PMC

(I) Set-up for generating sudden perturbations to the upper part of the subject's trunk. The wire is fastened to a rigid bar fastened by a vest at the upper part of the trunk. The movement of the trunk is measured by a potentiometer mounted on a reel. (II) Details of the perturbation apparatus (right top: 90 degrees rotation) and standing position (left): (a) cylinder, (b) 5 kg load, (c) gripping device, (d) solenoid for activating gripping device, (e) holding magnets, (f) load-bearing construction, (g) 5 kg load, (h) anchor plate for magnet, (i) holding magnet, (j) bearing construction, (k) vertical adjustable reel with potentiometer, and (l) horizontal adjustable reel to adjust wire length to individual subject height. Generation of sudden unloading ((III) left): first the computer activates the magnet (i) and releases the load (b, h, and g) causing the weight of the load applied to the wire to suddenly decrease from 5.4 kg (a) to 0.1 kg (i). Generation of the sudden loading ((III) right): first the computer activates the solenoid (d) causing the gripping device (c) to fix the load (b) to the cylinder (a) and secondly deactivates the holding magnets (e). This releases the load (b, h, and g) causing the weight of the load applied to the wire to suddenly increase from 5.4 kg (a) to 10.9 kg (a–d).

The blue line indicates the load displacement curve for a single knotted loop that slipped without breaking. Note the gradual steady increase in load with extension, a plateauing of load with further extension, then a slight increase and then decrease as further extension is applied. The knots in this loop eventually came apart as one suture end slipped over the other, resulting in the sudden, rapid decrease in load.

Experimental setup of sudden load experiment. Preloads to generate external flexion moment and sudden flexion moment perturbations were applied through a cable system. Trunk kinematics, kinetics, and EMG were recorded throughout the disturbance and response. Identical experimental trials were collected before and after extension muscle fatigue.

Schematic drawings of force application in various testing set-ups and typical force–displacement curves of mechanical tests in which root failure occurred. (A) Test of root clusters. The load is applied to a shoot segment of English ivy, normal to the surface. (A.1) Typical force–displacement curve with a slightly convex force increase followed by a sudden and complete failure of the sample, (A.2) Typical force–displacement curve for samples that showed preliminary failure events in the decreasing part of the curve before complete failure. (A.3) Typical force–displacement curve with a stepwise force decline showing several preliminary failure events after root strength is reached. (B) Tensile test of complete individual attached roots; the load is applied normal to the substrate. (C) Test of complete individual attached roots; the load is applied parallel to the substrate. (D) Test of isolated intact roots with cortex; the load is applied parallel to the longitudinal axis of the root. (E) Test of isolated central cylinders; the load is applied parallel to the longitudinal axis of the root. ar, attachment root; cc, central cylinder; cs, substrate; is, ivy shoot segment; rc, root cortex. (This figure is available in colour at JXB online.)