SUMMARY1. Ankle joints were subjected to sinusoidal movements at a range of amplitudes and frequencies. Records were made ofelectromyograms (e.m.g.s) in calf muscles, and of the forces at the joints.2. When the leg is relaxed, the ankle joint resists an imposed sinusoidal movement with a small approximately sinusoidal force. It is stiffer in its resistance to small movements than to large ones, and this resistance is greater when the joint is dorsiflexed than when it is plantarflexed.3. If the subject exerts a steady mean flexing force, the imposed sinusoidal movement generates reflex activity which may be recorded as a modulation of the gastrocnemius and soleus e.m.g.s.4. The e.m.g. response to the sinusoidal movement occurs later in cycles of movement at high than at low frequencies, as one could expect of a reflex pathway that involves a delay. The results suggest that this delay is between 50 and 60 ms, and we conclude that under these circumstances spinal stretch reflexes are playing the important part.5. The relation of the resisting force to the movement has been displayed as a vector. As the frequency changes, this vector describes the circular path that is characteristic of a system which includes delays or lags; this path enables one to draw conclusions about the amplitude and timing of the reflex resistance to the movement.6. When a subject exerts a moderate flexing force against the sinusoidal movement for some minutes, the reflex response becomes progressively potentiated. A subject whose reflex responses are normally slight may then exhibit a vigorous reflex response to the movement of that ankle. This enhancement of spinal reflex activity was accompanied by an increase in the myotatic reflex response at the ankle.7. Reflex responses to sinusoidal movement were most clearly seen when the subject exerted a mean flexing force that amounted to about one-fifth of his maximum. C. M. EVANS AND OTHERSvaried from subject to subject and from time to time) the reflex mechanism appeared to 'saturate', and further increases in amplitude were not accompanied by comparable increases in the reflex response. 9. With movements at 10-15 Hz the e.m.g. response often became large in alternate cycles, with less activity in the intervening cycles. In extreme cases e.m.g. activity was confined to alternate cycles, with electrical silence in the intervening ones. This alternation ofthe e.m.g. signal was usually accompanied by similar changes in the force records, so that each cycle with vigorous e.m.g. activity was followed by one in which the force rose to a high level.

Plague is a potentially dangerous reemerging disease. Because modern outbreaks are relatively infrequent, data for epidemiologic study are best found in historical accounts. In 1905, the Rand Plague Committee published a report describing an explosive outbreak of 113 cases of pneumonic plague that occurred in Johannesburg, South Africa, in 1904. Using these data, we investigated social, spatial, and temporal dynamics and quantified transmissibility as measured by the time-varying reproduction number. Risk for transmission was highest when friends, family members, and caregivers approached the sick. Reproduction numbers were 2–4. Transmission rates rapidly diminished after implementation of control measures, including isolation and safer burial practices. A contemporaneous smaller bubonic plague outbreak associated with a low-key epizootic of rats also occurred. Clusters of cases of pneumonic plague were mostly limited to the Indian community; cases of bubonic plague were mostly associated with white communities and their black African servants.

A study of the parish records of St Peter upon Cornhill in London from 1580 to 1605 revealed that children suffered a greater increase in mortality than adults in the plague years of 1593 and 1603, and servants accounted for the majority of deaths within the 15–24 age group. Some family groups avoided the plague altogether, others suffered a single burial, however in some cases, individuals within the same family household were buried within a short period of each other. The epidemiological pattern is complex and is moderated by social and demographic networks. Comparisons are made with modern epidemics caused by Yersinia pestis.

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