Table 1.
Parameter baselines, increments, and ranges used for the sensitivity analysis of two muscle fatigue models (MFM).
Fig 1.
Representation of Exertion Level (EL), Duty Cycle (DC), and Cycle Time (CT).
Toff is the rest time, and Ton is the portion of time when an exertion is generated.
Fig 2.
Illustration of predicted endurance times (ET) for the XFL model, in a single loading condition (EL = 0.6; DC = 65; CT = 60) and for a range of fatigue (F) and recovery (R) parameters.
Methods to derive sensitivity for the F and R parameters, ΦF and ΦR, are also illustrated, and were determined using Eqs 4 and 5.
Fig 3.
Representation of a considerable change in endurance time (ET) that can occur due to relatively small alterations in MFM parameters.
Here, ET1 is substantially shorter than ET2 despite a small increment in F (from Fm to Fm+1).
Fig 4.
Example illustration of the effects of 2D smoothing of endurance time (ET) predictions and the derived recovery sensitivity parameter (ΦR).
Original values of both are depicted in the left figures, and after smoothing on the right. For this illustration, the same model and loading condition was used as in Fig 2.
Table 2.
Empirical studies involving hand/grip exertions that included intermittent isometric loading conditions with reported mean (SD) endurance times (ET).
Fig 5.
Endurance Time (ET) predictions of three MFMs for sustained isometric exertions.
Target values are from data reported by Frey Law and Avin [18].
Table 3.
Optimized MFM parameters obtained for prolonged and intermittent isometric exertions, and the combination of both.
Fig 6.
Representative examples of fatigue sensitivity parameters (i.e., ΦF) for the XFL (left) and MCBZ (right) MFMs.
ΦF values were determined using Eq 4, iterating the F and R parameters over a wide range (Table 1). Higher values of ΦF indicate larger relative sensitivity to changes in F values. Some ΦF values (>10) are not shown, to better illustrate patterns of responses.
Fig 7.
Representative examples of recovery sensitivity parameters (i.e., ΦR) for the XFL (left) and MCBZ (right) MFMs.
ΦR values were determined using Eq 5, iterating the F and R parameters over a wide range (Table 1). Higher values of ΦR indicate larger relative sensitivity to changes in R values. Some ΦR values (>5) are not shown, to better illustrate patterns of responses.
Fig 8.
Sensitivity values of one model parameter (F and R) at the midrange of the other parameter, for different values of task parameters in the XFL (left) and MCBZ (right) MFMs (CT = 60 s).
Note that in this figure the viewpoint is different for ΦF and ΦR, to better visualize the patterns of responses.