Linear and nonlinear optimal designs have been used abundantly to predict the forces exerted by individual muscles for everyday movements such as walking. Individual muscle force predictions for athletic movements, those involving large ranges of motion and fast velocities of muscle contractions, are almost nonexistent. The purpose of this paper is to illustrate some of the design characteristics that must be considered for predicting individual muscle forces in athletic movements. To do this, the load sharing between two muscles, derived from nonlinear optimal designs, is considered in two ways: (a) in hypothetical experiments of muscle contractions, and (b) in real experiments of knee extension movements performed by one subject. The results suggested that additional design considerations must be made when predicting forces in athletic movements compared to everyday movements.
Lynda Read and Walter Herzog
The purpose of this study was to determine resultant knee joint forces and moments during a specific movement in Alpine ski racers. The movement analyzed consisted of a landing from a bump and the initiation of recovery (if necessary). Resultant loads were obtained using an inverse dynamics approach. Results of two specific skiers are contrasted, one skier landing in good form, the second skier landing in poor form. The skier landing in poor form exhibited larger knee flexion, and larger knee joint resultant forces and moments than the skier landing in good form. The movement of the skier landing in poor form has been associated with isolated anterior cruciate ligament (ACL) injury. However, the data obtained in this study do not indicate that either skier was in danger of ACL injury.
Walter Herzog and Lynda Read
The purpose of this study was to estimate cruciate ligament forces in Alpine skiing during a movement that has been associated with anterior cruciate ligament (ACL) tears. Resultant knee joint forces and moments were obtained from two skiers during a World Cup Downhill race using an inverse dynamics approach and a 2-D bilaterally symmetric system model. It was found that ACL forces were typically small for both skiers throughout the movement analyzed because quadriceps forces prevented anterior displacement of the tibia relative to the femur at the knee joint angles observed. However, for about 10 ms, loading conditions in the knee joint of Subject 2 (who displayed poor form) were such that large ACL forces may have been present. These particular loading conditions were never observed in Subject 1, who displayed good form. Since neither of the skiers was injured, it is not possible to draw firm conclusions about isolated ACL tears in Alpine skiing from the data at hand.
Walter Herzog and Rachid Ait-Haddou
The target article by Dr. Prilutsky is based on three incorrectly derived mathematical rules concerning force-sharing among synergistic muscles associated with a cost function that minimizes the sum of the cubed muscle stresses. Since these derived rules govern all aspects of Dr. Prilutsky's discussion and conclusion and form the basis for his proposed theory of coordination between one-and two-joint muscles, most of what is said in the target article is confusing or misleading at best or factually wrong at worst. The aim of our commentary is to sort right from wrong in Dr. Prilutsky's article within space limitations that do not allow for detailed descriptions of mathematical proofs and explicit discussions of the relevant experimental literature.
Franziska Onasch, Anthony Killick, and Walter Herzog
The aim of this study was to determine the effects of pole length on energy cost and kinematics in cross country double poling. Seven sub-elite male athletes were tested using pole sets of different lengths (ranging between 77% and 98% of participants’ body height). Tests were conducted on a treadmill, set to a 2% incline and an approximate racing speed. Poling forces, contact times, and oxygen uptake were measured throughout the testing. Pole length was positively correlated with ground contact time (r = .57, p < .001) and negatively correlated with poling frequency (r = −.48, p = .003). Pole length was also positively correlated with pole recovery time and propulsive impulse produced per poling cycle (r = .36, p = .031; r = .35, p = .042, respectively). Oxygen uptake and pole length were negatively correlated (r = −.51, p = .004). This acute study shows that increasing pole length for double poling in sub-elite cross country skiers under the given conditions seems to change the poling mechanics in distinct ways, resulting in a more efficient poling action by decreasing an athlete’s metabolic cost.
Kevin Boldt, Anthony Killick, and Walter Herzog
A 1:1 locomotion–respiration entrainment is observed in galloping quadrupeds, and is thought to improve running economy. However, this has not been tested directly in animals, as animals cannot voluntarily disrupt this entrainment. The purpose of this study was to evaluate metabolic economy in a human gait involving all four limbs, cross-country skiing, in natural entrainment and forced nonentrainment. Nine elite cross-country skiers roller skied at constant speed using the 2-skate technique. In the first and last conditions, athletes used the natural entrained breathing pattern: inhaling with arm recovery and exhaling with arm propulsion, and in the second condition, the athletes disentrained their breathing pattern. The rate of oxygen uptake (VO2) and metabolic rate (MR) were measured via expired gas analysis. Propulsive forces were measured with instrumented skis and poles. VO2 and MR increased by 4% and 5% respectively when skiers used the disentrained compared with the entrained breathing pattern. There were no differences in ski or pole forces or in timing of the gait cycle between conditions. We conclude that breathing entrainment reduces metabolic cost of cross-country skiing by approximately 4%. Further, this reduction is likely a result of the entrainment rather than alterations in gait mechanics.
Esther Suter, Walter Herzog, and Robert Bray
This study assessed muscle inhibition in patients with chronic anterior cruciate ligament (ACL) deficiency or ACL reconstruction. A series of protocols were tested for their effectiveness in increasing activity of the individual knee extensor muscles and decreasing muscle inhibition of the whole quadriceps group. Quadriceps muscle inhibition was measured by superimposing an electrical twitch onto the quadriceps muscle during a maximal voluntary knee extension. The level of activation of the individual knee extensor and knee flexor muscles was assessed via electromyography (EMG). Patients with ACL pathologies showed strength deficits and muscle inhibition in the knee extensors of the involved leg and the contralateral leg. Muscle inhibition was statistically significantly greater in ACL-deficient patients compared to ACL-reconstructed patients. When a knee extension was performed in combination with a hip extension, there was a significant increase, p < 0.05, in activation of the vastus medialis and vastus lateralis muscles compared to isolated knee extension. The use of an anti-shear device, designed to help stabilize the ACL-deficient knee, resulted in increased inhibition in the quadriceps muscle. Furthermore, a relatively more complete activation of the vasti compared to the rectus femoris was achieved during a fatiguing isometric contraction. Based on the results of this study, it is concluded that performing knee extension in combination with hip extension, or performing fatiguing knee extensor contractions, may be more effective in fully activating the vasti muscles than an isolated knee extensor contraction. Training interventions are needed to establish whether these exercise protocols are more effective than traditional rehabilitation approaches in decreasing muscle inhibition and achieving better functional recovery, including equal muscle strength in the injured and the contralateral leg.
Markus Tilp, Simon Steib, Gudrun Schappacher-Tilp, and Walter Herzog
Force enhancement following muscle stretching and force depression following muscle shortening are well-accepted properties of skeletal muscle contraction. However, the factors contributing to force enhancement/depression remain a matter of debate. In addition to factors on the fiber or sarcomere level, fiber length and angle of pennation affect the force during voluntary isometric contractions in whole muscles. Therefore, we hypothesized that differences in fiber lengths and angles of pennation between force-enhanced/depressed and reference states may contribute to force enhancement/depression during voluntary contractions. The purpose of this study was to test this hypothesis. Twelve subjects participated in this study, and force enhancement/depression was measured in human tibialis anterior. Fiber lengths and angles of pennation were quantified using ultrasound imaging. Neither fiber lengths nor angles of pennation were found to differ between the isometric reference contractions and any of the force-enhanced or force-depressed conditions. Therefore, we rejected our hypothesis and concluded that differences in fiber lengths or angles of pennation do not contribute to the observed force enhancement/depression in human tibialis anterior, and speculate that this result is likely true for other muscles too.