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 EMG-force 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. 00.050.10.150.20.250.30.350 100 200 300 400EMG Amplitude (mV)Deltoid Force (N)00.050.10.150.20 100 200 300 400EMG Amplitude (mV)First Dorsal Interosseous Force (N)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. 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. Recording Electrodes 2‐3 mm apart, placed over the bulk of the biceps brachii muscle. Ground Electrode Placed over the lateral epicondyle (bony prominence) of the elbow (confluence of forearm extensor muscles). 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 2 3 4 Measure/Calculate these values: rjoint center to dumbbell = ____________ 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) 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 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