200 . the output of the instrumentation amplifier is fed via resistance 89 to a &# 34 ; peak freeze &# 34 ; or peak detection circuit 86 . this output is made available , for example , at a jack 88 , as an analog output suitable for recording and / or display . the analog representation represents force throughout the time of the test procedure and does not freeze at peak force . the peak detection circuit 86 includes a first operational amplifier 87 ( such as rca &# 39 ; s ca 3130 ). the amplifier is connected to have a gain of one . a pair of diodes 90 and 91 connect the output of the operational amplifier 87 to a 0 . 47 μf peak freeze capacitor 93 . the diodes prevent discharge of the peak reading or peak freeze capacitor . the peak freeze capacitor 93 and a 330 ohm resistor are connected in series between the zero volt reference line 68 and an input to a second operational amplifier 97 ( e . g ., national semiconductor lh0052 ). this amplifier 97 is provided in the feedback loop of the first operational amplifier 87 along with a 20kω resistance 98 . it is the output of this second operational amplifier 97 that , through the potentiometer 22 , supplies the digital meter that indicates input force . inclusion of the second amplifier 97 protects the peak freeze capacitor 93 from bleed - off by virtue of its high input impedance . use of the two diodes 90 and 91 allows a high resistance 101 to be connected between the feedback from the second operational amplifier to the junction of the two diodes . the resistance 101 , one mω for example , keeps the anode side of the diode 91 at the same high voltage as the output of amplifier 97 after the output voltage from the amplifier 87 has dropped . this prevents drain - off of the peak freeze capacitor 93 by slight reverse current back through the diodes 90 and 91 when the input force to the manual muscle tester has been reduced and the output of the operational amplifier 87 drops . connection of the two diodes 90 and 91 along with the operational amplifier 97 in the feedback loop of the amplifier 87 also removes the &# 34 ; knee &# 34 ; or nonlinear portion of the characteristic curve of the diodes to produce a much more linear relation between the input to the peak freeze circuit and its output . by virtue of the apparent lack of feedback the gain of the peak freeze circuit , which can be viewed as a single amplifier , increases dramatically in the low voltage region , where the diode &# 34 ; knee &# 34 ; is evident , until the output is clamped at the input voltage . a diode 99 connected from the output of the operational amplifier 87 to the feedback path connection that amplifier &# 39 ; s inverter terminal prevents the output of that amplifier from going negative during the zeroing and meter resetting described below . the adjustable arm 103 of the potentiometer 22 provides the input to the digital meter 47 . the meter is a digital volt meter ( e . g ., printed circuit international &# 39 ; s pcim 176 ) with reference connections connected between the line 67 at positive five volts and the line 73 at negative five volts . the potentiometer 22 calibrates the meter . the meter connection is ratiometric ; the meter reference voltage is the same ten volts as the bridge input . the meter compares the peak freeze output of the amplifier 97 with the ten volts across the bridge . changes in the input or reference voltage affect proportionally the force indicative voltage applied to the meter and do not affect the ratio that the meter reads . thus , such changes do not result in error in the displayed value . calibration of the meter is checked by closing a calibration switch 23 . the switch is in series with a calibration resistance 106 and connects the resistance 106 in parallel with one of the strain gage resistances of the bridge . this imparts a standard imbalance to the bridge . each time the calibrate switch 23 is closed , then , the same digital reading should appear on the meter . if this does not occur , correction is made at the potentiometer 22 . to reset the instrument to zero , having tested a patient and noted the force indicated by the meter , a double pole single throw reset switch 18 is closed . one pair of contacts 18 &# 39 ; connects the output of the amplifier 97 through a resistance 109 to an input of yet another operational amplifier 110 ( again , lh0052 , for example ). this amplifier is connected as an integrator . its storage capacitor 112 in its feedback path stores the voltage necessary to produce a zero meter reading when no force is applied to the actuator 13 . a variable resistance 20 is shown connected to the amplifier 110 in the manner that is conventional for adjusting offset voltage . this variable resistance has been adjusted initially , in the absence of any input force , to bring the meter 47 to zero . the adjusted offset voltage , stored by the capacitor 112 , is the slight voltage needed to compensate , for example , slight error voltages such as may appear from the bridge or as offset voltages of the instrumentation and further operational amplifiers . when the contacts 18 &# 39 ; close , the negative integral that is the output of the zeroing amplifier 110 is supplied to pin 11 of the instrumentation amplifier 85 . pin 11 has a gain of one relationship with the output pin 7 . hence , the negative integral is applied as an input to the first of the operational amplifiers , amplifier 87 . this drives the output of that amplifier 87 to zero and discharges the peak freeze capacitor 93 through a resistance 111 and the remaining contacts 18 &# 34 ; of the double pole single throw reset switch 18 . the output of the amplifier 97 becomes zero and the digital meter registers zero . thus it will be seen that a compact , easy to read , and accurate manual muscle tester is provided . the forces applied other than along the instrument actuator &# 39 ; s axis of movement are cancelled . the circuitry translating the input force to the digital output display enhances easy reading through its peak detection &# 34 ; peak freeze &# 34 ; operation . an analog voltage is also provided so that a complete time history of a test may be recorded , written on a strip chart recorder , or otherwise displayed . the circuit is especially adapted to be largely impervious to input or reference voltage variations through its ratiometric meter operation . the size , arrangement of the physical package , and the few adjustments necessary make the instrument easy to use by physicians , physical therapists , trainers , and coaches , unskilled in the operation of complex instruments . its manner of use is very much like the subjective manual muscle tests already familiar to many . although a specific , preferred embodiment has been described in detail , modifications within the spirit and scope of this invention will be readily apparent . for example , the analog peak reading circuiting of the &# 34 ; peak freeze &# 34 ; provisions can be accomplished digitally by , for example , analog to digital convention of the bridge output , storage of the peak value in memory and display or stored peak in units of force . accordingly , the foregoing description of the preferred embodiment is not to be construed as limiting the scope of this invention as defined in the appended claims .
a portable manual muscle tester has an external actuator , a case capable of being held in one hand and a digital display indicating peak force applied to the actuator . strain gages connected in a bridge circuit are located on a flexure contacted by the actuator . the flexure is a cantilever beam shaped to fit the small case and to prevent reflected or mounting stresses being registered in the force display . the circuit from the strain gage bridge to the display includes an instrumentation amplifier followed by a peak freeze circuit that includes operational amplifiers and a peak freeze capacitor . the peak freeze capacitor provides a voltage that is read by a digital volt meter . a further operational amplifier connected as an integrator feeds back from the peak freeze circuit to an input of the instrumentation amplifier to reset the meter to zero after the displayed force reading has been noted .