Endurance time characteristics of human ankle dorsiflexors and plantarflexors.

The endurance time provides a convenient means to assess muscle fatigue resistance. The purpose of the experiments was to study endurance time characteristics of human ankle dorsiflexion and plantarflexion as a function of level of contraction. Nine subjects (four men, five women) were examined. The experimental protocol consisted of determining each individual maximum voluntary contraction (MVC) before each fatiguing experiment and undertaking only one fatiguing isometric contraction (dorsiflexion or plantarflexion) per day. Each subject produced at least six plantarflexion and six dorsiflexion contractions chosen from 15% to 90% MVC. An exponential model was fit to data for each individual and was then fit to the pooled data. The variance accounted for was over 99% for both dorsiflexion and plantarflexion mean values. As expected, endurance time declined as the contraction level increased. Plantarflexing responses presented a higher variation from subject to subject than for dorsiflexing. Inter-subject variability primarily seemed to involve a change in the shape of the endurance curve rather than a shift of the curve. When the data were pooled, the exponential curve had parameters similar to the average of the individual fits. Human ankle dorsiflexion and plantarflexion endurance times as a function of level of contraction were found to be similar under the present experimental conditions.