Fatigability of the Quadriceps Muscle in Non-cooperating Subjects

Positioning of the subject: A special device, consisting of a rigid padded wooden board resting on metal bar, which was attached to the metal framing of the hospital bed, held the knee joint at a 90-degree angle and secured firm support of the entire thigh (Fig. 1). The positioning was chosen to reduce unnecessary movement and minimize risk for mechanical ventilated ICU patients. Isometric knee extension forces were therefore measured while subjects were placed in a supine position in a hospital bed with a 30-degree incline of the head and torso.

Electrical muscle stimulation: Two carbon electrode pads were placed distally over the motor point of the medial (vastus medialis) and lateral (vastus lateralis) heads of the quadriceps muscle. The motor point was defined as the location that corresponded to the lowest possible threshold current and the motor threshold current was defined as the lowest train stimulation current that resulted in visible muscle contraction. Another pair of electrodes (5x9 cm) was placed 5 cm distal to the inguinal fold (10). To optimize electrical conduction, the skin was shaved and rinsed before applying the electrodes. This approach was similar to previous studies (11).

Initially, two constant current high voltage stimulators delivered ten single stimuli twitches with biphasic square pulses at widths of 300 μs. A train generator was then switched on, triggering the delivery a 35 Hz current in bouts of 3 seconds periods separated by 1-second pause for a total of 40 tetanic contractions. In immediate succession to the tetanic contractions a second series of ten single twitch stimuli ended the protocol. Figure 2 show an example of an actual recording of the force response curve.

On the first experimental day testing current was adjusted to 75% above the motor threshold current. On the second testing day, training intensity was determined as the stimulation intensity corresponding to the same force output level (mV) as generated by the first tetanic contraction on the first testing day, while maintaining the same ratio between the medial and the lateral intensity levels.

The stimulation parameters and the two-channel stimulation method were chosen in order to increase the amount of stimulated muscle mass, ensure a safe, tetanic contraction in the non-cooperating subjects; to reduce risk of excessive muscle damage and to avoid recruitment of sensory fibers and thus minimize discomfort.