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Title: The role of short latency reflexes in the motor control of quadriceps in humans
Author: Khademi-Kalantari, Khosro
ISNI:       0000 0001 3597 9579
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2002
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A total of 34 volunteer subjects participated in a series of experiments. These experiments were designed to investigate the characteristics and functional role of an excitatory reflex pathway linking the pretibial muscles to quadriceps (CPQ reflex). The CPQ reflex was evoked by low intensity electrical stimulation of common peroneal nerve (CPN) at the level of caput fibulae. The reflex was observed in averaged EMG of rectus femoris (RF) and vastus medialis (VM). Voluntary contraction of Q was needed to reveal the reflex and no responses were detected in the recordings from relaxed muscle. The peak to peak amplitude and area of the responses were measured in non-rectified and rectified averaged EMG. The stability of the CPQ reflex was investigated in the first series of experiments. Nine trials were distributed across three sessions with the perceptive caution to keep the stimulating and recording conditions similar. A constant contraction (20% of maximum voluntary contraction) in quadriceps was maintained by the subjects during the tests. Constant stimulus intensity to evoke maximum reflex was applied in all trials. The result suggested that the peak- peak amplitude and the threshold of the CPQ reflex was extremely consistent with-in session and between sessions of the experiment. Graded stimulus intensities between 0.9 and 1.5xmotor threshold (MT) in tibialis anterior (TA) were applied to CPN in the second series of experiments while the quadriceps (Q) was contracted at a level of 20% of maximum voluntary contraction (MVC). The electrical stimulation of the CPN evoked excitatory responses in all the subjects with a latency of about 30+/-2.1ms (mean +/- ISD). The responses were regarded significant with amplitude beyond 2 standard deviation (SD) of the mean. The mean was calculated from 30ms pre-stimulus averaged EMG. In three subjects making contractions at 20% of MVC significant CPQ reflexes were recorded with stimulation at an intensity of 0.9xMT. The extrapolation of the pooled data also suggests an electrical threshold of around O.8xMT. This low threshold and the short latency strongly suggest the participation of group I afferent types in this reflex pathway. Maximum response was evoked by stimulus intensity of about 1.3XMT. The changes in the reflex magnitude in different knee and hip joint positions were also investigated in another series of experiments. The knee positioning was shown to affect the reflex amplitude in all subjects. The reflex became smaller towards more flexed position of the knee joint. Two patterns of reaction were recognised among the subjects. One group showed a very significant and abrupt reduction in the magnitude of the reflex after 10°-20° of knee flexed positioning and the second group was characterised with a gradual and progressive reduction in the reflex magnitude from 10° to 50° of flexed position. The hip extended position also produced a significant attenuation of reflex magnitude compared to the flexed position however this effect was less consistent than the knee positioning in the investigated subjects. The correlation between the reflex magnitude and the level of muscular activity in the quadriceps at both extended and flexed positions of the knee was investigated in the next series of experiments. The reflex showed a significant linear correlation to the background EMG at the extended knee position. This is probably related to the synaptic facilitation of the CPQ pathway by the central inputs. No significant changes in latency were detected by the increase in voluntary contraction. At 130° flexed position however, no correlation was found between the reflex magnitude and the level of muscular activity in Q. In fact even at muscular activity of about 40% of MVC no reflex was detected in any of the subjects at flexed position. It was concluded that this inhibitory effect interacts with the reflex at pre-motoneuronal level. The functional role of the CPQ reflex was investigated in two other series of experiments. The pattern of changes in the reflex magnitude was studied during gait. Stimuli that could produce maximum reflex magnitude were applied in a pseudo-random sequence while the subjects were walking on a treadmill. The stimulation was applied at different time delays from heel strike, triggered by a pressure sensor at the heel. The reflex magnitude and the EMG activity in the VM, RF, TA, semitendinosus and medial gastrocnemius were measured. The reflex was at its highest magnitude shortly after heel strike. No responses were detected during the major period of gait cycle from midstance to terminal swing phase. The reflex magnitude pattern of modulation was closely correlated with the pattern of activity in the RF and VM i.e. both showed their peak shortly after heel strike and remained quiescent during the rest of gait cycle. This suggests that the CPQ reflex can provide a positive feedback input to Q motoneurone during the early stance phase of gait. This could help to stabilise the knee joint at this period to overcome the load of the body. The nature of the areflexic period of the gait cycle was investigated in the fifth experiment. A modified knee orthosis was used to produce increased muscular activity in Q during this quiescent period. Pairs of springs were fixed into the orthosis to provide a flexor momentum to the knee joint. The muscular activity of RF and VM increased significantly throughout the gait cycle. Stimuli were applied at three instants at midstance, terminal stance and terminal swing phase. These three instants were all areflexic and EMG-silent previous to the application of the spring-loaded knee orthosis. Therefore if no inhibitory effect was affecting the reflex pathway during this areflexic period, with increase in the background EMG, the reflex was expected to reappear. The stimulation at midstance and terminal swing phase re-evoked the reflex and a positive correlation was observed between the reflex magnitude and the increase in the muscular activity in quadriceps. However, stimulation at the terminal stance phase was unable to re-evoke the CPQ reflex despite the significant increase in the background activity in Q. It was concluded that an active inhibition was imposed on the reflex pathway during this period. The critical role of the lower limb at this period is transition from stance to swing phase. This necessitates unloading of the lower limb. This unloading phenomenon is associated with knee flexion and ankle dorsiflexion to clear the foot from the ground. A strong excitatory reflex from ankle dorsiflexors to Q muscles at this period would hinder the knee flexion and the transition. It is argued that this inhibition could contribute to the unloading and transition of the lower limb to the swing phase.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physiology