Biomedical Engineering Reference
In-Depth Information
pulses (Garland, 1992), or the increased forces produced at higher frequencies
(Russ, 2002) by trying to understand what aspect of the stimulation frequency
accelerates fatigue, researchers hope to develop new stimulation patterns such as
variable frequency trains that will delay the occurrence of muscle fatigue (Bickel,
2003).
7.3.3 Intermittent Stimulation
Muscle stimulation during FES exercise can be continuous or intermittent.
Intermittent stimulation ( Figure 7.1 , lower panel) involves repeated rhythmic
trains of stimulation separated by periods without stimulation. During intermit-
tent stimulation the “period” of repetition denotes the time between the start of
consecutive stimulation bursts (Franken, 1993). The on-time is the length of a
sequence of stimulation pulses during one cycle. The “duty cycle” is the on-time
divided by the period. Both the duty cycle and the period of the intermittent
stimulation have significant effect upon FES muscle fatigue.
Initially, a greater duty cycle produces a greater force time-integral (Lieber,
1993). However, fatigue rate increases with stimulation duty cycle (Baker, 1988),
such that over an extended session a greater force-time-integral may not neces-
sarily be produced (Lieber, 1993). This relationship is not surprising because
increased contraction times require more energy and produce more metabolic
byproducts, and because there is less recovery time between contractions. How-
ever, since a large proportion of the energy cost of a contraction is used to attain
the initial muscle force (Russ, 2002) there must be an optimum duty cycle that
represents the best trade-off between force output, fatigue, and the biomechanics
of the task.
During intermittent stimulation for protocols with equivalent duty cycles,
fatigue rate decreases as stimulation period is prolonged (Bergstr om, 1988; Hogan,
1998). Bergstr om et al's (1988) research, which investigated isometric muscle
contractions, demonstrated that even though the total muscle contraction time
and number of stimulation pulses were the same, fatigue developed faster in the
protocols with shorter excitation periods. The metabolic cost of attaining a force
is greater than maintaining the force. Hence, it is energetically 'expensive' and
inefficient to produce short contractions (Bergstr om, 1988; Russ, 2002). However,
if the length of the contraction is too long, then the amount of fatigue experienced
will be greater (McTague, 1991). During long contractions ischemia may be
induced by intramuscular pressure causing occlusion of blood circulation and
an acceleration of fatigue (Petrofsky, 1975; Petrofsky, 2000). Higher contraction
velocities also accelerate fatigue rate (Franken, 1993).
7.3.4 Electrode Types and Placements
A pair of electrodes is required to complete the circuit for current to flow through
the muscles to evoke contractions. Spacing of the electrode pair effects the current
 
Search WWH ::




Custom Search