Fundamentals of Exercise Physiology – Week 3 Notes

Learning Objectives

• Identify and describe all skeletal-muscle fibre types (Type I, IIA, IIX)
• Explain biochemical & contractile properties that separate fibre types
• Describe sarcomere movements during concentric, eccentric and isometric actions
• Interpret classical force–length, force–velocity and force–length–velocity relationships
• Detail cross-bridge behaviour during isometric contractions
• Explain neural regulation of muscle force and the force–frequency relationship

Sarcomeres in Series & Range of Motion (ROM)

• Muscle ROM is directly proportional to the number of sarcomeres arranged in series within each myofibril
  $\text{ROM}\;\propto\;N_{\text{sarcomeres in series}}$
• Comparative examples
  • Soleus ≈ 9 000 sarcomeres / myofibril → “shorter” functional ROM
  • Rectus femoris ≈ 30 000 sarcomeres / myofibril → “longer” ROM
• Conceptual cartoons
  • 1-, 2-, and 4-sarcomere myofibrils illustrate how identical ∆L at each sarcomere sums to larger overall length change in longer myofibrils
• Practical implication: muscles built for large joint excursions (e.g., hamstrings, rectus femoris) possess more serial sarcomeres than postural muscles (e.g., soleus)

Direction of Forces & Types of Contractions

• Two simultaneous forces in every contraction
  1. Insertional (muscle) force – always directed toward the muscle centre
  2. External load – directed away from muscle centre
• The balance between these forces determines contraction “type”
  • Concentric: $F<em>{\text{muscle}}>F</em>{\text{load}}$ → muscle & limb shorten
  • Eccentric: $F<em>{\text{muscle}}<F</em>{\text{load}}$ → muscle lengthens
  • Isometric: $F<em>{\text{muscle}}=F</em>{\text{load}}$ → constant length
• Visual metaphors: raising vs. lowering a weight; starting & stopping when forces re-equilibrate
• Other descriptors (often misused)
  • Isotonic = constant tension throughout ROM
  • Isokinetic = constant angular velocity (requires dynamometer)

Sarcomere Behaviour Inside Each Contraction

• Concentric
  • Thin filaments slide toward M-line; sarcomere shortens
  • Myosin force exceeds external load at every cross-bridge → whole muscle shortens
• Eccentric
  • External load pulls Z-discs apart faster than cross-bridges can re-attach → sarcomere lengthens
  • Greater force needed to detach heads; some heads enter “super-strong” state
• Isometric
  • Sarcomere length constant, but full 4-step cross-bridge cycle still turns over
  • $F<em>{\text{myosin}} = F</em>{\text{load}}$ so S1 head generates torque yet does not rotate lever arm appreciably → no limb movement

Force–Length (Length–Tension) Relationship

• Classical single-sarcomere curve created by summing unit forces from individual cross-bridges
• Portions of the curve
  1. Ascending limb (long → optimal)
     • Increasing overlap → linearly more attached heads → force rises
  2. Plateau (~$2.0–2.2\,\mu m$ sarcomere length)
     • No bare-zone heads; maximum overlap; additional shortening cannot recruit extra heads → force potentiates then levels
  3. Descending limb (excessively short)
     • Actin–actin interference; opposing thin filaments block binding → fewer attached heads → force declines
     • Beyond ~$1.7\,\mu m$ thick filament compresses against Z-disc; force dissipates internally
• Whole-muscle length–tension mimics sarcomere trend when architectural & connective-tissue factors are considered
• In vivo example: Elbow flexors produce - Low torque in full flexion - Peak torque near mid-range - Low torque at full extension
  (Data: Tsunoda et al., 1993; moment arm changes also contribute)

Force–Velocity Relationship

• Concentric side (positive velocities) • Force falls hyperbolically with increasing shortening speed • Mechanisms
  1. “Drag” from heads that cannot detach rapidly
  2. Decreasing proportion of heads able to attach during brief overlap times
     • Heavily influenced by myosin ATPase rate; faster isoforms maintain higher force at given velocity
• Eccentric side (negative velocities)
  • Force exceeds isometric maximum; curve plateaus or rises slightly with speed of lengthening
  • Requires extra energy to forcibly detach heads; mechanical stretch may activate additional binding domains → “super-state”
• Hill-type equation often used to model concentric limb
  $\bigl(F+a\bigr)\,(v+b)=\bigl(F_{\text{max}}+a\bigr)\,b$

3-D Force–Length–Velocity Surface

• Combines both previous relationships
  • Each iso-velocity slice resembles a length–tension curve
  • Each iso-length slice resembles a force–velocity curve
  • Shows interactive dependence: e.g., at longer lengths muscle can produce more force at a given velocity than at shorter lengths

Myosin Structure & Isoforms

• Myosin molecule: dimer (MW ≈ 520 kDa) → 2 heavy chains (MHC, 220 kDa each) plus 4 light chains
• Functional regions
  • Tail (α-helical coil) anchors into thick filament
  • S2 neck lever arm + S1 head = cross-bridge; contains catalytic & actin-binding sites
• Small amino-acid sequence variations → distinct MHC isoforms despite ≈95 % homology (e.g., MHC-IIA vs. IIX)
• Human skeletal muscle expresses three primary isoforms
  1. MHC-I (slow)
  2. MHC-IIA (fast)
  3. MHC-IIX (fastest)
• Cross-bridge cycle always 4 steps regardless of isoform, but attachment & detachment dwell times differ (IIX<I ≈→ quickest overall cycle)
• Consequence: fibre-type specific force–velocity curves; at any load, fibres with faster isoforms shorten faster & generate lower economical force

Biochemical & Contractile Properties of Fibre Types

• Oxidative capacity
  • Mitochondrial density, capillary supply, myoglobin ≫ in Type I
• Myosin ATPase activity
  • Low in Type I → slow shortening; highest in Type IIX → rapid shortening
• Performance metrics
  • $V_{\max}$ hierarchy: Type IIX > IIA > I (Fig 8.12)
  • Specific tension (kN m$^{-2}$): Type IIX ≈ IIA > I (Fig 8.13)
  • Energetic efficiency (ATP cost per unit force): highest in Type I, lowest in Type IIX (Δ≈280 % ATP per force unit; Stienen et al., 1996)
• Summary table (adapted from Table 8.2)
  • Type I: many mitochondria, aerobic metabolism, high fatigue resistance, low specific tension
  • Type IIA: intermediate oxidative-glycolytic, moderate fatigue resistance, high tension
  • Type IIX: glycolytic, low fatigue resistance, highest tension & speed, lowest efficiency
• Colour/Histochemical markers
  • Type I – red; Type IIA – pink; Type IIX – white
  • Distinct myosin ATPase staining pH sensitivity; immunohistochemistry for MHC isoform

Fibre-Type Distribution in Humans

• Average mixed muscle ≈50 % Type I vs. 50 % Type II (“rule of thumb”)
• Sex comparison (Simoneau & Bouchard, 1989): no significant difference in relative distributions; men have larger fibre cross-sectional areas
• Large inter-individual variability (genetic)
  • Type I proportion ranges ≈15–80 %
• Muscle-specific specialisation
  • Soleus → predominantly slow
  • Triceps brachii → predominantly fast
  • Orbicularis oculi → extremely fast for rapid eyelid closure
• Athletic populations
  • Endurance athletes display high % Type I; power/sprint athletes high % Type II
  • Correlation between $\dot V!O_{2\,\max}$ and % slow fibres (McArdle et al.)
• Plasticity
  • Training can induce IIx→IIa and IIa→I shifts (especially with endurance) but large genetic ceiling exists; complete I↔II conversion rare in adult humans

Practical / Real-World Implications

• ROM specialisation informs rehab & strength-training exercise selection (e.g., stretching vs. hypertrophy targeted muscles)
• Force–length curve underpins optimal joint angle selection for maximum torque in powerlifting or isometric testing
• Force–velocity understanding crucial for velocity-based resistance training and injury-prevention eccentric protocols
• Fibre-type profiling (biopsy or non-invasive surrogates) can tailor training toward athlete’s genetic predisposition (e.g., sprint vs. endurance)