Individuals with intellectual disability have lower voluntary muscle activation level.
Adults with mild ID can fire only part of their muscles, so routine strength training is medical, not optional.
01Research in Context
What this study did
Borji et al. (2014) tested how well adults with mild intellectual disability can turn on their muscles. They used small electric pulses to see how much of the muscle the adults could use on purpose.
They compared the group with ID to adults without ID. They looked at leg muscle strength, electrical muscle signals, and how much of the muscle each person could activate.
What they found
Adults with mild ID could activate far less of their leg muscle than the control group. Their muscle strength and electrical activity were also lower.
The study showed the problem is both in the brain's signal and in the muscle itself. This explains why many clients seem weak even when they try hard.
How this fits with other research
Two earlier lab studies saw the same thing. Borji et al. (2013) found adults with ID tire faster during hard intervals because of these same neural gaps. Zafeiridis et al. (2010) showed men with ID start weaker and fatigue more slowly, matching the low activation numbers.
A 2023 RCT by C et al. offers hope. High-effort weight training in group homes raised strength and body composition in adults with mild-moderate ID. The trial builds on the 2014 deficit data and proves the gap can shrink with the right program.
Oppewal et al. (2015) link the lab finding to daily life. In older adults with ID, poor fitness predicted a faster loss of cooking, shopping, and phone skills over three years. Weak muscles today can mean lost independence tomorrow.
Why it matters
Low voluntary activation is not laziness. It is a built-in neuromuscular deficit. Screen every adult client with ID for strength and refer them to graded exercise. Use longer rest breaks during interval work. Track grip or leg strength yearly; a drop can flag risk for losing daily living skills. Pair these data with Oppewal et al. (2015) when you write support plans—fitness is a functional outcome, not a side goal.
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02At a glance
03Original abstract
The aim of this study was to explore the voluntary activation level during maximal voluntary contraction (MVC) in individuals with intellectual disability (ID) versus individuals without ID using the twitch interpolation technique. Ten individuals with mild ID (ID group) and 10 sedentary men without ID (control group) participated in this study. The evaluation of neuromuscular function consisted in three brief MVCs (3s) of the knee extension superimposed with electrical nerve stimulation (NES) to measure voluntary activation. Muscle activity levels were also measured with surface EMG. The root mean square (RMS) was extracted from the EMG signal. The RMS/Mmax ratio and the neuromuscular efficiency (NME) were calculated. Our results reported that individuals with ID present lower muscle strength (p < 0.001), lower voluntary activation level (p < 0.001), lower RMS values of vastus lateralis (p < 0.05), vastus medialis (p < 0.05), and rectus femoris (p < 0.001) muscles. In addition, our results showed lower RMS/Mmax values in the ID group than in the control group for the VM (0.05 ± 0.01 mV vs. 0.04 ± 0.01 mV; p < 0.05) and the RF (0.06 ± 0.02 mV vs. 0.05 ± 0.02 mV; p < 0.05) muscles. However, no significant difference was reported for the VL muscle (0.05 ± 0.02 mV vs. 0.05 ± 0.02 mV; p=0.463). Moreover, Individuals with ID present smaller potentiated twitch (p < 0.001). However, no significant difference was reported in the NME ratio. These results suggest that the lower muscle strength known in individuals with ID is related to a central nervous system failure to activate motor units and to some abnormal intrinsic muscle properties. It seems that the inactive lifestyle adopted by individuals with ID is one of the most important factors of their lower voluntary activation levels. Therefore, physical activities should be introduced in life style of individuals with ID to improve their neuromuscular function.
Research in developmental disabilities, 2014 · doi:10.1016/j.ridd.2014.08.038