We propose to continue using and extending our studies on the contractile properties of single human motor units, using the new microneurographic technique we have developed during the past three years. This involves stimulating individual motor axons at a site distant from the muscle. This method has provided the first accurate, stable and repeatable measurement of both twitches, and also tetanic responses to a wide range of stimulus frequencies, delivered before and after a standard fatigue test (Burke, 1973). Results, so far, have been obtained only for thenar motor units; but their contractile properties showed distinct differences compared with those of other mammalian species, when both were tested in the same way. For example, these units could not be classified physiologically into different types using conventional criteria. For example, twitch contractile speed measures were unimodally distributed between units, and not correlated with fatigue indices. Furthermore, no units were found with fatigue indices in the FF range, and most weak (small) units were not both slow and fatigue resistant. Thus, if similar results are obtained from other muscles this will raise questions about the validity of interpreting whole human muscle responses, and their changes with fatigue in terms of oat data; and also, about their recruitment order and rate coding behavior in voluntary contractions. Our results also confirmed previous reports, obtained from cat data (Calancie & Bawa, 1985), of substantial errors in twitch data, as usually recorded in man by spike-triggered averaging. We plan now to: (1) use our new techniques to make similar measurements from motor units in postural and other hand muscles; (2) seek particularly units with fatigue indices in the FF range, which may have been absent in our thenar unit population; and (3) examine changes in unit contractile properties following fatigue from voluntary contraction, if possible. Experiments will also be done on whole human muscle to examine motoneuron firing rate regulation during fatigue. Our data generally favor the view that fatigue initiates an inhibitory reflex mediated by group III/IV muscle afferents, triggered by metabolite accumulation in the muscle. In contrast, Hagbarth's group (1990) suggests that the decline in firing rates is caused mainly by withdrawal of motoneuron facilitation by spindles during fatigue. New experiments are planned, some also involving microneurography, to discriminate between these two hypothesis, and to assess the relative roles of both mechanisms.
| Thomas, C K; Johansson, R S; Bigland-Ritchie, B (2006) EMG changes in human thenar motor units with force potentiation and fatigue. J Neurophysiol 95:1518-26 |
| Thomas, Christine K; Johansson, Roland S; Bigland-Ritchie, Brenda (2002) Incidence of F waves in single human thenar motor units. Muscle Nerve 25:77-82 |
| Thomas, C K; Johansson, R S; Bigland-Ritchie, B (1999) Pattern of pulses that maximize force output from single human thenar motor units. J Neurophysiol 82:3188-95 |
| Thomas, C K; Tucker, M E; Bigland-Ritchie, B (1998) Voluntary muscle weakness and co-activation after chronic cervical spinal cord injury. J Neurotrauma 15:149-61 |
| Thomas, C K; Zaidner, E Y; Calancie, B et al. (1997) Muscle weakness, paralysis, and atrophy after human cervical spinal cord injury. Exp Neurol 148:414-23 |
| Bigland-Ritchie, B; Rice, C L; Garland, S J et al. (1995) Task-dependent factors in fatigue of human voluntary contractions. Adv Exp Med Biol 384:361-80 |
| Howell, J N; Fuglevand, A J; Walsh, M L et al. (1995) Motor unit activity during isometric and concentric-eccentric contractions of the human first dorsal interosseus muscle. J Neurophysiol 74:901-4 |
| Bigland-Ritchie, B R; Furbush, F H; Gandevia, S C et al. (1992) Voluntary discharge frequencies of human motoneurons at different muscle lengths. Muscle Nerve 15:130-7 |
| Bigland-Ritchie, B; Thomas, C K; Rice, C L et al. (1992) Muscle temperature, contractile speed, and motoneuron firing rates during human voluntary contractions. J Appl Physiol 73:2457-61 |
| Rice, C L; Vollmer, T L; Bigland-Ritchie, B (1992) Neuromuscular responses of patients with multiple sclerosis. Muscle Nerve 15:1123-32 |
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