Can muscle strength asymmetry impair functional performance?

New research suggests that strength asymmetries in key muscle groups could impair performance in sports where jump performance is important

Strength imbalances between limbs are common in sports where players are exposed to high volumes of training and competition, and where there is a preferred limb dominance – eg right/left handed, right/left-foot kicking in tennis and soccer respectively. Some degree of asymmetry is inevitable; many skill training sessions in these kinds of sports require athletes to frequently repeat the execution of unilateral and asymmetric movements, and over time, this may lead to ‘between-limb’ differences in strength, power, and joint range of motion.

How much asymmetry matters?

Previous research has established that asymmetry in athletes is a potential risk factor for injury, most likely as a result of additional loading placed on the soft tissue structures of the non-dominant limb(1). There’s also evidence that limb asymmetry may negatively impact certain aspects of performance in some sports (see this article on the impact of asymmetry in cyclists)(2,3). However, what’s far less understood – particularly in athletes who have a dominant limb side – is how significant limb asymmetry needs to be before it impacts performance, what aspects of the asymmetry have the most negative impact and how athletes and coaches can determine whether the asymmetry present needs addressing. But now a newly published study on soccer players has thrown some light on these questions(4).

The research

A total of 203 professional male soccer players were investigated for strength and range of motion asymmetries. The strength tests compared left-right asymmetry in the hamstrings, quadriceps, and hip adductor and hip abductor muscles. Once these had been completed, the players were divided into 4 quartiles (Q1-Q4) based on the magnitude of their asymmetry for each test, Q1 being the lowest degree of asymmetry, and Q4 the highest asymmetry (largest strength difference between left and right limbs). There then followed bilateral and unilateral jumping tests to assess performance, along with range of motion tests to establish flexibility levels in each limb. The researchers then looked to see to what degree asymmetry affected potential performance.

The findings

The main findings were as follows:

  • The players exhibited a large range of left-right asymmetry (5.2% to 14.5%) across the different strength tests and performance scores.
  • Forward displayed greater asymmetry in concentric quadriceps and eccentric hip-abduction strength than did midfield and defense players.
  • When the hamstring and quadriceps strength asymmetry scores were combined, players with the greatest degree of asymmetry (ie in Q4) performed worse in the functional tests, particularly during in the single-legged countermovement jump and 10-second hop test.
  • Bilateral jump performance did not seem to be affected by quadriceps/hamstring asymmetry.
  • Hip adduction and abduction strength did not seem to affect single-legged countermovement jump and 10-second hop test performance.

Practical implications

This is yet another study to demonstrate that limb asymmetry in sport is common and very variable between individuals. In this study however, only the players in the 4th quartile – ie those with greatest asymmetry – seemed to be affected in terms of performance in the jump tests (sprinting and change of direction testing was not carried out). In terms of soccer players, those with left-right asymmetries in hamstring and quadriceps strength and those who were playing in forward positions seemed to be most affected. Reducing hamstring and quadriceps asymmetry in these players therefore seems desirable.

To do this, screening tests for limb strength should be conducted then unilateral leg strength exercises should be performed (eg single leg press, lunges etc) and the weaker leg worked first. The stronger leg should only be trained at the loadings/reps achieved by the weaker leg; this will allow the weaker leg to gain relatively more strength, thereby reducing the asymmetry. How much asymmetry should be tolerated? To date, no clear guidance exists. However, for the purposes of injury rehab, a limb strength asymmetry of less than 10% is recommended, and this may be a good starting point(5). Another way of determining tolerable levels of asymmetry is with functional testing on each limb. Previous research determined that an asymmetry level of 12.6% between limbs was detrimental for vertical jump performance while an asymmetry of just 3.9% was detrimental for standing broad jump performance(6).

 References

  1. Strength Cond J. 2012;34(1):82–86
  2. J Sports Sci. 2018;36(10):1135–1144
  3. Sports (Basel). 2019;7(3):58
  4. J Athl Train. 2020 Dec 2. doi: 10.4085/1062-6050-0013.20. Online ahead of print
  5. J Sports Med. 2011;45(9):709–714
  6. J Strength Cond Res. 2014;28(12):3557–3566

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