BIOMECHANICS OF SKELETAL MUSCLE KINE 458 BIOMECHANICS TERMS

BIOMECHANICS OF SKELETAL MUSCLE KINE 458 BIOMECHANICS

TERMS • Extensibility – ability to be stretched • Elasticity – ability to return to normal after a stretch • Viscoelastic property – Allows a muscle to gradual lengthen over time when stretched

TERMS • Contractile component – part of muscle that generates tension via muscle contraction • Series elastic component – tendons which store elastic energy when a muscle is stretched https: //www. youtube. com/watch? v=0 q 1 Vc. Tf. D 42 s • Parallel elastic component – muscle membranes which store elastic energy when a muscle is stretched https: //www. youtube. com/watch? v=H_FD 54 Mc. BMQ

TERMS • Motor unit • Twitch • Summation • Tetanus

TWITCH: CONTRACTILE RESPONSE BY NEURAL VOLLEY THAT REACHES LEVEL SUFFICIENT TO PRODUCE ACTION POTENTIAL TENSION DEVELOPED IN MUSCLE FROM SUBSEQUENT ACTION POTENTIALS IS SUMMATIVE…

TERMS • Fast-twitch fiber • Slow-twitch fiber • • Recruited first • Note: With exhaustive exercise, as slowtwitch fibers fatigue, relatively more fast twitch fibers will be recruited. *There is a continuum of fiber characteristics, so the distinctions are a bit artificial. The distinctions allow for better understanding of the features of muscle fibers* • Thus, exhaustive endurance-type activity is a viable strategy for stimulating higher threshold motor units when heavy loading or powerful movements are not possible.

TERMS • Parallel fiber arrangement • (also known as fusiform) • Pennate fiber arrangement • https: //www. youtube. com/watch? v=LMZLt. Upai. Nk

MUSCULAR STRENGTH • Ability of a muscle to generate torque at a particular joint. • Tension generating capacity of a muscle depends on: • • 1. Cross-sectional area 2. Training state • • *to some degree, these features are interrelated However, early adaptations (approx. first 12 weeks) to resistance/strength training are primarily neural • Via increased neuro-motor excitability, increase neural drive from central nervous system, and decreased inhibition to trained muscles.

FORCE-VELOCITY CURVE

FORCE VELOCITY RELATIONSHIP • Power = Force x Velocity Muscular Power = net torque x angular velocity at the joint • From previous figure, we see that maximal concentric power in a stereotypical muscle occurs at 1/3 maximum shortening velocity and 1/3 maximum concentric force generation. • The force – velocity relationship is thought to be related to time requirements of actin myosin cross-bridge cycling

ACTIVE & PASSIVE TENSION

STRETCH-SHORTENING CYCLE (SSC) Unique contraction type that employs stretch reflex and series elastic component (SEC) of the tensioned and prestretched muscle to produce a more forceful and metabolically efficient concentric contraction. *SSC is vitally important to human movement, from basic locomotion to high level athletic performance. * • Basic introduction to stretch shortening cycle exercises: • http: //davidpotach. com/stretch-shortening-cycle/ • Examples of SSC exercises: jumping, running, throwing, swinging, kettlebell swing*, even walking (to some extent, takes advantage of the SSC-see Komi (2000), next slide) • Kettlebell swing is a unique SSC exercise for the lower extremities, as the athlete remains in contact with the ground (Closed kinetic chain), which may be advantageous in certain situations. . • • When limiting # of jump / landing cycles due to injury When deficiencies in neuromuscular control / dynamic alignment / balance preclude safe use of jump / landing plyometric exercise

BIOMECHANICAL CONSIDERATIONS FOR SSC EXERCISE • Stretch shortening cycle exercise has a finite work capacity and have unique recovery demands… • Assigned reading for this module: • • Komi PV. Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics. 2000; 33: 1197 -1206. Optional reading on the kettlebell swing (with EMG data) : I will attach, but will not hold you responsible for this article: Mc. Gill S, Marshall LW. Kettlebell swing, snatch, and bottoms-up carry: back and hip muscle activation, motion, and low back loads. J Strength Cond Res. 2012; 26: 16 -27.

From: Komi PV. Stretch-shortening cycle: a powerful model to study normal and fatigued muscle. Journal of Biomechanics. 2000; 33: 1197 -1206.

ELECTROMYOGRAPHY (EMG)

SURFACE EMG (SEMG) • What it is… • measurement of the electrical activity of muscles during exercise via small electrodes placed over muscles of interest. • • Recording of the nervous system activity as it attempts to generate muscle force What it isn’t… • Not a reading of muscle tension/force • • However, during isometric contraction, these values should be related (with tension development being very slightly delayed) EMG is more “true” to tension/force in less dynamic movements • • Surface EMG signal varies linearly with muscle tension generation when the muscle is at a constant length. When muscle changes length during activity, surface EMG – tension relationship is less reliable. • Recall contribution of passive and elastic elements to muscle tension during dynamic exercise, which directly influence tension/force development in dynamic contractions. • • Not muscle “recruitment, ” per se, yet related • • Recruitment is the activation of additional motor units, while surface EMG measures coding patterns activity of motor units of interest Absolute indicator of the quality of an exercise • Since various mechanisms of muscle hypetrophy/strengthening exist (such as micro trauma and subsequent repair, metabolic stress, and time under tension), the EMG of a given exercise does not necessarily reflect the “quality” of an exercise. • • EMG does give insight into the active musculature within a given exercise and may certainly be used to assess the appropriateness of a given exercise, when taken into account with other features such as biomechanical considerations (kinetics and kinematics), functional analysis (how much carryover between exercise movement pattern and the movement you desire to improve), nature of subjective feedback (where does athlete/patient “feel the burn, ” presence of delayed onset muscle soreness? ), etc. . EMG data is especially valuable clinically for selecting “activation exercises” or “neuromuscular re-education, ” which may be desirable to improve coordination of movement, stability, or alignment • • See Hall page 161, Figure 6 -18 to see how passive tension in a lengthened muscle impacts total tension Mechanism of these interventions may be changes to the neuromuscular system (short term potentiation or long term plastic changes to corticospinal tracts) Note that surface EMG (electrodes on skin) is more reliable for measuring activity of superficial musculature, while more precise, intramuscular (fine wire/needle) EMG may be needed to measure deep muscles or very small muscles

EMG TERMS • MVC (maximum voluntary contraction): Highest EMG value obtained in isometric contraction. After measuring, subsequent exercises are compared to MVC and expressed as a percentage (i. e. 85% MVC) • • Peak MVC: The highest activation recorded (as percentage of isometric MVC) Mean MVC: The average muscle activity throughout interval of interest / exercise (as percentage of isometric MVC) • • muscle activity varies greatly throughout various joint angles of dynamic movement. For example, within the squat exercise, glute max is less active at top of movement but highly active at bottom of movement • • • Glute max EMG during a heavy squat may exhibit a high peak MVC at bottom of movement; however, the mean MVC may be much lower for the entire movement pattern. Therefore, consideration of only one of the above measurements may be a disservice. When interpreting EMG research, we must know which value is being reported and assess appropriateness before making conclusions or generalizations from the data. Can EMG exceed MVC? !? • Of course! • Since MVC is obtained during maximal voluntary isometric contraction, it is very possible, even likely, that certain dynamic and strength training exercises will exceed MVC due to task demand (load), stretch reflex, or irradiation from other muscles in kinetic chain.

EMG • Uses: • “SEMG is considered an acceptable tool for kinesiologic analysis of movement disorders because it is a method for recording and quantifying clinically important muscle-related activity with the least interference on the clinical picture. SEMG may also be useful in differentiating the many types of tremors, myoclonus, and dystonia; for evaluating gait and posture; and for evaluating psychophysical measurements of reaction and movement timing” • Pullman SL, et al. The Clinical Utility of Surface EMG. Nuerology. 2000; 55(2): 171 -177. http: //www. neurology. org/content/55/2/171 • *Surface EMG is not appropriate for diagnosis of neuromuscular disorders.

EMG DEMONSTRATION • Video by biomechanics researcher Bret Contreras Ph. D recording lower extremity EMG values for various muscle groups within 6 strength training exercises. . • https: //www. youtube. com/watch? v=v. RAY 4 f. Yt. F 2 Y • Although various software brands are available, EMG collection process and analysis is similar.

MUSCULAR FATIGUE • Muscle Fatigue: • “exercise-induced reduction in the maximal force capacity of a muscle” (Hall, p 167) • “a loss in the capacity for developing force and/or velocity of a muscle, resulting from muscle activity under load and which is reversible by rest” Gandevia S. C. Spinal and supraspinal factors in human muscle fatigue. Physiological Reviews. 2001; 81(4): 1725 -1789. http: //physrev. physiology. org/content/81/4/1725 • Why do we fatigue? • … Several proposed mechanisms, ultimately influenced by type of contraction (concentric, isometric, eccentric, stretch shortening), pattern of muscle activation, muscle fiber composition, external environment (temperature, altitude), and internal conditions (substrate availability, metabolite accumulation, dehydration, etc…) • The role of central fatigue is increasingly appreciated • Central fatigue: progressive failure of Central nervous system to drive motor neurons

MUSCLE / PERI-MUSCULAR INJURIES • On your own, please familiarize yourself with Strains, Contusions, Cramp, DOMS (Hall, p 168 -169). • Compartment syndrome: • Swelling/ pressure within compartment increases above perfusion pressure, resulting in ischemia / diminished distal pulses, loss of sensation, loss of motor function • Acute compartment syndromes are a medical emergency due to risk of nerve damage, tissue necrosis. May require fasciotomy/ release.

Scar status-post fasciotomy for volar forearm compartment…

• Chronic / exertional compartment syndrome is a reversible increase in compartment pressure corresponding with physical activity. • • • Most common: anterior and lateral compartments of lower leg. 70% are runners, similar rates among recreational and elite runners. Intra-compartment pressure test (invasive) is standard for diagnosis • • Normal resting intracompartmental pressure: 0 -8 mm Hg. Diagnostic criteria: • Resting compartment pressure greater than 15 mm Hg -OR-post-exercise compartment pressure (measured one minute after exercise) greater than 30 mm Hg -OR-post-exercise compartment pressure (measured five minutes after exercise) of greater than 20 mm Hg • Schubert AG. EXERTIONAL COMPARTMENT SYNDROME: REVIEW OF THE LITERATURE AND PROPOSED REHABILITATION GUIDELINES FOLLOWING SURGICAL RELEASE. International Journal of Sports Physical Therapy. 2011; 6(2): 126 -141.

CHRONIC/ EXERTIONAL COMPARTMENT SYNDROME MANAGEMENT

MUSCLE MODULE TASKS • Please read the Komi 2000 article from the Journal of Biomechanics on the stretch shortening cycle. It is a review article, so it covers a lot of ground and dives pretty deep in some areas. Read for practical content and do your best to interpret the Figures. • Also posted is the Kettlebell article by Stewart Mc. Gill. His research subject (wearing the EMG equipment is Pavel Tsatsuline, who is widely credited for popularizing the kettlebell as a training modality in the United States. • • Hall reading for this module is Chapter 6. The Kinetics Quiz is posted. Due Thursday, October 6 at 11: 59 PM.