As the human body ages, key physiological changes take place that affect a person’s ability to perform daily functional tasks such as walking. Some of these changes associated with aging are loss of muscle mass, atrophy, and generalized muscle weakness. Studies have shown that muscle weakness in the lower extremity due to aging make it more difficult for individuals to walk. Additionally, it is known that elderly adults have altered gait kinematics and kinetics. However, no studies have looked at how lower extremity muscles contribute to gait and the effects of muscle weakness on gait in elderly adults.
Another contributor to increased difficulty walking is knee osteoarthritis. Osteoarthritis is a degenerative disease that most often affects the cartilage in joints. Some of the major side effects associated with knee osteoarthritis include pain, swelling, and loss of motion in the affected joint. One of the most disabling limitations associated with knee osteoarthritis is weakness in the quadriceps, which can in turn affect how an individual walks. Together, all of these side effects can contribute to decreased walking speed or increased difficulty walking. The quadriceps muscles are known to contribute to vertical support and slowing of the body in the early part of the stance phase during gait in healthy, young adults. However, it is unknown how lower extremity muscles in healthy, elderly adults contribute to normal gait. While weakened quadriceps have been strongly correlated with functional limitations in patients with knee osteoarthritis, the important cause-effect relationships between abnormal lower extremity muscle function and patient function remain unknown. This study has three purposes: to 1) characterize the gait kinematics and kinetics of healthy, older adults, 2) determine how individual lower extremity muscles produce force during gait in healthy, older subjects, and 3) determine how individual lower extremity muscles compensate for simulated lower extremity weakness during gait in healthy, older subjects.
I used OpenSim, an open source software package that can be used to generate inverse dynamic simulations, to simulate weakened quadriceps, plantarflexors, and gluteus muscles in gait trials from healthy, older subjects. As I weakened the quadriceps to 40% of their original strength, the gluteus maximus increased its peak force by 10.9%. As I weakened the plantarflexors to 40% of their original strength, the soleus, iliopsoas, knee flexors, hamstrings, and minor ankle plantarflexors increased their peak force in late stance and the gastrocnemius, rectus femoris, tibialis anterior, and minor ankle dorsiflexors decreased their peak force in late stance. Additionally, as the gluteus muscles were weakened, the gluteus muscles, iliopsoas, and hip adductors produced less force and the hip external rotators, sartorius, quadriceps, tensor fasciae latae, hamstrings, and minor ankle plantarflexors and dorsiflexors produced more force. The results from these simulations have determined which lower extremity muscles naturally increase their force produced during gait in response to weakened lower extremity muscles, which are characteristic of knee osteoarthritis and aging. This information can then be used to inform physical therapy programs to specifically target certain muscles to compensate for weak quadriceps muscles.