Joaquin Andres Hoffer

Biomechanical Energy Harvesting: Generating Electricity During Walking with Minimal User Effort

We have developed a biomechanical energy harvester that generates electricity during human walking with little extra effort. Unlike conventional human-powered generators that use positive muscle work, our technology assists muscles in performing negative work, analogous to regenerative braking in hybrid cars, where energy normally dissipated during braking drives a generator instead. The energy harvester mounts at the knee and selectively engages power generation at the end of the swing phase, thus assisting deceleration of the joint. Test subjects walking with one device on each leg produced an average of 5 watts of electricity, which is about 10 times that of shoe-mounted devices. The cost of harvesting—the additional metabolic power required to produce 1 watt of electricity—is less than one-eighth of that for conventional human power generation. Producing substantial electricity with little extra effort makes this method well-suited for charging powered prosthetic limbs and other portable medical devices.

Roles of muscle activity and load on the relationship between muscle spindle length and whole muscle length in the freely walking cat.

The objective of this research was to compare the length of muscle spindles to the length of the whole muscle, during normal movements. Pairs of piezoelectric crystals were implanted near the origin and insertion of muscle fibres in the medial gastrocnemius (MG) muscle of cats. The distance between crystals was measured with pulsed ultrasound, the origin-to-insertion length of the MG muscle was measured with a transducer made of saline-filled silicone tubing, MG force was measured with a tendon force transducer and EMG activity was selectively recorded in the vicinity of implanted crystals. These signals were simultaneously recorded during posture or locomotion on a motorized treadmill. Three periods were identified in the step cycle, during which the relation between muscle length and spindle length changed dramatically. In period I (roughly corresponding to the late F and E1 phases of swing), the MG muscle and spindles followed similar length changes: both were stretched and then shortened by about 6 mm. In period II (corresponding to the stance phase, E2-E3) the MG muscle yielded under the weight of the body and was stretched by 1-3 mm, whereas the MG spindles typically continued shortening. In period III, the MG muscle shortened rapidly by 6-8 mm after the foot left the ground and then stretched again by about the same amount, whereas the spindles could remain nearly isometric. We attribute these large discrepancies in muscle and spindle length to the architecture of the MG muscle and the compliance of long tendinous elements in series with the spindles. We conclude that the length changes imposed on muscle spindles during voluntary movements are not simply related to the parent muscle length changes and cannot be estimated without taking into account the muscle architecture, the location of the spindle within the muscle, the level of muscle activation and the external load.

Skin contact force information in sensory nerve signals recorded by implanted cuff electrodes

When functional neuromuscular stimulation (FNS) is used to restore the use of paralyzed limbs after a spinal cord injury or stroke, it may be possible to control the stimulation using feedback information relayed by natural sensors in the skin. In this study the authors tested the hypothesis that the force applied on glabrous skin can be extracted from the electroneurographic (ENG) signal recorded from the sensory nerve. They used the central footpad of the cat hindlimb as a model of the human fingertip and recorded sensory activity with a cuff electrode chronically implanted around the tibial nerve. Their results showed that the tibial ENG signal, suitably filtered, rectified, and smoothed carries detailed static and dynamic information related to the force applied on the footpad. The authors derived a mathematical model of the force-ENG relation that provided accurate estimates of the ENG signal for a wide range of force profiles, amplitudes, and frequencies. Once fitted to data obtained in one recording session, the model could be made to fit data obtained in other sessions from the same cat, as well as from other cats, by simply adjusting its overall gain and offset. However, the model was noninvertible; i.e., the force could not be similarly predicted from the ENG signal, unless additional assumptions or restrictions were introduced. The authors discuss the reasons for these findings and their implications on the potential use of nerve signals as a source of continuous force feedback information suitable for closed-loop control of FNS. >

Restoration of use of paralyzed limb muscles using sensory nerve signals for state control of FES-assisted walking

A real-time functional electrical stimulation (FES) state controller was designed that utilized sensory nerve cuff signals from the cat forelimb to control the timing of stimulation of the Palmaris Longus (PalL) muscle during walking on the treadmill. Sensory nerve signals from the median and superficial radial nerves provided accurate, reliable feedback related to foot contact and lift-off which, when analyzed with single threshold Schmitt triggers, produced valuable state information about the step cycle. The study involved three experiments: prediction of the timing of muscle activity in an open-loop configuration with no stimulation, prediction of the timing of muscle activity in a closed-loop configuration that included stimulation of the muscle over natural PaIL electromyogram (EMG), and temporary paralysis of selected forelimb muscles coupled with the use of the state controller to stimulate the PalL in order to return partial support function to the anesthetized limb. The FES state controller was tested in a variety of walking conditions, including different treadmill speeds and slopes. The results obtained in these experiments demonstrate that nerve cuff signals can provide a useful source of feedback to FES systems for control of limb function.

Artifact-free sensory nerve signals obtained from cuff electrodes during functional electrical stimulation of nearby muscles

Restoration of the voluntary use of paralyzed limbs using functional neuromuscular stimulation (FNS) is limited by complex muscle properties and unpredictable load behaviors; closed-loop control of FNS would improve performance but requires reliable sensory feedback modalities. Sensory nerve signals recorded by cuff electrodes provide accurate information about forces acting on the skin in anesthetized animals; however, nerve cuff signals are very small (approximately 10 /spl mu/V), and during FNS they become contaminated with large stimulation artifacts and synchronous EMG potentials from nearby muscles. The authors show in this study that it is possible to record neural signals from the cat tibial nerve without interference from distributed stimulation of four calf muscles surrounding the recording electrode by use of high-pass filtering and synchronized bin-integration. Nerve signals sampled in this way retained all the information about footpad contact force that was normally obtained in the absence of muscle stimulation. The authors propose that this approach has wide applicability for rehabilitation of paralyzed people with neural prostheses. >

Gait phase information provided by sensory nerve activity during walking: applicability as state controller feedback for FES

In this study, the authors extracted gait-phase information from natural sensory nerve signals of primarily cutaneous origin recorded in the forelimbs of cats during walking on a motorized treadmill. Nerve signals were recorded in seven cats using nerve cuff or patch electrodes chronically implanted on the median, ulnar, and/or radial nerves. Features in the electroneurograms that were related to paw contact and lift-off were extracted by threshold detection. For four cats, a state controller model used information from two nerves (either median and radial, or ulnar and radial) to predict the timing of palmaris longus activity during walking. When fixed thresholds were used across a variety of walking conditions, the model predicted the timing of EMG activity with a high degree of accuracy (average error=7.8%, standard deviation=3.0%, n=14). When thresholds were optimized for each condition, predictions were further improved (average error=5.5%, standard deviation=2.3%, n=14). The overall accuracy with which EMG timing information could be predicted using signals from two cutaneous nerves for two constant walking speeds and three treadmill inclinations for four cats suggests that natural sensory signals may be implemented as a reliable source of feedback for closed-loop control of functional electrical stimulation (FES).

Biomechanical energy harvesting: Apparatus and method

A biomechanical energy harvester is presented that generates electricity during human walking. The key feature of this device is that the power generation adds only a minimal extra effort to the user. The knee-mounted devices accomplish this by selectively engaging power generation at the end of the swing phase when knee flexor muscles act to brake knee motion. Analogous to regenerative braking in hybrid cars, the device assists deceleration of each leg within each stride while generating electrical power. We developed a control system to engage/disengage power generation based on the measured knee kinematics during a gait cycle. Experimental results show that generative braking generated 4.8 plusmn 0.8 W of electrical power with a minimal increase in metabolic cost.

The effect of positioning on the biomechanical performance of soft shell hip protectors

Wearable hip protectors represent a promising strategy for reducing risk for hip fracture from a sideways fall. However, small changes in pad positioning may influence their protective benefit. Using a mechanical hip impact simulator, we investigated how three marketed soft shell hip protectors attenuate and redistribute the impact force applied to the hip, and how this depends on displacement from their intended position by 2.5 or 5 cm superiorly, posteriorly, inferiorly or anteriorly. For centrally-placed protectors, peak pressure was reduced 93% below the unpadded value by a 16 mm horseshoe-shaped protector, 93% by a 14 mm horseshoe protector, and 94% by a 16 mm continuous protector. In unpadded trials, 83% of the total force was applied to the skin overlying the proximal femur (danger zone). This was lowered to 19% by the centrally placed 16 mm horseshoe protector, to 34% by the 14 mm horseshoe, and to 40% by the 16 mm continuous protector. Corresponding reductions in peak force delivered to the femoral neck (relative to unpadded) were 45%, 38%, and 20%, respectively. The protective benefit of all three protectors decreased with pad displacement. For example, displacement of protectors by 5 cm anteriorly caused peak femoral neck force to increase 60% above centrally-placed values, and approach unpadded values. These results indicate that soft shell hip protectors provide substantial protective benefits, but decline in performance with small displacements from their intended position. Our findings confirm the need for correct and stable positioning of hip protectors in garment design.

A measure of peripheral nerve stimulation efficacy applicable to H-reflex studies.

BACKGROUND When H-reflexes are recorded during movement in human subjects, the stimulator current output is not a good indicator of sensory stimulation efficacy because of unavoidable nerve movement relative to the stimulus electrodes. Therefore, the M-wave amplitude has been used by researchers as an indicator of the efficacy of the stimulus. In this study we have examined the general validity of the hypothesis that the M-wave amplitude is directly proportional to the group I sensory afferent volley evoked by the stimulus. METHODS A nerve recording cuff, stimulating electrodes, and EMG recording electrodes were implanted in cats. Nerve cuff recordings of centrally propagating volleys evoked by electrical stimuli were directly compared to M-waves produced by the same stimuli. Compound action potentials (CAPs) recorded in the sciatic nerve were compared with soleus M-waves during either tibial nerve or soleus muscle nerve stimulation. CAPs in the ulnar nerve were correlated with flexor carpi ulnaris M-waves during ulnar nerve stimulation. RESULTS AND CONCLUSIONS Our findings indicate that for mixed nerve stimulation (e.g., tibial or ulnar nerve) the M-wave can be a reliable indicator of the centrally propagating sensory volley. Due to the high correlation between CAP and M-wave amplitude in these nerves, a small number of M-waves can give a good estimate of the size of the group I sensory volley. On the other hand, when nerves with only partially overlapping fibre diameter populations are stimulated (e.g., the soleus muscle nerve), the M-wave is not well correlated with the group I sensory volley and thus may not be used as a measure of the size of the input volley for H-reflex studies.

Mitigation of Ventilator-Induced Diaphragm Atrophy by Transvenous Phrenic Nerve Stimulation.

Rationale: Ventilator‐induced diaphragm dysfunction is a significant contributor to weaning difficulty in ventilated critically ill patients. It has been hypothesized that electrically pacing the diaphragm during mechanical ventilation could reduce diaphragm dysfunction. Objectives: We tested a novel, central line catheter‐based, transvenous phrenic nerve pacing therapy for protecting the diaphragm in sedated and ventilated pigs. Methods: Eighteen Yorkshire pigs were studied. Six pigs were sedated and mechanically ventilated for 2.5 days with pacing on alternate breaths at intensities that reduced the ventilator pressure‐time product by 20‐30%. Six matched subjects were similarly sedated and ventilated but were not paced. Six pigs served as never‐ventilated, never‐paced control animals. Measurements and Main Results: Cumulative duration of pacing therapy ranged from 19.7 to 35.7 hours. Diaphragm thickness assessed by ultrasound and normalized to initial value showed a significant decline in ventilated‐not paced but not in ventilated‐paced subjects (0.84 [interquartile range (IQR), 0.78‐0.89] vs. 1.10 [IQR, 1.02‐1.24]; P = 0.001). Compared with control animals (24.6 &mgr;m2/kg; IQR, 21.6‐26.0), median myofiber cross‐sectional areas normalized to weight and sarcomere length were significantly smaller in the ventilated‐not paced (17.9 &mgr;m2/kg; IQR, 15.3‐23.7; P = 0.005) but not in the ventilated‐paced group (24.9 &mgr;m2/kg; IQR, 16.6‐27.3; P = 0.351). After 60 hours of mechanical ventilation all six ventilated‐paced subjects tolerated 8 minutes of intense phrenic stimulation, whereas three of six ventilated‐not paced subjects did not (P = 0.055). There was a nonsignificant decrease in diaphragm tetanic force production over the experiment in the ventilated‐paced and ventilated‐not paced groups. Conclusions: These results suggest that early transvenous phrenic nerve pacing may mitigate ventilator‐induced diaphragm dysfunction.