Relative force-control precision worsens as the fraction of maximum force rises

What it is

How finely a person can control force isn't uniform across their full force range. The closer the target sits to that person's maximum voluntary contraction (percent MVC), the larger the relative control error becomes — force output at moderate levels is steadier and easier to hold precisely, while at levels close to full effort, the same absolute error is proportionally larger, and the output genuinely does feel shakier and harder to keep near target.

Why it happens

Producing greater force requires recruiting more, larger motor units and raising their firing rate, and the neural drive noise involved in this process isn't constant — it grows roughly in proportion to the demanded force. This is the same underlying neuromuscular noise source described in the previous entry, but viewed here across different points on the force scale: absolute noise really is larger at high force levels, and that larger absolute noise also represents a bigger fraction of the force level the user is trying to hold — the two effects compound, leaving high-force ranges noticeably less controllable in relative terms than moderate ones.

Studying it

A common design asks participants to hold a constant output at several target levels defined relative to their own maximum voluntary contraction (say, 20%, 50%, and 80% MVC), and measures the coefficient of variation (standard deviation divided by mean) of the output over time. The finding that this coefficient rises with target %MVC is the most common empirical form of this relationship.

Methodological note: maximum voluntary contraction varies widely between individuals, so comparing performance at the same absolute force level across people conflates individual strength differences with force-control precision. Normalizing each person's target to their own maximum before comparing across %MVC levels is usually necessary to see this relationship cleanly.

Where it stops holding

This relationship is clearest near the top of the force range, but precision also worsens at the opposite end, near the very low forces close to sensory and control thresholds — for a different reason: the force itself is too small to be reliably sensed and held, not a case of noise dominating a large drive signal. Controllability across the full force axis is therefore roughly poorer at both extremes and steadiest in the middle, not a simple "lower force is always steadier" relationship.

Applying it

• For interactions requiring fine, sustained force control (say, holding a given pressure level over time), place the target force in the middle of that person's usable range rather than requiring output near full effort for extended periods.
• For force interactions spanning users of different strength (weaker and stronger populations together), avoid a single absolute force threshold — set thresholds relative to the typical population's strength range so weaker users aren't forced to operate in their own high-%MVC zone.
• To verify: have a representative sample try to hold a constant force in the low, middle, and high segments of their own range, compare the coefficient of variation across the three, and confirm the interaction's required force level falls in the segment with the least variation.

Related

• Same group: A8.11.1 People have a limited ability to grade output force · A8.11.3 Force reproduction error grows sharply without feedback · A8.11.4 The usable number of pressure levels falls far short of sensor resolution · A8.11.5 Sustaining a given force level is harder than reaching it
• Nearby: A8.20 Static versus dynamic load · A11.06 Anthropometric data and sizing
• Search terms: force steadiness · percent MVC · maximum voluntary contraction