PMC4667789_01
Step-by-step stair descent achieved by functional neuromuscular stimulation, including description of phases: (a) double support, (b) leg off, (c)-(d) controlled lowering, (e) weight acceptance, (f)-(g) leg pull through, and (h) double support. Right limb begins descent cycle (leading) while left limb ends it (trailing).
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F1
Thus, the FNS-driven descent cycle was shortened to four phases: LO, controlled lowering phase (CL), weight acceptance, and LP (Figure 1). Stair descent was divided into five phases comprising distinct movements at each joint within each phase (Figure 1). Stair descent was initiated from quiet standing during double support (Figure 1(a)). LO was triggered by the user with a finger switch and began with hip and knee flexion under FNS control (Figure 1(b)). This relatively short period of flexion was followed by automatic activation of the lead leg knee extensors coincident with deactivation of ipsilateral hip and knee flexors and contralateral hip and knee extensors (Figure 1(c)). Lead limb hip extensors were automatically activated as hip flexors turned off (Figure 1(d)), preparing the limb for loading after ground contact. Trailing limb hip extensors were activated shortly after leading limb hip extensors to prevent forward trunk tilt and aid in stability after weight acceptance (Figure 1(e)). Once this occurred, trailing limb hip extensors were ramped off as knee and hip flexors were turned on to facilitate the first phase of LP (Figure 1(f)). Similar to LO, this flexion period lasted for a short time, after which hip and knee extensors were activated while flexors were deactivated, allowing extension of the trailing limb prior to double support (Figure 1(g)). At this point, knee and hip extensors remained on bilaterally to provide body weight support during double stance (Figure 1(h)), returning to the initial state in the sequence in preparation for descent to the next step.
Synthesized 14-channel stimulation pattern for stair descent with hybrid neuroprostheses including variable impedance knee mechanism as function of time. Phases of stair descent (Figure 1) are indicated on top of figure. Stimulus pulse width (in micro-seconds) of each muscle is plotted. Shading represents two fre-quencies of stimulus pulse train, 33.33 Hz (black) and 16.67 Hz (gray). Pattern is triggered by user pressing finger switch; dotted line (~2.7 s) is point at which stimulation is held between states 2 and 3 while waiting for second finger switch input (see Figure 3). Electrodes for stimulation of tibialis anterior (TA) were implanted bilaterally; however, ankles were locked using ankle-foot orthoses for stair descent and thus TA stimulation was omitted. CL = controlled lowering, DS = double support, Gl Max = gluteus maximus, Grac = gracilis, Int = vastus intermedius, L = left, Lat = vastus lateralis, LO = leg off, LP = leg pull through, Med = vastus medialis, P Add = posterior portion of adductor magnus, Quad = quadriceps, R = right, Sart = sartorius, Tensor = tensor fasciae latae, WA = weight acceptance.
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F2
The subject-specific stimulation pattern for stair descent is shown in Figure 2. The periods of high and low frequency are denoted in Figure 2 by black and light gray areas, respectively. The stimulation pattern in Figure 2 was generated using a procedure similar to one previously developed for walking. The resulting pattern of descent is shown in Figure 2, while muscle activity during each phase is described in detail subsequently.
Hybrid neuroprosthesis including variable impedance knee mechanism (VIKM-HNP) finite state machine showing status of leading right (R) and trailing left (L) limb VIKM damper and knee extensor stimulation (KE Stim) within each state with transition between states indicated. Note that only knee extensor muscles are modulated by controller; all other muscles remain unaffected by finite state machine. Above each state, resistance level of VIKM for leading (R) and trailing (L) knee is depicted in schematic. Black circle indicates highest duty cycle (lock knee to prevent motion), gray circle indicates moderate duty cycle to regulate knee motion, and white circle indicates no activity to allow unencumbered knee motion.
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F3
Conceptual operation of the controller for stair descent led by the right leg proceeded as follows (Figure 3).
Experimental setup for evaluation of stair descent with hybrid neuroprosthesis including variable impedance knee mechanism. Four six-axis load cells were placed below mounting points of hand rails to monitor upper-limb force. Force platforms were installed at each level to measure ground reaction forces. Once user completed descent, he transferred support to walker and stepped away from platform to sit. Spotter was present to ensure safety.
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F4
Because a step-by-step strategy was employed, an experimental staircase composed of a single step was constructed by positioning a platform 8 in. above the floor (Figure 4). For safety, a handle was placed on the back of the trunk corset and a spotter remained in close proximity to the subject at all times to provide extra support if needed (Figure 4).