15 Musculoskeletal Dynamics
In this chapter, we will explore the concepts of open and closed kinetic chains and the methods of assessing upper limb and lower limb musculoskeletal dynamics during a selection of functional tasks. This chapter presents techniques to investigate joint performance and control and provides a framework for functional joint assessment.
This battery of methods can provide useful information in the assessment of joint movement and control; however, as we will highlight, much care needs to be taken to determine which of these methods are appropriate as many parameters and variables could be measured. So the questions that we should ask before any tests are conducted are “What do we need to know?,” “What does this method of analysis actually tell us?,” and “How does this inform clinical practice?” With these questions in mind, musculoskeletal dynamics may be used to investigate important clinical questions and provide a significant contribution to clinical research.
Key Concepts: Kinetic Chains
Before we consider specific functional tasks, it is worth considering the concept of open and closed kinetic chains. A closed kinetic chain occurs when a distal segment meets considerable resistance (e.g., the foot on the ground in stance phase during walking). An open kinetic chain occurs when the distal segment is free to move in space with little or no resistance (e.g., the foot in swing phase during walking or the hand during reaching tasks). 1 There are a number of functional differences between open and closed kinetic chains that include differences in movement patterns and muscle activity. For example, during an open kinetic chain seated leg extension exercise, the proximal tibia rotates around a stationary distal femur and shear forces across the tibiofemoral joint are relatively high. During a closed kinetic chain standing squat, both femur and tibia move simultaneously under compression and shearing forces are reduced. This is an oversimplification as most joints have a significant triplanar motion.
15.1 Closed Kinetic Chain Lower Limb Tasks
15.1.1 Gait
Gait may be described as both closed chain during stance phase and open chain during swing phase. Many of the techniques of studying human gait have been applied to clinical practice leading to more detailed clinical assessment of therapeutic and surgical intervention. Parameters usually investigated in a laboratory gait analysis include: joint and segment kinematics, kinetics (ground reaction forces), inverse dynamics (joint moments and powers), and electromyography.
Jargon Simplified: Kinematics, Kinetics, and Inverse Dynamics
Kinematics is the study of the motion of the body without regard to the forces acting to produce the motion. The most commonly reported kinematic parameters in gait reports are joint angles, which can give us important information on the functional range of motion of a joint. Less common is the inclusion of joint angular velocities, which can often give us more information about joint control.
Kinetics is the study of forces that act on the body but without any reference to the position of the joints. These are usually assessed using force platforms, which allow the study of ground reaction forces. These are often used to determine abnormal loading and push off patterns, but can also give useful information about the movement of the body over the stance limb and mediolateral stability by considering the center of pressure.
Inverse dynamics combines the study of kinetics and kinematics. This may be used to calculate moments about joints and the power absorption/generation about those joints. This in turn may be used to indicate which muscles are active (e.g., by considering if the moments are trying to flex or extend a joint).
In addition to external measures, it is also possible to determine which muscles are active during different tasks and the level of electrical activity within the muscles using electromyography, often referred to as motor unit action potential. This, in combination with the kinematic and inverse dynamics data, allows us to determine which muscles/muscle groups are controlling the joint movement and whether they are working concentrically or eccentrically. It is also possible, with careful modeling, to obtain useful estimates of muscle forces.
In addition to the parameters mentioned previously, many other parameters have been identified as being clinically relevant for different patient groups. This has led to the development of modeling joints from considering joints to be simple hinges to more complex methods of considering the movement in six degrees of freedom and from the consideration of the foot as a single segment to multisegment analysis. In all cases, it is vital that clinicians agree on what factors are important for a given patient group and that bioengineers take this into consideration when conducting gait analysis.
Clinical relevance
Human walking allows a smooth and efficient progression of the body′s center of mass. To achieve this, there are a number of different movements of the joints in the lower limb. The correct functioning of the movement patterns of these joints allows a smooth and energy-efficient progression of the body. The relationship between the movements of the joints of the lower limb is critical. If there is any deviation in the coordination of these patterns, the energy cost of walking may increase and also the shock absorption at impact and propulsion may not be as effective, possibly leading to dysfunction and pain.
15.1.2 Stairs and Step Down
Stairs were developed over 6,000 years ago to add semipermanency to steep paths. Stair ambulation is often unavoidable and changes the joint kinematics in comparison to level walking. Stair ascent and descent each present unique challenges from the context of joint kinematics and muscle effort. When ascending, humans are required to raise their center of gravity during the pull up and then actively carry it forward to the next step. This is achieved through concentric muscular contraction, which displaces the center of gravity vertically. When descending, humans must actively carry their center of gravity forwards and then resist gravity during the controlled lowering phase. This is achieved through eccentric muscular contraction, which controls the rate of lowering of the center of gravity by absorbing kinetic energy. Eccentric muscle action predominates during descent, and the quadriceps must provide the majority of muscle force for controlled lowering of body mass while stepping down. If strong eccentric contractions were not employed, the center of gravity would accelerate under the influence of the gravitational pull of the earth. Fig. 15.1 shows a comparison of the knee movement between walking, stair ascent, and descent. 2
Clinical Relevance
The rehabilitation implications for stair ambulation are to restore adequate joint range of motion, motor control, muscle strength, and appropriate balance. The point of greatest instability during stair activities occurs when the support limb moves into single support with all three joints in a flexed position, this is particularly important when descending. Eccentric activity of the quadriceps controls the lowering of the body mass during descent, and any reduction in motor control from arthrogenic inhibition may lead to the person falling. Activities that promote controlled loading of the quadriceps during stair descent should progress gradually in rehabilitation. For example, minisquats and small step ups/downs should be employed prior to introducing the patient to stairs of normal height. Step up/down height can be adjusted until the person has enough strength and muscle control to attempt stair ambulation.
15.1.3 Squats and Dips
Squat exercises are a popular multiple joint workout and form an integral part of most rehabilitation programs. The use of eccentric squat activities for rehabilitation associated with tendinopathy has been well documented. 3 The exact etiology of tendinopathy is unknown; however, evidence suggests that a biochemical and biomechanical combination contribute. The biomechanical factors include an apparent disorganization of the collagen fibers that leads to a general thickening of the tendon. In addition, neovascularization has been identified around the area affected by the tendinopathy. Eccentric exercises have been shown to have a significant effect on the rate of recovery from tendinopathy, 4 but the physiological and biomechanical effect of the exercises is somewhat unknown. It has been suggested that the eccentric exercise induced a remodeling within the injured tendon that reduced the neovascularization and realigned the collagen fibers.
Many eccentric exercises and techniques used have little scientific background. Purdam et al 5 identified this as an area for further investigation and proposed a conservative management technique for patella tendinopathy. The technique was based on performing a single limb squat with the eccentrically controlling limb placed on a 25-degree decline. The basis for using a 25-degree decline was that by forcing the ankle into plantarflexion, passive and active calf tensions are reduced, which in turn reduces the work done around the ankle and produces a more focused exercise that targets the knee extensors. However, the reasoning for a 25-degree decline angle was not clearly identified. Zwerver et al 6 confirmed increased knee flexion moments at the deepest point of the squat with increasing angles of declination (up to 30 degree). Richards et al7 showed the differences between flat squats and squats performed on an 8-, 16-, and 24-degree decline (Fig. 15.2). Richards et al7 and Zwerver et al6 determined that the joint moments and muscle work done at the ankle may be controlled by altering the orientation of the foot on the squat platform by changing the angle of declination. By increasing the decline angle, a reduction in the loads and muscle activity were seen at the ankle while increasing the knee moments and muscle activity.
Clinical Relevance
Using a graduated decline squat angle offers a knee rehabilitation that allows a graduated increase in the load applied to the knee and graduated reduction in ankle moments and forces as the decline angle increases. Using a range of decline squat angles allows clinicians to be able to offer a controlled graduated rehabilitation environment for squatting tasks.
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