Interaction between walking speed and spasticity in children with spastic hamstrings

Abstract

Introduction:Walking velocity (WV) is known to influence different gait parameters [1,2]. Muscle length (ML) and especially lengthening velocity (MLV) increase with WV. Spasticity on the otherhand is associatedwith reducedMLandMLV [1]. Spasticmuscles show an increased coupling between EMG activity and MLV [3]. Therefore, we described the effect of walking speed on spastic hamstrings, with a focus on ML, MLV and EMG. Patients/materials and methods: 49 Children with CP (9.8±3 years)with aModified Ashworth Score≥1.5 for the hamstrings and 14 TD children (10.5±2.4 years) received a 3D lower limb gait analysis, including kinematics, kinetics (8 camera Vicon system), EMG of 8 muscles bilateral (Cometa, Wave EMG) and a clinical examination. They were instructed to walk at self-selected speed, faster and as fast as possible without running. To compare both groups, a linear regression model was created which resulted in two nondimensional gait velocities (v1, v2), used to compare a large set of parameters. Difference scores (DS) between both velocities were calculated. Statistical analysis was done using the Mann-Whitney U test. Results: During stance at both velocities, the CP group showed more knee flexion at IC (p<0.001) and duringmid stance (p<0.01), increasing with WV. The knee ROM during shock absorption was higher in the TD group. EMG activity in early stance phase was significantly higher in the CP group at v2 (p<0.05), corresponding to an increasing maximum hip moment (H1) in CP children, which was not seen in the TD group (DS: p=0.001). During swing, the maximum angular extension velocity was significantly higher in the TD group compared to the CP group at v1 and v2 (p<0.005), increasing with WV. Maximum MLV showed similar results, with a significant DS (p<0.005) indicating a higher increase in the TD group. No difference was seen in the maximum ML between both groups,while total range ofMLwas significantly higher in TD group at both speeds, with increasing length range at higher WV. EMG activity in swing showed significantly higher activity at v1 for the CP group, but not at v2. Discussion and conclusions: The effect of WV on knee kinematics at IC and mid stance are in accordance to Schwartz [2] and similar for TD and CP children. The increasing hamstrings activity in CP in early stance may be explained by the biomechanical demands (hip moment). In contrast with TD children who mainly use the gastrocnemius at late stance (push-off) to increase WV. In swing, TD children are capable of increasing their angular velocity and MLV more than children with CP. Van der Krogt [1] reported similar results. This confirms the fact that, although CP children are able to increase their MLV, they prefer to stay under their spastic threshold velocity during self-selectedWV. The TD group increases their EMG swing phase activity similar as CP children. This can be explained by a higher angular velocity due to the increased WV, demanding more eccentric contraction of the hamstrings to slow down knee extension motion. The difference in influence of WV on spastic hamstrings compared to TD children seems to be most pronounced on MLV, and less on EMG activity. Compared to the influence of WV on the spastic gastrocnemius, the latter showed an increase in EMG activity in swing, which was not seen in the TD group [4].