Abstract
Purpose
Distal lower limb motor impairment impacts gait mechanics in individuals with cerebral palsy (CP), however, the contribution of impairments of muscle activation to reduced gross motor function (GMF) is not clear. This study aimed to investigate deficits in plantar flexion voluntary activation capacity in CP compared to typically developed (TD) peers, and evaluate relationships between voluntary activation capacity, strength and GMF.
Methods
Fifteen ambulant individuals with spastic CP (23 ± 6 years, GMFCS I–III) and 14 TD (22 ± 2 years) people participated. Plantar- and dorsiflexion strength were assessed with a dynamometer. Voluntary activation capacity was assessed using the interpolated twitch technique via single twitch supramaximal tibial nerve stimulation. GMF was assessed using the timed upstairs test, 10 m walk test, muscle power sprint test and six-minute walk test.
Results
Plantar- and dorsiflexion strength were 55.6% and 60.7% lower in CP than TD (p < 0.001). Although voluntary activation capacity was 17.9% lower on average for CP than TD (p = 0.039), 46.7% of individuals with CP achieved a sufficiently high activation to fall within one standard deviation of the TD mean. Plantar flexion voluntary activation capacity did not correlate with strength (R2 = 0.092, p = 0.314) or GMF measures in the high functioning CP group (GMFCS I–II).
Conclusion
In contrast to previous research, plantar flexion activation capacity did not strongly predict weakness or reduced GMF. We propose that muscle size contributes more to weakness than voluntary activation capacity in high functioning individuals with CP and that relationships between muscle activation and functional capacity are complicated by effects at multiple joints.
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Abbreviations
- 10mWT:
-
10m walk test
- BMI:
-
Body mass index
- CI:
-
Confidence interval
- CP:
-
Cerebral palsy
- GMF:
-
Gross motor function
- GMFCS:
-
Gross motor function classification scale
- ICC:
-
Intra-class correlation coefficient
- iMVC:
-
Isometric maximum voluntary contraction
- ITT:
-
Interpolated twitch technique
- MPST:
-
Muscle power sprint test
- sEMG:
-
Surface electromyography
- TD:
-
Typically developed
- TUS:
-
Timed upstairs
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Acknowledgements
We gratefully acknowledge the support of Mr Jarred Gillett, PhD, for technical assistance during data collection.
Funding
This work was supported by a National Health and Medical Research Council Postgraduate Scholarship, co-funded by the Cerebral Palsy Alliance Research Foundation [APP1114651].
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All authors contributed to the study conception and design. Data collection and analysis were performed by SO’B. SO’B wrote the first manuscript draft. All authors contributed to data interpretation and editing of the manuscript. All authors read and approved the final manuscript.
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O’Brien, S.M., Carroll, T.J., Barber, L.A. et al. Plantar flexor voluntary activation capacity, strength and function in cerebral palsy. Eur J Appl Physiol 121, 1733–1741 (2021). https://doi.org/10.1007/s00421-021-04638-z
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DOI: https://doi.org/10.1007/s00421-021-04638-z