Contribution of sensory feedback to soleus muscle activity during voluntary contraction in humans.

Sensory feedback through spinal interneurons contributes to plantar flexor muscle activity during walking, but it is unknown whether this is also the case during nonlocomotor movements. Here, we explored the effect of temporary reduction of sensory feedback to ankle plantar flexors during voluntary contraction in sitting subjects. Thirteen healthy adults (mean age 32 yr) were seated with the right leg attached to a foot plate which could be moved in dorsi- or plantarflexion direction by a computer-controlled motor. EMG was recorded from the tibialis anterior (TA) and soleus (Sol) muscles. During static plantar flexion, while the plantar flexors were slowly stretched, a sudden plantar flexion caused a decline in Sol EMG at the same latency as the stretch reflex. This decline in EMG activity was still observed when transmission from dorsiflexors was blocked. It disappeared when transmission from ankle plantar flexors was also blocked. The same quick plantarflexion failed to produce a decline in EMG activity at the latency of the stretch reflex in the absence of slow stretch of the plantar flexors. Instead, a decline in EMG activity was observed 15-20 ms later. This decline disappeared following block of transmission from antagonists, suggesting that reciprocal inhibition was involved. These findings show that unload of ankle plantar flexors does not cause a similar drop in Sol EMG during voluntary contraction as during walking. This implies that sensory feedback through spinal interneurons only contributes little to the neural drive to plantar flexor muscles during human voluntary contraction in sitting subjects.NEW & NOTEWORTHY Sensory feedback through spinal reflex pathways makes only a minor contribution to neural drive to muscles during voluntary ankle plantar flexion. This differs distinctly from observations during walking and suggests that the neural drive to ankle plantar flexors during voluntary contraction do not rely on sensory feedback through similar spinal interneuronal networks as during walking. In line with animal studies this suggests that the integration of sensory feedback in CNS is task specific.

Muscle Contractures in Adults With Cerebral Palsy Characterized by Combined Ultrasound-Derived Echo Intensity and Handheld Dynamometry Measures.

We used ultrasound-derived echo intensity and hand-held dynamometry to characterize plantar flexor muscle contractures in adults with cerebral palsy (CP). Eleven adults with CP (aged 41 ± 12 y, Gross Motor Function Classification System I-II) and 11 neurologically intact adults (aged 35 ± 10 y) participated in the study. Echo intensity was measured from the medial gastrocnemius muscle using brightness mode ultrasound. Hand-held dynamometry was used to quantify plantar flexor passive muscle stiffness and ankle joint passive range of motion (pROM). Echo intensity correlated with both passive muscle stiffness (r = 0.57, p = 0.006) and pROM (r = -0.56, p = 0.006). Ultrasound echo intensity (p = 0.02, standardized mean difference [SMD] = 1.13) and passive muscle stiffness (p < 0.001, SMD = 1.99) were higher and ankle joint pROM (p < 0.001, SMD = 2.69) was lower in adults with CP than in neurologically intact adults. We conclude that combined ultrasound-derived echo intensity and hand-held dynamometry may be used to provide an objective characterization of muscle contractures.

Quantitative MRI and Clinical Assessment of Muscle Function in Adults With Cerebral Palsy.

Aim: To relate quantitative magnetic resonance imaging (MRI) of ankle plantar flexor muscles to clinical functional tests in adults with cerebral palsy (CP) and neurologically intact (NI) adults. Methods: Eleven adults with CP (aged 41 ± 12, GMFCS level I-II) and 11 NI adults (aged 35 ± 10) participated in this case-control study. We used MRI to assess muscle volume and composition of the triceps surae muscles. We quantified muscle function as maximal voluntary plantarflexion (MVC) torque and countermovement jump (CMJ) height. Results: Compared to NI adults, the MRI intramuscular fat fraction estimate was significantly higher and MRI muscle volume and functional abilities (MVC and CMJ) significantly lower in adults with CP. In NI adults, but not adults with CP, MRI muscle volume correlated significantly with MVC and CMJ. In adults with CP, the estimate of intramuscular fat levels correlated significantly with jump height in a CMJ. Discussion: This study shows reduced muscle volume and altered muscle composition in adults with CP. Muscle composition appears to provide a better marker than muscle volume of reduced muscle function and impaired performance in this population. Measurements of muscle composition could be used in the assessment of neuromuscular impairments and in the determination of rehabilitation protocols in individuals with neurological disorders.