Letter to the Editor: On "Advantages and disadvantages of stiffness instructions when studying postural control" by C.T. Bonnet: Taking a step towards a broader perspective on quiet standing instructions.

s / Gait & Posture 46 (2016) 208–220 213 COP registrations are often complemented with EMG measures and/or measures of cortical activity, allowing a better understanding of how intermuscular coordination of balance is regulated [17–21]. Alsodirect and indirectmeasures of the attentional investment in postural control are often used, such as stimulus-response reaction times or the regularity of COP excursions [10,22–25]. For example, if human subjects do indeed adopt an attentiondemanding stiffness strategy when instructed to stand as still as possible, as suggested by Bonnet [1], this should be evident in higher co-contraction, greater cortical (prefrontal) activity, longer stimulus-response reaction times andmore regular COP excursions compared to a neutral control instruction. Note that the amount of sway may be comparable between the two types of instructions. To fully understand “natural postural control”, one should rely on a comprehensive set of DVs of postural control, instead of simply equating postural control with the amount of postural sway. [DV] = f([IDV]): The well-defined role instruction plays in the study of postural control In our view, an encompassing study of postural control requires an examination of the effects of various IDVs on a comprehensive set of DVs of postural control, as outlined in Fig. 1. The role of instruction in this scheme is well-defined and functions as one particular IDV (tasks) within the broader set of IDVs (task, environment, subject, time). That is, an instruction may function as a within-subject factor to study the effects of specific task instructions on postural control DVs. This could involve–in the context of Bonnet’s Letter to theEditor–anexperimental contrast between the task instructions “to stand as still as possible” and “to stand naturally upright” (cf. [2,3]). Experiments that manipulate instruction are widely available in the postural control literature, such as studies on internal vs. external foci of attention [26–28]. Researchers should of course explain the rationale behind their instructions as IDV in their papers, and as well specify the employed instructions. Outside the [DV] = f([IDV]) scheme, instruction may act as a controlled study parameter, similar to parameters such as the type of equipmentand the sampling frequency,whichareheld constant. As alreadymentionedbyBonnet (2015), such instructionsmaybeused for data quality control, for example by instructing participants to keep the feet in place and to prevent excessive head and armmovements while standing upright. Such instructions are convenient for both researchers and participants. In conclusion, we do not see anymerit in disregarding a specific instruction from being used in the lab, as proposed by Bonnet [1] regarding the stiffness instruction, provided that the instructions are explicitly specified in the paper. We fully agree with Bonnet [1] that it is worthwhile to study “natural postural control”, which in our view requires an examination of the effects of various IDVs on a comprehensive set of postural control DVs (Fig. 1). Hence, we posit that maintaining an upright posture (inside and outside the lab) always takes place within a particular context, and that the instruction to the participant represents just one factor (i.e., task constraint) in an even broader set of IDVs influencing “natural postural control”. Because of the well-defined role instructions play in the study of postural control, we see no reason to a priori exclude a particular type of instruction from the scientific investigation of postural control.