Biomechanics Laboratory G Muscle Force Electromyography Introduction One of the major forces influencing human movement is muscle force The central nervous system modulates the force produced by muscle via two mechanisms recruitment varying the number of active motor units and rate coding varying the stimulation frequency of active motor units The electrical potentials associated with recruitment and rate coding can be measured using electromyography EMG techniques and this gives an indication of the activation of the muscle The mechanical effects of muscle force production can be quantified as the torque they produced about the joints Previous researchers have shown that in isometric static contractions there is a direct relationship between EMG signal amplitude and muscle force or torque production the higher the EMG amplitude the greater the force or torque produced by the muscle In some muscles the relationship between EMG amplitude and force appears to be nearly linear first dorsal interosseous muscle see bottom of the page while in other muscles a non linear relationship has been reported deltoid muscle see bottom of the page In the case of the non linear trends the amplitude of the EMG signal increases disproportionately more than muscle force i e the line curves upwards The most common explanation is 1 muscles which rely on both recruitment and rate coding over the full force range yield linear EMG force relations while 2 muscle which fully recruit all motor units at submaximal force have to rely on rate coding exclusively in the higher force range yielding a disproportionate increase in EMG amplitude at higher forces EMG can be measured easily and non invasively but direct muscle force measurement in humans is rarely possible it is too invasive However in the case of incremental isometric muscle contractions such as in holding increasingly heavy weights muscle force should change proportionally with the torque that must be produced to support the weights Thus it is possible to determine the features of the EMGforce relationship for various muscles or muscle groups even if muscle force is not measured directly In this lab you will determine the nature of the EMG force relationship for one of the muscles biceps brachii that generates flexion torque at the elbow joint You will determine the muscle torque necessary to hold increasingly heavy weights in your hand with your elbow fixed at 90 using the static equilibrium principle 0 35 EMG Amplitude mV EMG Amplitude mV 0 2 0 15 0 1 0 05 0 3 0 25 0 2 0 15 0 1 0 05 0 0 0 100 200 300 400 First Dorsal Interosseous Force N 0 100 200 300 Deltoid Force N 400 Methods Set up Biopac System Click on the desktop icon KIN430 EMG Lab and then follow the instructions within the program Set Up Subject Select one person in your group to be the subject Place the adhesive electrodes on the subject s biceps dominant arm on the locations indicated in the figure below The ground electrode which represents the baseline or zero electrical signal needs to be placed on the bony prominence of the elbow to ensure little to no electrical activity from nearby muscles Then connect the clips on the leads wires to the posts on the electrodes Connect the black clip to the electrode on the elbow and the red and white clips to the electrodes on the biceps muscle Ground Electrode Placed over the lateral epicondyle bony prominence of the elbow confluence of forearm extensor muscles Recording Electrodes 2 3 mm apart placed over the bulk of the biceps brachii muscle Recording data At the end of this process you should have a series of EMG amplitude measurements entered into the table below while holding 0 2 5 5 10 15 and 20 lbs weights in your hand Bend your arm such that the elbow joint is at 90 degrees forearm parallel to the floor Holding this position record the EMG signal for 2 seconds Calculate the EMG amplitude for this task using the computer software and record the value in the table below Grab the first dumbbell 2 5 lbs and put your elbow in the same static position elbow joint at 90 degrees record the EMG signal and calculate the EMG amplitude Repeat this process for the 5 10 15 and 20 lbs dumbbell weights Data Analysis The EMG activity will be collected and compared to the amount of torque and muscle force being produced at the elbow joint at each weight The data will be graphed to perform further analysis on the type of neuromodulation that occurs with each weight Elbow flexor force calculation Below is a diagram showing some of the important variables in this lab Using this information measure or calculate the required distances r or moment arms force and torque highlighted in bold Use the calculated and known values to solve for biceps muscle torque Tflexors and force Fflexors 1 Center of elbow joint rotation 2 Biceps muscle force Fflexors 3 Forearm hand center of mass COM 4 Dumbbell COM or point where Fdumbbell acts 1 2 3 4 Measure Calculate these values rjoint center to dumbbell 38 cm m distance from elbow joint center to dumbbell COM or distance from 1 to 4 rforearm hand COM 0 682 rjoint center to dumbbell m distance from elbow joint center to forearm hand COM or distance from 1 to 3 Fforearm hand Weightforearm hand 0 022 Total Body Weight N N Tforearm hand Fforearm hand rforearm hand COM Nm Known values Fdumbbell Weight of dumbbell N relbow flexors 0 045 m distance from elbow joint center to biceps muscle force or distance from 1 to 2 Use these equations to calculate for biceps muscle torque Tflexors and force Fflexors 1 2 3 Fdumbbell lbs 0 0 2 5 5 0 10 0 15 0 20 0 Fdumbbell N Tflexors Nm Fflexors N EMG Amplitude mV 0 1047 0 1984 0 2853 0 3398 0 4533 0 7427 Lab Report Prepare a brief lab report that contains the results and answers the questions described below You will submit the lab report through Turnitin on the course Moodle site 1 Calculate the force Fflexors that must be produced by the elbow flexor muscle group in order to maintain static equilibrium for each of the conditions as demonstrated in the free body diagram pictured above Include a table similar to the one above as well as written work showing how you calculated Tflexors and Fflexor for a dumbbell weight of 5 lbs in your lab report 2 Include a graph of EMG versus flexor muscle force Fflexors in your report You can make these using Excel or a similar program see example graphs on first page Describe the relationship you found between the EMG amplitude and the biceps muscle force