The primary purpose of this study was to examine the effects of contralateral throwing practice in the non-dominant arm on accuracy and the Accuracy:Velocity Ratio in the dominant limb during a season in well-trained baseball players. The second purpose was to examine if cross-education generalizes to other related but not specific parameters to baseball such as range of motion, strength, and other kinetic variables. We hypothesized that contralateral throwing meant to improve accuracy would improve accuracy in the dominant arm. We also hypothesized improvement in non-general parameters. Albeit, to a lesser extent. The primary hypothesis was supported in that dominant arm accuracy increased by 19% and the Accuracy:Velocity Ratio by 18.2%. The second hypothesis was generally not supported. We found that improvements did not occur in any parameters except in grip strength in the non-dominant arm and that this did not transfer to the dominant arm.
The overwhelming majority of research in cross-education literature has taken place in general strength parameters. A meta-analysis of research demonstrated an 18% average increase in strength in the non-trained arm when exercising in a contralateral manner (Green et al., 2018 A). However, the improvement of motor skills is an important clinical aspect that has been widely overlooked in contralateral training literature. Our research was the first that we are aware of that investigated the cross-education effects of a highly complex sports specific skill such as throwing accuracy. We found strong agreement with strength literature demonstrating a 19% increase in throwing accuracy in an otherwise asymptotic group of athletes as demonstrated in the control condition. The 40% increase in non-dominate throwing performance indicates that these athletes were likely ‘thrown’ back into the non-verbal rapid learning phase of Fitts and Posner’s learning model. It is in this stage where theoretical cortical activation is at its greatest and performance improvements are substantial as learning increases (Bertram et al., 2017; Sawyer et al. 2000).
We posited that underlying measurable physiological parameters such as improved torque and range of motion and/or general strength might underlie changes in performance. However, this generalized hypothesis was not supported. While we did not examine other explanations, scientists have proposed two major frameworks to explain cross-education effects (Ruddy et al., 2013). The first include “cross-activation models.” These models are centered on the observation that contralateral movements result in bilateral increases in corticospinal excitability (Ruddy et al., 2013). The associated speculation is that such generalized activity, when present during contralateral practice, leads to concurrent adaptations in neural circuits that project to the untrained arm; thus, improving in this case throwing accuracy. Alternatively, “bilateral access” models entail that motor programs formed during contralateral practice, may subsequently be utilized on the other limb—that is, by the motor program that constitutes the control centers for movements of both limbs (Park et al., 2003). Considering that cortical activity is greatest during the cognitive verbal phase of learning, it is possible that greater activation improved performance on the dominant arm. However, to date, this is speculative in our throwing experiment and remains to be studied.
With the exception of strength, we did not see a generalized increase in nonspecific parameters in the non-dominant arm that was trained in a contralateral manner. As such, transfer would not be expected in those parameters. These results agreed with past literature which suggested that transfer of skills and strength are task specific (Behm et al., 1993; Bachman et al., 1961). Moreover, research suggests that in less complex and simple tasks like grip strength that neural adaptations are plateaued at an early phase of learning (the first few months) (Gabriel et al., 2006). Thus, it may be that transfer of the non-dominant-to-dominant arms did not occur due to maxed out neural adaptations in the dominant arm.
In baseball throwing activities, it has been well described and demonstrated that accuracy is greater at lower velocities (Freeston et al., 2015). In the current study, we observed an improvement in accuracy without a sacrifice in velocity following contralateral throwing practice. This was indicated by an improvement in the ratio of accuracy to velocity (Fig. 4). Indeed, since the time of Fitts (see Kovacs et al., 2008 for explanation) it has been known that there is a strong inverse relationship between velocity and accuracy. Researchers believe that a hallmark of expertise is the ability to sustain high velocities while demonstrating greater than average or excellent improvements in accuracy (Englehorn, 1997). While we are uncertain of why this ratio improved, previous research in strength literature has shown a decrease in force variation that transferred from contralateral training to the untrained limb (Green et al. 2018A). Future research will need to investigate if this underlies the changes we see in this study.
One reason to potentially account for the lack of changes in non-throwing specific parameters such as range of motion and grip strength is task specificity in training. The primary focus of the training was accuracy; therefore, we would expect improvements in throwing accuracy but not velocity or non-specific throwing parameters. Hubbard et al. (2009) stated that increases in learning are task specific as learning is maximal in the task that is being trained. If the training regimen focused on improving velocity over the 4 weeks then we would expect to see velocity and potentially strength improvements as training effects are confined to the practiced task (Schubert et al., 2008). Including a strength training protocol and maximal intent throws to enforce strength and velocity in addition to accuracy training could be a solution to improving all aspects of baseball throwing. However, future research will need to investigate the impact of contralateral strength and accuracy training on complex and open skills such as baseball throwing.
Although this study did not focus in this area, future research should investigate the efficacy of a contralateral randomized training in rehabilitation and injury prevention (i.e. prehab). Such a training program could assist athletes returning from injury to minimize the losses in strength and accuracy due to inactivity, or aid in prevention of injury as performance in the dominant limb, which is commonly overused, could improve through training of the neglected, non-dominant limb