Samuel C. K. Lee

Voluntary muscle activation, contractile properties, and fatigability in children with and without cerebral palsy.

Cerebral palsy (CP) may lead to profound weakness in affected portions of the extremities and trunk. Knowing the mechanisms underlying muscle weakness will help to better design interventions for increasing force production in children with CP. This study quantified voluntary muscle activation, contractile properties, and fatigability of the quadriceps femoris and triceps surae in children with and without CP. Twelve children with CP (7–13 years) and 10 unaffected children (controls, 8–12 years) were assessed for (1) voluntary muscle activation during maximum voluntary isometric contractions (MVICs); (2) antagonist coactivation during agonist MVICs; (3) contractile properties, and (4) fatigability using electrically elicited tests. Children with CP were significantly weaker, had lower agonist voluntary muscle activation, and greater antagonist coactivation. In children with CP, the quadriceps normalized force–frequency relationship (FFR) was shifted upward at low frequencies and was less fatigable than controls. No differences were seen between groups in the normalized FFR and fatigability of the triceps surae. In addition, no differences were seen in the sum of the time to peak tension and half‐relaxation times between groups for either muscle. Because children with CP demonstrated large deficits in voluntary muscle activation, using voluntary contractions for strength training may not produce forces sufficient to induce muscle hypertrophy. Techniques such as enhanced feedback and neuromuscular electrical stimulation may be helpful for strengthening muscles that cannot be sufficiently recruited with voluntary effort. Muscle Nerve, 2005

Measurement of central activation failure of the quadriceps femoris in healthy adults

The purpose of this investigation was to describe the relationship between the central activation ratio (CAR) and the percent maximum voluntary effort (% MVE) during isometric quadriceps femoris contractions. Twenty‐one healthy, young adults participated in three test sessions. During each session, one of three train types was tested: a 100‐Hz 120‐ms train, a 100‐Hz 250‐ms train, or a 50‐Hz 500‐ms train. Subjects were seated on a force dynamometer and stabilized to perform a 3–5‐s isometric knee extension at MVE. Force targets were set at 25, 50, 75, and 100% of the MVE. With 5 min rest between efforts, subjects produced forces at the specified target levels. When each target was reached, the test train was delivered to quantify the amount of central activation. There were no significant differences in CARs across train types during maximal efforts, but during submaximal efforts at 25 and 50%, the 100‐Hz 250‐ms and 50‐Hz 500‐ms trains produced significantly lower CARs than the 100‐Hz 120‐ms train. The relationship between the CAR and the %MVE was curvilinear and best described by a second‐order polynomial for all three train types. If tests of central activation are going to be used clinically, it is important to know the relationship between the CAR and voluntary effort; however, further study will be required to extend these results to specific patient populations.

Effects of activation pattern on human skeletal muscle fatigue

Variable‐frequency stimulation trains (VFTs) that take advantage of the catchlike property of skeletal muscle have been shown to augment the force production of fatigued muscles compared with constant‐frequency trains (CFTs). The present study is the first to report the force augmentation produced by VFTs after fatiguing the muscle with VFTs versus fatiguing the muscle with CFTs. Data were obtained from the human quadriceps femoris muscles of 12 healthy subjects. Each subject participated in three experimental sessions. Each session fatigued the muscle with one of three protocols: CFTs with 70‐ms interpulse intervals (CFT70); CFTs with 55.5‐ms interpulse intervals (CFT55.5); or VFTs. Following each fatiguing protocol the muscles were tested with all three stimulation patterns (i.e., CFT55.5, CFT70, and VFT). At the end of the fatiguing protocol the VFT produced force–time integrals and peak forces ∼18% and 32% greater than the CFT70, respectively. The testing trains showed that the VFT produced ∼25–35% greater force–time integrals than either CFT and ∼35–47% greater peak forces than the CFT70. For each testing train, ∼10–15% greater force–time integrals were seen when the muscles were fatigued with the CFTs than when fatigued with the VFTs. These results support suggestions that VFTs may be useful during clinical applications of electrical stimulation.

Reduction of the fatigue-induced force decline in human skeletal muscle by optimized stimulation trains

OBJECTIVE To identify the stimulation pattern that optimizes the force-time integral produced during isometric contractions of fatigued human skeletal muscle. DESIGN Twelve healthy subjects with no history of lower extremity orthopedic problems voluntarily participated. RESULTS The primary findings were that (1) the optimized trains showed augmentation only from fatigued muscles and (2) a simple stimulation pattern, containing one brief (5msec) initial interpulse interval, produced the greatest force-time integrals and rates of rise of force. With muscle fatigue, the rate of rise of force of the constant-frequency train slowed, whereas the rate of rise of force of the optimized trains remained unchanged. This difference in the rate of rise of force may explain why the optimized trains, which take advantage of the catchlike property of skeletal muscle, are able to augment forces from fatigued muscles when compared with the constant-frequency train. CONCLUSIONS These results may have important clinical implications when using brief trains of electric stimulation to aid patients in performing functional movements and contribute to our understanding of the relationship between the activation pattern of a muscle and the force output produced.

Neuromuscular Electrical Stimulation Versus Volitional Isometric Strength Training in Children With Spastic Diplegic Cerebral Palsy: A Preliminary Study

Background. To date, no reports have investigated neuromuscular electrical stimulation (NMES) to increase muscle force production of children with cerebral palsy (CP) using high-force contractions and low repetitions. Objective. The aims of this study were to determine if isometric NMES or volitional training in children with CP could increase muscle strength and walking speed and to examine the mechanisms that may contribute to increased force production. Methods. Eleven children with spastic diplegia were assigned to an NMES training group or to a volitional training group. Participants in the NMES group had electrodes implanted percutaneously to activate the quadriceps femoris and triceps surae muscles. The volitional group trained with maximal effort contractions. Both groups performed a 12-week isometric strength-training program. Maximum voluntary isometric contraction (MVIC) force, voluntary muscle activation, quadriceps and triceps surae cross-sectional area (CSA), and walking speed were measured pre- and post-strength training. Results . The NMEStrained group had greater increases in normalized force production for both the quadriceps femoris and triceps surae. Similarly, only the NMES group showed an increase in walking speed after training. Changes in voluntary muscle activation explained approximately 67% and 37% of the changes seen in the MVIC of the NMES and volitional groups, respectively. Quadriceps femoris maximum CSA increased significantly for the NMES group only. Conclusions. This study was the first to quantitatively show strength gains with the use of NMES in children with CP. These results support the need for future experimental studies that will examine the clinical effectiveness of NMES strength training.

Trunk and Hip Muscle Activation Patterns Are Different During Walking in Young Children With and Without Cerebral Palsy

Background Poor control of postural muscles is a primary impairment in people with cerebral palsy (CP). Objective The purpose of this study was to investigate differences in the timing characteristics of trunk and hip muscle activity during walking in young children with CP compared with children with typical development (TD). Methods Thirty-one children (16 with TD, 15 with CP) with an average of 28.5 months of walking experience participated in this observational study. Electromyographic data were collected from 16 trunk and hip muscles as participants walked at a self-selected pace. A custom-written computer program determined onset and offset of activity. Activation and coactivation data were analyzed for group differences. Results The children with CP had greater total activation and coactivation for all muscles except the external oblique muscle and differences in the timing of activation for all muscles compared with the TD group. The implications of the observed muscle activation patterns are discussed in reference to existing postural control literature. Limitations The potential influence of recording activity from adjacent deep trunk muscles is discussed, as well as the influence of the use of an assistive device by some children with CP. Conclusions Young children with CP demonstrate excessive, nonreciprocal trunk and hip muscle activation during walking compared with children with TD. Future studies should investigate the efficacy of treatments to reduce excessive muscle activity and improve coordination of postural muscles in CP.

Trunk and hip muscle activity in early walkers with and without cerebral palsy--a frequency analysis.

Poor control of postural muscles is a primary impairment in cerebral palsy (CP), yet core trunk and hip muscle activity has not been thoroughly investigated. Frequency analysis of electromyographic (EMG) signals provides insight about the intensity and pattern of muscle activation, correlates with functional measures in CP, and is sensitive to change after intervention. The objective of this study was to investigate differences in trunk and hip muscle activation frequency in children with CP compared to children with similar amounts of walking experience and typical development (TD). EMG data from 31 children (15 with CP, 16 with TD) were recorded from 16 trunk and hip muscles bilaterally. A time-frequency pattern was generated using the continuous wavelet transform and instantaneous mean frequency (IMNF) was calculated at each interval of the gait cycle. Functional principal component analysis (PCA) revealed that IMNF was significantly higher in the CP group throughout the gait cycle for all muscles. Additionally, stride-to-stride variability was higher in the CP group. This evidence demonstrated altered patterns of trunk and hip muscle activation in CP, including increased rates of motor unit firing, increased number of recruited motor units, and/or decreased synchrony of motor units. These altered muscle activation patterns likely contribute to muscle fatigue and decreased biomechanical efficiency in children with CP.

Mathematical model that predicts isometric muscle forces for individuals with spinal cord injuries

The ideal functional electrical stimulation (FES) system requires a mathematical model to provide feedforward control of the stimulation parameters such that they are optimal for different individuals across a range of physiological conditions, muscles, and tasks. Recently we tested and validated such a model using able‐bodied subjects. The purpose of this study was to determine whether this model applied to persons with spinal cord injuries (SCI). To this end, the isometric force responses of the paralyzed quadriceps femoris muscles of 14 adolescents and young adults were tested. For each subject, the force responses to two six‐pulse stimulation trains were used to identify the parameter values of the model and then the model was used to predict the force responses to three train patterns across a range of frequencies in both a nonfatigued and fatigued condition. The intraclass correlation coefficients (ICCs) between the experimental and predicted force–time integrals and peak forces were above 0.90 for 12 of the 13 stimulation trains tested in the nonfatigued condition and all 13 trains tested in the fatigued condition. The success of our model with SCI subjects leads us to believe that our model may be useful for designing optimal stimulation parameters for standing and ambulation in patients who use FES. Muscle Nerve, 2005

A NOVEL STIMULATION PATTERN IMPROVES PERFORMANCE DURING REPETITIVE DYNAMIC CONTRACTIONS

The purpose of this study was to determine the effect of three different stimulation patterns on repetitive knee movements. Each subjects quadriceps femoris was stimulated with: (1) a constant‐frequency train (CFT) with an interpulse interval (IPI) of 50 ms; (2) a variable‐frequency train (VFT)—similar to the CFT, except with an initial doublet with an IPI of 5 ms; and (3) a doublet‐frequency train (DFT) with multiple doublets (doublet IPI 5 ms) separated by 50 ms, while the muscle was resisted by a load equal to 10% of the muscles maximum voluntary isometric contraction. The muscle was stimulated while the knee moved through a 50° arc of motion (90° to 40° of flexion). Testing was stopped when the subject failed to reach the target three consecutive times. Results showed that DFTs reached the target (mean ± SD) 36.4 ± 14.4 times, followed by VFTs (25.4 ± 17.9) and CFTs (17.4 ± 11.9). The DFT was the best pattern for producing shortening contractions. The results suggest that DFTs may have significant benefits during clinical functional electrical stimulation.

Variability and symmetry of gait in early walkers with and without bilateral cerebral palsy

PURPOSE Investigating gait characteristics during the early stages of walking in CP may contribute to the understanding of the development of impaired gait. The objective of this study was to investigate differences in the variability and symmetry of spatiotemporal gait characteristics during the early years of walking in children with bilateral spastic CP compared to children with similar amounts of walking experience and typical development (TD). METHODS The spatiotemporal gait parameters of 31 children (15 with spastic CP, 16 with TD) who had an average of 28.5 (18.1 SD) months of walking experience were collected using an instrumented walkway. RESULTS All primary spatiotemporal parameters were reduced in the CP group, who also demonstrated greater stride-to-stride variability, compared to the TD group. There were no statistically significant differences in side-to-side symmetry between groups. IMPLICATIONS Clinical trials investigating gait interventions during the early years of walking in children with CP should be conducted to determine if treatment can reduce the functional limitations that are present during the emergence of walking skills. Further investigation should examine variability and symmetry in the kinematics, kinetics, and muscle activity patterns of early walkers with CP, and the effect of treatment on the variability and symmetry of walking characteristics.