Memorial University of Newfoundland

School of Human Kinetics and Recreation

HKR 3310: Exercise Physiology Lab 5

EMG-force relationship, ITT-force relationship


1.  A subject relaxes, contracts and then relaxes his/her quadriceps while EMG activity is measured. Is there exactly 0 millivolts of activity when the muscle is at rest? If there is some electrical activity (EMG), what does the EMG represent, and why or how are the muscle fibres activated?





Resting EMG amplitude                                    = ~ 30 microvolts

EMG amplitude during the contraction   = 4.98 millivolts
2. Using the BIOPAC system, a subject with EMG electrodes attached to the quadriceps will perform isometric contractions of increasingly greater resistance. Measure the EMG integrated signal for each contraction. Graph the integrated values with their associated weights. Explain the neuromuscular mechanisms underlying the EMG-force relationship.



First contraction:           Force = 362 Newtons              EMG amplitude = 3.1 mV

Second contraction:      Force = 575 Newtons              EMG amplitude = 4.1 mV

Third contraction:          Force = 772 Newtons              EMG amplitude = 4.9 mV
3. Interpolated Twitch technique (ITT)

            The subject will perform a series of contractions at 25%, 50% and 100% of MVC. During the contractions a maximal twitch will be elicited. In addition another twitch will be evoked following the contraction.

            Explain what the extra or superimposed evoked force above the voluntary force represents? The twitch following the contraction should be potentiated (larger) than the resting twitch. What are the mechanisms underlying this potentiation?



ITT at 25% MVC


Notice in the following figures how the superimposed (interpolated) twitch becomes smaller with increasing voluntary force.



ITT at 50% MVC



Near maximal contraction ITT with two superimposed (interpolated) twitches.

Notice the anti-dromic effect following the superimposed twitches.


4. Both EMG and ITT are supposed to represent muscle activation. What are the advantages and disadvantages of the two techniques?