Conventionally assessed voluntary activation does not represent relative voluntary torque production
Open Access
- 15 March 2007
- journal article
- research article
- Published by Springer Science and Business Media LLC in European Journal of Applied Physiology
- Vol. 100 (3), 309-320
- https://doi.org/10.1007/s00421-007-0425-8
Abstract
The ability to voluntarily activate a muscle is commonly assessed by some variant of the twitch interpolation technique (ITT), which assumes that the stimulated force increment decreases linearly as voluntary force increases. In the present study, subjects (n = 7) with exceptional ability for maximal voluntary activation (VA) of the knee extensors were used to study the relationship between superimposed and voluntary torque. This includes very high contraction intensities (90–100%VA), which are difficult to consistently obtain in regular healthy subjects (VA of ∼90%). Subjects were tested at 30, 60, and 90° knee angles on two experimental days. At each angle, isometric knee extensions were performed with supramaximal superimposed nerve stimulation (triplet: three pulses at 300 Hz). Surface EMG signals were obtained from rectus femoris, vastus lateralis, and medialis muscles. Maximal VA was similar and very high across knee angles: 97 ± 2.3% (mean ± SD). At high contraction intensities, the increase in voluntary torque was far greater than would be expected based on the decrement of superimposed torque. When voluntary torque increased from 79.6 ± 6.1 to 100%MVC, superimposed torque decreased from 8.5 ± 2.6 to 2.8 ± 2.3% of resting triplet. Therefore, an increase in VA of 5.7% (from 91.5 ± 2.6 to 97 ± 2.3%) coincided with a much larger increase in voluntary torque (20.4 ± 6.1%MVC) and EMG (33.9 ± 6.6%max). Moreover, a conventionally assessed VA of 91.5 ± 2.6% represented a voluntary torque of only 79.6 ± 6.1%MVC. In conclusion, when maximal VA is calculated to be ∼90% (as in regular healthy subjects), this probably represents a considerable overestimation of the subjects’ ability to maximally drive their quadriceps muscles.Keywords
This publication has 46 references indexed in Scilit:
- Neuromuscular fatigue development during maximal concentric and isometric knee extensionsJournal of Applied Physiology, 2006
- Knee angle-dependent oxygen consumption during isometric contractions of the knee extensors determined with near-infrared spectroscopyJournal of Applied Physiology, 2005
- Strength and endurance of knee extensors in subjects after paralytic poliomyelitisDisability and Rehabilitation, 2005
- Muscle activation and blood flow do not explain the muscle length-dependent variation in quadriceps isometric enduranceJournal of Applied Physiology, 2005
- Twitch potentiation is greater after a fatiguing submaximal isometric contraction performed at short vs. long quadriceps muscle lengthJournal of Applied Physiology, 2005
- Initial phase of maximal voluntary and electrically stimulated knee extension torque development at different knee anglesJournal of Applied Physiology, 2004
- Influence of tendon slack on electromechanical delay in the human medial gastrocnemius in vivoJournal of Applied Physiology, 2004
- Comparison of interpolation and central activation ratios as measures of muscle inactivationMuscle & Nerve, 2001
- Temperature Effect on the Rates of Isometric Force Development and Relaxation in the Fresh and Fatigued Human Adductor Pollicis MuscleExperimental Physiology, 1999
- Clinical and experimental application of the percutaneous twitch superimposition technique for the study of human muscle activation.Journal of Neurology, Neurosurgery & Psychiatry, 1986