Functional effects of active muscle stiffness on movement in the elderly

In old age, people commonly have difficulty making everyday movements, such as standing up from a chair. The reason for this decline is not fully understood. Although people lose strength as they age, this does not fully explain their problem. Research has shown that older adults tense muscles as they support their body against gravity and as they start to move. We suggest this represents a primary deficit that makes them stiffer in a way that hampers whole-body movement. However, there is a basic lack of knowledge about the effects of muscle stiffness that is primarily due to technical difficulties in measuring stiffness during movement. Our proposed project will develop and test a new method for measuring muscle stiffness as people stand up from a chair, which elderly people find challenging. We will build a device that slightly pushes on people?s legs, and measures how much resistance there is present. This will enable us to determine where in their body and when in the movement they are stiff. We will also push on people?s legs while they are sitting quietly. This will help us understand whether the way people support themselves against gravity causes their muscles to be stiff. Once we have built and tested this device, we will perform experiments to see whether older people are indeed stiffer than young people, in a way that is detrimental, when they sit and stand up. We will also examine whether stiff people have more difficulty standing up from a chair than people who are less stiff. If they do, this would mean that it will be important to teach older adults who have difficulties moving to use less tension as they move. If our hypothesis is true, future studies will use our protocol to examine and validate possible retraining methods.

Ageing can cause functional disability for everyday motor tasks, which can severely affect mobility and quality of life. Our hypothesis is that inappropriate agonist/antagonist muscle co-contraction is a primary motor deficit that underlies this functional disability. Increased co-contraction decreases movement efficiency and requires greater exertion of the agonist muscle to overcome the antagonist activity. Therefore co-contraction could limit the net joint torques and generation of momentum necessary to meet the balance and movement constraints of whole-body tasks. This is particularly relevant for the elderly who approach their strength limits during everyday activities. Inappropriate co-contraction may arise in two ways: 1) through poor coordination of flexion and extension movement phases during complex whole-body actions requiring balance as well as movement control; 2) through excessive and/or non-adaptable postural muscle activity that is normally present to prevent the body collapsing under gravitational forces. The presence of co-contraction conventionally is determined using electromyography. However, electromyography does not reveal the mechanical consequences of co-contraction, which is needed to understand its impact on functional disability. Increased joint stiffness is the primary mechanical effect of co-contraction, but no method is available to measure stiffness during dynamic whole-body movement. To test our hypothesis, therefore, we propose to design and build a device that will enable us to measure hip and knee stiffness during unconstrained whole-body posture and movement tasks. We will study the model task of rising from a chair, which elderly people often find challenging. We will construct two machines to impose small push-pull sagittal displacements of the feet and lateral displacements of the shank in quiet sitting and during the critical periods of momentum generation and weight shift during rising from a chair. We will use a biomechanical model to determine knee and hip joint stiffness from the resulting torques, displacements and segment motions. After validating our stiffness measurement, we will study the effect of age on stiffness. We will also examine the correlation of stiffness with standard measures of functional ability in the elderly, such as the repeated standing up test, the capacity to generate momentum and net joint torque, as well as measures of coordination during weight shift. Our proposed methodology can be utilized in future studies to examine the potential of different interventions to reduce co-contraction, which could improve functional ability in the elderly, as well as to study postural and movement-related stiffness in populations with neurological and musculoskeletal disorders.