24 Mar Force Production in Relation to Muscle Fiber Types
Muscles facilitate locomotion of the skeletal system. This process is achieved through the ability of a muscle to actively shorten and produce tension (McGinnis, 2020). The production of tension occurs through the active contractile unit of each muscle fiber. This basic contractile unit of a single muscle fiber is the sarcomere (Haff and Triplett, 2016).
The process by which a single muscle fiber contracts to create tension is termed the sliding filament theory (Haff and Triplett, 2016). The sliding filament theory indicates a process occurs in correlation with the myosin and actin filaments within the sarcomere at the crossbridge of the fiber to create a contractile action (Haff and Triplett, 2016).
The collective action of a group of muscle fibers contraction or change in length develops tension and a corresponding pull at its’ attachment point (McGinnis, 2020). The points of attachment of the muscle fibers to the articular structures of the body is via the insertion of a tendon (McGinnis, 2020). The attachment of the tendon to the bone is the transition from the collagen fibers to fibrocartilage to bone (McGinnis, 2020). Typically, a muscle will have two attachment points on opposite sides of a joint (McGinnis, 2020). At the point in time where tension occurs within the muscle fibers a line of pull will occur at each attachment point (McGinnis, 2020). The muscle tension will pull with equal force on each attachment and the arrangement of the fibers within the corresponding muscle may affect function, amount of force generated, and the time over which force is generated (McGinnis, 2020).
Muscles and their corresponding fibers can be classified in terms of alignment. Long muscles are characterized by all fibers aligning parallel to the line of pull (McGinnis, 2020). The muscles are called longitudinal, strap, or fusiform muscles (McGinnis, 2020). The alignment of fibers within long muscles provides the opportunity to shorten over a greater distance compared to muscles not aligned in a longitudinal manner (McGinnis, 2020).
Muscles with fibers which are not aligned parallel to the line of pull area are called pennate or penniform (McGinnis, 2020). Pennate muscles are characterized by shorter fibers inserted at an angle to the tendon (McGinnis, 2020). Pennate muscles may be unipennate, bipennate, or multipennate and in general are able to generate greater forces in comparison to long muscles of similar size (McGinnis, 2020).
Force production of muscle fibers are based upon several components. The maximum force generated under the precursor of all fibers are stimulated within a muscle are based upon several physiological components. These factors include the cross-sectional area, arrangement, length, and velocity characteristics of the fibers (McGinnis, 2020).
Muscle fibers attached end to end are described as being in a series, whereas fibers alongside one another are parallel (McGinnis, 2020). Longer muscles in a series have the ability to elongate and shorten over a greater distance in comparison to shorter muscles (McGinnis, 2020). Even with a greater length of action, these fibers are no stronger in comparison (McGinnis, 2020). Strength increases within muscle fibers and correlating force outputs are associated with width (McGinnis, 2020). The increase in cross sectional area in a perpendicular arrangement (width) to line of pull provides an indication of the force longitudinal muscle is able to product (McGinnis, 2020).
The cross-sectional area of a muscle provides an indication of the maximum tensile force a muscle may produce, though this maximum is dependent upon length during contraction (McGinnis, 2020). The attachment of the myosin filaments to the actin filament at the cross bridge within the sarcomere are the basis for tensile force during active contraction (McGinnis, 2020). This is apparent in reference to a longitudinal muscle in relation to pennate muscle as the longitudinal muscle can create tension through a greater range (McGinnis, 2020).
Passive tension can be created in a muscle through the stretching of connective tissue (McGinnis, 2020). The passive stretching of these structures allows a muscle to develop tension beyond its resting length (McGinnis, 2020). This allows a muscle to develop tension beyond the 160% of its resting length (McGinnis, 2020). As a result, the collective tension created by a muscle is based upon the both the passive and active components. The active tension being the contractile units within the muscle and passive referring to the stretch beyond resting length.
The length-tension relationship of a muscle directly affects the tension generated (Haff and Triplett, 2016). Additionally, the functional range of a muscle dictated by the associated joints range of motion will limit the length a muscle can achieve and its’ force abilities (Haff and Triplett, 2016). This component is evident in single joint compared to multi-joint muscles. A single joint muscle is limited by the range of motion of the joint crossed (McGinnis, 2020). A single joint muscle will operate within the 60-160% of resting length, whereas a multi-joint is not as constrained by the range of motion about a single joint. A multi-joint muscle may stretch beyond the 160% of its’ resting length, allowing for passive tension to be added, and in turn increasing the total tension of the muscle (McGinnis, 2020).
In addition to length, the maximum force a muscle generates is dependent upon contractile velocity (McGinnis, 2020). The contractile velocity of the muscle if slow, the number of cross bridges associated with the contraction in the initial steps are less, resulting in greater tension developed (McGinnis, 2020). Whereas if the velocity of the muscle is rapid, a larger portion of cross bridges are recruited in the initial phases, resulting in less tension generated overall (McGinnis, 2020).
Outside of velocity, the type of muscle contractile activity directly corresponds to the force a muscle can generate. The eccentric, concentric, and isometric activity is associated with varying degrees of force generation. An eccentric or isometric muscular contraction can generate more force compared to a concentric contraction (McGinnis, 2020).
Additional factors associated with the force generating capacities are the stretch-shortening cycle, fatigue, muscle fiber type, length of moment arm, and rate of doing work (McGinnis, 2020). The stretch-shortening cycle associated with plyometric type exercises where an eccentric stretch of a muscle prior to a short amortization phase and coinciding rapid concentric action affects force production (Haff and Triplett, 2016). Fatigue levels associated with continuous muscle stimulation adversely affects force production and an eventual decline in tension production (McGinnis, 2020). Fiber type in relation to Type I and Type II with varying aerobic and anaerobic capacities affect force production (Haff and Triplett, 2016). The Type IIa and Type IIb fiber types have a higher anaerobic capacity in comparison to Type I fibers (Haff and Triplett, 2016). This higher level of anaerobic capacity directly correlates to the potential for higher levels of force production in a short amount of time (Haff and Triplett, 2016). The length of moment arm will directly affect the amount of torque generated by a force as the mathematical equation for torque is force times length of moment arm (McGinnis, 2020). Finally, the rate of doing work which the product of force times velocity will affect force production (McGinnis, 2020).
Haff, G. Triplett, T. (Ed.). (2016) Essentials of strength and conditioning 4th edition, Champaign, IL: Human Kinetics.
McGinnis, P (2020) Biomechanics of Sport and Exercise, Champaign, IL: Human Kinetics.
Shaik, K. Kamaldeen, D. Maruthy, K. Johnson, P. Kumar, S. (2020) A newer approach for quantitative assessment of patellar tendon reflex response using biomechanical data of foot movement by a digital method. Journal of Sports Orthopedics and Traumatology, 36 (3) 271-277.