Within-subject consistency of unimodal and bimodal force application during the countermovement jump

Countermovement jump (CMJ) force data are often time-normalized so researchers and practitioners can study the effect that sex, training status, and training intervention have on CMJ strategy, the so-called force-time curve shape. Data are often collected on an individual basis and then averaged across groups of interest. However, little is known about the within-subject agreement of the CMJ force-time curve shape, and this formed the aim of this study. Fifteen men performed 10 CMJs on in-ground force plates, force-time curves were plotted, and their shape categorized as exhibiting either a single peak (unimodal) or a double peak (bimodal). Percentage agreement and the kappa coefficient were used to assess within-subject agreement. Over two and three trials 13% demonstrated a unimodal shape, 67% exhibited a bimodal shape, and 20% were inconsistent. When five trials were considered the unimodal shape was not demonstrated consistently, 67% demonstrated a bimodal shape, and 33% were inconsistent. Over 10 trials none demonstrated the unimodal shape, 60% demonstrated the bimodal shape, and 40% were inconsistent. The results of this study suggest that researchers and practitioners should ensure within-subject consistency before group averaging CMJ force-time data to help avoid errors that not doing so may cause

Practical applications of biomechanical principles in resistance training: moments and moment arms

Exercise professionals routinely prescribe resistance training to clients with varied goals. Therefore, they need to be able to modify the difficulty of a variety of exercises and to understand how such modifications can alter the relative joint loading on their clients so to maximise the potential for positive adaptation and to minimise injury risk. This paper is the first in a three part series that will examine how a variety of biomechanical principles and concepts have direct relevance to the prescription of resistance training for the general and athletic populations as well as for musculoskeletal injury rehabilitation. In this paper, we start by defining the terms moment (torque), moment arms, compressive, tensile and shear forces as well as joint stress (pressure). We then demonstrate how an understanding of moments and moment arms is integral to the exercise professionals’ ability to develop a systematic progression of variations of common exercises. In particular, we examine how a variety of factors including joint range of motion, body orientation, type of external loading, the lifter’s anthropometric proportions and the position of the external load will influence the difficulty of each exercise variation. We then highlight the primary results of several selected studies which have compared the resistance moment arms and joint moments, forces or stresses that are encountered during selected variations of common lower body resistance training exercises. We hope that exercise professionals will benefit from this knowledge of applied resistance training biomechanics and be better able to systematically progress exercise difficulty and to modify joint loading as a result. The two remaining articles in this series will focus on the neuromechanical properties of the human musculoskeletal system and better understanding the biomechanical implications of a variety of alternative resistance training techniques, respectively

Practical applications of biomechanical principles in resistance training: The use of bands and chains

In recent years, it has become popular for athletes and recreational trainers to perform resistance training with the addition of bands and chains. In this paper, we consider the advantages of manipulating an exercise to match the resistance provided with the force capabilities of the lifter, which generally change throughout the movement. We explain that bands and chains can be used to manipulate a variety of exercises that have the potential to enhance performance in sport and in many daily tasks. Whilst there are many similarities between the use of bands and chains for resistance training, we note that there are key differences and discuss the biomechanics of each material separately. In particular, we discuss that chains provide resistance primarily in the vertical plane and the resistance is linearly related to the displacement of the barbell. In contrast, bands can be set up to produce substantial horizontal forces in addition to the primary resistance force that often acts in the vertical direction. Also, research has demonstrated that bands provide a resistance force that is related in a curvilinear fashion to the displacement of the barbell. After introducing the main biomechanical features associated with each type of resistance material, we present findings from the strength and conditioning literature that has demonstrated the potential for bands and chains to improve the stimulus associated with strength and power training. At present, a more compelling evidence base has emerged for the use of bands in resistance training, particularly with regard to the development of power. It is not known whether this asymmetry reflects the greater number of studies conducted with bands or is influenced by methodological differences between studies. However, we also discuss the possibility that different inertial properties of bands compared with chains may make the former a more effective choice for the development of power. We hope that exercise professionals will benefit from this knowledge and obtain insight into how an understanding of biomechanical principles can assist with prescribing contemporary training regimes

Comparison of Different Minimal Velocity Thresholds to Establish Deadlift One Repetition Maximum

The aim of this study was to compare the actual deadlift one repetition maximum (1RM) and the deadlift 1RM predicted from individualised load-velocity profiles. Twelve moderately resistance-trained men participated in three deadlift sessions. During the first, 1RM was assessed; during the second, load-velocity profiles were recorded with six loads (65% to 90% 1RM) using a linear position transducer recording at 1000 Hz; and during the third, minimal velocity thresholds (MVT) were recorded from the velocity of the last repetition during sets to volitional fatigue with 70% and 80% 1RM with a linear position transducer recording at 1000 Hz. Regression was then used to generate individualised load-velocity profiles and the MVT was used as a cut-off value from which to predict deadlift 1RM. In general, velocity reliability was poor to moderate. More importantly, predicted deadlift 1RMs were significantly and meaningfully less than actual deadlift 1RMs (p < 0.05, d = 1.03–1.75). The main practical application that should be taken from the results of this study is that individualized load-velocity profiles should not be used to predict deadlift 1RM. Practitioners should not use this method in combination with the application of MVT obtained from the last repetition of sets to volitional fatigue

Practical applications of biomechanical principles in resistance training: Neuromuscular factors and relationships

This paper is the second in our three part series examining how a variety of biomechanical principles and concepts\ud have direct relevance to the prescription of resistance training for the general and athletic populations as well as for\ud musculoskeletal injury rehabilitation. In this paper, we considered different neuromuscular characteristics of resistance\ud exercise. We started by defining the causes of motion, discussing force and Newton’s second law of linear motion. This\ud led to discussion of impulse, and how its relationship with momentum can be used to study force-time curves recorded\ud from different ground-based resistance exercises. This enables the sports biomechanist to derive movement velocity,\ud which enables study of the relationship between force and velocity, and we concluded that as the force required to\ud cause movement increases the velocity of movement must decrease. This relationship is critical because it enables\ud strength and conditioning coaches and exercise professionals to manipulate resistance-training loads to maximise\ud training gains for sports performance. We described representative force-time curves from basic human movements\ud to provide a foundation for discussion about how different resistance-training gains can be achieved. This focused on\ud exercise technique, including use of the stretch-shortening cycle, magnitude of load, ballistic resistance exercise, and\ud elastic band and chain resistance (although elements of this will receive greater attention in our final article). Finally, we\ud defined and explained the concept of mechanical work and power output, examining the effect that load has on power\ud output by considering the load-power relationships of different common resistance exercises. We hope that exercise\ud professionals will benefit from this knowledge of applied resistance training biomechanics. Specifically, we feel that\ud the take home message of this article is that resistance exercise load and technique can be manipulated to maximise\ud resistance-training gains, and that this can be particularly useful for athletes trying to improve sporting performance

Practical applications of biomechanical principles in resistance training: Moments and moment arms

Exercise professionals routinely prescribe resistance training to clients with varied goals. Therefore, they need to be able to modify the difficulty of a variety of exercises and to understand how such modifications can alter the relative joint loading on their clients so to maximise the potential for positive adaptation and to minimise injury risk. This paper is the first in a three part series that will examine how a variety of biomechanical principles and concepts have direct relevance to the prescription of resistance training for the general and athletic populations as well as for musculoskeletal injury rehabilitation. In this paper, we start by defining the terms moment (torque), moment arms, compressive, tensile and shear forces as well as joint stress (pressure). We then demonstrate how an understanding of moments and moment arms is integral to the exercise professionals’ ability to develop a systematic progression of variations of common exercises. In particular, we examine how a variety of factors including joint range of motion, body orientation, type of external loading, the lifter’s anthropometric proportions and the position of the external load will influence the difficulty of each exercise variation. We then highlight the primary results of several selected studies which have compared the resistance moment arms and joint moments, forces or stresses that are encountered during selected variations of common lower body resistance training exercises. We hope that exercise professionals will benefit from this knowledge of applied resistance training biomechanics and be better able to systematically progress exercise difficulty and to modify joint loading as a result. The two remaining articles in this series will focus on the neuromechanical properties of the human musculoskeletal system and better understanding the biomechanical implications of a variety of alternative resistance training techniques, respectively

Phase specific changes in the countermovement jump occur without change in peak metrics following training

The countermovement jump (CMJ) is routinely used to assess changes in strength-power qualities. Common measures derived from this test include jump height, peak power and peak velocity. However, valuable information on training induced changes in CMJ performance may be missed if phase and subphase variables are not included in the analysis also. The objective of this investigation was to determine whether significant performance changes can occur in the CMJ in the absence of changes in jump height or peak-form metrics. Sixteen recreationally trained males undertook 10-weeks of resistance training consisting of weightlifting, ballistic and plyometric actions with heavy and light loads. The CMJ was performed pre- and post-test with both peak-form metrics and mean phase/subphase metrics analysed. Mean velocity (p < 0.01) and mean power (p < 0.01) significantly improved following training while peak velocity (p = 0.18), peak power (p = 0.29), and jump height (p = 0.24) did not. Work, countermovement depth, eccentric duration and total movement duration significantly improved too (p < 0.01 to 0.03). Practitioners should consider using CMJ variables beyond jump height and instantaneous metrics to more thoroughly diagnose performance changes of the leg extensors following training

Do the peak and mean force methods of assessing vertical jump force asymmetry agree?

The aim of this study was to assess agreement between peak and mean force methods of quantifying force asymmetry during the countermovement jump (CMJ). Forty-five men performed four CMJ with each foot on one of two force plates recording at 1000 Hz. Peak and mean were obtained from both sides during the braking and propulsion phases. The dominant side was obtained for the braking and propulsion phase as the side with the largest peak or mean force and agreement was assessed using percentage agreement and the kappa coefficient. Braking phase peak and mean force methods demonstrated a percentage agreement of 84% and a kappa value of 0.67 (95% confidence limits: 0.45 to 0.90), indicating substantial agreement. Propulsion phase peak and mean force methods demonstrated a percentage agreement of 87% and a kappa value of 0.72 (95% confidence limits: 0.51 to 0.93), indicating substantial agreement. While agreement was substantial, side-to-side differences were not reflected equally when peak and mean force methods of assessing CMJ asymmetry were used. These methods should not be used interchangeably, but rather a combined approach should be used where practitioners consider both peak and mean force to obtain the fullest picture of athlete asymmetry

Force and acceleration characteristics of military foot drill: implications for injury risk in recruits

Background: Foot drill involving marching and drill manoeuvres is conducted regularly during basic military recruit training. Characterising the biomechanical loading of foot drill will improve our understanding of the contributory factors to lower limb overuse injuries in recruits. Aim: Quantify and compare forces, loading rates and accelerations of British Army foot drill, within and between trained and untrained personnel. Methods: 24 trained soldiers (12 men and 12 women; TRAINED) and 12 civilian men (UNTRAINED) performed marching and five drill manoeuvres on force platforms; motion capture recorded tibial position. Peak vertical impact force (PF), peak vertical loading rate (PLR), expressed as multiples of body weight (BW) and peak tibial impact acceleration (PTA) were recorded. Results: Drill manoeuvre PF, PLR and PTA were similar, but higher in TRAINED men (PF, PLR: p<0.01; PTA: p<0.05). Peak values in TRAINED men were shown for the halt (mean (SD); PF: 6.5 (1.5) BW; PLR: 983 (333) BW/s PTA; PTA: 207 (57) m/s2) and left turn (PF: 6.6 (1.7) BW; PLR: 928 (300) BW/s; 184 (62) m/s2). Marching PF, PLR, PTA were similar between groups and lower than all drill manoeuvres (PF: 1.1–1.3 BW; PLR: 42–70 BW/s; p<0.01; PTA: 23–38 m/s2; p<0.05). Conclusions: Army foot drill generates higher forces, loading rates and accelerations than activities such as running and load carriage, while marching is comparable to moderate running (10.8 km/h). The large biomechanical loading of foot drill may contribute to the high rate of overuse injuries during initial military training, and strategies to regulate/reduce this loading should be explored

The bilateral deficit during jumping tasks: Relationship with speed and change of direction speed performance

Research to date has investigated the phenomenon of the bilateral deficit (BLD); however, limited research exists on its association with measures of athletic performance. The purpose of the present study was to investigate the magnitude of the BLD and examine its relationship with linear speed and change of direction speed (CODS) performance. Eighteen physically active and healthy university students performed double and single leg countermovement jumps (CMJ), drop jumps (DJ) and standing broad jumps (SBJ), to calculate the BLD across jump tasks. Subjects also performed 10m and 30m sprints and a 505 CODS test, which were correlated with all BLD metrics. Results showed varying levels of BLD across CMJ metrics (jump height, peak force, eccentric impulse, concentric impulse, peak power), DJ metrics (ground contact time, flight time), and the SBJ (distance). However, a bilateral facilitation (BLF) was shown for jump height and reactive strength index (RSI) during the DJ test. The main findings of the present study were that: 1) a larger BLD in CMJ jump height related to a faster 505 change of direction (COD) (left leg) (r = -0.48; p = 0.04), 505 COD (right leg) (r = -0.53; p = 0.02) and COD deficit (right leg) (r = -0.59; p = 0.01), 2) a larger BLD in CMJ concentric impulse related to faster 505 COD (left leg) (r = -0.51; p = 0.03), 505 COD (right leg) (r = -0.64, p = 0.01) and COD deficit (right leg) (r = -0.60; p = 0.01), 3) a larger BLD in DJ flight time related to a faster 505 COD (left leg) (r = -0.48; p = 0.04). These results suggest that a larger BLD is associated with faster CODS performance, but not linear speed. This highlights the individual nature of the BLD and may support the notion of developing movement competency on one limb for enhanced CODS performance