The Influence of Active Hamstring Stiffness on Markers of Isotonic Muscle Performance

Abstract

Background: Previous research demonstrates hamstring muscle-tendon stiffness (HMTS) influences isometric strength, landing biomechanics and architectural tissue properties. However, the influence on kinetics & kinematics during other modes of strength testing (isotonic dynamometry) has yet to be established.

Purpose: Investigate how HMTS influences kinetics and kinematics during a novel isotonic muscle performance test which has never been done for the hamstrings. Previous work using dynamometry has been limited to isometric or isokinetic contractions, so the novelty arises from our custom isotonic protocol which allows quantitative assessment of the stretch-shortening cycle.

Methods: Twenty-six recreationally active individuals (15 males, 11 females, 23.8 ± 2.5 years) completed baseline testing for anthropometry and maximum isometric hamstring strength (MVIC). At least 48 h later, subjects completed a measure of HMTS (damped oscillation technique) followed by an isotonic knee flexion test (eccentric velocity 180°/s; concentric torque 25% of MVIC). Separate linear regression models with examination of residuals were conducted between HMTS and each muscle performance variable. Standardized coefficients determined the magnitude of the relationships.

Results: Significance was found for all outcome variables tested. HMTS and rate of torque development demonstrated the strongest relationship followed by isotonic concentric peak torque. The weakest relationship observed was with isometric peak torque.

Conclusions: These findings build off previous work quantifying HMTS by showing HMTS more strongly relates to dynamic versus static muscle testing and identifies the potential clinical utility of isotonic dynamometry.

Keywords: biomechanics; modeling; musculoskeletal; sports medicine; tendon.

Figure 1

Experimental setup for assessment of active hamstring muscle-tendon stiffness. Subjects were fitted with a rigid ankle splint, a load corresponding to 25% MVIC, and a tri-axial accelerometer (circled in red). A member of the research team applied a downward manual perturbation (indicated by the downward black arrow) to obtain the damped oscillation which was viewed and analyzed offline.

Figure 2

Step-by-step sequence of the isotonic hamstring test. (A) Participants began in 90° knee flexion; (B) eccentric knee flexion at 180°/s; (C) end range knee extension; (D) concentric knee flexion against a relative torque of 25% MVIC.

Figure 3

Graphical representation of the isotonic test computations from the torque (solid black line), velocity (solid gray line) and position (dotted line) to obtain concentric peak torque, rate of torque development, rate of velocity development, and rebound time.

Figure 4

Scatter plots displaying natural log of hamstring muscle-tendon stiffness on the x axess and rate of torque development (A), relative peak torque (B), rate of velocity development at 100 ms (C), and rebound time (D) on the vertical axes. For the relative peak torques (B) the solid line and filled data points (•) represent isotonic peak torque, while dotted line and open data points (◦) represent isometric peak torque. It should be noted that isotonic values were log transformed due to normality violation and thus do not represent peak torque values in their original units. For rebound time, a more negative value represents a faster rebound time (faster transition between eccentric and concentric contractions).