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Joint contact forces or joint reaction forces are the internal forces experienced by bones and articular cartilage at the contact surface of the joint. These forces are used to express the load on the joint. Understanding and quantifying these forces has implications for diseases and gait abnormalities as well as rehabilitation and neural control.

Definition & History
Joint contact forces are caused by a combination of 1. external forces traced up the kinematic chain to the joint of interest (for lower limbs this is typically the ground reaction force), 2. induced forces from accelerations of the respective segments, and 3. internal forces created by muscle contraction across the joint. Initial studies focused on the joint intersegmental force which only incorporates the first two components; however, this underestimates the contact force due to the missing component of muscle co-contraction which is often the highest contributor to joint contact forces. Most studies examining these forces in humans focus on weight-bearing joints within the lower limbs, specifically the knee as well as the hip. These forces have been quantified in animals as well as humans.

Measurement Techniques
Quantification of joint contact forces is a challenging problem and can be split into direct measurement methods and estimation methods.

Direct Measurement
Joint contact forces have been measured both in vitro and in vivo.

In Vitro
In vitro contact forces have been measured with instrumented cadaver limbs; however, these measures are inherently difficult to evaluate due to the challenges of replicating muscle forces in vitro. Since this measure is in cadavers, strain gauges and load cells can be placed within the joint to directly measure these forces.

In Vivo
In vivo joint contact forces are difficult to measure. In animals, joint contact forces have been measured directly by sensate scaffolds which are instrumented polymer structures that replace a section of articular cartilage. These forces have not been measured directly in intact normal human joints. The current accepted gold-standard measurement has been instrumented joint replacements that allow direct measurement of these forces. These are a relatively new development and they can only measure forces in joints that were already degraded and surgically altered.

Estimation Methods
Due to the difficulty of direct measurement, current research has focused on estimating joint contact forces using musculoskeletal models or surrogate measures.

Surrogate Measures
Surrogate measures are alternate physical values that are more easily measurable than contact force and have shown correlation with those forces. These measures such as knee adduction moment for the knee have been used in some studies; however, they do not reflect all aspects of loading and only show mild task specific correlations, thus missing out on critical aspects of loading.

Musculoskeletal Modeling
Musculoskeletal modeling approaches have used several techniques to estimate muscle force necessary for calculating contact forces: static optimization and electromyography (EMG)-driven models. Static optimization (or dynamic optimization with activation dynamics) replaces the nervous system with the assumption that the nervous system attempts to minimize the magnitude of muscle activation across all muscles with different weightings. EMG-driven models use estimated muscle activation from EMG. A third method combines these two into a hybrid model requiring fewer EMG sensors and still incorporating deep muscles that cannot be measured with EMG. A significant challenge for these models is validating that they are achieving accurate results for both muscle forces and contact forces due to the lack of a true direct in vivo measurement.

Other Methods
Recent research has also discovered a link between joint contact forces and joint acoustic emissions, which are acoustic signals originating within the joint that can be picked up through vibrations on the skin. These acoustic signals have shown correlation with changes in joint contact forces and have also been used for estimating contact force using machine learning algorithms. This has the potential to offer a wearable method of quantifying joint contact force.

Implications
Joint contact forces are a critical metric for both researchers and clinicians for several reasons.

Neural Control
Research using a rat model has discovered that the central nervous system (CNS) may actively control joint contact forces during gait. Due to the rat anatomy, the effect of a change in contact loading was alterable by changes in muscle activation that did not also alter the overall knee torque. When this intervention was introduced, the rat altered muscle activation patterns to decrease joint contact forces, implying that the CNS directly controls joint load. This has implications for neuromuscular modeling as well as conventional thoughts on neural control strategies.

Clinical Implications
Joint contact forces and their surrogate measures have been connected to several clinically relevant outcomes:
 * Progression of diseases such as osteoarthritis
 * Injury risk such as ACL tears
 * Pain and cartilage degradation

There are also rehabilitation measures that could be associated to joint contact loading such that monitoring load could provide information on recovery progress as well as alerting clinicians of overuse or potential for re-injury. The above references have noted the hazards of increased joint loading; however, decreased loading can also be detrimental. Increasing joint load can benefit bone and cartilage and counteract bone loss.

Current Technologies for Altering Contact Forces
There are several strategies to minimize joint contact forces in order to mitigate the detrimental effects mentioned above.

Invasive Methods
Invasive methods include:
 * Joint distraction
 * Osteotomy

Non-invasive methods
Non-invasive methods include:
 * Weight loss
 * Footwear modification
 * Bracing
 * Gait Retraining

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