| Animals must maintain stability when running over rough, uneven terrain, but the methods by which they successfully do so are largely unknown, particularly at high speeds. "Drop-down" studies are a useful way to examine how animals cope with an unexpected change in terrain height. To investigate joint function during and following a drop and make predictions about the likely preflex and reflex responses necessary to compensate for altered mechanical conditions, I ran helmeted guinea fowl Numida meleagris over a trackway with a camouflaged drop in substrate height. 2-D inverse dynamics were used to calculate the net moments, powers, and angular trajectory at each joint. The drop dramatically altered limb posture. The ankle, knee and hip all showed significantly increased extension during the drop with differences in angle of 26.5° +/- 2.5°, 13.3o +/- 3.9°, and 20.4° +/- 2.4°, respectively. Net moments at the tarsometatarso-phalangeal joint (TMP) and ankle during the subsequent stance period were reduced but predictable, whereas net moments at the hip and knee were variable and unpredictable, particularly in early stance. Based on these results I predicted reflex alteration of muscle activity at the proximal joints would be necessary to cope with variations in net moment by increasing mechanical impedance. I also predicted muscle shortening of the digital flexors due to ankle extension would reduce force production by these muscles, requiring an increase in muscle activation to compensate and provide enough force to support the stance phase moments. The ankle may require increased activation to maintain the observed extension moment and potentially co-contraction to prevent hyperextension.;To test these predictions, I sampled twelve muscles during both level and drop runs using intra-muscular electromyography (EMG) electrodes. The average EMG amplitude was compared between level and drop runs during 3 time periods: Period 1, the "drop," the time between 4 expected foot contact and the average time of actual contact, Period 2, "stance," the additional period following actual contact during which changes in EMG amplitude would on average be predicted to influence stance phase mechanics, and Period 3, "early swing," the period of time during which changes in EMG amplitude would be expected to influence the mechanics of pulling the limb from the hole. Two muscles acting at the TMP joint and one acting at the knee/hip showed significant alterations in EMG amplitude during Period 1, indicating that both distal and proximal muscles show the fastest reflex responses. A number of other muscles at the ankle, knee, and hip also showed alterations in muscle recruitment with only a slightly longer delay during Periods 2 and 3. In support of my predictions, increased activation was found in the digital flexors and increased co-contraction was found at the ankle and hip. Increased activation was also found in the knee extensors, likely to compensate for altered contractile conditions. My findings suggest there is a coordinated reflex response across multiple joints in response to an unexpected drop in terrain height. |
