Torque-onset determination: Unintended consequences of the threshold method.

Area of Science:

• Biomechanics
• Human Physiology
• Motor Control

Background:

• Visual torque-onset-detection (TOD) is a reliable method for assessing muscle function.
• Threshold-based TOD methods can introduce onset bias, particularly with lower torques or rates of torque development (RTD).
• This bias can lead to inaccuracies when comparing contractile parameters between different groups.

Purpose of the Study:

• To compare the impact of differential TOD bias on contractile parameters.
• To investigate how fixed (4-Nm) and relative (5% MVC) thresholds affect TOD bias in torque-disparate groups (boys vs. men).
• To evaluate the accuracy of different TOD methods in representing muscle contractile properties.

Main Methods:

• Maximal, explosive, isometric knee-extensions were performed by 15 boys and 12 men.
• Torque and EMG data were collected, with best contractions selected by peak torque (MVC) and peak RTD.
• Visual-TOD, 4-Nm-threshold TOD, and 5% MVC-threshold TOD were compared for torque-time traces, electromechanical delays (EMD), and times to peak RTD (tRTD).

Main Results:

• Fixed 4-Nm thresholds showed markedly different biases between boys and men (40.3±14.1 vs. 18.4±7.1ms), unlike 5% MVC thresholds.
• Electromechanical delay (EMD) differences were significantly amplified by the 4-Nm threshold (5.0ms visual vs. 26.9ms 4Nm).
• The 4-Nm threshold erroneously indicated boys were faster than men at any given %MVC, contradicting visual TOD findings.

Conclusions:

• Threshold-based TOD methods can misrepresent muscle contractile properties when comparing dissimilar groups (e.g., children vs. adults).
• Fixed thresholds introduce significant group-specific biases, leading to erroneous conclusions.
• Relative thresholds (5% MVC) reduce confounding group comparisons, though some error may persist.