These are electromyograph (EMG) from a biceps muscle recordings taken during A and P I lab.  Remember that electrodes are placed along the muscle and they sense the voltage change as the muscle cell membranes depolarizes with an action potential.  The voltage difference (the y-axis or up and down trace) show the sum of all of the muscle cells whose membranes are depolarizing.  The x-axis (or moving across the screen) is just time passing.  The small      voltage change was bending the elbow with no weight.  The large voltage change   was lifting a heavy book. 

Here, the first EMG recording is in "concentric" firing of the biceps as the elbow bends and the second EMG is eccentric firing as the elbow is extended.

This is an isometric contraction. The arm was held immobile as the test subject tried to bend at the elbow.

Here, the left contraction of the muscle was just bending at the elbow while the right contracting was bending while resisting a strong pushing force against the hand. 

These small repeated firings of the muscle are isometric contractions--no movement at the elbow, while the biceps muscle holds the arm still as the test subject was snapping their fingers.

This represents the end of a long period--a full minute--as the test subject pulled up, isometrically without moving the arm--against the lab bench.  When a muscle fires for such a long time, it often keeps on showing electrical activity after the subject relaxes--a situation called "tetanus."  in this case, the test subject relaxed at the first blue line, but the muscle kept firing for a short time.

Biology Department                                                                                                                         In Ecuador:

Yavapai College                                                                                                                      Casilla 10-01-699

1100 East Sheldon Street                                                                                                            Ibarra-Ecuador

Prescott, AZ  86301                                                                                                            Tel:  593-62-608-789

Office:  4-233A                                                                                                                              Yahoo E-mail

Phone:  (928) 717-7628; (800)-922-6787

Frolich E-mail