Bilateral Improvements Following Unilateral Home-Based Training in Plantar Flexors: A Potential for Cross-Education in Rehabilitation

Abstract

Context: Cross-education (CE) refers to neuromuscular gains in the untrained limb upon contralateral limb training. To date, only laboratory-based exercise programs have demonstrated CE. Home-based exercise prescription eliciting CE could have greater clinical applicability. Objective: To determine the effect of an 8-week, home-based unilateral strength training intervention on isokinetic muscle strength, muscular excitation, and power in trained and untrained plantar flexors. Design: Randomized controlled trial. Methods: Thirty-four healthy participants were randomized to intervention (n = 20) or control (n = 14). The intervention group completed 3 sets of 12 repetitions of progressively loaded unilateral calf raises 3 days per week. Concentric and eccentric peak torque were measured using isokinetic dynamometry at 30°/s and 120°/s. Maximal electromyogram amplitude was simultaneously measured. Power was measured using a jump mat. All variables were measured at preintervention, midintervention, and postintervention. Results: Strength significantly increased bilaterally pre–post at both velocities concentrically and eccentrically in intervention group participants. Maximal electromyogram amplitude significantly increased pre–post bilaterally at both velocities in the medial gastrocnemii of the intervention group. Power significantly increased bilaterally pre–post in the intervention group, with a dose–response effect demonstrated in the untrained plantar flexors. The CE effects of strength, power, and electromyogram activation were 23.4%, 14.6%, and 25.3%, respectively. All control group values were unchanged pre–post. Conclusion: This study shows that a simple at-home unilateral plantar flexor exercise protocol induces significant increases in contralateral strength, muscular excitation, and power. These results suggest the applicability of CE in home rehabilitation programs aiming to restore or maintain neuromuscular function in inactive individuals or immobilized ankles.

The Effect of Contraction Type and Training Volume in Unilateral Exercises on Cross-Education: A Narrative Review Study

“…This review showed mixed exercise in the form of traditional, cluster, or other types of exercises in an intensity range (705% 1 RM-100% 1 RM) could produce cross-education as much as 6.5-45% (13,17,21,31,36,38). This review has also shown that if exercises are applied with a high intensity, such as 90% 1 RM, but in low volume (5 set/1 rep), could not significantly produce cross-education (31).…”

“…Cross-education is limited to the homologous and heterologous muscles (5) of the untrained limb because the effect of cross-education requires the neural contributions of the trained muscles responsible for maintaining cross-education (10). While there is much evidence about cross-education, in recent years, most studies have shown that different training protocols created varied cross-education results (1,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). It has shown that the rate of increase in strength of the untrained limb varied from 45.2% (26), 30% (15), and 11% (16) to 5% (12) in the untrained limb.…”

“…Regarding the different protocols of unilateral exercise, including the type of contraction and volume of exercises (number of sets, sessions, frequency, and repetitions of training), two meta-analyses conducted in 2017 showed that the type of contractions and the volume of exercises can affect the occurrence and rate of cross-education (27,28). On the other hand, studies published from 2017 until now have used different training volumes with contradictory results about the rate and occurrence of cross-edu-cation (1,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). Some studies used ten weeks of training in 20 repetitions (26), and some used 4 to 6 weeks of training in 5 to 8 repetitions (12,15,30) or several days of training (11) in their training protocol.…”

“…Some studies used ten weeks of training in 20 repetitions (26), and some used 4 to 6 weeks of training in 5 to 8 repetitions (12,15,30) or several days of training (11) in their training protocol. It appeared that studies about the rate and occurrence of cross-education used different protocols in their training programs yielded contradictory results (1,(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25). Combining and investigating the results of these studies can help us deduce the best conclusion about the effect of type of contraction and volume of training in cross-education.…”