Effects of age and muscle length parameters on single- versus multiple-joint lower-body strength expression

The elderly population of 65 years or greater is continuing to grow all over the world, with the majority experiencing at least one functional limitation for performing daily tasks. Researchers have then discovered age-related changes in neuromuscular physiology leads to muscle dysfunction that causes impaired mobility, thus making strength the primary measurement tool for muscular impairment. There is much research out there on age-associated changes in strength-related capacities through the use of isolated-joint testing protocols. However, the majority of human performance is heavily based off of multi-joint movements, such as getting out of a chair or walking. It is very possible that there are greater declines in strength across the lifespan than originally perceived from single-joint testing. Not to mention, traditionally most single-joint studies evaluated the maximal joint force/torque at a single optimal joint angle whereas, muscle force production is dependent upon the length of the muscle. The purpose of this study to examine the age related differences in single joint versus multi-joint testing at different muscle lengths. Approximately 20 healthy young participants (18-30 years) and 20 apparently healthy elderly participants (65-85 years) with no regular, structured exercise routine will be recruited through the local community to came into the lab for 2 visits 48-96 hours apart. The first visit involved the following: a) participants will be thoroughly screened to ensure all inclusion criteria are met, b) completion of all paperwork/questionnaires, and c) participants undergoing practice trials of all strength assessments. The second visit had the participants only perform the experimental strengths assessments via the Biodex System 3, Biodex Medical Systems, Shirley, NY, USA, Exerbotics Squat, Exerbotics, Tulsa, OK, USA; at random. Isometric testing was preformed at 90°, 60°, and 20° using a goniometer with participants performing 2 maximal voluntary contractions for the leg flexors and extensors. Those same positions were also applied to the squat machine with once again, participants doing 2 MVCs for each. The data was then examined using a three-way mixed factorial analyses of variance. There were no significant differences in height (P = 0.886) nor in body mass (P = 0.692). For all torque values, the young were stronger than the elderly. There were no significant interactions of Age x Mode x Angle for knee extensors (P = 0.337), the knee flexors (P = .947), and combination of them both (P =. 440). However, there were also no significant interactions of Age x Mode (P = 0.39), but in Mode x Angle (P <0.001) for knee extensors. For knee flexors there was only a significant interaction between Age x Mode (P <0.001). The combination of knee flexors and knee extensors only yielded a significant interaction of Mode x Angle (P<0.001).Follow up pair wise comparisons for knee extensors showed there was a significant difference between groups (P<0.001) and modality (P<0.001), but not angle (P = 0.101). The hamstrings had significant differences in the age groups (P<0.001), modality (P<0.001), and in the angle (P<0.001). When the average leg extensors and flexors were combined there were observations made in only the age groups (P<0.001) and in the modality (P = 0.013), but not in the angle (P = 0.60). In conclusion, there were not significant differences in all angles for a given modality and type of muscle group. However, it appears that angle does play a role on strength. Further research is needed to determine the role of aging on muscular strength.