Adrian Lees

The biomechanics of soccer: a review

This review considers the biomechanical factors that are relevant to success in the game of soccer. Three broad areas are covered: (1) the technical performance of soccer skills; (2) the equipment used in playing the game; and (3) the causative mechanisms of specific soccer injuries. Kicking is the most widely studied soccer skill. Although there are many types of kick, the variant most widely reported in the literature is the maximum velocity instep kick of a stationary ball. In contrast, several other skills, such as throwing-in and goalkeeping, have received little attention; some, for example passing and trapping the ball, tackling, falling behaviour, jumping, running, sprinting, starting, stopping and changing direction, have not been the subject of any detailed biomechanical investigation. The items of equipment reviewed are boots, the ball, artificial and natural turf surfaces and shin guards. Little of the research conducted by equipment manufacturers is in the public domain; this part of the review therefore concentrates on the mechanical responses of equipment, player-equipment interaction, and the effects of equipment on player performance and protection. Although the equipment has mechanical characteristics that can be reasonably well quantified, the player-equipment interaction is more difficult to establish; this makes its efficacy for performance or protection difficult to predict. Some soccer injuries may be attributable to the equipment used. The soccer boot has a poor protective capability, but careful design can have a minor influence on reducing the severity of ankle inversion injuries. Performance requirements limit the scope for reducing these injuries; alternative methods for providing ankle stability are necessary. Artificial surfaces result in injury profiles different from those on natural turf pitches. There is a tendency for fewer serious injuries, but more minor injuries, on artificial turf than on natural turf pitches. Players adapt to surface types over a period of several games. Therefore, changing from one surface to another is a major aetiological factor in surface-related injuries. Heading the ball could lead to long-term brain damage. Simulation studies suggest the importance of ball mass, ball speed and player mass in affecting the severity of impact. Careful instruction and skill development, together with the correct equipment, is necessary for young players. Most applications of biomechanical techniques to soccer have been descriptive experimental studies. Biomechanical modelling techniques have helped in the understanding of the underlying mechanisms of performance, although their use has been limited. It is concluded that there are still many features of the game of soccer that are amenable to biomechanical treatment, and many opportunities for biomechanists to make a contribution to the science of soccer.

Adjustments in gait symmetry with walking speed in trans-femoral and trans-tibial amputees

The effect of increased walking speed on temporal and loading asymmetry was investigated in highly active trans-femoral and trans-tibial amputees. With increasing walking speed, temporal gait variables reduced in duration, particularly on the prosthetic limb, while vertical ground reaction force (vGRF) increased in magnitude, particularly on the intact limb. Thus, temporal asymmetry reduced and loading asymmetry increased with walking speed. The greater force on the intact limb may reflect the method by which the amputees achieve greater temporal symmetry in order to walk fast, and could possibly account for greater instances of joint degeneration in the intact limb reported in the literature.

A comparison of muscle strength and flexibility between the preferred and non-preferred leg in English soccer players

Most soccer players have a favoured foot for kicking the ball, and it is believed that this preference may lead to an asymmetry in the strength and flexibility of the lower extremities. This study was designed to determine whether asymmetry in strength and flexibility are present in the legs of soccer players. Forty-one elite and sub-elite soccer players (age 23.4 ± 3.8 years; height 1.81 ± 0.06 m; body mass 81.7 ± 9.9 kg) were studied (data are presented as mean ± SD). The dynamic strength of knee flexors (hamstrings) and knee extensors (quadriceps) was measured using an isokinetic dynamometer at angular velocities of 1.05, 2.09, 5.23 rad/s (in a concentric mode) and 2.09 rad/s (in an eccentric mode). The concentric strength ratio (hamstringsconc/quadricepsconc) and the dynamic control ratio (hamstringsecc/quadricepsconc) were computed. Hip joint flexibility (in flexion) was measured using a goniometer. A significant difference between the preferred and non-preferred leg was found in the knee flexors at 2.09 rad/s (119 ± 22 versus 126 ± 24 Nm; P < 0.05) and for the dynamic control ratio (0.79 ± 0.13 versus 0.84 ± 0.16 Nm; P < 0.05). In both cases the knee flexors of the preferred leg were weaker than those of the non-preferred leg. A total of 28 of the 41 players (68%) had significant musculoskeletal abnormality (imbalance >10%) in one or more specific muscle groups. No significant differences were found in flexibility of the hip joint between the preferred and non-preferred leg (P > 0.05). It is concluded that the lower strength of the knee flexor muscles of the preferred leg may be associated with the differential use of these muscle during the kicking action and thus constitutes a unique training effect associated with soccer. This in turn can lead to muscular imbalance which is generally regarded as an injury risk factor.

Muscle fatigue induced by exercise simulating the work rate of competitive soccer

Fatigue represents a reduction in the capability of muscle to generate force. The aim of the present study was to establish the effects of exercise that simulates the work rate of competitive soccer players on the strength of the knee extensors and knee flexors. Thirteen amateur soccer players (age 23.3±3.9 years, height 1.78±0.05 m, body mass 74.8±3.6 kg; mean±s) were tested during the 2000–2001 soccer season. Muscle strength of the quadriceps and hamstrings was measured on an isokinetic dynamometer. A 90 min soccer-specific intermittent exercise protocol, incorporating a 15 min half-time intermission, was developed to provide fatiguing exercise corresponding in work rate to a game of soccer. The exercise protocol, performed on a programmable motorized treadmill, consisted of the different intensities observed during soccer match-play (e.g. walking, jogging, running, sprinting). Muscle strength was assessed before exercise, at half-time and immediately after exercise. A repeated-measures analysis of variance showed significant reductions (P <0.001) in peak torque for both the quadriceps and hamstrings at all angular velocities (concentric: 1.05, 2.09, 5.23 rad · s−1; eccentric: 2.09 rad · s−1). The peak torque of the knee extensors (KE) and knee flexors (KF) was greater before exercise [KE: 232±37, 182±34, 129±27, 219±41 N · m at 1.05, 2.09 and 5.23 rad · s−1 (concentric) and 2.09 rad · s−1 (eccentric), respectively; KF: 126±20, 112±19, 101±16, 137±23 N · m] than at half-time (KE: 209±45, 177±35, 125±36, 214±43 N · m; KF: 114±31, 102±20, 92±15, 125±25 N · m) and greater at half-time than after exercise (KE: 196±43, 167±35, 118±24, 204±43 N · m; KF: 104±25, 95±21, 87±13, 114±27 N · m). For the hamstrings : quadriceps ratio, significant changes were found (P <0.05) for both legs, the ratio being greater before than after exercise. For fast : slow speed and left : right ratios, no significant changes were found. We conclude that there is a progressive reduction in muscle strength that applies across a range of functional characteristics during exercise that mimics the work rate in soccer.

Science and the major racket sports: a review

Abstract The major racket sports include badminton, squash, table tennis and tennis. The growth of sports science and the commercialization of racket sports in recent years have focused attention on improved performance and this has led to a more detailed study and understanding of all aspects of racket sports. The aim here, therefore, is to review recent developments of the application of science to racket sports. The scientific disciplines of sports physiology and nutrition, notational analysis, sports biomechanics, sports medicine, sports engineering, sports psychology and motor skills are briefly considered in turn. It is evident from these reviews that a great deal of scientific endeavour has been applied to racket sports, but this is variable across both the racket sports and the scientific disciplines. A scientific approach has helped to: implement training programmes to improve players’ fitness; guide players in nutritional and psychological preparation for play; inform players of the strategy and tactics used by themselves and their opponents; provide insight into the technical performance of skills; understand the effect of equipment on play; and accelerate the recovery from racket-arm injuries. Racket sports have also posed a unique challenge to scientists and have provided vehicles for developing scientific methodology. Racket sports provide a good model for investigating the interplay between aerobic and anaerobic metabolism and the effect of nutrition, heat and fatigue on performance. They have driven the development of mathematical solutions for multi-segment interactions within the racket arm during the performance of shots, which have contributed to our understanding of the mechanisms of both performance and injury. They have provided a unique challenge to sports engineers in relation to equipment performance and interaction with the player. Racket sports have encouraged developments in notational analysis both in terms of analytical procedures and the conceptualization of strategy and tactics. Racket sports have provided a vehicle for investigating fast interceptive actions, hand-eye coordination and perception-action coupling in the field of motor control. In conclusion, science has contributed considerably to our knowledge and understanding of racket sports, and racket sports have contributed to science by providing unique challenges to researchers.

The biomechanics of kicking in soccer: A review

Abstract Kicking is the defining action of soccer, so it is appropriate to review the scientific work that provides a basis of our understanding of this skill. The focus of this review is biomechanical in nature and builds on and extends previous reviews and overviews. While much is known about the biomechanics of the kicking leg, there are several other aspects of the kick that have been the subject of recent exploration. Researchers have widened their interest to consider the kick beginning from the way a player approaches the ball to the end of ball flight, the point that determines the success of the kick. This interest has encapsulated characteristics of overall technique and the influences of the upper body, support leg and pelvis on the kicking action, foot–ball impact and the influences of footwear and soccer balls, ball launch characteristics and corresponding flight of the ball. This review evaluates these and attempts to provide direction for future research.

A biomechanical analysis of good and poor performers of the vertical jump

The vertical jump is widely used as a field test of performance capability, particularly in games like soccer. Invariably some players perform better than others and, while this is usually put down to greater strength or ‘explosive power’, there is no detailed information to explain how the muscles around the major joints contribute to this performance and what the nature of this contribution is, or indeed whether aspects of technique are important to performance. Detailed knowledge of this type would be useful to help understand which muscle characteristics are important in successful performance of jumping and may enable insights to be gained in terms of strength training for players. The aim of this study was to investigate the contribution made by the lower limb joints to vertical jump performance by good and poor performers of the counter-movement jump. Two groups of players were selected who were found to be good and poor jumpers, respectively. Each player was required to perform three maximal vertical counter-movement jumps with, and three jumps without, an arm swing. The jump performance was recorded simultaneously by means of a force platform and a ProReflex automatic motion analysis system at 240 Hz. Values at the ankle, knee and hip were computed from these data for joint moments and power. Generally, better jumpers demonstrated greater joint moments, power and work done at the ankle, knee and hip, and as a result jumped higher under both conditions. It appears that the superior performance of the better jumpers was due to greater muscle capability in terms of strength and rate of strength development in all lower limb joints rather than to technique, which differed less noticeably between the groups. It is concluded that the muscle strength characteristics of the lower limb joints are the main determinant of vertical jump performance with technique playing a smaller role.

An application of neural networks for distinguishing gait patterns on the basis of hip-knee joint angle diagrams

In this study neural networks were applied to perform automated diagnosis of gait patterns. The three conditions of gait used were normal gait, a simulation of leg length difference, and a simulation of leg weight difference. Kinematic temporal changes were recorded by an on-line motion recording system. Hip-knee joint angle diagrams were obtained from eight subjects under the three conditions. After pre-processing, the hip-knee joint angle diagrams were presented to neural networks, which learned to distinguish the three conditions. Subsequent to training, unknown gait patterns were presented to the neural networks, which assigned those patterns into the right class with a correct assignment ratio of 83.3%. The results suggest that neural networks could be applied successfully in the automated diagnosis of gait disorders in a clinical context. Keywork Gait analysis; Neural network; Automated diagnosis; Angle-angle diagram

The functional demands on the intact limb during walking for active trans-femoral and trans-tibial amputees.

The aim of this study was to investigate the loading demands placed on the intact limb in terms of joint moments and power for active transfemoral and transtibial amputees in comparison to a group of ablebodied subjects. Four (4) transtibial, 4 transfemoral amputees and 10 ablebodied subjects walked at 1.2m.s1 along a walkway whilst kinematic data from both the intact and prosthetic limbs, and kinetic data from the intact limb only were collected. A Panasonic VHS video camera was used to film subjects walking in the sagittal plane with simultaneous force data collected from a Kistler force platform. The amputees were found to compensate for the functional loss of one or more joints by increasing net joint moments and power output on their intact limb compared to ablebodied subjects. At the intact limb ankle, the range of motion, peak dorsiflexor moment and power generation at toeoff increased. At the intact limb knee, power generation during stance and extensor moments and power absorption at toeoff increased. At the intact limb hip, extensor moment and power absorption during stance, and hip flexor moment and power generation at toeoff increased. These findings were partly attributed to the prostheses used but mainly to adaptation mechanisms displayed by transfemoral and transtibial amputees. They have implications for the mobility of amputees and the long term health of their joints. It was recommended that prosthesis design, prosthesis fitting and training in the use of the prosthesis were all factors which could be investigated with a view to minimising intact limb loading.

Understanding and measuring coordination and control in kicking skills in soccer: Implications for talent identification and skill acquisition

In this review, we explore the role of motor control and biomechanics in developing an understanding of soccer skills using kicking as the main vehicle. The links between these sub-disciplines of sport science have not been well established in the past because of an emphasis on cognitive processes in traditional accounts of motor behaviour. We argue that a dynamical systems interpretation of the processes of coordination and control in movements with multiple degrees of freedom signals a new era in the relationship between the sub-disciplines of motor control and biomechanics. Although research on coordination and control of soccer skills is currently sparse, there are indications that the relationship between motor control and biomechanics could form a significant component of scientific programmes in talent identification and skill development. Further interdisciplinary work is needed to enhance understanding of coordination and control of soccer skills.