Position sense at the human forearm in the horizontal plane during loading and vibration of elbow muscles

Abstract

When blindfolded subjects match the position of their forearms in the vertical plane they rely on signals coming from the periphery as well as from the central motor command. The command signal provides a positional cue from the accompanying effort sensation required to hold the arm against gravity. Here we have asked, does a centrally generated effort signal contribute to position sense in the horizontal plane, where gravity cannot play a role? Blindfolded subjects were required to match forearm position for the unloaded arm and when flexors or extensors were bearing 10%, 25% or 40% of maximum loads. Before each match the reference arm was conditioned by contracting elbow muscles while the arm was held flexed or extended. For the unloaded arm conditioning led to a consistent pattern of errors which was attributed to signals from flexor and extensor muscle spindles. When elbow muscles were loaded the errors from conditioning converged, presumably because the spindles had become coactivated through the fusimotor system during the load-bearing contraction. However, this convergence was seen only when subjects supported a static load. When they moved the load differences in errors from conditioning persisted. Muscle vibration during load bearing or moving a load did not alter the distribution of errors. It is concluded that for position sense of an unloaded arm in the horizontal plane the brain relies on signals from muscle spindles. When the arm is loaded, an additional signal of central origin contributes, but only if the load is moved.

Figure 1. Experimental set-up and muscle conditioning

A, experimental set-up. Subjects sat with their forearms supported by cradles which were hinged at a point coaxial with the elbow joint. The arms could be moved in the horizontal plane across a surface that was subdivided into angular positions in degrees, the locations of the arms being indicated by a pointer below each hand. The horizontal movement was almost frictionless and in the unloaded condition required no effort to maintain a given elbow angle. To load the arm, a pulley system with a cable was connected to the apparatus, allowing weights to be attached (Load Flexors and Load Extensors). B, muscle conditioning. A simplified diagram of an arm with one elbow flexor and one extensor. Contracting the muscles while holding the arm flexed and then moving it to an intermediate test angle leaves flexors taut and extensors slack. Similarly, contracting the arm while it is extended and then moving it to the test angle leaves extensors taut and flexors slack. The conditioning positions have been indicated by the dashed outlines, the arrows indicating the direction of movement to the test angle.

Figure 2. Effect of loading extensors

Data for single subject. Matching errors for 6 matching trials with the reference arm flexion conditioned (•, FC) or extension conditioned (^, EC). When the matching arm adopted a more extended position this was assigned a positive value, if it was more flexed it was negative. A, no load; B, 10% MVC load on the extensor muscles; C, pooled errors for a single subject following flexion conditioning (•, continuous line, FC) and extension conditioning (^, dashed line, EC) for the relaxed arm (0% load), and extensor loads representing 10% and 25% MVC.

Figure 3. Effect of holding a static load

A, pooled data for 9 subjects undertaking loading of the extensors showing mean position error (±s.e.m.) for 0%, 10% and 25% loading of the extensor muscles. Flexion conditioning (•, continuous line), extension conditioning (^, dashed line). B, pooled data for 9 subjects undertaking loading of the flexors showing mean position error (±s.e.m.) for 0%, 10% and 25% loading conditions. Flexion conditioning (•, continuous line), extension conditioning (^, dashed line).

Figure 4. Effect of moving a loaded arm

A, pooled data for 10 subjects, showing mean position error (±s.e.m.) for 0%, 10% and 25% loading of the extensor muscles in a matching task where subjects moved the load to the target themselves. Flexion conditioning, •, continuous line; extension conditioning, ^, dashed line. Asterisks indicate significant differences between the two forms of conditioning. Dotted line indicates zero error. B, pooled data for 10 subjects where they moved the load themselves, showing mean position error (±s.e.m.) for 0%, 10% and 25% loading of the flexor muscles. Flexion conditioning, •, continuous line; extension conditioning, ^, dashed line. Asterisks indicate significant differences between the two forms of conditioning. Dotted line indicates zero error.

Figure 5. Effect of vibration

A, data for a single subject of the effect of vibration of elbow flexors on position sense when the reference arm was unloaded (left-hand panel) and when the flexors were supporting a load (25% MVC, right-hand panel). Control, ^; vibration, •. Dotted line indicates zero error. B, position errors (means ±s.e.m.) for 11 subjects expressed relative to the unloaded, non-vibrated condition (No Load). Errors were large during vibration of the unloaded muscle (No Load + vib) and small during vibration of the loaded muscle (Load + vib). Asterisk indicates significant difference.