Dear Editor-in-Chief,

We are concerned with an important confusion about data unit of measurement presented by Girard et al. when evaluating the changes in Lower Limbs Stiffness (LLS) during prolonged tennis playing (1). Evaluation of LLS is of great interest in a sport like tennis where speed and reactivity takes a great part. LLS is an important element of the muscle biomechanical characteristics and the optimal utilization of the stretch-shortening cycle requires some level of stiffness (2). Then an appropriate level of stiffness is necessary for optimal performance and prevention of injury (2). To evaluate LLS, Girard et al. used a standardized procedure proposed by Dalleau et al. (3). This method allows evaluation of LLS (expressed in N/m or in kN/m) in field conditions via the measure of both flight and contact times during a multi-rebound test.

Such a procedure has been previously used in several sport activities and results reported (4-5). However, in the article published by the British Journal of Sports Medicine, the results are presented expressed in N/m/kg while the correct unit is N/m or kN/m, as mentioned above. Comparison with the range of previous LLS data reported in the literature (4-5) confirms the confusion in the choice of unit by Girard et al. and suggests that it should likely be in kN/m. Although not affecting the conclusion of the authors concerning the relative changes in stiffness during prolonged tennis playing, we would like the scientists interested in LLS evaluation to be aware of this mistake in order to avoid a wrong interpretation of the data presented and to permit their use for comparison with other results.

By Sylvain Durand and Abdel Rahmani, Université du Maine, France.