Muscle Size Symmetry

Differences in strength and power levels between corresponding left and right-sided muscles of the body have been reported in professional athletes. These differences have been associated with increased injury risk and performance deficits. Muscle thickness is linked to the ability of a muscle to exert both strength and power.

Definition of Muscle Size Symmetry

The symmetry (balance) of muscle thickness scores between contralateral muscles.

Importance of Muscle Size Symmetry

  • Muscle Size Symmetry can act as a surrogate for traditional methods of assessing contralateral differences in strength and power, since the thickness of a muscle contributes significantly to the strength and force of its contraction.
  • Differences in strength and power levels between contralateral muscles of the body have been associated with increased injury risk and performance deficits1
  • Research has suggested that
    • Strength deficits in the weakest leg should be restored to within 10% of that of the unaffected leg2
    • Contralateral isokinetic strength deficits of greater than 15% increase the risk of future hamstring muscle strains by up to 2.6 times2
    • Professional soccer players with untreated muscle imbalances were 4–5 times more likely to sustain a hamstring injury than their counterparts with symmetrical muscle strength3
  • Field tests have been reported as a reliable and effective means for predicting a person’s lower extremity strength and power4. However, these often take some time to organize, measure and analyze.
    • Muscle Size Symmetry scans can be conducted rapidly, accurately and reliably. It provides coaches and/or nutritionists an opportunity to quickly screen for potential lower extremity strength deficits in athletic populations. This can act as an early warning of potential injury risk.
    • Muscle Size Symmetry can provide additional supplementary information to the rehab process: inadequate rehabilitation and premature return to play after injury have been identified as risk factors for injury recurrence.
    • Muscle Size Symmetry can be used to develop and monitor training routines designed to reduce any observed contralateral deficits.

Take Home Message

Muscle Size Symmetry is a non-invasive, easily acquired surrogate for traditional measures of contralateral muscle differences. When contralateral differences - or negative trends - in Muscle Size Symmetry are found, this provides an early - and easily acquired -  “heads up” for coaches and trainers. Potential strength/power deficits can be quickly screened for, and a much earlier determination made by the athlete/coach as to the need for further testing and evaluation.


The illustration below displays the symmetry of muscle thickness scores for Right and Left Thigh -  Rectus Femoris (RF). The Left RF score is 7% thinner than the Right RF score.

MuscleSound also tracks Muscle Size Symmetry over time, and against the baseline assessment to identify trends that can be used to fine tune training or rehab regimens

  1. Brughelli, M, Cronin, J, Mendiguchia, J, Kinsella, D, and Nosaka, K. Contralateral leg deficits in kinetic and kinematic variables during running in Australian rules football players with previous hamstring injuries. J Strength Cond Res 24(9): 2539–2544, 2010
  2. Keeley, DW, Plummer, HA, and Oliver, GD. Predicting asymmetrical lower extremity strength deficits in college aged men and women using common horizontal and vertical power field tests: A possible screening mechanism. J Strength Cond Res 25(6): 1632–1637, 2011
  3. Croisier, J-L., Ganteaume, S., Binet, J., Genty, M., and Ferret, J-M. Strength Imbalances and Prevention of Hamstring Injury in Professional Soccer Players. The American Journal of Sports Medicine 36(8):1469-1475, 2008
  4. Hamilton, RT, Shultz, SJ, Schmidt, RJ, and Perrin, DH. Triple-hop distance as a valid predictor of lower limb strength and power. J Athl Train 43: 144–151, 2008. 

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