Typically strength has been measured via single-joint strength curves for various joints throughout the body. Single-joint strength curves are useful due to the connection between muscle tension and muscle length, and muscle length and joint angle. As a result, single joint strength curves take on one of the three different shapes: ascending, descending, or ascending-descending. To some degree, the characteristic shapes of single-joint strength curves reflect the portion of the length-tension curve over which the muscle or muscle group operates. Therefore, single-joint strength curves might provide a means for optimizing musculoskeletal model outpu.
Despite the importance of single-joint strength curves, many activities of daily living are multi-joint efforts.measured multi-joint leg extension capacity and found that endpoint force increases as leg extension increases. Similar results have been found in research aimed at athletic population.as well as in workplace environment estimated joint torques during maximum effort isometric leg extensions. Strong correlations were found between joint torques and endpoint force when the knee was flexed 60 degrees or more, indicating stronger associations between joint torques and endpoint force when force capacity is decreased. While statistical perspectives on joint contributions to lower extremity endpoint force are useful, a clear mechanical description of joint contributions to endpoint force across postures is lacking.
Clinical research has also utilized statistical models to understand the relationship between multi-joint force capacity and functional tasks. Endpoint force in a lower extremity multi-joint effort correlates moderately with functional tasks such as gait or sit to stand. Contribution of individual joints to activities of daily living was examined by Hasegawa by utilizing lower extremity single-joint strength measures to statistically discriminate between functionally independent and dependent groups of elderly adults.