Chris M. Gregory

Impact of varying pulse frequency and duration on muscle torque production and fatigue.

Neuromuscular electrical stimulation (NMES) involves the use of electrical current to facilitate contraction of skeletal muscle. However, little is known concerning the effects of varying stimulation parameters on muscle function in humans. The purpose of this study was to determine the extent to which varying pulse duration and frequency altered torque production and fatigability of human skeletal muscle in vivo. Ten subjects underwent NMES‐elicited contractions of varying pulse frequencies and durations as well as fatigue tests using stimulation trains of equal total charge, yet differing parametric settings at a constant voltage. Total charge was a strong predictor of torque production, and pulse trains with equal total charge elicited identical torque output. Despite similar torque output, higher‐ frequency trains caused greater fatigue. These data demonstrate the ability to predictably control torque output by simultaneously controlling pulse frequency and duration and suggest the need to minimize stimulation frequency to control fatigue. Muscle Nerve, 2007

Nonlinear Neuromuscular Electrical Stimulation Tracking Control of a Human Limb

A high-level objective of neuromuscular electrical stimulation (NMES) is to enable a person to achieve some functional task. Towards this goal, the objective of the current effort is to develop a NMES controller to produce a knee position trajectory that will enable a human shank to track any continuous desired trajectory (or constant setpoint). A nonlinear control method is developed to control the human quadriceps femoris muscle undergoing nonisometric contractions. The developed controller does not require a muscle model and can be proven to yield asymptotic stability for a nonlinear muscle model in the presence of bounded nonlinear disturbances (e.g., spasticity, delays, fatigue). The performance of the controller is demonstrated through a series of closed-loop experiments on human subjects. The experiments illustrate the ability of the controller to enable the leg shank to track single and multiple period trajectories with different periods and ranges of motion, and also track desired step changes with changing loads.

Metabolic enzymes and phenotypic expression among human locomotor muscles.

Percutaneous biopsies were taken from the right vastus lateralis (VL), tibialis anterior (TA), soleus (Sol), and lateral gastrocnemius (LG) muscles of eight recreationally active adult males. Approximately 60 fibers in each sample were analyzed for their type (I, IIa, or IIx), cross‐sectional area (CSA), and succinic dehydrogenase (SDH), alpha glycerol phosphate dehydrogenase (GPDH) and calcium‐activated actomyosin adenosine triphosphatase (qATPase) activities. This was done to test the hypothesis that metabolic enzyme activities are more reflective of the functional diversity among human locomotor muscles than fiber type composition. The results showed that enzymatic characteristics differed more or less than expected between muscles of the same or different fiber type. For example, the relative CSA occupied by fast fibers was only about 50% greater in the mixed (LG and VL) than in the slow (Sol and TA) muscles (57 vs. 38%). At the same time, average fiber SDH activity and fiber type specific SDH:qATPase*%CSA, both used as estimates of fatigue resistance, were greater in Sol and LG than in TA and VL. As a result, the two slow muscles and the two mixed muscles had different values, and a mixed muscle (LG) had higher values than a slow muscle (TA). The findings suggest that differences in enzymatic profile, more than fiber type composition, afford human locomotor muscles the capacity to perform their purportedly divergent functional tasks.

Lower extremity skeletal muscle function in persons with incomplete spinal cord injury

Study design:A cross-sectional study design.Objectives:To characterize and specifically quantify impairments in muscle function after chronic incomplete spinal cord injury (SCI).Setting:University of Florida, Gainesville, FL, USA.Methods:Voluntary and electrically elicited contractile measurements were performed and voluntary activation deficits were quantitatively determined in the knee extensor and ankle plantar flexor muscle groups in 10 individuals with chronic incomplete SCI (C5-T8, ASIA C or D) and age-, gender-, height- and body weight matched healthy controls.Results:Persons with incomplete-SCI were able to produce only 36 and 24% of the knee extensor torque and 38 and 26% of the plantar flexor torque generated by noninjured controls in the self-reported less-involved and more-involved limbs, respectively (P<0.05). In addition, both indices of explosive or instantaneous muscle strength, torque200 (absolute torque reached at 200 ms) and the average rate of torque development (ARTD) were dramatically reduced in the ankle plantar flexor and knee extensor muscle groups in persons with incomplete-SCI. However, the deficit in instantaneous muscle strength was most pronounced in the ankle plantar flexor muscles, with an 11.7-fold difference between the torque200 measured in the self-reported more involved limb and a 5-fold difference in the less-involved limb compared to control muscles. Voluntary activation deficits ranged between 42 and 66% in both muscle groups. Interestingly, electrically elicited contractile properties did not differ between the groups.Conclusion:The resultant impact of incomplete-SCI is that affected muscles not only become weak, but slow to develop voluntary torque. We speculate that the large deficit in torque200 and ARTD in the ankle plantar flexors muscles of persons with incomplete-SCI may limit locomotor function. The results presented in this study provide a quantitative and sensitive assessment of muscle function upon which future research examining rehabilitation programs aimed at restoring muscle function and promoting functional recovery after incomplete-SCI may be based.

Closed-Loop Neural Network-Based NMES Control for Human Limb Tracking

Closed-loop control of skeletal muscle is complicated by the nonlinear muscle force to length and velocity relationships and the inherent unstructured and time-varying uncertainties in available models. Some pure feedback methods have been developed with some success, but the most promising and popular control methods for neuromuscular electrical stimulation (NMES) are neural network (NN)-based methods. Efforts in this paper focus on the use of a NN feedforward controller that is augmented with a continuous robust feedback term to yield an asymptotic result (in lieu of typical uniformly ultimately bounded stability). Specifically, an NN-based controller and Lyapunov-based stability analysis are provided to enable semi-global asymptotic tracking of a desired limb time-varying trajectory (i.e., non-isometric contractions). The developed controller is applied as an amplitude modulated voltage to external electrodes attached to the distal-medial and proximal-lateral portion of the quadriceps femoris muscle group in non-impaired volunteers. The added value of incorporating a NN feedforward term is illustrated through experiments that compare the developed controller with and without the NN feedforward component.

Resistance training and locomotor recovery after incomplete spinal cord injury: a case series.

Study design:Longitudinal intervention case series.Objective:To determine if a 12-week resistance and plyometric training program results in improved muscle function and locomotor speed after incomplete spinal cord injury (SCI).Setting:University research setting.Methods:Three ambulatory individuals with chronic (18.7±2.2 months post injury) motor incomplete SCI completed 12 weeks of lower extremity resistance training combined with plyometric training (RPT). Muscle maximum cross-sectional area (max-CSA) of the knee extensor (KE) and plantar flexor (PF) muscle groups was determined using magnetic resonance imaging (MRI). In addition, peak isometric torque, time to peak torque (T 20–80), torque developed within the initial 220 ms of contraction (torque220) and average rate of torque development (ARTD) were calculated as indices of muscle function. Maximal as well as self-selected gait speeds were determined pre- and post-RPT during which the spatio-temporal characteristics, kinematics and kinetics of gait were measured.Results:RPT resulted in improved peak torque production in the KE (28.9±4.4%) and PF (35.0±9.1%) muscle groups, as well as a decrease in T20–80, an increased torque220 and an increase ARTD in both muscle groups. In addition, an increase in self-selected (pre-RPT=0.77 m/s; post-RPT=1.03 m/s) and maximum (pre-RPT=1.08 m/s; post-RPT=1.47 m/s) gait speed was realized. Increased gait speeds were accompanied by bilateral increases in propulsion and hip excursion as well as increased lower extremity joint powers.Conclusions:The combination of lower extremity RPT can attenuate existing neuromuscular impairments and improve gait speed in persons after incomplete SCI.

Predictor-Based Compensation for Electromechanical Delay During Neuromuscular Electrical Stimulation

Electromechanical delay (EMD) is a biological artifact that arises due to a time lag between electrical excitation and tension development in a muscle. EMD is known to cause degraded performance and instability during neuromuscular electrical stimulation (NMES). Compensating for such input delay is complicated by the unknown nonlinear muscle force-length and muscle force-velocity relationships. This paper provides control development and a mathematical stability analysis of a NMES controller with a predictive term that actively accounts for EMD. The results are obtained through the development of a novel predictor-type method to address the delay in the voltage input to the muscle. Lyapunov-Krasovskii functionals are used within a Lyapunov-based stability analysis to prove semi-global uniformly ultimately bounded tracking. Experiments on able-bodied volunteers illustrate the performance and robustness of the developed controller during a leg extension trajectory following task.

Locomotor Training and Muscle Function After Incomplete Spinal Cord Injury: Case Series

Abstract Background/Objective: To determine whether 9 weeks of locomotor training (LT) results in changes in muscle strength and alterations in muscle size and activation after chronic incomplete spinal cord injury (SCI). Study Design: Longitudinal prospective case series. Methods: Five individuals with chronic incomplete SCI completed 9 weeks of LT. Peak isometric torque, torque developed within the initial 200 milliseconds of contraction (Torque200), average rate of torque development (ARTD), and voluntary activation deficits were determined using isokinetic dynamometry for the knee-extensor (KE) and plantar-flexor (PF) muscle groups before and after LT. Maximum muscle crosssectional area (CSA) was measured prior to and after LT. Results: Locomotor training resulted in improved peak torque production in all participants, with the largest increases in the more-involved PF (43.9% ± 20.0%), followed by the more-involved KE (21.1% ± 12.3%). Even larger improvements were realized in Torque200 and ARTD (indices of explosive torque), after LT. In particular, the largest improvements were realized in the Torque200 measures of the PF muscle group. Improvements in torque production were associated with enhanced voluntary activation in both the KE and ankle PF muscles and an increase in the maximal CSA of the ankle PF muscles. Conclusion: Nine weeks of LT resulted in positive alterations in the KE and PF muscle groups that included an increase in muscle size, improved voluntary activation, and an improved ability to generate both peak and explosive torque about the knee and ankle joints.

Effects of testosterone replacement therapy on skeletal muscle after spinal cord injury

Study design: Randomized control.Objective: To examine the effects of testosterone replacement therapy (TRT) on skeletal muscle 11 weeks after complete SCI.Setting: Athens, Georgia USA.Methods: Soleus (SOL), gastrocnemius (GA), tibialis anterior (TA), vastus lateralis (VL) and triceps brachii (TRI) muscles were taken from twelve young male Charles River rats 11 weeks after complete SCI (T-9 transection, n=8) or sham surgery (n=4). Rats received either TRT (two 5 cm capsules, n=4) or empty capsules (n=8) implanted at surgery. Muscle samples were sectioned and fibers analyzed qualitatively for myosin ATPase and quantitatively for succinate dehydrogenase (SDH), α-glycerol-phosphate dehydrogenase (GPDH) and actomyosin ATPase (qATPase) activities using standard techniques.Results: SCI decreased average fiber size (49±4%) in affected muscles and the percentage of slow fibers in SOL (93±3% to 17±2%). In addition, there was a decrease in SDH and an increase in GPDH and qATPase activities across the four hind-limb muscles of the SCI animals. Fiber size in the TRI was increased (31±2%) by SCI while enzyme activities were not altered. Average fiber size across the four hind limb muscles was decreased by only 30% in TRT SCI animals and their SOL contained 39±2% slow fibers. TRT also attenuated changes in enzyme activities. There was no effect of TRT on the TRI relative to SCI.Conclusions: TRT was effective in attenuating alterations in myofibrillar proteins during 11 weeks of SCI in affected skelatal muscles.Sponsorship: Supported by a grant from The National Institutes of Health (HD-33738) and HD-37645 to KV, and HD-39676 to GAD.

An extremum seeking method for non-isometric neuromuscular electrical stimulation

An optimal extremum seeking approach is developed in this paper to identify frequency and voltage modulation parameters for a neuromuscular electrical stimulation control objective. The control objective is to externally apply optimally varied voltage or frequency modulation parameters to a human quadriceps muscle to generate a desired knee joint angle. Experimental results are provided to illustrate the limb positioning performance of a real-time extremum seeking routine (i.e., Brents Method).