Introduction: In elderly individuals, the ability to stand up from surfaces of various heights is fundamental to functional independence, and its measure has been considered an index of thigh muscle strength. However, previous findings on the relationship between sit-to-stand time and the force-producing capacity of the knee extensor muscles are still in controversy. From the findings of Linderman et al. (2003), knee extension strength and leg extension power showed good correlations with mechanical power during the rising phase in sit-to-stand movement, but not to chair rise time. This suggests that the ability to perform sit-to-stand should be expressed as mechanical power developed during the task, when one wants to examine how it can be related to the measures of the force generation capacity of the thigh muscles. The prominent advantage of the sit-to-stand test as a field test is that it requires minimum instrumentation, i.e., a stopwatch and standard armless chair. One can conventionally calculate the power during sit-to-stand movement using the measured variables; body mass, leg length, and time required to perform the task. The present study aimed to examine how the estimated power could be related to measures of the force-generation capacity of the knee extensor muscles.
Methods: A total of 38 men (63.9 ± 8.1 yrs, 1.67 ± 0.06 m, 66.0 ± 7.0 kg, M ± SD) and women (63.0 ± 7.7 yrs, 1.53 ± 0.04 m, 51.2 ± 5.6 kg) performed a 10-repeated sit-to-stand as fast as possible. A steel molded chair 0.40 m high and 0.36 m deep was used for the test. The time during the prescribed action (Tsit-stand) was determined with a manual stopwatch. The leg length (L), defined as the distance from the great trochanter of the femur to the malleolus lateralis, was measured using a steel measure. The power index during the action (Psit-stand) was calculated using the following equation; Psit-stand = (L - 0.4) × body mass × 9.8 × 10/Tsit-stand. The volume of quadriceps femoris muscle (QFMV) and isometric knee extension torque (KE) were measured using MRI and a static myometer, respectively.
Results: There was no significant correlation between Tsit-stand and each of QFMV, KE, and KE relative to QFMV (KE/QFMV). On the other hand, Psit-stand was highly correlated with QFMV (r = .799, p < .001) and its value relative to body mass (r = .644, p < .001). In addition, KE was also significantly correlated to Psit-stand (r = .742, p < .001). However, there was no significant relationship between Psit-stand and KE/QFMV.
Discussion: The main finding of the current study was that QFMV and KE were highly correlated to Psit-stand rather than Tsit-stand. Among the variables used for calculating Psit-stand, body mass was significantly correlated to QFMV and KE, suggesting that the variable has influenced the observed relationships between Psit-stand and either QFMV or KE. Even when the influence of body mass was controlled statistically, however, the corresponding relationships were still significant. These results support the assumption that, regardless of body mass, Psit-stand can be a useful index to assess the force-generation capacity of the knee extensor muscles.