Postural muscle responses and adaptations to backward platform perturbations in young people with and without intellectual disability

Highlights

• Postural responses were studied in young people with and without intellectual disability (ID).

• No differences were found in muscle onset time, synergies and strategies.

• Young people with ID adapted their muscle responses to perturbations poorly.

• Young people with ID may have reduced ability to use somatosensory information.

Abstract

This study examines postural muscle responses to backward perturbations in young people (16–20 years) with and without intellectual disability (ID). The study included 56 young people with ID and 43 age-matched without ID volunteers. The subjects stood on a platform that was moved backwards in a surface translation. Lower and upper leg muscles and lower back spine muscles were recorded with surface electromyography (EMG). Muscle onset latency, time to peak amplitude (EMG), adaptation of muscle responses to repeated perturbations (using integrated EMG (IEMG) for epochs), and synergies and strategies were assessed. The result showed no differences between the two groups in muscle onset latency, synergies, and strategies. Young people with ID reduced their time to peak amplitude in investigated muscles, a response that was different from the group without ID. Also, young people with ID tended to adapt their IEMG less compared to the controls. These findings suggest that young people with ID have limited ability to use somatosensory information and adapt their postural muscle responses to repeated external perturbations.

Introduction

Young and old individuals with intellectual disability (ID) have reduced postural balance compared to age-matched individuals without ID [1], [2], [3], [4] and they fall more often than their peers [5]. Researchers have found that people with ID have limited ability to anticipate postural adjustments when they move from the sitting to standing position and when they stand on one leg [6]. In addition, these researchers have found that ID-limited people exhibit general stability problems (Forward Reach Test) [4], [7] and gait stability issues (Timed Up and Go Test) [7], [8], but the reason for these problems is unclear. One reason for the reduced postural balance could be that individuals with ID have slower postural muscle responses compared to individuals without ID.

The postural balance system uses principally four different motor control strategies after external perturbations have been attended: ankle, hip, mixed of ankle/hip or step strategies [9], [10]. Ankle strategy restore the CoM primarily around the ankle joints, hip strategy restore the CoM primarily by motions at the hip joints and step strategy restore the CoM by taking a step to reposition the base of support [11]. If the platform on which the subject is standing moves backwards, the subject's body sways forward. If the duration and velocity of the backwards translation is not too difficult, the subject uses an ankle strategy to control the movement in the ankle and knee joint. The muscle synergy for an ankle strategy begins in the gastrocnemius muscles and then proceeds to the hamstrings and then to the paraspinal muscles [12], [13]. For more challenging perturbations, the other strategies are used.

After a perturbation, the first muscle response is a stretch reflex (latency 40–50 ms), followed by automatic postural responses (latency 70–150 ms), and then voluntary reaction responses (180–250 ms) [14]. Delays in the activation could occur from slowed sensory or motor conduction or decreased central processing and can lead to reduced balance control [15].

Studies have shown that persons with decreased central processing – such as in traumatic brain injury [16], Huntington's disease [17], ageing [15], and in children with Down's syndrome [18] – have delayed muscle onset latencies, but no studies have examined muscle onsets for young people with ID. To this end, this study investigates muscle onset latencies from external perturbations. This study included young people with ID and age-matched peers without ID. The use of aged-matched peers presented several questions: Do young people with ID have delayed muscle onset latencies? Are their times to peak amplitude (EMG) slower? Do they exhibit different muscle synergies and strategies? Do young people with ID have a slower adaptation of their postural muscle responses?