Performance Fatigability: Mechanisms and Task Specificity

Abstract

Performance fatigability is characterized as an acute decline in motor performance caused by an exercise-induced reduction in force or power of the involved muscles. Multiple mechanisms contribute to performance fatigability and originate from neural and muscular processes, with the task demands dictating the mechanisms. This review highlights that (1) inadequate activation of the motoneuron pool can contribute to performance fatigability, and (2) the demands of the task and the physiological characteristics of the population assessed, dictate fatigability and the involved mechanisms. Examples of task and population differences in fatigability highlighted in this review include contraction intensity and velocity, stability and support provided to the fatiguing limb, sex differences, and aging. A future challenge is to define specific mechanisms of fatigability and to translate these findings to real-world performance and exercise training in healthy and clinical populations across the life span.

Figure 1.

Sites of fatigue. Schematic of the sites and processes that lead to force and power generation at the muscle and can contribute to performance fatigability. Processes include activation of the motor cortex, descending drive, spinal activation of the motoneuron pool, neuromuscular propagation across the neuromuscular junction, muscle perfusion (blood flow) and processes at the muscle fiber including excitation–contraction coupling, metabolism, and cross-bridge kinetics. Shown within the spinal cord schematic are selected excitatory and inhibitory pathways to the spinal cord that have been found to modulate the output of the motoneuron pool during fatiguing contractions. Motoneurons receive inhibitory inputs from a variety of sources that are mediated through inhibitory interneurons (represented as the shaded neuron and synapse in the circuitry). However, the dominant inhibitory effect from group III–IV afferents during fatiguing contractions involves a reduction in excitatory input by presynaptic inhibition of the group Ia afferents (Hunter et al. 2004d). Note that excitatory pathways are shown as white triangles and inhibitory pathways as the dark-shaded triangles.

Figure 2.

Task dependency of performance fatigability. (A) Power-task duration relation for cycling in young men and women. Shown is the hyperbolic relation for cycling between power (relative to maximum power achieved during a 3-sec test at 80 rpm) and time to failure (trial duration) for multiple cycling tests performed at various power outputs. Over several days of testing, active young men (n = 7) and women (n = 7) performed trials to obtain the peak 3-sec power output (100%) followed by 11 to 14 constant-load tests to elicit failure between 3 and 300 sec. The time constant for the power-duration relationships was 0.0207/sec (R² = 0.96) so that the greater the relative power the shorter the time to failure. The time constant and shape of the hyperbolic relation did not differ between the sexes. (Figure and data adapted with permission from Sundberg et al. 2017.) (B) Force and position task. Time to failure (mean ± SEM) of sustained isometric submaximal contractions performed with the ankle dorsiflexor muscles for a force task and two positions tasks that differed in load compliance and foot support. Data are extracted from two studies performed on a total of 23 young (18–30 yr) men (n = 12) and women (n = 11) (Hunter et al. 2008b; Yoon et al. 2009a). The subjects were seated with knee at 90 deg of flexion, the ankle in neutral position (0 deg dorsiflexion) and with a load supported at the forefoot equivalent to 20% of maximal voluntary contraction (MVC). The load compliance and support provided to the foot varied between the three tasks: There was no support provided under the foot for the “position unsupported” task, a position task with foot support (position supported) that allowed one degree of freedom of movement at the ankle in the sagittal plane, and a force task (force) in which the forefoot was rigidly attached to a force transducer.

Figure 3.

Sex differences in fatigability for isometric voluntary contractions. Represented are mean data from 46 isometric contraction studies (intermittent and sustained) that assessed fatigability of men and women. Plotted is the percentage sex difference in fatigability in each study, calculated as the mean difference in fatigability between men and women as a percent of the women’s value. The fatigability values used for the calculation were either the fatigue index or time to task failure for the sustained or intermittent isometric fatiguing contractions. The x-axis represents the contraction intensity (percentage of maximal voluntary contraction [MVC]) at which the fatiguing contractions were performed. Upper limb muscles are represented in closed symbols and lower limb muscles in open symbols. Back and neck muscles are represented as gray symbols. Most data points are above the line, indicating that women were less fatigable than men for many of the muscle groups. There was a significant negative relation between the relative contraction intensity and the magnitude of the sex difference for the isometric contractions when all muscle groups were included (r² = 0.19). (From Hunter 2016a; reprinted, with permission.)

Figure 4.

Sex differences in fatigability. Shown are potential mechanisms that may contribute to the sex differences in fatigability and when women are less fatigable than men during fatiguing contractions. Mechanisms in the muscle, central nervous system, and feedback between them are featured. The contribution of potential mechanisms, however, can vary with the task demands, environmental conditions, and muscle group. A negative sign indicates that the physiological variable or process is less in women than men and, conversely, a positive sign indicates that it is greater in women than men. (From Hunter 2014; adapted, with permission.)

Figure 5.

Age differences in fatigability with dynamic contractions. Peak power with a load equivalent to 20% maximal voluntary isometric contraction (MVIC) torque at baseline (in Watts [W], left y-axis) and during 90 maximal-effort velocity contractions (normalized to baseline [%], right y-axis) for the knee extensor (A), and elbow flexor (B) muscles. The absolute power (W) was the average of six contractions performed before the fatiguing task by 35 young adults (21.0 ± 2.6 yr; 16 men and 19 women) and 32 old adults (71.3 ± 6.3 yr; 18 men and 14 women). Participants performed 90 maximal-effort, fast, concentric, isotonic contractions (one contraction/3 sec, three sets of 30 contractions separated by ∼10 sec) with a 20% MVIC load with the elbow flexor and knee extensor muscles on separate days. Shown are means (±SEM) of five contractions at the start and end of the three sets of contractions and normalized to the baseline values. Old adults had greater reductions in power than young adults with a larger age difference for the knee extensor (36% age difference) than the elbow flexor muscles (10% age difference) (age difference is shown by a daggar, P < 0.05). MVCC, Maximal voluntary concentric contraction. (Data adapted from Senefeld et al. 2016.)