Comparison of lengthening and shortening stiffness tests in single skeletal muscle fibres

Studies of skeletal muscle fibre function often incorporate stiffness tests to provide information on contractile performance. These tests are most often performed by applying rapid small (<0.5 % fibre length) lengthening or shortening steps and measuring the corresponding change in force. Despite these stiffness tests being regularly performed in studies of contractile function, their repeatability during contractions has not been evaluated, and reported differences in stiffness measured from lengthening versus shortening tests have not been fully evaluated. Single muscle fibres were chemically permeabilized and maximally activated at three different lengths. During maximal activation at each length three lengthening and three shortening tests were performed; these were then repeated with the fibre relaxed. Both stiffness and force measures were normalized to fibre size (stiffness normalized to modulus and force normalized to stress (i.e. specific force)) to best represent the intrinsic properties of the fibres. Active modulus tests were highly repeatable with mean coefficient of variations (CoV) less than 0.028 (2.8 %). Active modulus was on average 28 % higher in all fibres in response to the lengthening compared to the shortening tests. Interestingly, correlations between specific force and active modulus were significantly (p < 0.01) higher for the shortening (r = 0.86) compared to lengthening (r = 0.78) tests. Relaxed modulus tests were less repeatable with mean CoVs ranging from 0.089 to 0.151 (8.9 to 15.1 %). Relaxed modulus was not significantly affected by the direction (lengthening versus shortening) of the test.