Effects of forward tilted seating and foot-support on postural adjustments in children with spastic cerebral palsy: An EMG-study

Highlights

• FW seat inclination does not affect postural EMG of reaching children with CP.

• Foot-support in seated children with CP increases tonic trunk extensor activity regardless of CP type or seat inclination.

• Increased tonic trunk extensor activity was associated with improved kinematics of reaching.

Abstract

Objective

To evaluate the effect of 15° forward (FW) seat inclination and foot-support in children with cerebral palsy (CP) on postural adjustments during reaching.

Design

Observational study repeated-measures design; step two of two-step-project.

Setting

Laboratory unit within University Hospital and two special education schools.

Participants

19 children (ten unilateral spastic CP (US-CP); nine bilateral spastic CP (BS-CP); Gross Motor Function Classification System levels I-III; 6–12 years old). Participants were able to take part for one one-hour session.

Intervention

Reaching while sitting in four seating conditions (FW or horizontal seat; with or without foot-support) applied in randomized order.

Outcome measures

Simultaneously, surface electromyography (EMG) of neck, trunk and arm muscles and kinematics of head and reaching arm (step one of two-step-project) were recorded. Primary outcome parameters were the ability to modulate EMG-amplitudes at baseline and during reaching (phasic muscle activity). Other EMG-parameters were direction-specificity (1st control level), and 2nd level of control parameters: recruitment order, and anticipatory postural activity. Motor behaviour measures: ability to modulate EMG-amplitudes to kinematic characteristics of reaching and head stability.

Results

Only foot-support was associated with increased tonic background EMG-amplitudes and decreased phasic EMG-amplitudes of the trunk extensors in children with US-CP and BS-CP (mixed-models analyses; p-values <0.01). The foot-support effect was also associated with better kinematics of reaching (Spearman's Rho; p-values <0.01).

Conclusion

In terms of postural adjustments during forward reaching, foot-support enhanced the children's capacity to modulate trunk extensor activity, which was associated with improved reaching quality. FW-tilting did not affect postural muscle activity.

Introduction

Children with cerebral palsy (CP) often exhibit postural dysfunctions during reaching while sitting.1, 2 Adaptive seating is recommended to improve postural control.3, 4, 5, 6 In children with CP functioning at Gross Motor Function Classification System (GMFCS) level I-III⁷ seat-surface inclination is often used,8, 9 but also debated.10, 11, 12, 13, 14, 15, 16, 17

Postural control is the basis of motor function. To maintain posture and balance in space or in correspondence to gravity, such as during sitting while reaching, a complex interaction of the musculoskeletal and neural systems is required.¹ In terms of motor control, the nervous system creates postural synergies to deal with the problem of many degrees of freedom. In the neural control of postural synergies which involves the control of postural muscle activity, two functional levels may be distinguished.¹⁸ The first level consists of direction-specificity, implying for instance that during forward reaching the dorsal postural muscles are primarily activated. The second level involves the fine-tuning of the direction-specific adjustments, for example by (a) selecting the recruitment order of the agonist muscles (e.g., top-down or bottom-up recruitment); (b) presence of anticipatory postural activity19, 20, 21; and (c) modulation of the degree of postural muscle contraction (reflected by surfaced electromyography (EMG)-amplitude). The latter (c) is the most subtle form of postural fine-tuning.1, 18, 21

The kinematics of reaching movements furnishes information on the control of the movements. In adults a typical reaching movement is programmed in a feedforward way and consists of one acceleration and one deceleration; the combination of one acceleration and one deceleration is called a movement unit (MU).²² During early development, reaching movements are performed with trial and error during which inaccurately feedforward programmed movements are corrected multiple times by feedback. As a result, early reaching movements consist of multiple MUs and they have a curved and relative long movement trajectory. With increasing age and increasing experience feedforward control improves, and major part of the movement is covered by 1 MU (the transport MU). In other words, a better kinematic movement quality, that mostly relies on feedforward control, is especially reflected by fewer MUs and a relatively long transport MU. It also results in movements with a less curved and shorter trajectory.1, 22, 23

School-age children with CP, GMFCS level I-III, can generate the basic level of control during reaching but have impaired fine-tuning.¹ Presumably the children's major problem is their reduced capacity to modulate EMG-amplitude to the specifics of the task, e.g. reaching velocity.² This problem is more severe in children with bilateral spastic CP (BS-CP) than in children with unilateral (US-CP).1, 2 Also, reaching movements of school-age children with CP show impairments, for example, they consist less often of one MU and have a smaller transport MU than those of age-matched typically developing peers.²⁴

Most likely, the debate on the most appropriate seat inclination in ambulatory children with CP is caused by the many factors playing a role: different outcome measures used to evaluate the adequacy of seating,10, 12, 13, 16, 25, 26, 27 heterogeneity in CP, the degree of postural support provided, and variation in seat-angle (5°,17, 26 10°,13, 26 and 15°12, 13, 17, 26, 27, 28). Yet, in children with GMFCS levels I-III, a 10° and 15° forward tilting of the seat surface (FW-tilting) is generally recommended.12, 13, 26 As 15° FW-tilting was associated with better postural stability (reflected by an improved ratio between anterior-posterior and medial-lateral sway) during forward reaching and better reaching kinematics (reflected by shorter movement times) than the 10° configuration,²⁶ we decided to further evaluate the 15° FW-tilting.

Previously, only Hadders-Algra et al.²⁸ specified CP-subtypes. Their study included 58 children with US-CP and BS-CP, functioning at GMFCS level I-IV. Hadders-Algra et al. demonstrated that only in children with US-CP, the 15° FW-tilting improved postural efficiency and quality of reaching. The improved postural efficiency was reflected by a reduced phasic activity of the postural muscles, which was associated with a better kinematic quality of reaching. The improved reaching quality was reflected by reaching movements during which a greater part of the movement was covered by the transport MU. In contrast, in children with BS-CP, the horizontal seat surface was associated with less sway of the head and a more mobile trunk compared to the FW tilting position.²⁸

However, Hadders-Algra et al.²⁸ did not apply foot-support – due to the fact that their research line has a developmental approach, starting in early infancy when foot-support during sitting is uncommon. Others evaluated the effect of seat inclinations in the presence of foot-support but did not address its contribution.11, 12, 13, 14, 16, 17, 26, 27 Based on studies in adults, it is generally acknowledged that the lower limbs have an essential role in balancing the body in seated reaching tasks; they prevent falling forward and provide postural stability by means of a load through the feet typically occurring around the end of the reach. The forces acting at the feet facilitate the return of the upper body to the upright position.²⁹ In addition, foot-support furnishes sensory information that may be used to control posture. In this respect it is interesting to note that part of children with BS-CP are known to have sensorimotor perceptual impairments that are associated with impaired postural adjustments and decreased efficiency of reaching while being seated.³⁰ As we lacked information on the effect of foot-support in addition to seat surface inclination in children with CP, we recently replicated the Hadders-Algra et al. study²⁸ while also evaluating the effect of foot-support as a possible factor affecting postural control. We embarked on a two-step-project.

In step one of the project, we evaluated the effect on the kinematics of the head in space and reaching quality.³¹ The data confirmed the differential effect of FW-inclination on kinematic reaching quality: only children with US-CP benefitted from FW-tilting, the children with BS-CP performed better on a horizontal seat surface.³¹ No effect of FW-tilting on head stability was found, presumably because the children in our study³¹ were less severely affected than those in the Hadders-Algra (2007) study²⁸ (GMFCS levels I-III and I-IV, respectively). Interestingly, in all children with CP, foot-support in the FW-tilted position was associated with higher reaching velocity, a similar effect was absent in the horizontal condition. In children with US-CP, foot-support in the FW-tilted position also was associated with a shorter reaching duration. However, in the children with BS-CP foot-support in the FW-tilted position had a deteriorating effect: it induced longer total path lengths and longer reaching durations.³¹

In step two, the current study, we address the effect of the seating modifications on postural adjustments. To the best of our knowledge, no other study focused on the specific effect of foot-support as a possible factor affecting postural muscular adjustments during forward reaching. We aim to evaluate the effect of 15° FW-tilting of the seat surface in combination with the effect of foot-support in children with spastic CP, i.e. US-CP and BS-CP, GMFCS levels I-III, on postural adjustments while reaching. We address the following questions:

• Does FW-tilting or horizontal seating, with or without foot-support affect EMG-parameters of postural control during reaching? The effect is studied at both levels of postural control, but we hypothesized that a potential effect is best expressed in the EMG-amplitudes, i.e. the amplitudes at baseline and during reaching. Therefore, the EMG-amplitudes are our primary outcome parameters.

• Does seating condition affect the capacity to modulate EMG-amplitudes during reaching in terms of stronger correlations between EMG-amplitudes and kinematics of reaching and head stability?

• Do the putative effects depend on the type of CP, or GMFCS levels?