The simplest form of artificial muscle is arguably an artificial muscle. Tendon actuation connects a motor to a fiber that can be pushed or pulled with a motor to drive deformation (similar to how a muscle pulls to curl a finger). This approach is the simplest to integrate but one of the least seamless. That being said, there are a number of great works that look at digital fabrication of these actuators. Albaugh et. al from CMU shows fabulous work using machine knitting to integrate tendons into knit structures. 
This is the first type of artificial that comes to most people’s minds and is also one of the most accessible. Itis due to a solid-phase change when heated from martensite to austenite and back to martensite when deformed.
However, it is also one of the worst. It is difficult to program, stiff, one-way actuation that degrades quickly when cycled. That being said, there are a lot of cool ideas to come from this area, especially with the use of design tools.
KnitDermis: Knitted Tactile On-Body Interfaces by The Hybrid Body Lab from Hybrid Body Lab on Vimeo.
Flapping origami crane from Jie Qi on Vimeo.
This actuator went viral in 2014 as it took a relatively common and boring material (nylon fishing line) and produced inexpensive artificial muscles with 100x the strength of the human muscle (by weight) and large contraction strokes up to 50%. [3, 7]< This works leverages spring dynamics and some basics of knot theory that allows a twisting fiber to become a contracting one. I have tons of experience with this fiber, even doing a clothing collection. It is trivial to make but not as trivial to use.
I have a paper on these from undergraduate, where I add a silicone coating to make the transformation two-way.
ModiFiber: Two-Way Morphing Soft Thread Actuators for Tangible Interaction from Jack Forman on Vimeo.
During fiber drawing, the polymer chains of the thread become extended in the draw direction, causing anisotropy (directionally dependent properties). When heated, the extended polymer chains contract to a lower energy state (Figure 4E), resulting in a reduction in fiber length and expansion in fiber diameter. If twisted, the fiber untwists when heated. Alone, these twisted fibers function as twisting actuators but continued twisting causes the fiber to form a coiled spring-like structure. These muscles are considered homochiral (twist direction and coil direction are the same), and provide significant shrinking and twisting actuation . This is because writhe (in-plane loop) and twist can be freely converted as long as the linking number (the sum of twist and writhe) is conserved.
Dielectric elastomers, also known as Electroactive Polymers, are another class of artificial muscle that are receiving a lot of interest. When fabricated correctly, they have large, fast deformations and are extremely lightweight. However, they are difficult to fabricate robustly. 
I helped on a paper where we used these to make a untethered soft robotic jellyfish!
If you want high forces and frequency of actuation, thin McKibben muscles are your best bet - as long as you don't mind including a pump and controller platform. These muscles are named after Joseph Laws Mckibben but were invented by Dr. Kenneth Landauer. These muscles work similarly to a Chinese finger trap with an air balloon.
Bibliography  Tingyu Cheng, Guori Li, Yiming Liang, Mingqi Zhang, Bangyuan Liu, Tuck-Whye Wong, Jack Forman, Mianhong Chen, Guanyun Wang, Ye Tao, and Tiefeng Li. 2018. Untethered soft robotic jellyfish. Smart Mater. Struct. 28, 1 (November 2018), 015019. https://doi.org/10.1088/1361-665X/aaed4f  Jack Forman, Taylor Tabb, Youngwook Do, Meng-Han Yeh, Adrian Galvin, and Lining Yao. 2019. ModiFiber: Two-Way Morphing Soft Thread Actuators for Tangible Interaction. In Proceedings of the 2019 CHI Conference on Human Factors in Computing Systems (CHI ’19), 1–11. https://doi.org/10.1145/3290605.3300890  Carter S. Haines, Márcio D. Lima, Na Li, Geoffrey M. Spinks, Javad Foroughi, John D. W. Madden, Shi Hyeong Kim, Shaoli Fang, Mônica Jung de Andrade, Fatma Göktepe, Özer Göktepe, Seyed M. Mirvakili, Sina Naficy, Xavier Lepró, Jiyoung Oh, Mikhail E. Kozlov, Seon Jeong Kim, Xiuru Xu, Benjamin J. Swedlove, Gordon G. Wallace, and Ray H. Baughman. 2014. Artificial Muscles from Fishing Line and Sewing Thread. Science 343, 6173 (February 2014), 868–872. https://doi.org/10.1126/science.1246906  Digital Fabrication of Soft Actuated Objects by Machine Knitting. https://dl.acm.org/doi/fullHtml/10.1145/3290607.3311767  Stretching the Bounds of 3D Printing with Embedded Textiles | Proceedings of the 2017 CHI Conference on Human Factors in Computing Systems. https://dl.acm.org/doi/10.1145/3025453.3025460  Get an idea about shape memory alloys and their applications. https://www.skyfilabs.com/project-ideas/shape-memory-effect-intelligent-alloys  New twist on artificial muscles | PNAS. https://www.pnas.org/doi/10.1073/pnas.1605273113  Shape Changing Surfaces and Structures | Proceedings of the 2019 CHI Conference on Human Factors in Computing Systems. https://dl.acm.org/doi/10.1145/3290605.3300355  Electroactive Polymers – materiability. https://materiability.com/portfolio/electroactive-polymers/