Effects of Limb-Specific Fatigue on Motor Learning during an Upper Extremity Proprioceptive Task

Objective: To investigate the impact of limb-specific fatigue on acquisition and retention of an upper extremity proprioceptive task. Methods: Twenty-two right-handed participants were randomly and equally assigned to either fatigued or non-fatigued protocols. Acquisition phase for the upper extremity task consisted of 5 blocks each with 12 trials. After 48 hours, all participants performed 1 block retention test (12 trials) with the left arm followed by 1 block transfer test (12 trials) with the right arm. Performance for each block was analyzed using a one-way analysis of variance (ANOVA). Performance differences between groups for acquisition was analyzed using a 2 x 5 (group x block) ANOVA with repeated measures on the blocks. The performance on retention-transfer was analyzed by separate ANOVAs. Statistical significance set at p<0.05.


Introduction
Both intrinsic and extrinsic information will shape how the motor system plans, coordinates, and executes purposeful movement (Wolpert, Pearson, & Ghez, 2013). Moreover, motor learning (i.e. the cognitive processes that occur as a result of practice) assumes that acquiring a motor skill relies on the synchronization of the CNS, PNS, and neuromuscular system (Brooks, 1983). Therefore, deficits in motor ability may be attributed to acute or chronic dysfunction of one of these systems (Seidler et al., 2010).
Additionally, early research has primarily used aerobic exercise to induce fatigue (Paillard, 2012).
These studies have primarily focused on the effects of generalized exercise instead of localized, limbspecific, exercise. Moreover, only a few studies have shown that limb-specific fatigue impacts performance (Forestier & Nougier, 1998;Huysmans et al., 2008).
However, the effects of limb-specific fatigue on motor skill acquisition and learning are not clear. Therefore, the purpose of this study was twofold; 1-to investigate the effect of limbspecific-fatigue on motor skill acquisition; 2 -to measure this effect on motor skill retention. We hypothesized that limb-specific fatigue would inhibit motor skill acquisition and long-term retention of this specific motor task.

Method Participants
College-aged (19-30) right-handed participants (n = 22) were randomly and equally assigned to either the fatigued or non-fatigued protocols.
Participants had no prior experience with the protocol. The investigation was approved by the University's Institutional Review Board. Informed consent was obtained from all participants prior to the experiment.

Apparatus and task
Prior to completing the testing protocol, chair Overall acquisition performance for each block was analyzed using a one-way analysis of variance (ANOVA). Performance differences between groups for acquisition was analyzed using a 2 x 5 (group x block) ANOVAs with repeated measures on the blocks. The performance on retention-transfer was analyzed by separate ANOVAs.
Statistical analysis was conducted with SAS 9.2 (SAS Institute Inc., Cary, NC) with statistical significance set at p < 0.05.

Acquisition
One-way ANOVA demonstrated both groups decreased E as a result of practice across blocks, F

Discussion
The primary purpose of this study was to examine the effect of limb-specific upper extremity fatigue on motor skill acquisition and retention.
Both the fatigued and non-fatigued groups learned the upper extremity proprioceptive task but the extent to which the task was learned was dependent upon fatigue status. In agreement with previous research, E was most affected by practice inconsistency as measured by VE (Paillard, 2012).
Due to fatigue, the participant's ability to accurately judge position during acquisition was affected. Furthermore, fatigue interfered with optimizing task-specific memory which was demonstrated by reduced performance during retention and transfer blocks.
The findings from this study support previous research that found localized muscular fatigue is detrimental to performance (Davey et al., 2002;Evans, Scoville, Ito, & Mello, 2003;Lyons, Al-Nakeeb, & Nevill, 2006). This was evident by greater E exhibited by the fatigued group.
Researchers have reported that a fatiguing task prior to the practice bout has been shown to reduce learning (Carron & Ferchuk, 1971). It has also been stated that local fatigue (i.e. limb-specific) introduced before and maintained throughout early practice may significantly depress motor learning (Whitley, 1975). However, the degree of impairment depends on why and where the fatigue was produced (Kanekar, Santos, & Aruin, 2008).
This investigation found limb-specific fatigue interpolated throughout practice to be a variable that affects acquisition and retention (Carron, 1969;Carron & Ferchuk, 1971;Davey et al., 2002;Huysmans et al., 2008;Masters, Poolton, & Maxwell, 2008). Previous experiments have allowed participants to recover during practice by either providing a single fatigue bout (low disturbance to the system) or numerous practices after the participant has recovered (Alderman, 1965;Schmidt, 1969). By allowing a recovery period, it is difficult to know if fatigue is still present throughout the acquisition process.
Additionally, allowing long and continuous practice trials may evoke active recovery rather than inducing fatigue (Benson, 1968;Carron & Ferchuk, 1971;Schmidt, 1969). The time required to perform this task was brief and therefore less susceptible to active recovery. Moreover, the interpolated nature of the fatiguing task increased the likelihood that the task was truly performed during a fatigued state.
The body inherently contains methods of ongoing compensation to counteract fatigue (Paillard, 2012). Compensatory mechanisms at various levels of the neuromuscular system may act to delay the effects of fatigue, thus prolonging the accuracy of the motor activity (Enoka et al., 2011).
This study utilized a localized, guided task, eliminated visual compensation, controlled body position, and maintained the vestibular reference (i.e. head stayed in the same position) to limit such compensation. By controlling for compensatory motor strategies, we can better conclude that taskspecific fatigue negatively affected motor skill acquisition and retention.

Conclusions
The findings from this study support that when learning a discrete upper extremity task, it is not recommended to do so in a fatigued state for optimal learning to occur. Our investigation indicated that (1) acute effects of fatigue are not limited to the lower body and (2) limb-specific fatigue affects task acquisition and retention.
Future research should investigate relative workloads, duration of workloads, duration of rest intervals, and type of skill being learned before generalizing practical implications of such findings.