Automated postural responses are modified in a functional manner by instruction

Abstract

The restoration of upright balance after a perturbation relies on highly automated and, to a large extent, stereotyped postural responses. Although these responses occur before voluntary control comes into play, previous research has shown that they can be functionally modulated on the basis of cognitive set (experience, advanced warning, instruction, etc.). It is still unknown, however, how the central nervous system deals with situations in which the postural response is not necessarily helpful in the execution of a task. In the present study, the effects of instruction on automated postural responses in neck, trunk, shoulder, and leg muscles were investigated when people were either instructed to recover balance after being released from an inclined standing posture [balance recovery (BR) trials], or not to recover at all and fall onto a safety mattress in the most comfortable way [fall (F) trials], in both backward and leftward directions. Participants were highly successful in following the instructions, consistently exhibiting stepping responses for balance recovery in BR trials, and suppressing stepping in the F trials. Yet EMG recordings revealed similar postural responses with onset latencies between 70 and 130 ms in both BR and F trials, with slightly delayed responses in F trials. In contrast, very pronounced and early differences were observed between BR and F trials in response amplitudes, which were generally much higher in BR than in F trials, but with clear differentiation between muscles and perturbation directions. These results indicate that a balance perturbation always elicits a postural response, irrespective of the task demands. However, when a specific balance recovery response is not desired after a perturbation, postural responses can be selectively downregulated and integrated into the motor output in a functional and goal-oriented way.

Fig. 1

a Schematic diagram of the experimental setup (backward perturbation position illustrated). Subjects stood supported at an angle of 15° to the vertical. The tether was released unexpectedly, inducing a balance perturbation. b Raw data from a typical backward balance recovery trial (dark gray area and black, dashed lines) and a fall trial (light grey area and gray, solid lines), showing left sternocleidomastoid EMG (SCL), anterior deltoid EMG (DAL), rectus femoris EMG (RFL), right tibialis anterior (TAR), lateral movement of the left elbow marker, and upward movement of the right foot marker. Tether release is at time = 0 ms

Fig. 2

Average onset latencies (± SE) in response to backward and leftward perturbations for left and right sternocleidomastoid (SCL and SCR), anterior deltoid (DAL and DAR), posterior deltoid (DPL and DPR), rectus abdominis (ABL and ABR), rectus femoris (RFL and RFR), and tibialis anterior (TAL and TAR). Data from balance recovery trials are shown as black diamonds, and data from fall trials are shown as gray squares. * P < 0.01

Fig. 3

EMG amplitudes of balance recovery (BR) trials minus the amplitudes in fall (F) trials in response to backward and leftward perturbations. Average differences (±SE) are shown as a function of time after tether release for bilateral sternocleidomastoid (SC), anterior deltoid (DA), posterior deltoid (DP), rectus abdominis (AB), rectus femoris (RF), and tibialis anterior (TA). Muscles on the left side of the body are shown as black dashed lines, muscles on the right are shown as gray solid lines. The arrows indicate at which instant EMG amplitudes started to deviate significantly between BR and F trials

Fig. 4

Average changes in flexion and abduction angles (±SE) of the left (L) and right (R) arm between tether release and 200 ms post-release for balance recovery (diamonds) and fall trials (squares). * P < 0.05