Stuart A. Binder-Macleod

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

Effects of stimulation intensity on the physiological responses of human motor units

Quadriceps femoris muscles were studied in 50 healthy subjects to determine the physiological responses of the motor units recruited at different force levels during transcutaneous electrical stimulation. During one set of experiments force-frequency relationships were compared at stimulation intensities that produced tetanic contraction of 20%, 50%, or 80% of the maximum voluntary isometric contraction (MVC). No differences in the normalized force-frequency relationship were observed between the 20% and 50% of MVC conditions and only a slight shift to the left was observed at 80% of MVC. The other set of experiments measured the responses to electrically elicited fatigue tests using frequencies of 20, 40, or 60 pps and, at each frequency, intensities that produced 20% or 50% of MVC. Fatigue was greater for the 50% than 20% MVC force conditions. Within each force level fatigue increased with increasing frequency. However, though the differences in the level of recruitment needed to produce the two forces varied for each frequency, the differences in the amount of fatigue produced at each force did not vary between the three stimulation frequencies. This suggests that the fatigue characteristics of the recruited motor units were similar at all intensities tested. We posit, therefore, that the physiological recruitment order during transcutaneous electrical stimulation is less orderly than previously suggested.

Mechanisms underlying quadriceps weakness in knee osteoarthritis.

PURPOSE To identify determinants of quadriceps weakness among persons with end-stage knee osteoarthritis (OA). METHODS One-hundred twenty-three individuals (mean age 64.9 +/- 8.5 yr) with Kellgren/Lawrence grade IV knee OA participated. Quadriceps strength (MVIC) and volitional muscle activation (CAR) were measured using a burst superimposition test. Muscle composition (lean muscle cross-sectional area (LMCSA) and fat CSA (FCSA)) were quantified using magnetic resonance imaging. Specific strength (MVIC/LMCSA) was computed. Interlimb differences were analyzed using paired-sample t-tests. Regression analysis was applied to identify determinants of MVIC. An alpha level of 0.05 was adopted. RESULTS The OA limb was significantly weaker, had lower CAR, and had smaller LMCSA than the contralateral limb. CAR explained 17% of the variance in the contralateral limbs MVIC compared with 40% in the OA limb. LMCSA explained 41% of the variance in the contralateral limbs MVIC compared with 27% in the OA limb. CONCLUSION Both reduced CAR and LMCSA contribute to muscle weakness in persons with knee OA. Similar to healthy elders, the best predictor of strength in the contralateral, nondiseased limb was largely determined by LMCSA, whereas CAR was found to be the primary determinant of strength in the OA limb. Deficits in CAR may undermine the effectiveness of volitional strengthening programs in targeting quadriceps weakness in the OA population.

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.

Fatigability of human quadriceps femoris muscle following anterior cruciate ligament reconstruction

The responses of quadriceps femoris muscles to an electrically elicited fatigue test were recorded from both lower extremities of 18 patients who had recently undergone unilateral, anterior cruciate ligament reconstruction. The fatigue test consisted of 40 pps, 13-pulse electrical trains that were repeated once per second for 3 min. The intensity of stimulation was set for each extremity to produce 20% of the maximum voluntary isometric contraction of the uninvolved muscle. The uninvolved quadriceps femoris muscle showed a significantly greater rate of decline in force over the first minute than the involved muscle (0.803%.s-1 for uninvolved muscle vs 0.620%.s-1 for involved muscle). Similarly, the average forces produced over the last minute were significantly lower for the uninvolved than the involved quadriceps femoris muscle (uninvolved = 42.6%, involved = 50.4% of their original forces). These surprising results showed that the involved quadriceps femoris muscles were more endurant than the uninvolved muscles. It is suggested that the increases in endurance of the involved muscle may have been due, in part, to greater recruitment of Type I fibers with electrical stimulation or selective Type II fiber atrophy in the involved muscle.

Functional Electrical Stimulation of Ankle Plantarflexor and Dorsiflexor Muscles. Effects on Poststroke Gait

Background and Purpose— Functional electrical stimulation (FES) is a popular poststroke gait rehabilitation intervention. Although stroke causes multijoint gait deficits, FES is commonly used only for the correction of swing-phase foot drop. Ankle plantarflexor muscles play an important role during gait. The aim of the current study was to test the immediate effects of delivering FES to both ankle plantarflexors and dorsiflexors on poststroke gait. Methods— Gait analysis was performed as subjects (N=13) with chronic poststroke hemiparesis walked at their self-selected walking speeds during walking with and without FES. Results— Compared with delivering FES to only the ankle dorsiflexor muscles during the swing phase, delivering FES to both the paretic ankle plantarflexors during terminal stance and dorsiflexors during the swing phase provided the advantage of greater swing-phase knee flexion, greater ankle plantarflexion angle at toe-off, and greater forward propulsion. Although FES of both the dorsiflexor and plantarflexor muscles improved swing-phase ankle dorsiflexion compared with noFES, the improvement was less than that observed by stimulating the dorsiflexors alone, suggesting the need to further optimize stimulation parameters and timing for the dorsiflexor muscles during gait. Conclusions— In contrast to the typical FES approach of stimulating ankle dorsiflexor muscles only during the swing phase, delivering FES to both the plantarflexor and dorsiflexor muscles can help to correct poststroke gait deficits at multiple joints (ankle and knee) during both the swing and stance phases of gait. Our study shows the feasibility and advantages of stimulating the ankle plantarflexors during FES for poststroke gait.

A mathematical model that predicts skeletal muscle force

This study demonstrates the validity of a mathematical model that predicts the force generated by rat skeletal muscles during brief subtetanic and tetanic isometric contractions. The model consists of three coupled differential equations (ODEs). The first two equations represent the calcium dynamics and the third equation represents force dynamics. The model parameters were identified from brief trains of regularly spaces pulses [constant-frequency trains (CFTs)] that produce subtetanic muscle responses. Using these parameters, the model was able to predict isometric forces from other stimulation patterns. For the gastrocnemius muscles predictions were made for responses to CFTs with interpulse intervals (IPIs) ranging from 10 to 50 ms and variable-frequency trains (VFTs), where the initial IPI=10 ms and the remaining IPIs were identical to those used for the CFTs. For the soleus muscles predictions were made for 10-100-ms CFTs. The shape of the predicted responses closely match the experimental data. Comparisons between experimental and modeled force-time integrals, peak forces, and time-to-peak also suggest excellent agreement between the model and the experiment data. Many physiological parameters predicted by the model agree with values obtained independently by others. In conclusion, the model accurately predicts isometric forces generated by rat gastrocnemius and soleus muscles produced by brief stimulation trains.

Are voluntary muscle activation deficits in older adults meaningful

The relationship between the central activation ratio (CAR) and contraction force is curvilinear, not linear as was previously believed. Voluntary quadriceps femoris muscle activation from previously collected data sets in 46 older adults (64–84 years) and 46 young adults (18–32 years) were therefore reexamined using a curvilinear model of the voluntary muscle activation–percent maximum voluntary force relationship. This method revealed lower voluntary muscle activation in older adults (0.868 ± 0.018) than younger subjects (0.978 ± 0.005). The mean difference between older and younger adults was 11%, which may be more meaningful than previous reports of 2–4% because it could explain the greater rate of strength loss as compared to loss of muscle mass as humans age. Muscle Nerve 27: 99–101, 2003

Novel Patterns of Functional Electrical Stimulation Have an Immediate Effect on Dorsiflexor Muscle Function During Gait for People Poststroke

Background Foot drop is a common gait impairment after stroke. Functional electrical stimulation (FES) of the ankle dorsiflexor muscles during the swing phase of gait can help correct foot drop. Compared with constant-frequency trains (CFTs), which typically are used during FES, novel stimulation patterns called variable-frequency trains (VFTs) have been shown to enhance isometric and nonisometric muscle performance. However, VFTs have never been used for FES during gait. Objective The purpose of this study was to compare knee and ankle kinematics during the swing phase of gait when FES was delivered to the ankle dorsiflexor muscles using VFTs versus CFTs. Design A repeated-measures design was used in this study. Participants Thirteen individuals with hemiparesis following stroke (9 men, 4 women; age=46–72 years) participated in the study. Methods Participants completed 20- to 40-second bouts of walking at their self-selected walking speeds. Three walking conditions were compared: walking without FES, walking with dorsiflexor muscle FES using CFTs, and walking with dorsiflexor FES using VFTs. Results Functional electrical stimulation using both CFTs and VFTs improved ankle dorsiflexion angles during the swing phase of gait compared with walking without FES (X̅±SE=−2.9°±1.2°). Greater ankle dorsiflexion in the swing phase was generated during walking with FES using VFTs (X̅±SE=2.1°±1.5°) versus CFTs (X̅±SE=0.3±1.3°). Surprisingly, dorsiflexor FES resulted in reduced knee flexion during the swing phase and reduced ankle plantar flexion at toe-off. Conclusions The findings suggest that novel FES systems capable of delivering VFTs during gait can produce enhanced correction of foot drop compared with traditional FES systems that deliver CFTs. The results also suggest that the timing of delivery of FES during gait is critical and merits further investigation.

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.