Endurance time is joint-specific: a modelling and meta-analysis investigation

Abstract

Static task intensity-endurance time (ET) relationships (e.g. Rohmert's curve) were first reported decades ago. However, a comprehensive meta-analysis to compare experimentally-observed ETs across bodily regions has not been reported. We performed a systematic literature review of ETs for static contractions, developed joint-specific power and exponential models of the intensity-ET relationships, and compared these models between each joint (ankle, trunk, hand/grip, elbow, knee, and shoulder) and the pooled data (generalised curve). 194 publications were found, representing a total of 369 data points. The power model provided the best fit to the experimental data. Significant intensity-dependent ET differences were predicted between each pair of joints. Overall, the ankle was most fatigue-resistant, followed by the trunk, hand/grip, elbow, knee and finally the shoulder was most fatigable. We conclude ET varies systematically between joints, in some cases with large effect sizes. Thus, a single generalised ET model does not adequately represent fatigue across joints. STATEMENT OF RELEVANCE: Rohmert curves have been used in ergonomic analyses of fatigue, as there are limited tools available to accurately predict force decrements. This study provides updated endurance time-intensity curves using a large meta-analysis of fatigue data. Specific models derived for five distinct joint regions should further increase prediction accuracy.

Figure 1

The general power (R² = 0.81) and exponential (R² = 0.78) ET models are shown with their 95% confidence intervals (CIs) overlaid on the full data set (N = 194 studies, 369 task intensities).

Figure 2

The joint-specific power models, 95% CIs, and their corresponding experimental data points are shown for the A) Ankle; B) Grip/Hand; C) Knee; D) Elbow; E) Trunk; and F) Shoulder. Each symbol represents the mean endurance time reported for each task intensity. Note the variations in y-axis scaling across panels.

Figure 3

Joint-specific power fatigue models are plotted to demonstrate relative differences in fatigue resistance (endurance time, ET) as a function of contraction intensity: ankle (solid, dashed); trunk (solid, grey); grip (short-dash, grey); elbow (long-dash, black); knee (solid, black); shoulder (dash-dot, grey). The general model is also shown (dash-dot-dot, black). Note, greater fatigue-resistance is predicted by longer ETs at a given intensity (e.g. ankle versus shoulder).

Figure 4

The mean (95% CIs) power fatigue models for six of the 15 joint pairs demonstrating similar endurance time (ET) predictions: A) Ankle vs. Trunk; B) Knee vs. Shoulder; moderate ET differences: C) Ankle vs. Elbow; D) Grip vs. Knee; and large ET differences: E) Ankle vs. Knee; and F) Elbow vs Shoulder. For each pair, the more fatigue-resistant joint is shown with black circles, the more fatigable joint with open circles. Pooled weighted means (95% CIs) based on reported SD are shown for each joint. Note the varying scales used for ET. *p < 0.05 for the experimental data consistent with model.

Figure 5

Within joint comparisons of power fatigue models and the corresponding mean experimental data points are shown for ankle dorsiflexion and plantarflexion. No significant differences were observed between model predictions for ankle torque.