The total force exerted by two limbs is less than the sum of the forces exerted by homonymous muscles when acting alone is known as bilateral deficit (BLD). This phenomenon is thought to be influenced by some type of neural adaptation facilitated by the long-term patterns of muscle activation that affect the descending drive to the interneuronal poolswhich presumably is originated at the supraspinal level. Taniguchi (1998) has found that unilateral training resulted in an increase in BLD whereas bilateral training demonstrated a reduction in the BLD. According to Vandervoort, Sale and Moroz (1984) bilateral contractions appear to reduce motor unit activation relative to unilateral maximal voluntary contractions and suggest that this is due to due to a reduced utilization of the fast-twitch motor units. These findings are in accordance with the study of Koh, Grabiner and Clough (1993) which found a 17.0% to 24.6% decrease in maximum torques produced bilaterally compared with unilateral tasks and suggests that this decreased in fast twitch motor units might be accountable for the BLD.
Example of bilateral deficit
Another possible explanation is that BLD occurs due to differences in antagonist muscle coactivation between unilateral and bilateral contractions. Kuruganti, Murphy and Pardy (2011) have tested this hypothesis and have found no alterations in antagonist muscle patterns between unilateral and bilateral contractions suggesting that that BDL is not related to antagonist muscle coactivation.
According to Zatsiorskys (2000) the extent of BDL is usually between 5-10%, however when fast contractions are executed it can reach 25-45 %. Conversely, during a knee extension study performing submaximal and maximal isometric contractions, the electromyography (EMG) results have shown the same amount of sub-maximal force between unilateral and bilateral groups. Janzen, Chilibeck and Davison (2006) have found that bilateral training reduced the BLD, whereas unilateral training had an insignificant effect. This is in line with a study of 6 weeks performing BT 3 times per week which shows that the BLD was reduced for the knee extension between young and old adults. However, the same author did not find changes on knee flexion.
Interesting, in a study with sprinters comparing counter movement jumps (CMJ), unilateral jumps with sprinting starting from the blocks has shown that the sum of ground reaction force of the unilateral jump was (2405 N) which is 33.9% (p=<,01) higher comparing to the bilateral jump (1589 N) showing a tendency for BLD. Conversely, take off velocity of two-leg jumps were significantly higher than one-leg jumps and because of the force velocity relationship, the impulse by the one leg jump as higher 43.4% (p<.01). The same study shows that sprinters with the highest BLD were not necessarily the faster ones, as the total impulse (r=-.550) and the peak forces from the block (r=-.630) negatively correlated with subjects BLD. Consequently, sprinters with high BLD were not able to produce a greater impulse from the blocks at high velocity. Bračič et al., (2010) related to high ground reaction forces on the one-leg jump and a possible association with the force/velocity ratio is studied by Bobbert, De Graaf, Jonk & Casius (2006) which tested unilateral and bilateral squat jump and found a 20% decrease of work in the bilateral jump as well as a reduction of 5% on EMG of the rectus femoris. The author also found that the vertical velocity of the center of mass was higher on the bilateral jump. Therefore, it appears that the force velocity relationship is the reason of BLD on jumps and not the reduction of neural drive. Moreover, Tanigushi (1998) has found largest improvements in power after tests following the same regime of training and shows that bilateral training improve bilateral power test and unilateral training improve unilateral power while no improvement were found testing bilateral training group for the unilateral power test and vice versa. It appears that the magnitude of functional strength is related to adaptations to specificity as the increase in function seems to be specific to the mode of training.
Bobbert, M. F., De Graaf, W. W., Jonk, J. N. & Casius, L. J. R. (2006). Explanation of the bilateral deficit in human vertical squat jumping. Journal of Applied Physiology, 100(2), 493–9.
Bračič, M., Supej, M., Peharec, S., Bračič, P. & Čoh, M. (2010). An investigation of the influence of bilateral deficit on the counter-movement jump performance in elite sprinters. Kinesiologija, 42(1), 73–81.
Häkkinen, K., Kallinen, M., Linnamo, V., Pastinen, U. M., Newton, R. U., & Kraemer, W. J. (1996). Neuromuscular adaptations during bilateral versus unilateral strength training in middle-aged and elderly men and women. Acta Physiologica Scandinavica, 158(1), 77-88.
Jakobi, J. M., & Cafarelli, E. (1998). Neuromuscular drive and force production are not altered during bilateral contractions. Journal of Applied Physiology, 84(1) 200-6.
Janzen, C. L., Chilibeck, P. D. & Davison, K. S. (2006). The effect of unilateral and bilateral strength training on the bilateral deficit and lean tissue mass in post-menopausal women. European Journal of Applied Physiology, 97(3), 253–60.
Khodiguian, N., Cornwell, A., Lares, E., DiCaprio, P, A. & Hawkins, S, A. (2003). Expression of the bilateral deficit during reflexively evoked contractions. Journal of Applied Physiology, 94(1), 171–8.
Koh, T. J., Grabiner, M. D. & Clough, C. A. (1993). Bilateral deficit is larger for step than for ramp isometric contractions. Journal of Applied Physiology, 74(3), 1200-1205.
Kuruganti, U., Murphy, T. & Pardy, T. (2011). Bilateral deficit phenomenon and the role of antagonist muscle activity during maximal isometric knee extensions in young, athletic men. European Journal of Applied Physiology, 111(7), 1533-1539.
Kuruganti, U., Parker, P., Rickards, J., Tingley, M. & Sexsmith, J. (2005). Bilateral isokinetic training reduces the bilateral leg strength deficit for both old and young adults. European Journal of Applied Physiology, 94(1-2), 175–9.
Semmler, J, G. & Enoka, R, M. (2000). Neural contributions changes in muscle strength. In: V. Zatsiorsky (Eds.), Biomechanics in Sport. (pp.03-20) Australia: Blackwell Science.
Taniguchi, Y. (1998). Relationship between the modifications of bilateral deficit in upper and lower limbs by resistance training in humans. European Journal of Applied Physiology and Occupational Physiology. 78(3), 226-30.
Vandervoort, A. A., Sale, D. G. & Moroz, J. (1984). Comparison of motor unit activation during unilateral and bilateral leg extension. Journal of Applied Physiology, 56(1), 46–51.