ISOMETRIC STRENGTH MEASUREMENTS AS PREDICTORS OF
PHYSICAL PERFORMANCE IN COLLEGE MEN
by
Merek Guy, Crystal Piatt, Leah Himmelberg, Kate Ballmann, and J. L. Mayhew
Human Performance Laboratory, Truman State University, Kirksville, MO

Original Publication Information:
IAHPERD Journal Volume 30. No.1 Fall, 1996

INTRODUCTION
Strength is a fundamental quality necessary for achieving optimum physical performance. It is typically defined as the maximum amount of force that can be exerted against a resistance. However, there are several ways to measure maximal force. Currently the most common ways of measurement are isotonic, isometric, and isokinetic. The isotonic technique usually requires lifting as much weight as possible through a full range of motion. The isometric technique requires the individual to push or pull maximally against a recording device without movement taking place. The isokinetic technique controls the speed of movement during maximal contraction while measuring force application.

Most physical performances are usually dynamic and exist along a continuum from anaerobic to aerobic in nature. Anaerobic performance usually require force to be exerted quickly, while aerobic power demands a lower force generation over a longer time period. Although isometric strength measurement has been used widely, the degree to which isometric strength contributes to various performances remains controversial. The purpose of this study was to determine the relationship between commonly used isometric strength measurements and selected physical performance tests.

METHODS
Eighty-six men enrolled in a required fitness class volunteered to serve as subjects. Their physical characteristics were typical of college male students (Table 1).

Table 1. Physical And Performance Characteristics Of The Subjects (n= 86).
Varable Mean SD
Age (y) 19.5 1.6
Height (cm) 179.0 7.1
Weight (kg) 74.0 11.0
% Fat 12.5 5.9
R Grip Str (N) 478.7 70.8
L Grip Str (N) 438.0 80.2
Back Str (N) 1,379.7 250.4
Leg Str (N) 4,341.4 1,300.6
Total Str (N) 6,589.8 1,455.8
Margaria-Kalamen (W) 1,379.9 205.1
Lewis Power (W) 1,103.9 184.5
Vertical Jump (cm) 54.0 8.7
Standing Long Jump (m) 2.15 0.22
40-yd Dash (s) 5.4 0.4
VO2max (ml/kg/min) 38.5 8.8

Anaerobic power was determined from the vertical jump (VJ), Lewis power jump (LPJ), standing long jump (SLJ), Margaria-Kalamen stair run (M-K), and 40-yd dash.

Aerobic power was assessed from a VO2max test predicted from two 6-min bicycle ergometer rides. Heart rate was taken during the last two minutes of each ride, and the rides were separated by a 48-hr recovery period. The workloads and heart rates were used to estimate VO2max, and the average of the two trials was used to represent aerobic capacity (Astrand & Rodahl, 1970).

Strength was determined isometrically for right and left grip, back pull, and leg lift (Clarke, 1967). Two trials were given for each measurement, and the higher trial used for analysis. Total strength was calculated as the sum of the four measurements. Relative strength was calculated by dividing total strength by body weight.

RESULTS
All strength measurements had significant but low relationships with power measurements (Table 2). Most of the strength measurements had nonsignificant relationships with aerobic capacity. The sum of the strength measurements produced no greater correlation with power performances than did the individual strength measurements. Expressing strength relative to body weight reduced the relationships with M-K, VJ, and LPJ tests but slightly increased the correlation with SLJ. The relationship with 40-yd dash was unaltered. Leg strength had lower correlations with jump tests than did the other strength measures.

*r = 0.21 significant at p<0.05.

Table 2. Relationship Of Strength Tests
To Power Tests In College Men (n= 86).

Power Performances*
Isometric M-K LPJ VJ SLJ 40 VO2max
R Grip (N) 0.38 0.45 0.30 0.20 - 0.23 - 0.23
L Grip (N) 0.40 0.43 0.30 0.21 - 0.25 - 0.17
Back Str (N) 0.44 0.45 0.35 0.28 - 0.31 - 0.14
Leg Str (N) 0.42 0.23 0.21 0.27 - 0.45 0.16
Total Str (N) 0.49 0.32 0.28 0.31 - 0.48 0.10
Str/kg (N/kg) 0.07 0.03 0.22 0.40 - 0.49 0.04

DISCUSSION
While the isometric strength measurements used in this study were significantly related to the power performances, they accounted for no more than 24% of the common variance among the tests. This is approximately equivalent to the value noted by Wilson and Murphy (1996) in their review of isometric testing as a predictor of dynamic performance.

The angle at which isometric testing is performed is known to affect force measurement (Murphy et al., 1995). For the sake of testing convenience, isometric strength is usually measured at only one angle in the range of motion. Murphy et al. (1995) have recently suggested that the best angle to use for comparison with performance scores is the one which produces peak force. In the current study, the angles used for back lift and leg pull corresponded as nearly as possible to the optimum angle. The fact that the correlations still accounted for so small an amount of the performance variance points again to measurement specificity and the limited application of isometric testing.

Perhaps the best explanation as to why isometric testing does not account for any more of the variance in dynamic performance than it does may be neuromuscular. Recent research has shown that different neural activation patterns are present for isometric and dynamic performances (Murphy & Wilson, 1996; Ter Haar Romeny et al., 1982). Therefore, it is possible that the individual who has the better neuro-motor recruitment pattern can produce greater dynamic performance despite lower isometric strength scores.

REFERENCES