Strength training for injury prevention

STRENGTH

Strength reflects the ability to apply force and overcome resistance. It is a function of the neuro-musculo-skeletal system and closely related to muscle cross-sectional area. Its development is indispensable for success in sports performed against high resistance including combat sports, rock-climbing and rowing. It is a pre-requisite for activities with high power demands as shown by the muscular physique in top throwers, jumpers, sprinters and gymnasts. Muscularity and strength also favour performance in contact sports where the rigours of competition are more easily endured without injury. Manifestation of strength is found under various conditions.

Isometric strength

Maximum isometric strength is defined operationally as the force of reaction achieved when the greatest possible effort is intentionally brought to bear in a static muscle contraction from two to six seconds duration. The muscle length stays constant and no movement occurs. Isometric strength is important when allied to the requisite sport skills where performance is periodically under static conditions or movement initiated by overcoming large resistances. This applies in rugby scrummaging, tug-of-war, weightlifting and wrestling, for example. The hammer thrower needs to counter great centrifugal forces as he rotates and the soccer player balances statically on one leg while place-kicking.

Explosive strength

Explosive strength as defined by Fleishman (1964) denotes the ability to expend energy in one explosive act, as in jumping or projecting some object as far as possible. The force generated may be sub-maximal since force and velocity are interrelated. Muscle shortening velocity is greatest when externally unloaded while force is maximal at zero velocity or isometric conditions. In rapid contractions there is limited time for liberation of chemical energy and interconnections of myofibrilar filaments. Forces developed in eccentric contractions are greater at all velocities than those of concentric work. Power output and mechanical efficiency are improved if muscle is pre-stretched allowing storage of elastic energy for immediate utilisation in concentric action (Thys et al, 1972). This is achieved in appropriate leg musculature by sinking the hips prior to jumping and in arm muscles by winding up before bowling. As explosive strength is applied rapidly under dynamic conditions it is imperative that technique is perfected to avoid injury. Absolute strength provides inadequate compensation for poor coordination when muscle or articulo-skeletal damage is promoted. In team games explosive strength must be allied to good timing. The rugby forward must strive to outjump his line-out opponent, win ball possession and avoid collisions at the same time.

Dynamic strength

The strength of the limb muscles applied to moving or supporting body mass repeatedly over a given period of time was known as dynamic strength by Fleishman (1964). It reflects the strength endurance of the organism or its ability to withstand fatigue under sustained strength expenditure exemplified in performing press-ups (or holding maximum isometric tension to exhaustion). Dynamic strength is especially important in maximal efforts of half-minute to three minutes duration. It is distinct from muscle endurance which indicates the ability of a muscle group to contract continually against a light load and which depends on the circulation. When muscle repeatedly contracts in a vigorous fashion fatigue is shown by its inability to continue delivering the same mechanical work. Maximal power output declines more rapidly due to a shift in the substrate subserving contraction. Highest power is produced in the first few seconds of activity, values up to six horse power being found (Davies, 1971). Fuel here is furnished by intramuscular phosphagens. When all-out effort is extended beyond these first seconds energy is supplied from glycogen within muscle anaerobically. This store increases the work capacity at a reduced power production. Eventually maximal effort is limited within a minute or so of onset by accumulation of metabolites and lowered pH within muscle, or lack of metabolic substrate due to retarded adenosine triphosphate (ATP) synthesis (Gollnick and Hermansen, 1973). Under such fatigued conditions skill may become disjoined, judgement impaired and injury promoted. Even in sustained activities where the organism operates with almost limitless though subdued power supply through oxidative phosphorylation, added strength facilitates both performance and individual safety since the relative strain on working muscles imposed by a fixed work load is now reduced. The poor strength and muscularity of endurance competitors leaves them vulnerable when responding to spurts from opponents or encountering steep uphill gradients.

Strength and muscle size

In general the larger the muscle the greater its strength, though the relationship has qualifications. Differences between muscles in strength capability arise from their pennation or manner of tendonous attachment. Muscles with parallel fibres, e.g. sartorius, have poor force production but great mobility compared to muscles whose fibres converge obliquely on the tendon. Surface area of contact is increased where muscle fibres approach the tendon from one side, e.g. soleus, both sides, e.g. rectus femoris, or multi-pennately, e.g. deltoid, rather than the end. Distributions of fibre types similarly account for variability between muscles and between individuals. Thigh and calf muscles of top sprinters are endowed with high proportions of white or fast-twitch fibres compared with the abundance of red or slow-twitch fibres in endurance athletes. Responses of muscles and individuals to work-induced hypertrophy may vary because of differences in fibre-type distributions. Finally, endurance training may reduce muscle size by decreasing intramuscular lipids or temporarily increase muscle diameter by combining exhaustive training and diet to boost muscular glycogen stores as done by marathon runners the week before racing.

STRENGTH FOR INJURY PREVENTION

Muscles act as agonists, antagonists and synergists to cause, permit and assist movement. Unwanted actions are prevented by muscles acting as stabilisers. The orderliness of patterned muscular involvement is regulated by the nervous system. Greater strength in stabilising musculature improves individual tolerance to those external exigencies promoting imbalance in various sports. The gymnast needs great leg strength to control landing from a vault as well as for the more explosive operations. In field-invasive games, greater leg strength assists the manifold manoeuvres such as turning, accelerating, decelerating, tackling and avoiding tackles without injury. In contact games great roborisation is needed and players with poor strength have greater difficulty in surviving prolonged competitive periods without being injured (Reilly, 1975; Reilly and Thomas, 1977). Cahill and Griffith (1978) showed that pre-season total body conditioning significantly reduces the frequency and severity of knee injuries in American university footballers.

Imperfect strength development in particular muscle groups may predispose to local injury. This is because muscles may secure the integrity of joints by crossing the joint or having their tendons inserted around the capsule. Knee stability, for example, is considerably enhanced by strengthening the quadriceps which safeguard the joint in conjunction with the cruciate and collateral ligaments. Quadriceps exercises are especially recommended in rehabiUtation of knee injuries as the joint is further unstable if their strength subsides. Return to contact games should be delayed until adequate knee strength is restored. Quadriceps development can allow professional athletes to mask ligament or meniscus damage and osteoarthrosis (Smilhe, 1970). Similarly, shoulder stability is enhanced by training surrounding musculature and many back problems are avoided by developing the erector spinae. Improving the strength of abdominal and back muscles not only helps prevent lumbar pain but serves also to remedy low back pain conditions (Corbin, 1971). Bodnar (1977) reported a relationship between the incidence of shoulder and neck injuries and weakness of the cervical muscles in American college footballers and recommended strengthening cervical and trunk muscles as protective measures.

Uneven distribution of strength is likely to predispose to injury. This may be manifested in disproportionate development in one of the agonist/ antagonist pair. The hamstrings, antagonists to the quadriceps during the first 160 to 165 ° of leg extension, must equally be considered when devising training programmes for quadriceps. A greater susceptibility to hamstring strains is found when individuals have an inappropriate flexor-extensor strength ratio (Burkett, 1970). The hamstring/quadriceps strength ratio recommended by Klein and Allman (1969) is 0.60.

Uneven distribution is also evident in contra-lateral differences in strength acquisition. This inequality is seen in the girth of the serving arm in top tennis players compared with the non-playing limb and is a logical outcome of repetitive practice in throwing events and racket sports which impose unilateral demands. Imbalance can have serious consequences in the lower extremities as it is then likely to prompt assymetrical locomotion. Liemohn (1978) found that all hamstring injuries in a group of track and field athletes were to the non-dominant leg, substantiating that the weaker side is the more susceptible. Though specific unilateral demands are imposed in certain athletic performances, preparation should still involve bilateral strength training to avoid uneven gains. Some contra-lateral effects do occur in the non-exercised limb but are of insufficient magnitude to excuse not training both. Cross-training effects can however be exploited to reduce muscle atrophy and for reconditioning after immobilisation. Isotonic exercise has been found preferable to isometric for producing cross-training (Clarke, 1973) though both methods have shown equal effectiveness when equated for load and duration (Coleman, 1969). Action potentials occur in contralateral musculature, demonstrating the irradiation of nerve impulses to extremities other than those directly engaged in activity (Bowers, 1966). Seemingly, innervation of descending pyramidal fibres that cross sides before supplying muscles overflows to the smaller number of uncrossed fibres, so that movement occurs in an activated muscle and isometric contractions in its symmetrical partner. Sufficient overflow occurs only under strong volition so high intensity is desirable for cross-education (De Vries, 1977). The effect is supported by histological findings (Reitsma, 1969) and underlines nervous system involvement in force application.

Neural control of maximal voluntary effort results in a strength reserve not normally expressed. This reservoir may amount to 25 per cent and is released in exceptional circumstances and emergencies. It serves normally to protect the muscle, its tendon and bony attachment from over-exertion. This inhibition is eliminated under extreme motivational conditions (Ikai and Steinhaus, 1961). The effect is attributed to adrenalin release and reduced nervous inhibition which permit greater motor unit recruitment. This safety reserve is harnessed under strong motivation by athletes prior to strength events. It may also be tapped in training muscle by electrical stimulation though attendant risks make it inadvisable. Electrotherapy is valuable in rehabilitation to stimulate muscle which has temporarily lost its effective innervation since the motor pathways are minimally involved in electrical training. Strength training reduces the effect of the inhibitory mechanism and permits greater tension to occur. This may be due to greater antagonist relaxation as the movement proceeds or to increased shielding of Golgi sensory organs by muscle connective tissue thickened from training, allowing innervation of the muscles to progress further uninhibited.

A basic programme of general muscle conditioning is advocated to avoid leaving local weaknesses in the ath- lete’s make-up. A broad basis of general strength sets a foundation on which specific strength can be safely developed. Improved arm and back strength allows the legs to be overloaded in power cleans and half-squats when leg strength is being specifically developed. An outstanding single benefit to a strengthened muscle is that greater stresses can be borne without tearing. Developed muscle also provides a fleshy shield to cushion the effect of blows on bone or abdominal organs, thereby affording some protection in combat sports.

STRENGTH TRAINING Law of use and disuse

The organism habituates to the load placed on it indicating that structure is modified by function. Connective tissue and bone adapt as well as skeletal muscle. The muscle fibre and its surrounding sarcolemma thicken and the connective tissue of the muscle broadens and toughens. Primary cellular multiplication occurs in ligaments and tendons which grow and increase in tensile strength. Bones adapt to suit stresses and strains within their tolerance by forming new supportive trabeculae. Calcium phosphate and calcium carbonate in bone increase in response to training providing greater sturdiness. Severe repetitive strain may result in inflammation and osteoporosis as occurs in stress fractures. With prolonged rest, bones de-mineralise and weaken and muscles atrophy. The trainer’s quandary is the recognition of the thin line separating harmful overload from the maximal stimulant for physiological accommodation. The triple principles of overload, reversibility and specificity contain a framework for regulating training regimes to permit continuous adaptation.

Principle of overload

The principle of overload indicates that physiological systems improve in function when challenged to work at supra-normal levels. Muscle hypertrophies only when forced to operate beyond customary intensities. The stimulus threshold represents the work intensity below which no effect accrues. The ability to train muscle is greatest when strength training begins though some muscles may be in poorer condition than others and respond more readily. The load must be progressively increased to elicit further adaptive reactions as training develops and the training stimulus gradually raised. Gains become increasingly difficult to obtain as the muscle approaches its theoretical potential.

Overload is accomplished by emphasising the duration or intensity of exercise. Many repetitions of low intensity work improve muscular endurance while few repetitions at high intensity develop strength. High intensity work at speed promotes muscular power as well as strength. Many sportsmen wish to develop strength without accompanying hypertrophy. Boxers, wrestlers and oarsmen want to avoid excessive increases in muscle bulk which necessitate moving to a higher competitive weight category. Belated attempts to shed weight by dehydration are counter-productive since strength abates with fluid loss. Hurdlers and jumpers whose extra muscle constitutes an additional load to be lifted against gravity similarly seek to avoid undue weight gains. Alternatively throwers and rugby forwards can utilise beneficially the extra mass acquired. Though the demarcation line for non-hypertrophy is unclear, speed of contraction appears important in reducing the extent of hypertrophy. Experience suggests that repeated sets of four to six repetitions at maximal intensity invokes hypertrophy while fewer sets of six to ten repetitions short of maximal effort promotes strength without concomitant muscle growth. Maximum can be conveniently determined by De Lorme’s (1945) repetition maximum (RM) criterion which indicates the maximum load that can be lifted a given number of repetitions and varies with the number attempted. MacQueen (1954) reported that when body builders employed less than RM intensity for considerably more than 10 repetitions and more than four or five sets hypertrophy did not result. Hypertrophy is regulated largely by testosterone which explains why pronounced muscle growth is not found when female athletes undertake severe strength training regimes (Brown and Wilmore, 1974). Hypertrophy is observed when training is supplemented by synthetic sex hormones or anabolic steroids.

Bouts of progressive exercise overload should be interspersed by regular rest periods to allow recovery. The common practice of weight training three or four times a week seems reasonable to avoid overuse but a minimum of twice-weekly strength sessions is recommended to provide adequate training stimulus. Ryan (1968) acknowledged insufficient rest as a factor contributing to injury and intermittent illness. De Lorme and Watkins (1948) found that five days per week was usually the heaviest schedule that could be employed without developing serious signs of delayed recovery. Clarke (1973) concluded that training at 6RM for three sets, three times a week seemed an optimal combination. Progression involves an evolving upward spiral of overload-fatigue-recovery, with recovery improving as the individual gets fitter. Frequency and intensity may be reduced during the peak of the competitive season when the objective is to maintain the strength level already acquired and concentrate on speed and skill.

The principle of reversibility

Reversibility indicates that a training effect subsides if training is discontinued. Strength gains are lost at about one-third their rate of acquisition (Jensen and Fisher, 1972). Athletes who abruptly terminate a crash course of pre-season strength training are likely to lose form in mid-season due to the relatively rapid strength loss. Isotonic training leads to greater retention than isometric (Rasch and Morehouse, 1957). Strength loss after daily isometric training equalled the rate at which it was acquired in Muller’s (1959) investigations. Hettinger (1961) and Morehouse (1967) claimed that strength gained can be retained with one high intensity session every two weeks. At least a weekly work-out seems preferable for retention in top athletes through the competitive period though fortnightly sessions probably serve to retard strength loss. Strength gained is not entirely lost, a portion of it remaining indefinitely (McMorris and Elkins, 1954). Retraining, when commenced, is also considerably easier than initially (Waldeman and Stull, 1959). Special care is needed during the early weeks of retraining to avoid muscle pulls as athletes are often reluctant to accept their diminished function through inactivity.

The principle of specificity

Specificity suggests that strength training effects may be limited to the pattern of muscular involvement in the conditioning exercise. Different types of motor units exist within muscle so that a given type of exercise recruits a specific combination of motor units best adapted to that demand. It is desirable that strength training routines should simulate closely the motor programme of the event and employ the specific muscle groups involved. This invites preliminary analysis of competitive skills. Specificity is also shown in isometric exercise in that strength gains may be restricted to the angle at which training occurred (Belka, 1968), so that isometrics should be undertaken at a series of joint angles. Where great strength is needed only at the beginning of the movement, as in ballistic actions, exercises in that range may suffice. Specificity is further corroborated in that isometric programmes increase isometric strength more than they do isotonic strength, the reverse being also true (Berger, 1962). A strength training schedule should evidently be planned to suit individual needs. This must vary with the sport and be adjusted for age and sex. The curves showing average strength values in relation to age and sex can be altered by training which not only arrests the normal gradual decline after mid-20s but brings further improvement for another 10 to 15 years. Though the difference between the sexes varies with muscle groups, it is common after adolescence to expect two-thirds of male strength in females. The difference is largely attributable to men’s greater muscle mass though on average their bones, tendons and ligaments are also more robustly constructed. Female athletes are often unwilling to undertake strength regimes for social reasons and in consequence suffer more injuries in sports requiring explosive efforts (Klaus, 1964).

Evaluation and measurement

Regular strength testing provides an objective basis for rescheduling regimes to apply progressive resistance. It serves also to evaluate the conditioning programme and give valuable feedback on the athlete’s progress. It helps to identify weaknesses and devise individual schedules.

Performance tests provide convenient criteria for strength measurement. Selected weight lifts can be used for 1RM, 6RM or 10RM. Tests involving moving a heavy load to exhaustion are also employed since muscular strength and absolute endurance are highly correlated (Baumgartner and Jackson, 1975). Strength measurement has typically employed instruments capable of recording the. maximum force of an isometric contraction. These include cable tensio-meters, dynamometers, spring balances and electrical strain gauges (Clarke, 1967) and administration is hardly feasible outside a sports science laboratory. Peak force during dynamic movements may be recorded using isokinetic machinery. Static and dynamic measurement of rotational movements can be achieved by a dynamometer based on a rotary torque activator (Jensen, 1976).

The vertical and standing broad jumps conveniently indicate explosive strength and leg power. The softball throw or shot putt for distance have been used as indicants of muscle power in arm work. As power is a function of the rate of force application, strength is a major component. A power lever, based on a wheel and axle lever system (Glencross, 1966), permits direct measurement of the horsepower in a single explosive movement. The stair run test of Margaria et al (1966) was designed to indicate the horsepower output of the phosphagens or maximum alactic anaerobic power. A comparable test for the bicycle ergometer has also been designed (Pirnay and Crielaard, 1978). Though these tests may be used for monitoring conditioning progress, the ultimate test of the validity of the training is the competitive performance.

STRENGTH TRAINING METHODS

Isometrics

Isometrics implicate muscular tension without corresponding limb movement. Body segments or external objects may provide the resistance. Special isometric racks have been designed for use in diverse fashions in gymnasia. The scrummaging rack used by rugby forwards in training outdoors affords another example.

Isometric programmes gained widespread appeal from the pioneer studies of Muller (1957). The magnitude of the training effects reported initially were not replicated in later studies of fitter subjects. For these the work load had to be substantially elevated beyond the advocated daily contraction held for six seconds at 66 per cent of maximal isometric capacity to get similar results. Consequently the claims attributed earlier to isometrics were considerably qualified. Furthermore improvement may be restricted to the angle at which the contraction is held and this may not appreciably assist dynamic performance. Thirdly, the concomitant compression of the vascular bed with occlusion of blood supply to the muscles under tension and the rise in blood pressure this precipitates make isometrics unsuitable for sedentary individuals undergoing exercise for prevention of coronary heart disease, particularly if they already possess predisposing factors.

Isometric training is recommended where competition places specific demands for isometric strength. It may be especially valuable to athletes desiring increased muscle strength without corresponding hypertrophy since isometrics do not provoke muscle growth (Falls et al, 1970). For the majority, isometrics can be judiciously incorporated into a broader conditioning programme.

Mechanical and physical resistances

Weight training presents a convenient method of overloading muscle and is commonly employed in conditioning for competition. Loads are provided by dumb-bells or weighted barbells and are easily adjustable for progressive resistance. Eccentric work can be incorporated by weight lowering in addition to the more usual lifting exercises so elastic as well as contractile elements in muscle are trained. Spring loaded devices allow eccentric arm work though they do not have inherent progressive resistance.

Pulley systems permit alteration of the direction of force application and are frequently employed in land conditioning of swimmers. Hydraulics have been effectively incorporated in the design of rowing machines. Isokinetic equipment allows constant speed of contraction and maximal force exertion throughout the complete range of movement. Multi-station apparatus affords an opportunity of training various muscle groups and engaging a squad of players on a single machine.

Strength training can employ partner work if equipment is unavailable. Routines may involve pushing against an opponent’s shoulder who reciprocates or performing half-squats with partner supported supine on the back. A whole team can be occupied in relay races, each member carrying a colleague on his back during his effort, adding variety to training. Resistance work using a partner is useful in sports where a partner or opponent is actually supported during performance as in ice-skating or wrestling. Where facilities are impoverished, a bench, box or chair is still easily obtained for step-ups. This can be useful where different strength requirements are juxtaposed. The long jumper, for instance, who needs great leg strength for take off and isometric back strength to stabilise his upper body in flight, can train both simultaneously by holding a medicine ball while stepping-up.

Bench stepping has long been used for fitness testing as well as training as it permits calculation of mechanical work done. Other ergometric modes present suitable strength training methods under precisely controlled conditions. Cyclists might use friction braked bicycles which are also valuable in rehabilitation since body weight is not supported. Whole body and rowing ergometers are useful in conditioning oarsmen while motor driven treadmills can be modified for skiers. Flumes could be used for preparation of swimmers, canoeists and rowers under exact control. These facilities are expensive and could reasonably be made available only to the elite at centres of sporting excellence.

Natural resistance

Muscles may be overloaded in working against supra-normal resistance by carefully exploiting natural conditions. Running up sand dunes was vindicated in the performances of Australian middle-distance runners in the early 1960s. Flat soled shoes are recommended as barefoot athletes may tread on broken glass concealed in the sand. Weekly sessions are sufficient in incorporating sandhill training into the overall schedule in the early phase to avoid Achilles tendon trouble. Running along a flat beach or soft sand is another form of resistance training. In inland areas grassy hills provide a satisfactory alternative to sand dunes .

Running on soft ground is also recommended provided conditions are not so slippery that hamstring tears are possible. All these methods are eminently suitable for training dynamic strength. Other variations depending on climate and location include running on snow or ankle deep in water, as long as the normal running motion is not severely altered. Work in water may be especially useful for rehabilitation of leg muscles as the strain from lifting bodyweight against gravity is absent. Cyclists may take advantage of otherwise inclement windy conditions by repetitive short sprints against the wind.

Man built environments may similarly be exploited. If conditions are too difficult for outdoor work, suitable indoor staircases can substitute for uphill gradients. Stadium terrace steps are frequently used by football coaches in the training of players.

Functional overload

Strength can be improved by overloading the individual in practices closely related to competition requirements. Running in heavy boots, jumping with ankle weights or jackets weighted with lead are examples. The obvious advantage of this approach is the high likelihood of strength transfer to the event trained for.

Functional overload can be applied to runners either by attachment of a harness held by a trainer at its ends , or drawing a sled. The vigorous running action needed should not be greatly modified from normal for optimal effects. Wrist weights used by gymnasts are another example while weight throwers use excessively heavy implements or attempt explosive release of medicine balls.

Various forms of rebounding and plyometric drills fall within this category. These emphasise elastic properties of muscle in their execution and tend to develop explosive strength. Drills include repetitive hopping on both legs together or separately, jumping decathlons or exaggerated bounding strides. In hopping, the hips should not sink below the level where the femur is parallel to the floor to safeguard the knee joint. Depth jumping drills involve explosive elevation onto or over a bench or box top on descent from a greater height. The quadriceps are stretched in lowering bodyweight prior to contracting powerfully and by storing elastic energy in the process increase the tension developed in positive work (Cavagna et al, 1968). Since the shock on landing is absorbed largely by the knee joints the complete drill must be smoothly controlled to avoid injury.

Strength training is considered an indispensable part of preparation for athletic competition and injury prevention. The programme should be geared to the individual’s specific needs and be based on progressive resistance. General and specific conditioning should be developed and uneven strength gains avoided. Retraining during rehabilitation is important before returning to competition. Testing provides a sound basis for monitoring progress and evaluating the programme. Strength training may be tapered during the competitive season in many sports to retain the capacity already developed so as to allow greater emphasis on technique and tactical aspects.

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